commit-reach(repo_in_merge_bases_many): report missing commits

[thirdparty/git.git] / merge-recursive.c

/*
 * Recursive Merge algorithm stolen from git-merge-recursive.py by
 * Fredrik Kuivinen.
 * The thieves were Alex Riesen and Johannes Schindelin, in June/July 2006
 */
#include "git-compat-util.h"
#include "merge-recursive.h"

#include "alloc.h"
#include "cache-tree.h"
#include "commit.h"
#include "commit-reach.h"
#include "config.h"
#include "diff.h"
#include "diffcore.h"
#include "dir.h"
#include "environment.h"
#include "gettext.h"
#include "hex.h"
#include "merge-ll.h"
#include "lockfile.h"
#include "match-trees.h"
#include "name-hash.h"
#include "object-file.h"
#include "object-name.h"
#include "object-store-ll.h"
#include "path.h"
#include "repository.h"
#include "revision.h"
#include "sparse-index.h"
#include "string-list.h"
#include "symlinks.h"
#include "tag.h"
#include "tree-walk.h"
#include "unpack-trees.h"
#include "xdiff-interface.h"

struct merge_options_internal {
        int call_depth;
        int needed_rename_limit;
        struct hashmap current_file_dir_set;
        struct string_list df_conflict_file_set;
        struct unpack_trees_options unpack_opts;
        struct index_state orig_index;
};

struct path_hashmap_entry {
        struct hashmap_entry e;
        char path[FLEX_ARRAY];
};

static int path_hashmap_cmp(const void *cmp_data UNUSED,
                            const struct hashmap_entry *eptr,
                            const struct hashmap_entry *entry_or_key,
                            const void *keydata)
{
        const struct path_hashmap_entry *a, *b;
        const char *key = keydata;

        a = container_of(eptr, const struct path_hashmap_entry, e);
        b = container_of(entry_or_key, const struct path_hashmap_entry, e);

        return fspathcmp(a->path, key ? key : b->path);
}

/*
 * For dir_rename_entry, directory names are stored as a full path from the
 * toplevel of the repository and do not include a trailing '/'.  Also:
 *
 *   dir:                original name of directory being renamed
 *   non_unique_new_dir: if true, could not determine new_dir
 *   new_dir:            final name of directory being renamed
 *   possible_new_dirs:  temporary used to help determine new_dir; see comments
 *                       in get_directory_renames() for details
 */
struct dir_rename_entry {
        struct hashmap_entry ent;
        char *dir;
        unsigned non_unique_new_dir:1;
        struct strbuf new_dir;
        struct string_list possible_new_dirs;
};

static struct dir_rename_entry *dir_rename_find_entry(struct hashmap *hashmap,
                                                      char *dir)
{
        struct dir_rename_entry key;

        if (!dir)
                return NULL;
        hashmap_entry_init(&key.ent, strhash(dir));
        key.dir = dir;
        return hashmap_get_entry(hashmap, &key, ent, NULL);
}

static int dir_rename_cmp(const void *cmp_data UNUSED,
                          const struct hashmap_entry *eptr,
                          const struct hashmap_entry *entry_or_key,
                          const void *keydata UNUSED)
{
        const struct dir_rename_entry *e1, *e2;

        e1 = container_of(eptr, const struct dir_rename_entry, ent);
        e2 = container_of(entry_or_key, const struct dir_rename_entry, ent);

        return strcmp(e1->dir, e2->dir);
}

static void dir_rename_init(struct hashmap *map)
{
        hashmap_init(map, dir_rename_cmp, NULL, 0);
}

static void dir_rename_entry_init(struct dir_rename_entry *entry,
                                  char *directory)
{
        hashmap_entry_init(&entry->ent, strhash(directory));
        entry->dir = directory;
        entry->non_unique_new_dir = 0;
        strbuf_init(&entry->new_dir, 0);
        string_list_init_nodup(&entry->possible_new_dirs);
}

struct collision_entry {
        struct hashmap_entry ent;
        char *target_file;
        struct string_list source_files;
        unsigned reported_already:1;
};

static struct collision_entry *collision_find_entry(struct hashmap *hashmap,
                                                    char *target_file)
{
        struct collision_entry key;

        hashmap_entry_init(&key.ent, strhash(target_file));
        key.target_file = target_file;
        return hashmap_get_entry(hashmap, &key, ent, NULL);
}

static int collision_cmp(const void *cmp_data UNUSED,
                         const struct hashmap_entry *eptr,
                         const struct hashmap_entry *entry_or_key,
                         const void *keydata UNUSED)
{
        const struct collision_entry *e1, *e2;

        e1 = container_of(eptr, const struct collision_entry, ent);
        e2 = container_of(entry_or_key, const struct collision_entry, ent);

        return strcmp(e1->target_file, e2->target_file);
}

static void collision_init(struct hashmap *map)
{
        hashmap_init(map, collision_cmp, NULL, 0);
}

static void flush_output(struct merge_options *opt)
{
        if (opt->buffer_output < 2 && opt->obuf.len) {
                fputs(opt->obuf.buf, stdout);
                strbuf_reset(&opt->obuf);
        }
}

__attribute__((format (printf, 2, 3)))
static int err(struct merge_options *opt, const char *err, ...)
{
        va_list params;

        if (opt->buffer_output < 2)
                flush_output(opt);
        else {
                strbuf_complete(&opt->obuf, '\n');
                strbuf_addstr(&opt->obuf, "error: ");
        }
        va_start(params, err);
        strbuf_vaddf(&opt->obuf, err, params);
        va_end(params);
        if (opt->buffer_output > 1)
                strbuf_addch(&opt->obuf, '\n');
        else {
                error("%s", opt->obuf.buf);
                strbuf_reset(&opt->obuf);
        }

        return -1;
}

static struct tree *shift_tree_object(struct repository *repo,
                                      struct tree *one, struct tree *two,
                                      const char *subtree_shift)
{
        struct object_id shifted;

        if (!*subtree_shift) {
                shift_tree(repo, &one->object.oid, &two->object.oid, &shifted, 0);
        } else {
                shift_tree_by(repo, &one->object.oid, &two->object.oid, &shifted,
                              subtree_shift);
        }
        if (oideq(&two->object.oid, &shifted))
                return two;
        return lookup_tree(repo, &shifted);
}

static inline void set_commit_tree(struct commit *c, struct tree *t)
{
        c->maybe_tree = t;
}

static struct commit *make_virtual_commit(struct repository *repo,
                                          struct tree *tree,
                                          const char *comment)
{
        struct commit *commit = alloc_commit_node(repo);

        set_merge_remote_desc(commit, comment, (struct object *)commit);
        set_commit_tree(commit, tree);
        commit->object.parsed = 1;
        return commit;
}

enum rename_type {
        RENAME_NORMAL = 0,
        RENAME_VIA_DIR,
        RENAME_ADD,
        RENAME_DELETE,
        RENAME_ONE_FILE_TO_ONE,
        RENAME_ONE_FILE_TO_TWO,
        RENAME_TWO_FILES_TO_ONE
};

/*
 * Since we want to write the index eventually, we cannot reuse the index
 * for these (temporary) data.
 */
struct stage_data {
        struct diff_filespec stages[4]; /* mostly for oid & mode; maybe path */
        struct rename_conflict_info *rename_conflict_info;
        unsigned processed:1;
};

struct rename {
        unsigned processed:1;
        struct diff_filepair *pair;
        const char *branch; /* branch that the rename occurred on */
        /*
         * If directory rename detection affected this rename, what was its
         * original type ('A' or 'R') and it's original destination before
         * the directory rename (otherwise, '\0' and NULL for these two vars).
         */
        char dir_rename_original_type;
        char *dir_rename_original_dest;
        /*
         * Purpose of src_entry and dst_entry:
         *
         * If 'before' is renamed to 'after' then src_entry will contain
         * the versions of 'before' from the merge_base, HEAD, and MERGE in
         * stages 1, 2, and 3; dst_entry will contain the respective
         * versions of 'after' in corresponding locations.  Thus, we have a
         * total of six modes and oids, though some will be null.  (Stage 0
         * is ignored; we're interested in handling conflicts.)
         *
         * Since we don't turn on break-rewrites by default, neither
         * src_entry nor dst_entry can have all three of their stages have
         * non-null oids, meaning at most four of the six will be non-null.
         * Also, since this is a rename, both src_entry and dst_entry will
         * have at least one non-null oid, meaning at least two will be
         * non-null.  Of the six oids, a typical rename will have three be
         * non-null.  Only two implies a rename/delete, and four implies a
         * rename/add.
         */
        struct stage_data *src_entry;
        struct stage_data *dst_entry;
};

struct rename_conflict_info {
        enum rename_type rename_type;
        struct rename *ren1;
        struct rename *ren2;
};

static inline void setup_rename_conflict_info(enum rename_type rename_type,
                                              struct merge_options *opt,
                                              struct rename *ren1,
                                              struct rename *ren2)
{
        struct rename_conflict_info *ci;

        /*
         * When we have two renames involved, it's easiest to get the
         * correct things into stage 2 and 3, and to make sure that the
         * content merge puts HEAD before the other branch if we just
         * ensure that branch1 == opt->branch1.  So, simply flip arguments
         * around if we don't have that.
         */
        if (ren2 && ren1->branch != opt->branch1) {
                setup_rename_conflict_info(rename_type, opt, ren2, ren1);
                return;
        }

        CALLOC_ARRAY(ci, 1);
        ci->rename_type = rename_type;
        ci->ren1 = ren1;
        ci->ren2 = ren2;

        ci->ren1->dst_entry->processed = 0;
        ci->ren1->dst_entry->rename_conflict_info = ci;
        if (ren2) {
                ci->ren2->dst_entry->rename_conflict_info = ci;
        }
}

static int show(struct merge_options *opt, int v)
{
        return (!opt->priv->call_depth && opt->verbosity >= v) ||
                opt->verbosity >= 5;
}

__attribute__((format (printf, 3, 4)))
static void output(struct merge_options *opt, int v, const char *fmt, ...)
{
        va_list ap;

        if (!show(opt, v))
                return;

        strbuf_addchars(&opt->obuf, ' ', opt->priv->call_depth * 2);

        va_start(ap, fmt);
        strbuf_vaddf(&opt->obuf, fmt, ap);
        va_end(ap);

        strbuf_addch(&opt->obuf, '\n');
        if (!opt->buffer_output)
                flush_output(opt);
}

static void repo_output_commit_title(struct merge_options *opt,
                                     struct repository *repo,
                                     struct commit *commit)
{
        struct merge_remote_desc *desc;

        strbuf_addchars(&opt->obuf, ' ', opt->priv->call_depth * 2);
        desc = merge_remote_util(commit);
        if (desc)
                strbuf_addf(&opt->obuf, "virtual %s\n", desc->name);
        else {
                strbuf_repo_add_unique_abbrev(&opt->obuf, repo,
                                              &commit->object.oid,
                                              DEFAULT_ABBREV);
                strbuf_addch(&opt->obuf, ' ');
                if (repo_parse_commit(repo, commit) != 0)
                        strbuf_addstr(&opt->obuf, _("(bad commit)\n"));
                else {
                        const char *title;
                        const char *msg = repo_get_commit_buffer(repo, commit, NULL);
                        int len = find_commit_subject(msg, &title);
                        if (len)
                                strbuf_addf(&opt->obuf, "%.*s\n", len, title);
                        repo_unuse_commit_buffer(repo, commit, msg);
                }
        }
        flush_output(opt);
}

static void output_commit_title(struct merge_options *opt, struct commit *commit)
{
        repo_output_commit_title(opt, the_repository, commit);
}

static int add_cacheinfo(struct merge_options *opt,
                         const struct diff_filespec *blob,
                         const char *path, int stage, int refresh, int options)
{
        struct index_state *istate = opt->repo->index;
        struct cache_entry *ce;
        int ret;

        ce = make_cache_entry(istate, blob->mode, &blob->oid, path, stage, 0);
        if (!ce)
                return err(opt, _("add_cacheinfo failed for path '%s'; merge aborting."), path);

        ret = add_index_entry(istate, ce, options);
        if (refresh) {
                struct cache_entry *nce;

                nce = refresh_cache_entry(istate, ce,
                                          CE_MATCH_REFRESH | CE_MATCH_IGNORE_MISSING);
                if (!nce)
                        return err(opt, _("add_cacheinfo failed to refresh for path '%s'; merge aborting."), path);
                if (nce != ce)
                        ret = add_index_entry(istate, nce, options);
        }
        return ret;
}

static inline int merge_detect_rename(struct merge_options *opt)
{
        return (opt->detect_renames >= 0) ? opt->detect_renames : 1;
}

static void init_tree_desc_from_tree(struct tree_desc *desc, struct tree *tree)
{
        parse_tree(tree);
        init_tree_desc(desc, tree->buffer, tree->size);
}

static int unpack_trees_start(struct merge_options *opt,
                              struct tree *common,
                              struct tree *head,
                              struct tree *merge)
{
        int rc;
        struct tree_desc t[3];
        struct index_state tmp_index = INDEX_STATE_INIT(opt->repo);

        memset(&opt->priv->unpack_opts, 0, sizeof(opt->priv->unpack_opts));
        if (opt->priv->call_depth)
                opt->priv->unpack_opts.index_only = 1;
        else {
                opt->priv->unpack_opts.update = 1;
                /* FIXME: should only do this if !overwrite_ignore */
                opt->priv->unpack_opts.preserve_ignored = 0;
        }
        opt->priv->unpack_opts.merge = 1;
        opt->priv->unpack_opts.head_idx = 2;
        opt->priv->unpack_opts.fn = threeway_merge;
        opt->priv->unpack_opts.src_index = opt->repo->index;
        opt->priv->unpack_opts.dst_index = &tmp_index;
        opt->priv->unpack_opts.aggressive = !merge_detect_rename(opt);
        setup_unpack_trees_porcelain(&opt->priv->unpack_opts, "merge");

        init_tree_desc_from_tree(t+0, common);
        init_tree_desc_from_tree(t+1, head);
        init_tree_desc_from_tree(t+2, merge);

        rc = unpack_trees(3, t, &opt->priv->unpack_opts);
        cache_tree_free(&opt->repo->index->cache_tree);

        /*
         * Update opt->repo->index to match the new results, AFTER saving a
         * copy in opt->priv->orig_index.  Update src_index to point to the
         * saved copy.  (verify_uptodate() checks src_index, and the original
         * index is the one that had the necessary modification timestamps.)
         */
        opt->priv->orig_index = *opt->repo->index;
        *opt->repo->index = tmp_index;
        opt->priv->unpack_opts.src_index = &opt->priv->orig_index;

        return rc;
}

static void unpack_trees_finish(struct merge_options *opt)
{
        discard_index(&opt->priv->orig_index);
        clear_unpack_trees_porcelain(&opt->priv->unpack_opts);
}

static int save_files_dirs(const struct object_id *oid UNUSED,
                           struct strbuf *base, const char *path,
                           unsigned int mode, void *context)
{
        struct path_hashmap_entry *entry;
        int baselen = base->len;
        struct merge_options *opt = context;

        strbuf_addstr(base, path);

        FLEX_ALLOC_MEM(entry, path, base->buf, base->len);
        hashmap_entry_init(&entry->e, fspathhash(entry->path));
        hashmap_add(&opt->priv->current_file_dir_set, &entry->e);

        strbuf_setlen(base, baselen);
        return (S_ISDIR(mode) ? READ_TREE_RECURSIVE : 0);
}

static void get_files_dirs(struct merge_options *opt, struct tree *tree)
{
        struct pathspec match_all;
        memset(&match_all, 0, sizeof(match_all));
        read_tree(opt->repo, tree,
                  &match_all, save_files_dirs, opt);
}

static int get_tree_entry_if_blob(struct repository *r,
                                  const struct object_id *tree,
                                  const char *path,
                                  struct diff_filespec *dfs)
{
        int ret;

        ret = get_tree_entry(r, tree, path, &dfs->oid, &dfs->mode);
        if (S_ISDIR(dfs->mode)) {
                oidcpy(&dfs->oid, null_oid());
                dfs->mode = 0;
        }
        return ret;
}

/*
 * Returns an index_entry instance which doesn't have to correspond to
 * a real cache entry in Git's index.
 */
static struct stage_data *insert_stage_data(struct repository *r,
                const char *path,
                struct tree *o, struct tree *a, struct tree *b,
                struct string_list *entries)
{
        struct string_list_item *item;
        struct stage_data *e = xcalloc(1, sizeof(struct stage_data));
        get_tree_entry_if_blob(r, &o->object.oid, path, &e->stages[1]);
        get_tree_entry_if_blob(r, &a->object.oid, path, &e->stages[2]);
        get_tree_entry_if_blob(r, &b->object.oid, path, &e->stages[3]);
        item = string_list_insert(entries, path);
        item->util = e;
        return e;
}

/*
 * Create a dictionary mapping file names to stage_data objects. The
 * dictionary contains one entry for every path with a non-zero stage entry.
 */
static struct string_list *get_unmerged(struct index_state *istate)
{
        struct string_list *unmerged = xmalloc(sizeof(struct string_list));
        int i;

        string_list_init_dup(unmerged);

        /* TODO: audit for interaction with sparse-index. */
        ensure_full_index(istate);
        for (i = 0; i < istate->cache_nr; i++) {
                struct string_list_item *item;
                struct stage_data *e;
                const struct cache_entry *ce = istate->cache[i];
                if (!ce_stage(ce))
                        continue;

                item = string_list_lookup(unmerged, ce->name);
                if (!item) {
                        item = string_list_insert(unmerged, ce->name);
                        item->util = xcalloc(1, sizeof(struct stage_data));
                }
                e = item->util;
                e->stages[ce_stage(ce)].mode = ce->ce_mode;
                oidcpy(&e->stages[ce_stage(ce)].oid, &ce->oid);
        }

        return unmerged;
}

static int string_list_df_name_compare(const char *one, const char *two)
{
        int onelen = strlen(one);
        int twolen = strlen(two);
        /*
         * Here we only care that entries for D/F conflicts are
         * adjacent, in particular with the file of the D/F conflict
         * appearing before files below the corresponding directory.
         * The order of the rest of the list is irrelevant for us.
         *
         * To achieve this, we sort with df_name_compare and provide
         * the mode S_IFDIR so that D/F conflicts will sort correctly.
         * We use the mode S_IFDIR for everything else for simplicity,
         * since in other cases any changes in their order due to
         * sorting cause no problems for us.
         */
        int cmp = df_name_compare(one, onelen, S_IFDIR,
                                  two, twolen, S_IFDIR);
        /*
         * Now that 'foo' and 'foo/bar' compare equal, we have to make sure
         * that 'foo' comes before 'foo/bar'.
         */
        if (cmp)
                return cmp;
        return onelen - twolen;
}

static void record_df_conflict_files(struct merge_options *opt,
                                     struct string_list *entries)
{
        /* If there is a D/F conflict and the file for such a conflict
         * currently exists in the working tree, we want to allow it to be
         * removed to make room for the corresponding directory if needed.
         * The files underneath the directories of such D/F conflicts will
         * be processed before the corresponding file involved in the D/F
         * conflict.  If the D/F directory ends up being removed by the
         * merge, then we won't have to touch the D/F file.  If the D/F
         * directory needs to be written to the working copy, then the D/F
         * file will simply be removed (in make_room_for_path()) to make
         * room for the necessary paths.  Note that if both the directory
         * and the file need to be present, then the D/F file will be
         * reinstated with a new unique name at the time it is processed.
         */
        struct string_list df_sorted_entries = STRING_LIST_INIT_NODUP;
        const char *last_file = NULL;
        int last_len = 0;
        int i;

        /*
         * If we're merging merge-bases, we don't want to bother with
         * any working directory changes.
         */
        if (opt->priv->call_depth)
                return;

        /* Ensure D/F conflicts are adjacent in the entries list. */
        for (i = 0; i < entries->nr; i++) {
                struct string_list_item *next = &entries->items[i];
                string_list_append(&df_sorted_entries, next->string)->util =
                                   next->util;
        }
        df_sorted_entries.cmp = string_list_df_name_compare;
        string_list_sort(&df_sorted_entries);

        string_list_clear(&opt->priv->df_conflict_file_set, 1);
        for (i = 0; i < df_sorted_entries.nr; i++) {
                const char *path = df_sorted_entries.items[i].string;
                int len = strlen(path);
                struct stage_data *e = df_sorted_entries.items[i].util;

                /*
                 * Check if last_file & path correspond to a D/F conflict;
                 * i.e. whether path is last_file+'/'+<something>.
                 * If so, record that it's okay to remove last_file to make
                 * room for path and friends if needed.
                 */
                if (last_file &&
                    len > last_len &&
                    memcmp(path, last_file, last_len) == 0 &&
                    path[last_len] == '/') {
                        string_list_insert(&opt->priv->df_conflict_file_set, last_file);
                }

                /*
                 * Determine whether path could exist as a file in the
                 * working directory as a possible D/F conflict.  This
                 * will only occur when it exists in stage 2 as a
                 * file.
                 */
                if (S_ISREG(e->stages[2].mode) || S_ISLNK(e->stages[2].mode)) {
                        last_file = path;
                        last_len = len;
                } else {
                        last_file = NULL;
                }
        }
        string_list_clear(&df_sorted_entries, 0);
}

static int update_stages(struct merge_options *opt, const char *path,
                         const struct diff_filespec *o,
                         const struct diff_filespec *a,
                         const struct diff_filespec *b)
{

        /*
         * NOTE: It is usually a bad idea to call update_stages on a path
         * before calling update_file on that same path, since it can
         * sometimes lead to spurious "refusing to lose untracked file..."
         * messages from update_file (via make_room_for path via
         * would_lose_untracked).  Instead, reverse the order of the calls
         * (executing update_file first and then update_stages).
         */
        int clear = 1;
        int options = ADD_CACHE_OK_TO_ADD | ADD_CACHE_SKIP_DFCHECK;
        if (clear)
                if (remove_file_from_index(opt->repo->index, path))
                        return -1;
        if (o)
                if (add_cacheinfo(opt, o, path, 1, 0, options))
                        return -1;
        if (a)
                if (add_cacheinfo(opt, a, path, 2, 0, options))
                        return -1;
        if (b)
                if (add_cacheinfo(opt, b, path, 3, 0, options))
                        return -1;
        return 0;
}

static void update_entry(struct stage_data *entry,
                         struct diff_filespec *o,
                         struct diff_filespec *a,
                         struct diff_filespec *b)
{
        entry->processed = 0;
        entry->stages[1].mode = o->mode;
        entry->stages[2].mode = a->mode;
        entry->stages[3].mode = b->mode;
        oidcpy(&entry->stages[1].oid, &o->oid);
        oidcpy(&entry->stages[2].oid, &a->oid);
        oidcpy(&entry->stages[3].oid, &b->oid);
}

static int remove_file(struct merge_options *opt, int clean,
                       const char *path, int no_wd)
{
        int update_cache = opt->priv->call_depth || clean;
        int update_working_directory = !opt->priv->call_depth && !no_wd;

        if (update_cache) {
                if (remove_file_from_index(opt->repo->index, path))
                        return -1;
        }
        if (update_working_directory) {
                if (ignore_case) {
                        struct cache_entry *ce;
                        ce = index_file_exists(opt->repo->index, path, strlen(path),
                                               ignore_case);
                        if (ce && ce_stage(ce) == 0 && strcmp(path, ce->name))
                                return 0;
                }
                if (remove_path(path))
                        return -1;
        }
        return 0;
}

/* add a string to a strbuf, but converting "/" to "_" */
static void add_flattened_path(struct strbuf *out, const char *s)
{
        size_t i = out->len;
        strbuf_addstr(out, s);
        for (; i < out->len; i++)
                if (out->buf[i] == '/')
                        out->buf[i] = '_';
}

static char *unique_path(struct merge_options *opt,
                         const char *path,
                         const char *branch)
{
        struct path_hashmap_entry *entry;
        struct strbuf newpath = STRBUF_INIT;
        int suffix = 0;
        size_t base_len;

        strbuf_addf(&newpath, "%s~", path);
        add_flattened_path(&newpath, branch);

        base_len = newpath.len;
        while (hashmap_get_from_hash(&opt->priv->current_file_dir_set,
                                     fspathhash(newpath.buf), newpath.buf) ||
               (!opt->priv->call_depth && file_exists(newpath.buf))) {
                strbuf_setlen(&newpath, base_len);
                strbuf_addf(&newpath, "_%d", suffix++);
        }

        FLEX_ALLOC_MEM(entry, path, newpath.buf, newpath.len);
        hashmap_entry_init(&entry->e, fspathhash(entry->path));
        hashmap_add(&opt->priv->current_file_dir_set, &entry->e);
        return strbuf_detach(&newpath, NULL);
}

/**
 * Check whether a directory in the index is in the way of an incoming
 * file.  Return 1 if so.  If check_working_copy is non-zero, also
 * check the working directory.  If empty_ok is non-zero, also return
 * 0 in the case where the working-tree dir exists but is empty.
 */
static int dir_in_way(struct index_state *istate, const char *path,
                      int check_working_copy, int empty_ok)
{
        int pos;
        struct strbuf dirpath = STRBUF_INIT;
        struct stat st;

        strbuf_addstr(&dirpath, path);
        strbuf_addch(&dirpath, '/');

        pos = index_name_pos(istate, dirpath.buf, dirpath.len);

        if (pos < 0)
                pos = -1 - pos;
        if (pos < istate->cache_nr &&
            !strncmp(dirpath.buf, istate->cache[pos]->name, dirpath.len)) {
                strbuf_release(&dirpath);
                return 1;
        }

        strbuf_release(&dirpath);
        return check_working_copy && !lstat(path, &st) && S_ISDIR(st.st_mode) &&
                !(empty_ok && is_empty_dir(path)) &&
                !has_symlink_leading_path(path, strlen(path));
}

/*
 * Returns whether path was tracked in the index before the merge started,
 * and its oid and mode match the specified values
 */
static int was_tracked_and_matches(struct merge_options *opt, const char *path,
                                   const struct diff_filespec *blob)
{
        int pos = index_name_pos(&opt->priv->orig_index, path, strlen(path));
        struct cache_entry *ce;

        if (0 > pos)
                /* we were not tracking this path before the merge */
                return 0;

        /* See if the file we were tracking before matches */
        ce = opt->priv->orig_index.cache[pos];
        return (oideq(&ce->oid, &blob->oid) && ce->ce_mode == blob->mode);
}

/*
 * Returns whether path was tracked in the index before the merge started
 */
static int was_tracked(struct merge_options *opt, const char *path)
{
        int pos = index_name_pos(&opt->priv->orig_index, path, strlen(path));

        if (0 <= pos)
                /* we were tracking this path before the merge */
                return 1;

        return 0;
}

static int would_lose_untracked(struct merge_options *opt, const char *path)
{
        struct index_state *istate = opt->repo->index;

        /*
         * This may look like it can be simplified to:
         *   return !was_tracked(opt, path) && file_exists(path)
         * but it can't.  This function needs to know whether path was in
         * the working tree due to EITHER having been tracked in the index
         * before the merge OR having been put into the working copy and
         * index by unpack_trees().  Due to that either-or requirement, we
         * check the current index instead of the original one.
         *
         * Note that we do not need to worry about merge-recursive itself
         * updating the index after unpack_trees() and before calling this
         * function, because we strictly require all code paths in
         * merge-recursive to update the working tree first and the index
         * second.  Doing otherwise would break
         * update_file()/would_lose_untracked(); see every comment in this
         * file which mentions "update_stages".
         */
        int pos = index_name_pos(istate, path, strlen(path));

        if (pos < 0)
                pos = -1 - pos;
        while (pos < istate->cache_nr &&
               !strcmp(path, istate->cache[pos]->name)) {
                /*
                 * If stage #0, it is definitely tracked.
                 * If it has stage #2 then it was tracked
                 * before this merge started.  All other
                 * cases the path was not tracked.
                 */
                switch (ce_stage(istate->cache[pos])) {
                case 0:
                case 2:
                        return 0;
                }
                pos++;
        }
        return file_exists(path);
}

static int was_dirty(struct merge_options *opt, const char *path)
{
        struct cache_entry *ce;
        int dirty = 1;

        if (opt->priv->call_depth || !was_tracked(opt, path))
                return !dirty;

        ce = index_file_exists(opt->priv->unpack_opts.src_index,
                               path, strlen(path), ignore_case);
        dirty = verify_uptodate(ce, &opt->priv->unpack_opts) != 0;
        return dirty;
}

static int make_room_for_path(struct merge_options *opt, const char *path)
{
        int status, i;
        const char *msg = _("failed to create path '%s'%s");

        /* Unlink any D/F conflict files that are in the way */
        for (i = 0; i < opt->priv->df_conflict_file_set.nr; i++) {
                const char *df_path = opt->priv->df_conflict_file_set.items[i].string;
                size_t pathlen = strlen(path);
                size_t df_pathlen = strlen(df_path);
                if (df_pathlen < pathlen &&
                    path[df_pathlen] == '/' &&
                    strncmp(path, df_path, df_pathlen) == 0) {
                        output(opt, 3,
                               _("Removing %s to make room for subdirectory\n"),
                               df_path);
                        unlink(df_path);
                        unsorted_string_list_delete_item(&opt->priv->df_conflict_file_set,
                                                         i, 0);
                        break;
                }
        }

        /* Make sure leading directories are created */
        status = safe_create_leading_directories_const(path);
        if (status) {
                if (status == SCLD_EXISTS)
                        /* something else exists */
                        return err(opt, msg, path, _(": perhaps a D/F conflict?"));
                return err(opt, msg, path, "");
        }

        /*
         * Do not unlink a file in the work tree if we are not
         * tracking it.
         */
        if (would_lose_untracked(opt, path))
                return err(opt, _("refusing to lose untracked file at '%s'"),
                           path);

        /* Successful unlink is good.. */
        if (!unlink(path))
                return 0;
        /* .. and so is no existing file */
        if (errno == ENOENT)
                return 0;
        /* .. but not some other error (who really cares what?) */
        return err(opt, msg, path, _(": perhaps a D/F conflict?"));
}

static int update_file_flags(struct merge_options *opt,
                             const struct diff_filespec *contents,
                             const char *path,
                             int update_cache,
                             int update_wd)
{
        int ret = 0;

        if (opt->priv->call_depth)
                update_wd = 0;

        if (update_wd) {
                enum object_type type;
                void *buf;
                unsigned long size;

                if (S_ISGITLINK(contents->mode)) {
                        /*
                         * We may later decide to recursively descend into
                         * the submodule directory and update its index
                         * and/or work tree, but we do not do that now.
                         */
                        update_wd = 0;
                        goto update_index;
                }

                buf = repo_read_object_file(the_repository, &contents->oid,
                                            &type, &size);
                if (!buf) {
                        ret = err(opt, _("cannot read object %s '%s'"),
                                  oid_to_hex(&contents->oid), path);
                        goto free_buf;
                }
                if (type != OBJ_BLOB) {
                        ret = err(opt, _("blob expected for %s '%s'"),
                                  oid_to_hex(&contents->oid), path);
                        goto free_buf;
                }
                if (S_ISREG(contents->mode)) {
                        struct strbuf strbuf = STRBUF_INIT;
                        if (convert_to_working_tree(opt->repo->index,
                                                    path, buf, size, &strbuf, NULL)) {
                                free(buf);
                                size = strbuf.len;
                                buf = strbuf_detach(&strbuf, NULL);
                        }
                }

                if (make_room_for_path(opt, path) < 0) {
                        update_wd = 0;
                        goto free_buf;
                }
                if (S_ISREG(contents->mode) ||
                    (!has_symlinks && S_ISLNK(contents->mode))) {
                        int fd;
                        int mode = (contents->mode & 0100 ? 0777 : 0666);

                        fd = open(path, O_WRONLY | O_TRUNC | O_CREAT, mode);
                        if (fd < 0) {
                                ret = err(opt, _("failed to open '%s': %s"),
                                          path, strerror(errno));
                                goto free_buf;
                        }
                        write_in_full(fd, buf, size);
                        close(fd);
                } else if (S_ISLNK(contents->mode)) {
                        char *lnk = xmemdupz(buf, size);
                        safe_create_leading_directories_const(path);
                        unlink(path);
                        if (symlink(lnk, path))
                                ret = err(opt, _("failed to symlink '%s': %s"),
                                          path, strerror(errno));
                        free(lnk);
                } else
                        ret = err(opt,
                                  _("do not know what to do with %06o %s '%s'"),
                                  contents->mode, oid_to_hex(&contents->oid), path);
        free_buf:
                free(buf);
        }
update_index:
        if (!ret && update_cache) {
                int refresh = (!opt->priv->call_depth &&
                               contents->mode != S_IFGITLINK);
                if (add_cacheinfo(opt, contents, path, 0, refresh,
                                  ADD_CACHE_OK_TO_ADD))
                        return -1;
        }
        return ret;
}

static int update_file(struct merge_options *opt,
                       int clean,
                       const struct diff_filespec *contents,
                       const char *path)
{
        return update_file_flags(opt, contents, path,
                                 opt->priv->call_depth || clean, !opt->priv->call_depth);
}

/* Low level file merging, update and removal */

struct merge_file_info {
        struct diff_filespec blob; /* mostly use oid & mode; sometimes path */
        unsigned clean:1,
                 merge:1;
};

static int merge_3way(struct merge_options *opt,
                      mmbuffer_t *result_buf,
                      const struct diff_filespec *o,
                      const struct diff_filespec *a,
                      const struct diff_filespec *b,
                      const char *branch1,
                      const char *branch2,
                      const int extra_marker_size)
{
        mmfile_t orig, src1, src2;
        struct ll_merge_options ll_opts = {0};
        char *base, *name1, *name2;
        enum ll_merge_result merge_status;

        ll_opts.renormalize = opt->renormalize;
        ll_opts.extra_marker_size = extra_marker_size;
        ll_opts.xdl_opts = opt->xdl_opts;

        if (opt->priv->call_depth) {
                ll_opts.virtual_ancestor = 1;
                ll_opts.variant = 0;
        } else {
                switch (opt->recursive_variant) {
                case MERGE_VARIANT_OURS:
                        ll_opts.variant = XDL_MERGE_FAVOR_OURS;
                        break;
                case MERGE_VARIANT_THEIRS:
                        ll_opts.variant = XDL_MERGE_FAVOR_THEIRS;
                        break;
                default:
                        ll_opts.variant = 0;
                        break;
                }
        }

        assert(a->path && b->path && o->path && opt->ancestor);
        if (strcmp(a->path, b->path) || strcmp(a->path, o->path) != 0) {
                base  = mkpathdup("%s:%s", opt->ancestor, o->path);
                name1 = mkpathdup("%s:%s", branch1, a->path);
                name2 = mkpathdup("%s:%s", branch2, b->path);
        } else {
                base  = mkpathdup("%s", opt->ancestor);
                name1 = mkpathdup("%s", branch1);
                name2 = mkpathdup("%s", branch2);
        }

        read_mmblob(&orig, &o->oid);
        read_mmblob(&src1, &a->oid);
        read_mmblob(&src2, &b->oid);

        /*
         * FIXME: Using a->path for normalization rules in ll_merge could be
         * wrong if we renamed from a->path to b->path.  We should use the
         * target path for where the file will be written.
         */
        merge_status = ll_merge(result_buf, a->path, &orig, base,
                                &src1, name1, &src2, name2,
                                opt->repo->index, &ll_opts);
        if (merge_status == LL_MERGE_BINARY_CONFLICT)
                warning("Cannot merge binary files: %s (%s vs. %s)",
                        a->path, name1, name2);

        free(base);
        free(name1);
        free(name2);
        free(orig.ptr);
        free(src1.ptr);
        free(src2.ptr);
        return merge_status;
}

static int find_first_merges(struct repository *repo,
                             struct object_array *result, const char *path,
                             struct commit *a, struct commit *b)
{
        int i, j;
        struct object_array merges = OBJECT_ARRAY_INIT;
        struct commit *commit;
        int contains_another;

        char merged_revision[GIT_MAX_HEXSZ + 2];
        const char *rev_args[] = { "rev-list", "--merges", "--ancestry-path",
                                   "--all", merged_revision, NULL };
        struct rev_info revs;
        struct setup_revision_opt rev_opts;

        memset(result, 0, sizeof(struct object_array));
        memset(&rev_opts, 0, sizeof(rev_opts));

        /* get all revisions that merge commit a */
        xsnprintf(merged_revision, sizeof(merged_revision), "^%s",
                  oid_to_hex(&a->object.oid));
        repo_init_revisions(repo, &revs, NULL);
        /* FIXME: can't handle linked worktrees in submodules yet */
        revs.single_worktree = path != NULL;
        setup_revisions(ARRAY_SIZE(rev_args)-1, rev_args, &revs, &rev_opts);

        /* save all revisions from the above list that contain b */
        if (prepare_revision_walk(&revs))
                die("revision walk setup failed");
        while ((commit = get_revision(&revs)) != NULL) {
                struct object *o = &(commit->object);
                int ret = repo_in_merge_bases(repo, b, commit);
                if (ret < 0) {
                        object_array_clear(&merges);
                        release_revisions(&revs);
                        return ret;
                }
                if (ret)
                        add_object_array(o, NULL, &merges);
        }
        reset_revision_walk();

        /* Now we've got all merges that contain a and b. Prune all
         * merges that contain another found merge and save them in
         * result.
         */
        for (i = 0; i < merges.nr; i++) {
                struct commit *m1 = (struct commit *) merges.objects[i].item;

                contains_another = 0;
                for (j = 0; j < merges.nr; j++) {
                        struct commit *m2 = (struct commit *) merges.objects[j].item;
                        if (i != j) {
                                int ret = repo_in_merge_bases(repo, m2, m1);
                                if (ret < 0) {
                                        object_array_clear(&merges);
                                        release_revisions(&revs);
                                        return ret;
                                }
                                if (ret > 0) {
                                        contains_another = 1;
                                        break;
                                }
                        }
                }

                if (!contains_another)
                        add_object_array(merges.objects[i].item, NULL, result);
        }

        object_array_clear(&merges);
        release_revisions(&revs);
        return result->nr;
}

static void print_commit(struct repository *repo, struct commit *commit)
{
        struct strbuf sb = STRBUF_INIT;
        struct pretty_print_context ctx = {0};
        ctx.date_mode.type = DATE_NORMAL;
        /* FIXME: Merge this with output_commit_title() */
        assert(!merge_remote_util(commit));
        repo_format_commit_message(repo, commit, " %h: %m %s", &sb, &ctx);
        fprintf(stderr, "%s\n", sb.buf);
        strbuf_release(&sb);
}

static int is_valid(const struct diff_filespec *dfs)
{
        return dfs->mode != 0 && !is_null_oid(&dfs->oid);
}

static int merge_submodule(struct merge_options *opt,
                           struct object_id *result, const char *path,
                           const struct object_id *base, const struct object_id *a,
                           const struct object_id *b)
{
        struct repository subrepo;
        int ret = 0, ret2;
        struct commit *commit_base, *commit_a, *commit_b;
        int parent_count;
        struct object_array merges;

        int i;
        int search = !opt->priv->call_depth;

        /* store a in result in case we fail */
        /* FIXME: This is the WRONG resolution for the recursive case when
         * we need to be careful to avoid accidentally matching either side.
         * Should probably use o instead there, much like we do for merging
         * binaries.
         */
        oidcpy(result, a);

        /* we can not handle deletion conflicts */
        if (is_null_oid(base))
                return 0;
        if (is_null_oid(a))
                return 0;
        if (is_null_oid(b))
                return 0;

        if (repo_submodule_init(&subrepo, opt->repo, path, null_oid())) {
                output(opt, 1, _("Failed to merge submodule %s (not checked out)"), path);
                return 0;
        }

        if (!(commit_base = lookup_commit_reference(&subrepo, base)) ||
            !(commit_a = lookup_commit_reference(&subrepo, a)) ||
            !(commit_b = lookup_commit_reference(&subrepo, b))) {
                output(opt, 1, _("Failed to merge submodule %s (commits not present)"), path);
                goto cleanup;
        }

        /* check whether both changes are forward */
        ret2 = repo_in_merge_bases(&subrepo, commit_base, commit_a);
        if (ret2 < 0) {
                output(opt, 1, _("Failed to merge submodule %s (repository corrupt)"), path);
                goto cleanup;
        }
        if (ret2 > 0)
                ret2 = repo_in_merge_bases(&subrepo, commit_base, commit_b);
        if (ret2 < 0) {
                output(opt, 1, _("Failed to merge submodule %s (repository corrupt)"), path);
                goto cleanup;
        }
        if (!ret2) {
                output(opt, 1, _("Failed to merge submodule %s (commits don't follow merge-base)"), path);
                goto cleanup;
        }

        /* Case #1: a is contained in b or vice versa */
        ret2 = repo_in_merge_bases(&subrepo, commit_a, commit_b);
        if (ret2 < 0) {
                output(opt, 1, _("Failed to merge submodule %s (repository corrupt)"), path);
                goto cleanup;
        }
        if (ret2) {
                oidcpy(result, b);
                if (show(opt, 3)) {
                        output(opt, 3, _("Fast-forwarding submodule %s to the following commit:"), path);
                        repo_output_commit_title(opt, &subrepo, commit_b);
                } else if (show(opt, 2))
                        output(opt, 2, _("Fast-forwarding submodule %s"), path);
                else
                        ; /* no output */

                ret = 1;
                goto cleanup;
        }
        ret2 = repo_in_merge_bases(&subrepo, commit_b, commit_a);
        if (ret2 < 0) {
                output(opt, 1, _("Failed to merge submodule %s (repository corrupt)"), path);
                goto cleanup;
        }
        if (ret2) {
                oidcpy(result, a);
                if (show(opt, 3)) {
                        output(opt, 3, _("Fast-forwarding submodule %s to the following commit:"), path);
                        repo_output_commit_title(opt, &subrepo, commit_a);
                } else if (show(opt, 2))
                        output(opt, 2, _("Fast-forwarding submodule %s"), path);
                else
                        ; /* no output */

                ret = 1;
                goto cleanup;
        }

        /*
         * Case #2: There are one or more merges that contain a and b in
         * the submodule. If there is only one, then present it as a
         * suggestion to the user, but leave it marked unmerged so the
         * user needs to confirm the resolution.
         */

        /* Skip the search if makes no sense to the calling context.  */
        if (!search)
                goto cleanup;

        /* find commit which merges them */
        parent_count = find_first_merges(&subrepo, &merges, path,
                                         commit_a, commit_b);
        switch (parent_count) {
        case 0:
                output(opt, 1, _("Failed to merge submodule %s (merge following commits not found)"), path);
                break;

        case 1:
                output(opt, 1, _("Failed to merge submodule %s (not fast-forward)"), path);
                output(opt, 2, _("Found a possible merge resolution for the submodule:\n"));
                print_commit(&subrepo, (struct commit *) merges.objects[0].item);
                output(opt, 2, _(
                       "If this is correct simply add it to the index "
                       "for example\n"
                       "by using:\n\n"
                       "  git update-index --cacheinfo 160000 %s \"%s\"\n\n"
                       "which will accept this suggestion.\n"),
                       oid_to_hex(&merges.objects[0].item->oid), path);
                break;

        default:
                output(opt, 1, _("Failed to merge submodule %s (multiple merges found)"), path);
                for (i = 0; i < merges.nr; i++)
                        print_commit(&subrepo, (struct commit *) merges.objects[i].item);
        }

        object_array_clear(&merges);
cleanup:
        repo_clear(&subrepo);
        return ret;
}

static int merge_mode_and_contents(struct merge_options *opt,
                                   const struct diff_filespec *o,
                                   const struct diff_filespec *a,
                                   const struct diff_filespec *b,
                                   const char *filename,
                                   const char *branch1,
                                   const char *branch2,
                                   const int extra_marker_size,
                                   struct merge_file_info *result)
{
        if (opt->branch1 != branch1) {
                /*
                 * It's weird getting a reverse merge with HEAD on the bottom
                 * side of the conflict markers and the other branch on the
                 * top.  Fix that.
                 */
                return merge_mode_and_contents(opt, o, b, a,
                                               filename,
                                               branch2, branch1,
                                               extra_marker_size, result);
        }

        result->merge = 0;
        result->clean = 1;

        if ((S_IFMT & a->mode) != (S_IFMT & b->mode)) {
                result->clean = 0;
                /*
                 * FIXME: This is a bad resolution for recursive case; for
                 * the recursive case we want something that is unlikely to
                 * accidentally match either side.  Also, while it makes
                 * sense to prefer regular files over symlinks, it doesn't
                 * make sense to prefer regular files over submodules.
                 */
                if (S_ISREG(a->mode)) {
                        result->blob.mode = a->mode;
                        oidcpy(&result->blob.oid, &a->oid);
                } else {
                        result->blob.mode = b->mode;
                        oidcpy(&result->blob.oid, &b->oid);
                }
        } else {
                if (!oideq(&a->oid, &o->oid) && !oideq(&b->oid, &o->oid))
                        result->merge = 1;

                /*
                 * Merge modes
                 */
                if (a->mode == b->mode || a->mode == o->mode)
                        result->blob.mode = b->mode;
                else {
                        result->blob.mode = a->mode;
                        if (b->mode != o->mode) {
                                result->clean = 0;
                                result->merge = 1;
                        }
                }

                if (oideq(&a->oid, &b->oid) || oideq(&a->oid, &o->oid))
                        oidcpy(&result->blob.oid, &b->oid);
                else if (oideq(&b->oid, &o->oid))
                        oidcpy(&result->blob.oid, &a->oid);
                else if (S_ISREG(a->mode)) {
                        mmbuffer_t result_buf;
                        int ret = 0, merge_status;

                        merge_status = merge_3way(opt, &result_buf, o, a, b,
                                                  branch1, branch2,
                                                  extra_marker_size);

                        if ((merge_status < 0) || !result_buf.ptr)
                                ret = err(opt, _("failed to execute internal merge"));

                        if (!ret &&
                            write_object_file(result_buf.ptr, result_buf.size,
                                              OBJ_BLOB, &result->blob.oid))
                                ret = err(opt, _("unable to add %s to database"),
                                          a->path);

                        free(result_buf.ptr);
                        if (ret)
                                return ret;
                        /* FIXME: bug, what if modes didn't match? */
                        result->clean = (merge_status == 0);
                } else if (S_ISGITLINK(a->mode)) {
                        result->clean = merge_submodule(opt, &result->blob.oid,
                                                        o->path,
                                                        &o->oid,
                                                        &a->oid,
                                                        &b->oid);
                        if (result->clean < 0)
                                return -1;
                } else if (S_ISLNK(a->mode)) {
                        switch (opt->recursive_variant) {
                        case MERGE_VARIANT_NORMAL:
                                oidcpy(&result->blob.oid, &a->oid);
                                if (!oideq(&a->oid, &b->oid))
                                        result->clean = 0;
                                break;
                        case MERGE_VARIANT_OURS:
                                oidcpy(&result->blob.oid, &a->oid);
                                break;
                        case MERGE_VARIANT_THEIRS:
                                oidcpy(&result->blob.oid, &b->oid);
                                break;
                        }
                } else
                        BUG("unsupported object type in the tree");
        }

        if (result->merge)
                output(opt, 2, _("Auto-merging %s"), filename);

        return 0;
}

static int handle_rename_via_dir(struct merge_options *opt,
                                 struct rename_conflict_info *ci)
{
        /*
         * Handle file adds that need to be renamed due to directory rename
         * detection.  This differs from handle_rename_normal, because
         * there is no content merge to do; just move the file into the
         * desired final location.
         */
        const struct rename *ren = ci->ren1;
        const struct diff_filespec *dest = ren->pair->two;
        char *file_path = dest->path;
        int mark_conflicted = (opt->detect_directory_renames ==
                               MERGE_DIRECTORY_RENAMES_CONFLICT);
        assert(ren->dir_rename_original_dest);

        if (!opt->priv->call_depth && would_lose_untracked(opt, dest->path)) {
                mark_conflicted = 1;
                file_path = unique_path(opt, dest->path, ren->branch);
                output(opt, 1, _("Error: Refusing to lose untracked file at %s; "
                                 "writing to %s instead."),
                       dest->path, file_path);
        }

        if (mark_conflicted) {
                /*
                 * Write the file in worktree at file_path.  In the index,
                 * only record the file at dest->path in the appropriate
                 * higher stage.
                 */
                if (update_file(opt, 0, dest, file_path))
                        return -1;
                if (file_path != dest->path)
                        free(file_path);
                if (update_stages(opt, dest->path, NULL,
                                  ren->branch == opt->branch1 ? dest : NULL,
                                  ren->branch == opt->branch1 ? NULL : dest))
                        return -1;
                return 0; /* not clean, but conflicted */
        } else {
                /* Update dest->path both in index and in worktree */
                if (update_file(opt, 1, dest, dest->path))
                        return -1;
                return 1; /* clean */
        }
}

static int handle_change_delete(struct merge_options *opt,
                                const char *path, const char *old_path,
                                const struct diff_filespec *o,
                                const struct diff_filespec *changed,
                                const char *change_branch,
                                const char *delete_branch,
                                const char *change, const char *change_past)
{
        char *alt_path = NULL;
        const char *update_path = path;
        int ret = 0;

        if (dir_in_way(opt->repo->index, path, !opt->priv->call_depth, 0) ||
            (!opt->priv->call_depth && would_lose_untracked(opt, path))) {
                update_path = alt_path = unique_path(opt, path, change_branch);
        }

        if (opt->priv->call_depth) {
                /*
                 * We cannot arbitrarily accept either a_sha or b_sha as
                 * correct; since there is no true "middle point" between
                 * them, simply reuse the base version for virtual merge base.
                 */
                ret = remove_file_from_index(opt->repo->index, path);
                if (!ret)
                        ret = update_file(opt, 0, o, update_path);
        } else {
                /*
                 * Despite the four nearly duplicate messages and argument
                 * lists below and the ugliness of the nested if-statements,
                 * having complete messages makes the job easier for
                 * translators.
                 *
                 * The slight variance among the cases is due to the fact
                 * that:
                 *   1) directory/file conflicts (in effect if
                 *      !alt_path) could cause us to need to write the
                 *      file to a different path.
                 *   2) renames (in effect if !old_path) could mean that
                 *      there are two names for the path that the user
                 *      may know the file by.
                 */
                if (!alt_path) {
                        if (!old_path) {
                                output(opt, 1, _("CONFLICT (%s/delete): %s deleted in %s "
                                       "and %s in %s. Version %s of %s left in tree."),
                                       change, path, delete_branch, change_past,
                                       change_branch, change_branch, path);
                        } else {
                                output(opt, 1, _("CONFLICT (%s/delete): %s deleted in %s "
                                       "and %s to %s in %s. Version %s of %s left in tree."),
                                       change, old_path, delete_branch, change_past, path,
                                       change_branch, change_branch, path);
                        }
                } else {
                        if (!old_path) {
                                output(opt, 1, _("CONFLICT (%s/delete): %s deleted in %s "
                                       "and %s in %s. Version %s of %s left in tree at %s."),
                                       change, path, delete_branch, change_past,
                                       change_branch, change_branch, path, alt_path);
                        } else {
                                output(opt, 1, _("CONFLICT (%s/delete): %s deleted in %s "
                                       "and %s to %s in %s. Version %s of %s left in tree at %s."),
                                       change, old_path, delete_branch, change_past, path,
                                       change_branch, change_branch, path, alt_path);
                        }
                }
                /*
                 * No need to call update_file() on path when change_branch ==
                 * opt->branch1 && !alt_path, since that would needlessly touch
                 * path.  We could call update_file_flags() with update_cache=0
                 * and update_wd=0, but that's a no-op.
                 */
                if (change_branch != opt->branch1 || alt_path)
                        ret = update_file(opt, 0, changed, update_path);
        }
        free(alt_path);

        return ret;
}

static int handle_rename_delete(struct merge_options *opt,
                                struct rename_conflict_info *ci)
{
        const struct rename *ren = ci->ren1;
        const struct diff_filespec *orig = ren->pair->one;
        const struct diff_filespec *dest = ren->pair->two;
        const char *rename_branch = ren->branch;
        const char *delete_branch = (opt->branch1 == ren->branch ?
                                     opt->branch2 : opt->branch1);

        if (handle_change_delete(opt,
                                 opt->priv->call_depth ? orig->path : dest->path,
                                 opt->priv->call_depth ? NULL : orig->path,
                                 orig, dest,
                                 rename_branch, delete_branch,
                                 _("rename"), _("renamed")))
                return -1;

        if (opt->priv->call_depth)
                return remove_file_from_index(opt->repo->index, dest->path);
        else
                return update_stages(opt, dest->path, NULL,
                                     rename_branch == opt->branch1 ? dest : NULL,
                                     rename_branch == opt->branch1 ? NULL : dest);
}

static int handle_file_collision(struct merge_options *opt,
                                 const char *collide_path,
                                 const char *prev_path1,
                                 const char *prev_path2,
                                 const char *branch1, const char *branch2,
                                 struct diff_filespec *a,
                                 struct diff_filespec *b)
{
        struct merge_file_info mfi;
        struct diff_filespec null;
        char *alt_path = NULL;
        const char *update_path = collide_path;

        /*
         * It's easiest to get the correct things into stage 2 and 3, and
         * to make sure that the content merge puts HEAD before the other
         * branch if we just ensure that branch1 == opt->branch1.  So, simply
         * flip arguments around if we don't have that.
         */
        if (branch1 != opt->branch1) {
                return handle_file_collision(opt, collide_path,
                                             prev_path2, prev_path1,
                                             branch2, branch1,
                                             b, a);
        }

        /* Remove rename sources if rename/add or rename/rename(2to1) */
        if (prev_path1)
                remove_file(opt, 1, prev_path1,
                            opt->priv->call_depth || would_lose_untracked(opt, prev_path1));
        if (prev_path2)
                remove_file(opt, 1, prev_path2,
                            opt->priv->call_depth || would_lose_untracked(opt, prev_path2));

        /*
         * Remove the collision path, if it wouldn't cause dirty contents
         * or an untracked file to get lost.  We'll either overwrite with
         * merged contents, or just write out to differently named files.
         */
        if (was_dirty(opt, collide_path)) {
                output(opt, 1, _("Refusing to lose dirty file at %s"),
                       collide_path);
                update_path = alt_path = unique_path(opt, collide_path, "merged");
        } else if (would_lose_untracked(opt, collide_path)) {
                /*
                 * Only way we get here is if both renames were from
                 * a directory rename AND user had an untracked file
                 * at the location where both files end up after the
                 * two directory renames.  See testcase 10d of t6043.
                 */
                output(opt, 1, _("Refusing to lose untracked file at "
                               "%s, even though it's in the way."),
                       collide_path);
                update_path = alt_path = unique_path(opt, collide_path, "merged");
        } else {
                /*
                 * FIXME: It's possible that the two files are identical
                 * and that the current working copy happens to match, in
                 * which case we are unnecessarily touching the working
                 * tree file.  It's not a likely enough scenario that I
                 * want to code up the checks for it and a better fix is
                 * available if we restructure how unpack_trees() and
                 * merge-recursive interoperate anyway, so punting for
                 * now...
                 */
                remove_file(opt, 0, collide_path, 0);
        }

        /* Store things in diff_filespecs for functions that need it */
        null.path = (char *)collide_path;
        oidcpy(&null.oid, null_oid());
        null.mode = 0;

        if (merge_mode_and_contents(opt, &null, a, b, collide_path,
                                    branch1, branch2, opt->priv->call_depth * 2, &mfi))
                return -1;
        mfi.clean &= !alt_path;
        if (update_file(opt, mfi.clean, &mfi.blob, update_path))
                return -1;
        if (!mfi.clean && !opt->priv->call_depth &&
            update_stages(opt, collide_path, NULL, a, b))
                return -1;
        free(alt_path);
        /*
         * FIXME: If both a & b both started with conflicts (only possible
         * if they came from a rename/rename(2to1)), but had IDENTICAL
         * contents including those conflicts, then in the next line we claim
         * it was clean.  If someone cares about this case, we should have the
         * caller notify us if we started with conflicts.
         */
        return mfi.clean;
}

static int handle_rename_add(struct merge_options *opt,
                             struct rename_conflict_info *ci)
{
        /* a was renamed to c, and a separate c was added. */
        struct diff_filespec *a = ci->ren1->pair->one;
        struct diff_filespec *c = ci->ren1->pair->two;
        char *path = c->path;
        char *prev_path_desc;
        struct merge_file_info mfi;

        const char *rename_branch = ci->ren1->branch;
        const char *add_branch = (opt->branch1 == rename_branch ?
                                  opt->branch2 : opt->branch1);
        int other_stage = (ci->ren1->branch == opt->branch1 ? 3 : 2);

        output(opt, 1, _("CONFLICT (rename/add): "
               "Rename %s->%s in %s.  Added %s in %s"),
               a->path, c->path, rename_branch,
               c->path, add_branch);

        prev_path_desc = xstrfmt("version of %s from %s", path, a->path);
        ci->ren1->src_entry->stages[other_stage].path = a->path;
        if (merge_mode_and_contents(opt, a, c,
                                    &ci->ren1->src_entry->stages[other_stage],
                                    prev_path_desc,
                                    opt->branch1, opt->branch2,
                                    1 + opt->priv->call_depth * 2, &mfi))
                return -1;
        free(prev_path_desc);

        ci->ren1->dst_entry->stages[other_stage].path = mfi.blob.path = c->path;
        return handle_file_collision(opt,
                                     c->path, a->path, NULL,
                                     rename_branch, add_branch,
                                     &mfi.blob,
                                     &ci->ren1->dst_entry->stages[other_stage]);
}

static char *find_path_for_conflict(struct merge_options *opt,
                                    const char *path,
                                    const char *branch1,
                                    const char *branch2)
{
        char *new_path = NULL;
        if (dir_in_way(opt->repo->index, path, !opt->priv->call_depth, 0)) {
                new_path = unique_path(opt, path, branch1);
                output(opt, 1, _("%s is a directory in %s adding "
                               "as %s instead"),
                       path, branch2, new_path);
        } else if (would_lose_untracked(opt, path)) {
                new_path = unique_path(opt, path, branch1);
                output(opt, 1, _("Refusing to lose untracked file"
                               " at %s; adding as %s instead"),
                       path, new_path);
        }

        return new_path;
}

/*
 * Toggle the stage number between "ours" and "theirs" (2 and 3).
 */
static inline int flip_stage(int stage)
{
        return (2 + 3) - stage;
}

static int handle_rename_rename_1to2(struct merge_options *opt,
                                     struct rename_conflict_info *ci)
{
        /* One file was renamed in both branches, but to different names. */
        struct merge_file_info mfi;
        struct diff_filespec *add;
        struct diff_filespec *o = ci->ren1->pair->one;
        struct diff_filespec *a = ci->ren1->pair->two;
        struct diff_filespec *b = ci->ren2->pair->two;
        char *path_desc;

        output(opt, 1, _("CONFLICT (rename/rename): "
               "Rename \"%s\"->\"%s\" in branch \"%s\" "
               "rename \"%s\"->\"%s\" in \"%s\"%s"),
               o->path, a->path, ci->ren1->branch,
               o->path, b->path, ci->ren2->branch,
               opt->priv->call_depth ? _(" (left unresolved)") : "");

        path_desc = xstrfmt("%s and %s, both renamed from %s",
                            a->path, b->path, o->path);
        if (merge_mode_and_contents(opt, o, a, b, path_desc,
                                    ci->ren1->branch, ci->ren2->branch,
                                    opt->priv->call_depth * 2, &mfi))
                return -1;
        free(path_desc);

        if (opt->priv->call_depth)
                remove_file_from_index(opt->repo->index, o->path);

        /*
         * For each destination path, we need to see if there is a
         * rename/add collision.  If not, we can write the file out
         * to the specified location.
         */
        add = &ci->ren1->dst_entry->stages[flip_stage(2)];
        if (is_valid(add)) {
                add->path = mfi.blob.path = a->path;
                if (handle_file_collision(opt, a->path,
                                          NULL, NULL,
                                          ci->ren1->branch,
                                          ci->ren2->branch,
                                          &mfi.blob, add) < 0)
                        return -1;
        } else {
                char *new_path = find_path_for_conflict(opt, a->path,
                                                        ci->ren1->branch,
                                                        ci->ren2->branch);
                if (update_file(opt, 0, &mfi.blob,
                                new_path ? new_path : a->path))
                        return -1;
                free(new_path);
                if (!opt->priv->call_depth &&
                    update_stages(opt, a->path, NULL, a, NULL))
                        return -1;
        }

        if (!mfi.clean && mfi.blob.mode == a->mode &&
            oideq(&mfi.blob.oid, &a->oid)) {
                /*
                 * Getting here means we were attempting to merge a binary
                 * blob.  Since we can't merge binaries, the merge algorithm
                 * just takes one side.  But we don't want to copy the
                 * contents of one side to both paths; we'd rather use the
                 * original content at the given path for each path.
                 */
                oidcpy(&mfi.blob.oid, &b->oid);
                mfi.blob.mode = b->mode;
        }
        add = &ci->ren2->dst_entry->stages[flip_stage(3)];
        if (is_valid(add)) {
                add->path = mfi.blob.path = b->path;
                if (handle_file_collision(opt, b->path,
                                          NULL, NULL,
                                          ci->ren1->branch,
                                          ci->ren2->branch,
                                          add, &mfi.blob) < 0)
                        return -1;
        } else {
                char *new_path = find_path_for_conflict(opt, b->path,
                                                        ci->ren2->branch,
                                                        ci->ren1->branch);
                if (update_file(opt, 0, &mfi.blob,
                                new_path ? new_path : b->path))
                        return -1;
                free(new_path);
                if (!opt->priv->call_depth &&
                    update_stages(opt, b->path, NULL, NULL, b))
                        return -1;
        }

        return 0;
}

static int handle_rename_rename_2to1(struct merge_options *opt,
                                     struct rename_conflict_info *ci)
{
        /* Two files, a & b, were renamed to the same thing, c. */
        struct diff_filespec *a = ci->ren1->pair->one;
        struct diff_filespec *b = ci->ren2->pair->one;
        struct diff_filespec *c1 = ci->ren1->pair->two;
        struct diff_filespec *c2 = ci->ren2->pair->two;
        char *path = c1->path; /* == c2->path */
        char *path_side_1_desc;
        char *path_side_2_desc;
        struct merge_file_info mfi_c1;
        struct merge_file_info mfi_c2;
        int ostage1, ostage2;

        output(opt, 1, _("CONFLICT (rename/rename): "
               "Rename %s->%s in %s. "
               "Rename %s->%s in %s"),
               a->path, c1->path, ci->ren1->branch,
               b->path, c2->path, ci->ren2->branch);

        path_side_1_desc = xstrfmt("version of %s from %s", path, a->path);
        path_side_2_desc = xstrfmt("version of %s from %s", path, b->path);
        ostage1 = ci->ren1->branch == opt->branch1 ? 3 : 2;
        ostage2 = flip_stage(ostage1);
        ci->ren1->src_entry->stages[ostage1].path = a->path;
        ci->ren2->src_entry->stages[ostage2].path = b->path;
        if (merge_mode_and_contents(opt, a, c1,
                                    &ci->ren1->src_entry->stages[ostage1],
                                    path_side_1_desc,
                                    opt->branch1, opt->branch2,
                                    1 + opt->priv->call_depth * 2, &mfi_c1) ||
            merge_mode_and_contents(opt, b,
                                    &ci->ren2->src_entry->stages[ostage2],
                                    c2, path_side_2_desc,
                                    opt->branch1, opt->branch2,
                                    1 + opt->priv->call_depth * 2, &mfi_c2))
                return -1;
        free(path_side_1_desc);
        free(path_side_2_desc);
        mfi_c1.blob.path = path;
        mfi_c2.blob.path = path;

        return handle_file_collision(opt, path, a->path, b->path,
                                     ci->ren1->branch, ci->ren2->branch,
                                     &mfi_c1.blob, &mfi_c2.blob);
}

/*
 * Get the diff_filepairs changed between o_tree and tree.
 */
static struct diff_queue_struct *get_diffpairs(struct merge_options *opt,
                                               struct tree *o_tree,
                                               struct tree *tree)
{
        struct diff_queue_struct *ret;
        struct diff_options opts;

        repo_diff_setup(opt->repo, &opts);
        opts.flags.recursive = 1;
        opts.flags.rename_empty = 0;
        opts.detect_rename = merge_detect_rename(opt);
        /*
         * We do not have logic to handle the detection of copies.  In
         * fact, it may not even make sense to add such logic: would we
         * really want a change to a base file to be propagated through
         * multiple other files by a merge?
         */
        if (opts.detect_rename > DIFF_DETECT_RENAME)
                opts.detect_rename = DIFF_DETECT_RENAME;
        opts.rename_limit = (opt->rename_limit >= 0) ? opt->rename_limit : 7000;
        opts.rename_score = opt->rename_score;
        opts.show_rename_progress = opt->show_rename_progress;
        opts.output_format = DIFF_FORMAT_NO_OUTPUT;
        diff_setup_done(&opts);
        diff_tree_oid(&o_tree->object.oid, &tree->object.oid, "", &opts);
        diffcore_std(&opts);
        if (opts.needed_rename_limit > opt->priv->needed_rename_limit)
                opt->priv->needed_rename_limit = opts.needed_rename_limit;

        ret = xmalloc(sizeof(*ret));
        *ret = diff_queued_diff;

        opts.output_format = DIFF_FORMAT_NO_OUTPUT;
        diff_queued_diff.nr = 0;
        diff_queued_diff.queue = NULL;
        diff_flush(&opts);
        return ret;
}

static int tree_has_path(struct repository *r, struct tree *tree,
                         const char *path)
{
        struct object_id hashy;
        unsigned short mode_o;

        return !get_tree_entry(r,
                               &tree->object.oid, path,
                               &hashy, &mode_o);
}

/*
 * Return a new string that replaces the beginning portion (which matches
 * entry->dir), with entry->new_dir.  In perl-speak:
 *   new_path_name = (old_path =~ s/entry->dir/entry->new_dir/);
 * NOTE:
 *   Caller must ensure that old_path starts with entry->dir + '/'.
 */
static char *apply_dir_rename(struct dir_rename_entry *entry,
                              const char *old_path)
{
        struct strbuf new_path = STRBUF_INIT;
        int oldlen, newlen;

        if (entry->non_unique_new_dir)
                return NULL;

        oldlen = strlen(entry->dir);
        if (entry->new_dir.len == 0)
                /*
                 * If someone renamed/merged a subdirectory into the root
                 * directory (e.g. 'some/subdir' -> ''), then we want to
                 * avoid returning
                 *     '' + '/filename'
                 * as the rename; we need to make old_path + oldlen advance
                 * past the '/' character.
                 */
                oldlen++;
        newlen = entry->new_dir.len + (strlen(old_path) - oldlen) + 1;
        strbuf_grow(&new_path, newlen);
        strbuf_addbuf(&new_path, &entry->new_dir);
        strbuf_addstr(&new_path, &old_path[oldlen]);

        return strbuf_detach(&new_path, NULL);
}

static void get_renamed_dir_portion(const char *old_path, const char *new_path,
                                    char **old_dir, char **new_dir)
{
        char *end_of_old, *end_of_new;

        /* Default return values: NULL, meaning no rename */
        *old_dir = NULL;
        *new_dir = NULL;

        /*
         * For
         *    "a/b/c/d/e/foo.c" -> "a/b/some/thing/else/e/foo.c"
         * the "e/foo.c" part is the same, we just want to know that
         *    "a/b/c/d" was renamed to "a/b/some/thing/else"
         * so, for this example, this function returns "a/b/c/d" in
         * *old_dir and "a/b/some/thing/else" in *new_dir.
         */

        /*
         * If the basename of the file changed, we don't care.  We want
         * to know which portion of the directory, if any, changed.
         */
        end_of_old = strrchr(old_path, '/');
        end_of_new = strrchr(new_path, '/');

        /*
         * If end_of_old is NULL, old_path wasn't in a directory, so there
         * could not be a directory rename (our rule elsewhere that a
         * directory which still exists is not considered to have been
         * renamed means the root directory can never be renamed -- because
         * the root directory always exists).
         */
        if (!end_of_old)
                return; /* Note: *old_dir and *new_dir are still NULL */

        /*
         * If new_path contains no directory (end_of_new is NULL), then we
         * have a rename of old_path's directory to the root directory.
         */
        if (!end_of_new) {
                *old_dir = xstrndup(old_path, end_of_old - old_path);
                *new_dir = xstrdup("");
                return;
        }

        /* Find the first non-matching character traversing backwards */
        while (*--end_of_new == *--end_of_old &&
               end_of_old != old_path &&
               end_of_new != new_path)
                ; /* Do nothing; all in the while loop */

        /*
         * If both got back to the beginning of their strings, then the
         * directory didn't change at all, only the basename did.
         */
        if (end_of_old == old_path && end_of_new == new_path &&
            *end_of_old == *end_of_new)
                return; /* Note: *old_dir and *new_dir are still NULL */

        /*
         * If end_of_new got back to the beginning of its string, and
         * end_of_old got back to the beginning of some subdirectory, then
         * we have a rename/merge of a subdirectory into the root, which
         * needs slightly special handling.
         *
         * Note: There is no need to consider the opposite case, with a
         * rename/merge of the root directory into some subdirectory
         * because as noted above the root directory always exists so it
         * cannot be considered to be renamed.
         */
        if (end_of_new == new_path &&
            end_of_old != old_path && end_of_old[-1] == '/') {
                *old_dir = xstrndup(old_path, --end_of_old - old_path);
                *new_dir = xstrdup("");
                return;
        }

        /*
         * We've found the first non-matching character in the directory
         * paths.  That means the current characters we were looking at
         * were part of the first non-matching subdir name going back from
         * the end of the strings.  Get the whole name by advancing both
         * end_of_old and end_of_new to the NEXT '/' character.  That will
         * represent the entire directory rename.
         *
         * The reason for the increment is cases like
         *    a/b/star/foo/whatever.c -> a/b/tar/foo/random.c
         * After dropping the basename and going back to the first
         * non-matching character, we're now comparing:
         *    a/b/s          and         a/b/
         * and we want to be comparing:
         *    a/b/star/      and         a/b/tar/
         * but without the pre-increment, the one on the right would stay
         * a/b/.
         */
        end_of_old = strchr(++end_of_old, '/');
        end_of_new = strchr(++end_of_new, '/');

        /* Copy the old and new directories into *old_dir and *new_dir. */
        *old_dir = xstrndup(old_path, end_of_old - old_path);
        *new_dir = xstrndup(new_path, end_of_new - new_path);
}

static void remove_hashmap_entries(struct hashmap *dir_renames,
                                   struct string_list *items_to_remove)
{
        int i;
        struct dir_rename_entry *entry;

        for (i = 0; i < items_to_remove->nr; i++) {
                entry = items_to_remove->items[i].util;
                hashmap_remove(dir_renames, &entry->ent, NULL);
        }
        string_list_clear(items_to_remove, 0);
}

/*
 * See if there is a directory rename for path, and if there are any file
 * level conflicts for the renamed location.  If there is a rename and
 * there are no conflicts, return the new name.  Otherwise, return NULL.
 */
static char *handle_path_level_conflicts(struct merge_options *opt,
                                         const char *path,
                                         struct dir_rename_entry *entry,
                                         struct hashmap *collisions,
                                         struct tree *tree)
{
        char *new_path = NULL;
        struct collision_entry *collision_ent;
        int clean = 1;
        struct strbuf collision_paths = STRBUF_INIT;

        /*
         * entry has the mapping of old directory name to new directory name
         * that we want to apply to path.
         */
        new_path = apply_dir_rename(entry, path);

        if (!new_path) {
                /* This should only happen when entry->non_unique_new_dir set */
                if (!entry->non_unique_new_dir)
                        BUG("entry->non_unique_new_dir not set and !new_path");
                output(opt, 1, _("CONFLICT (directory rename split): "
                               "Unclear where to place %s because directory "
                               "%s was renamed to multiple other directories, "
                               "with no destination getting a majority of the "
                               "files."),
                       path, entry->dir);
                clean = 0;
                return NULL;
        }

        /*
         * The caller needs to have ensured that it has pre-populated
         * collisions with all paths that map to new_path.  Do a quick check
         * to ensure that's the case.
         */
        collision_ent = collision_find_entry(collisions, new_path);
        if (!collision_ent)
                BUG("collision_ent is NULL");

        /*
         * Check for one-sided add/add/.../add conflicts, i.e.
         * where implicit renames from the other side doing
         * directory rename(s) can affect this side of history
         * to put multiple paths into the same location.  Warn
         * and bail on directory renames for such paths.
         */
        if (collision_ent->reported_already) {
                clean = 0;
        } else if (tree_has_path(opt->repo, tree, new_path)) {
                collision_ent->reported_already = 1;
                strbuf_add_separated_string_list(&collision_paths, ", ",
                                                 &collision_ent->source_files);
                output(opt, 1, _("CONFLICT (implicit dir rename): Existing "
                               "file/dir at %s in the way of implicit "
                               "directory rename(s) putting the following "
                               "path(s) there: %s."),
                       new_path, collision_paths.buf);
                clean = 0;
        } else if (collision_ent->source_files.nr > 1) {
                collision_ent->reported_already = 1;
                strbuf_add_separated_string_list(&collision_paths, ", ",
                                                 &collision_ent->source_files);
                output(opt, 1, _("CONFLICT (implicit dir rename): Cannot map "
                               "more than one path to %s; implicit directory "
                               "renames tried to put these paths there: %s"),
                       new_path, collision_paths.buf);
                clean = 0;
        }

        /* Free memory we no longer need */
        strbuf_release(&collision_paths);
        if (!clean && new_path) {
                free(new_path);
                return NULL;
        }

        return new_path;
}

/*
 * There are a couple things we want to do at the directory level:
 *   1. Check for both sides renaming to the same thing, in order to avoid
 *      implicit renaming of files that should be left in place.  (See
 *      testcase 6b in t6043 for details.)
 *   2. Prune directory renames if there are still files left in the
 *      original directory.  These represent a partial directory rename,
 *      i.e. a rename where only some of the files within the directory
 *      were renamed elsewhere.  (Technically, this could be done earlier
 *      in get_directory_renames(), except that would prevent us from
 *      doing the previous check and thus failing testcase 6b.)
 *   3. Check for rename/rename(1to2) conflicts (at the directory level).
 *      In the future, we could potentially record this info as well and
 *      omit reporting rename/rename(1to2) conflicts for each path within
 *      the affected directories, thus cleaning up the merge output.
 *   NOTE: We do NOT check for rename/rename(2to1) conflicts at the
 *         directory level, because merging directories is fine.  If it
 *         causes conflicts for files within those merged directories, then
 *         that should be detected at the individual path level.
 */
static void handle_directory_level_conflicts(struct merge_options *opt,
                                             struct hashmap *dir_re_head,
                                             struct tree *head,
                                             struct hashmap *dir_re_merge,
                                             struct tree *merge)
{
        struct hashmap_iter iter;
        struct dir_rename_entry *head_ent;
        struct dir_rename_entry *merge_ent;

        struct string_list remove_from_head = STRING_LIST_INIT_NODUP;
        struct string_list remove_from_merge = STRING_LIST_INIT_NODUP;

        hashmap_for_each_entry(dir_re_head, &iter, head_ent,
                                ent /* member name */) {
                merge_ent = dir_rename_find_entry(dir_re_merge, head_ent->dir);
                if (merge_ent &&
                    !head_ent->non_unique_new_dir &&
                    !merge_ent->non_unique_new_dir &&
                    !strbuf_cmp(&head_ent->new_dir, &merge_ent->new_dir)) {
                        /* 1. Renamed identically; remove it from both sides */
                        string_list_append(&remove_from_head,
                                           head_ent->dir)->util = head_ent;
                        strbuf_release(&head_ent->new_dir);
                        string_list_append(&remove_from_merge,
                                           merge_ent->dir)->util = merge_ent;
                        strbuf_release(&merge_ent->new_dir);
                } else if (tree_has_path(opt->repo, head, head_ent->dir)) {
                        /* 2. This wasn't a directory rename after all */
                        string_list_append(&remove_from_head,
                                           head_ent->dir)->util = head_ent;
                        strbuf_release(&head_ent->new_dir);
                }
        }

        remove_hashmap_entries(dir_re_head, &remove_from_head);
        remove_hashmap_entries(dir_re_merge, &remove_from_merge);

        hashmap_for_each_entry(dir_re_merge, &iter, merge_ent,
                                ent /* member name */) {
                head_ent = dir_rename_find_entry(dir_re_head, merge_ent->dir);
                if (tree_has_path(opt->repo, merge, merge_ent->dir)) {
                        /* 2. This wasn't a directory rename after all */
                        string_list_append(&remove_from_merge,
                                           merge_ent->dir)->util = merge_ent;
                } else if (head_ent &&
                           !head_ent->non_unique_new_dir &&
                           !merge_ent->non_unique_new_dir) {
                        /* 3. rename/rename(1to2) */
                        /*
                         * We can assume it's not rename/rename(1to1) because
                         * that was case (1), already checked above.  So we
                         * know that head_ent->new_dir and merge_ent->new_dir
                         * are different strings.
                         */
                        output(opt, 1, _("CONFLICT (rename/rename): "
                                       "Rename directory %s->%s in %s. "
                                       "Rename directory %s->%s in %s"),
                               head_ent->dir, head_ent->new_dir.buf, opt->branch1,
                               head_ent->dir, merge_ent->new_dir.buf, opt->branch2);
                        string_list_append(&remove_from_head,
                                           head_ent->dir)->util = head_ent;
                        strbuf_release(&head_ent->new_dir);
                        string_list_append(&remove_from_merge,
                                           merge_ent->dir)->util = merge_ent;
                        strbuf_release(&merge_ent->new_dir);
                }
        }

        remove_hashmap_entries(dir_re_head, &remove_from_head);
        remove_hashmap_entries(dir_re_merge, &remove_from_merge);
}

static struct hashmap *get_directory_renames(struct diff_queue_struct *pairs)
{
        struct hashmap *dir_renames;
        struct hashmap_iter iter;
        struct dir_rename_entry *entry;
        int i;

        /*
         * Typically, we think of a directory rename as all files from a
         * certain directory being moved to a target directory.  However,
         * what if someone first moved two files from the original
         * directory in one commit, and then renamed the directory
         * somewhere else in a later commit?  At merge time, we just know
         * that files from the original directory went to two different
         * places, and that the bulk of them ended up in the same place.
         * We want each directory rename to represent where the bulk of the
         * files from that directory end up; this function exists to find
         * where the bulk of the files went.
         *
         * The first loop below simply iterates through the list of file
         * renames, finding out how often each directory rename pair
         * possibility occurs.
         */
        dir_renames = xmalloc(sizeof(*dir_renames));
        dir_rename_init(dir_renames);
        for (i = 0; i < pairs->nr; ++i) {
                struct string_list_item *item;
                int *count;
                struct diff_filepair *pair = pairs->queue[i];
                char *old_dir, *new_dir;

                /* File not part of directory rename if it wasn't renamed */
                if (pair->status != 'R')
                        continue;

                get_renamed_dir_portion(pair->one->path, pair->two->path,
                                        &old_dir,        &new_dir);
                if (!old_dir)
                        /* Directory didn't change at all; ignore this one. */
                        continue;

                entry = dir_rename_find_entry(dir_renames, old_dir);
                if (!entry) {
                        entry = xmalloc(sizeof(*entry));
                        dir_rename_entry_init(entry, old_dir);
                        hashmap_put(dir_renames, &entry->ent);
                } else {
                        free(old_dir);
                }
                item = string_list_lookup(&entry->possible_new_dirs, new_dir);
                if (!item) {
                        item = string_list_insert(&entry->possible_new_dirs,
                                                  new_dir);
                        item->util = xcalloc(1, sizeof(int));
                } else {
                        free(new_dir);
                }
                count = item->util;
                *count += 1;
        }

        /*
         * For each directory with files moved out of it, we find out which
         * target directory received the most files so we can declare it to
         * be the "winning" target location for the directory rename.  This
         * winner gets recorded in new_dir.  If there is no winner
         * (multiple target directories received the same number of files),
         * we set non_unique_new_dir.  Once we've determined the winner (or
         * that there is no winner), we no longer need possible_new_dirs.
         */
        hashmap_for_each_entry(dir_renames, &iter, entry,
                                ent /* member name */) {
                int max = 0;
                int bad_max = 0;
                char *best = NULL;

                for (i = 0; i < entry->possible_new_dirs.nr; i++) {
                        int *count = entry->possible_new_dirs.items[i].util;

                        if (*count == max)
                                bad_max = max;
                        else if (*count > max) {
                                max = *count;
                                best = entry->possible_new_dirs.items[i].string;
                        }
                }
                if (bad_max == max)
                        entry->non_unique_new_dir = 1;
                else {
                        assert(entry->new_dir.len == 0);
                        strbuf_addstr(&entry->new_dir, best);
                }
                /*
                 * The relevant directory sub-portion of the original full
                 * filepaths were xstrndup'ed before inserting into
                 * possible_new_dirs, and instead of manually iterating the
                 * list and free'ing each, just lie and tell
                 * possible_new_dirs that it did the strdup'ing so that it
                 * will free them for us.
                 */
                entry->possible_new_dirs.strdup_strings = 1;
                string_list_clear(&entry->possible_new_dirs, 1);
        }

        return dir_renames;
}

static struct dir_rename_entry *check_dir_renamed(const char *path,
                                                  struct hashmap *dir_renames)
{
        char *temp = xstrdup(path);
        char *end;
        struct dir_rename_entry *entry = NULL;

        while ((end = strrchr(temp, '/'))) {
                *end = '\0';
                entry = dir_rename_find_entry(dir_renames, temp);
                if (entry)
                        break;
        }
        free(temp);
        return entry;
}

static void compute_collisions(struct hashmap *collisions,
                               struct hashmap *dir_renames,
                               struct diff_queue_struct *pairs)
{
        int i;

        /*
         * Multiple files can be mapped to the same path due to directory
         * renames done by the other side of history.  Since that other
         * side of history could have merged multiple directories into one,
         * if our side of history added the same file basename to each of
         * those directories, then all N of them would get implicitly
         * renamed by the directory rename detection into the same path,
         * and we'd get an add/add/.../add conflict, and all those adds
         * from *this* side of history.  This is not representable in the
         * index, and users aren't going to easily be able to make sense of
         * it.  So we need to provide a good warning about what's
         * happening, and fall back to no-directory-rename detection
         * behavior for those paths.
         *
         * See testcases 9e and all of section 5 from t6043 for examples.
         */
        collision_init(collisions);

        for (i = 0; i < pairs->nr; ++i) {
                struct dir_rename_entry *dir_rename_ent;
                struct collision_entry *collision_ent;
                char *new_path;
                struct diff_filepair *pair = pairs->queue[i];

                if (pair->status != 'A' && pair->status != 'R')
                        continue;
                dir_rename_ent = check_dir_renamed(pair->two->path,
                                                   dir_renames);
                if (!dir_rename_ent)
                        continue;

                new_path = apply_dir_rename(dir_rename_ent, pair->two->path);
                if (!new_path)
                        /*
                         * dir_rename_ent->non_unique_new_path is true, which
                         * means there is no directory rename for us to use,
                         * which means it won't cause us any additional
                         * collisions.
                         */
                        continue;
                collision_ent = collision_find_entry(collisions, new_path);
                if (!collision_ent) {
                        CALLOC_ARRAY(collision_ent, 1);
                        hashmap_entry_init(&collision_ent->ent,
                                                strhash(new_path));
                        hashmap_put(collisions, &collision_ent->ent);
                        collision_ent->target_file = new_path;
                } else {
                        free(new_path);
                }
                string_list_insert(&collision_ent->source_files,
                                   pair->two->path);
        }
}

static char *check_for_directory_rename(struct merge_options *opt,
                                        const char *path,
                                        struct tree *tree,
                                        struct hashmap *dir_renames,
                                        struct hashmap *dir_rename_exclusions,
                                        struct hashmap *collisions,
                                        int *clean_merge)
{
        char *new_path = NULL;
        struct dir_rename_entry *entry = check_dir_renamed(path, dir_renames);
        struct dir_rename_entry *oentry = NULL;

        if (!entry)
                return new_path;

        /*
         * This next part is a little weird.  We do not want to do an
         * implicit rename into a directory we renamed on our side, because
         * that will result in a spurious rename/rename(1to2) conflict.  An
         * example:
         *   Base commit: dumbdir/afile, otherdir/bfile
         *   Side 1:      smrtdir/afile, otherdir/bfile
         *   Side 2:      dumbdir/afile, dumbdir/bfile
         * Here, while working on Side 1, we could notice that otherdir was
         * renamed/merged to dumbdir, and change the diff_filepair for
         * otherdir/bfile into a rename into dumbdir/bfile.  However, Side
         * 2 will notice the rename from dumbdir to smrtdir, and do the
         * transitive rename to move it from dumbdir/bfile to
         * smrtdir/bfile.  That gives us bfile in dumbdir vs being in
         * smrtdir, a rename/rename(1to2) conflict.  We really just want
         * the file to end up in smrtdir.  And the way to achieve that is
         * to not let Side1 do the rename to dumbdir, since we know that is
         * the source of one of our directory renames.
         *
         * That's why oentry and dir_rename_exclusions is here.
         *
         * As it turns out, this also prevents N-way transient rename
         * confusion; See testcases 9c and 9d of t6043.
         */
        oentry = dir_rename_find_entry(dir_rename_exclusions, entry->new_dir.buf);
        if (oentry) {
                output(opt, 1, _("WARNING: Avoiding applying %s -> %s rename "
                               "to %s, because %s itself was renamed."),
                       entry->dir, entry->new_dir.buf, path, entry->new_dir.buf);
        } else {
                new_path = handle_path_level_conflicts(opt, path, entry,
                                                       collisions, tree);
                *clean_merge &= (new_path != NULL);
        }

        return new_path;
}

static void apply_directory_rename_modifications(struct merge_options *opt,
                                                 struct diff_filepair *pair,
                                                 char *new_path,
                                                 struct rename *re,
                                                 struct tree *tree,
                                                 struct tree *o_tree,
                                                 struct tree *a_tree,
                                                 struct tree *b_tree,
                                                 struct string_list *entries)
{
        struct string_list_item *item;
        int stage = (tree == a_tree ? 2 : 3);
        int update_wd;

        /*
         * In all cases where we can do directory rename detection,
         * unpack_trees() will have read pair->two->path into the
         * index and the working copy.  We need to remove it so that
         * we can instead place it at new_path.  It is guaranteed to
         * not be untracked (unpack_trees() would have errored out
         * saying the file would have been overwritten), but it might
         * be dirty, though.
         */
        update_wd = !was_dirty(opt, pair->two->path);
        if (!update_wd)
                output(opt, 1, _("Refusing to lose dirty file at %s"),
                       pair->two->path);
        remove_file(opt, 1, pair->two->path, !update_wd);

        /* Find or create a new re->dst_entry */
        item = string_list_lookup(entries, new_path);
        if (item) {
                /*
                 * Since we're renaming on this side of history, and it's
                 * due to a directory rename on the other side of history
                 * (which we only allow when the directory in question no
                 * longer exists on the other side of history), the
                 * original entry for re->dst_entry is no longer
                 * necessary...
                 */
                re->dst_entry->processed = 1;

                /*
                 * ...because we'll be using this new one.
                 */
                re->dst_entry = item->util;
        } else {
                /*
                 * re->dst_entry is for the before-dir-rename path, and we
                 * need it to hold information for the after-dir-rename
                 * path.  Before creating a new entry, we need to mark the
                 * old one as unnecessary (...unless it is shared by
                 * src_entry, i.e. this didn't use to be a rename, in which
                 * case we can just allow the normal processing to happen
                 * for it).
                 */
                if (pair->status == 'R')
                        re->dst_entry->processed = 1;

                re->dst_entry = insert_stage_data(opt->repo, new_path,
                                                  o_tree, a_tree, b_tree,
                                                  entries);
                item = string_list_insert(entries, new_path);
                item->util = re->dst_entry;
        }

        /*
         * Update the stage_data with the information about the path we are
         * moving into place.  That slot will be empty and available for us
         * to write to because of the collision checks in
         * handle_path_level_conflicts().  In other words,
         * re->dst_entry->stages[stage].oid will be the null_oid, so it's
         * open for us to write to.
         *
         * It may be tempting to actually update the index at this point as
         * well, using update_stages_for_stage_data(), but as per the big
         * "NOTE" in update_stages(), doing so will modify the current
         * in-memory index which will break calls to would_lose_untracked()
         * that we need to make.  Instead, we need to just make sure that
         * the various handle_rename_*() functions update the index
         * explicitly rather than relying on unpack_trees() to have done it.
         */
        get_tree_entry(opt->repo,
                       &tree->object.oid,
                       pair->two->path,
                       &re->dst_entry->stages[stage].oid,
                       &re->dst_entry->stages[stage].mode);

        /*
         * Record the original change status (or 'type' of change).  If it
         * was originally an add ('A'), this lets us differentiate later
         * between a RENAME_DELETE conflict and RENAME_VIA_DIR (they
         * otherwise look the same).  If it was originally a rename ('R'),
         * this lets us remember and report accurately about the transitive
         * renaming that occurred via the directory rename detection.  Also,
         * record the original destination name.
         */
        re->dir_rename_original_type = pair->status;
        re->dir_rename_original_dest = pair->two->path;

        /*
         * We don't actually look at pair->status again, but it seems
         * pedagogically correct to adjust it.
         */
        pair->status = 'R';

        /*
         * Finally, record the new location.
         */
        pair->two->path = new_path;
}

/*
 * Get information of all renames which occurred in 'pairs', making use of
 * any implicit directory renames inferred from the other side of history.
 * We need the three trees in the merge ('o_tree', 'a_tree' and 'b_tree')
 * to be able to associate the correct cache entries with the rename
 * information; tree is always equal to either a_tree or b_tree.
 */
static struct string_list *get_renames(struct merge_options *opt,
                                       const char *branch,
                                       struct diff_queue_struct *pairs,
                                       struct hashmap *dir_renames,
                                       struct hashmap *dir_rename_exclusions,
                                       struct tree *tree,
                                       struct tree *o_tree,
                                       struct tree *a_tree,
                                       struct tree *b_tree,
                                       struct string_list *entries,
                                       int *clean_merge)
{
        int i;
        struct hashmap collisions;
        struct hashmap_iter iter;
        struct collision_entry *e;
        struct string_list *renames;

        compute_collisions(&collisions, dir_renames, pairs);
        CALLOC_ARRAY(renames, 1);

        for (i = 0; i < pairs->nr; ++i) {
                struct string_list_item *item;
                struct rename *re;
                struct diff_filepair *pair = pairs->queue[i];
                char *new_path; /* non-NULL only with directory renames */

                if (pair->status != 'A' && pair->status != 'R') {
                        diff_free_filepair(pair);
                        continue;
                }
                new_path = check_for_directory_rename(opt, pair->two->path, tree,
                                                      dir_renames,
                                                      dir_rename_exclusions,
                                                      &collisions,
                                                      clean_merge);
                if (pair->status != 'R' && !new_path) {
                        diff_free_filepair(pair);
                        continue;
                }

                re = xmalloc(sizeof(*re));
                re->processed = 0;
                re->pair = pair;
                re->branch = branch;
                re->dir_rename_original_type = '\0';
                re->dir_rename_original_dest = NULL;
                item = string_list_lookup(entries, re->pair->one->path);
                if (!item)
                        re->src_entry = insert_stage_data(opt->repo,
                                        re->pair->one->path,
                                        o_tree, a_tree, b_tree, entries);
                else
                        re->src_entry = item->util;

                item = string_list_lookup(entries, re->pair->two->path);
                if (!item)
                        re->dst_entry = insert_stage_data(opt->repo,
                                        re->pair->two->path,
                                        o_tree, a_tree, b_tree, entries);
                else
                        re->dst_entry = item->util;
                item = string_list_insert(renames, pair->one->path);
                item->util = re;
                if (new_path)
                        apply_directory_rename_modifications(opt, pair, new_path,
                                                             re, tree, o_tree,
                                                             a_tree, b_tree,
                                                             entries);
        }

        hashmap_for_each_entry(&collisions, &iter, e,
                                ent /* member name */) {
                free(e->target_file);
                string_list_clear(&e->source_files, 0);
        }
        hashmap_clear_and_free(&collisions, struct collision_entry, ent);
        return renames;
}

static int process_renames(struct merge_options *opt,
                           struct string_list *a_renames,
                           struct string_list *b_renames)
{
        int clean_merge = 1, i, j;
        struct string_list a_by_dst = STRING_LIST_INIT_NODUP;
        struct string_list b_by_dst = STRING_LIST_INIT_NODUP;
        const struct rename *sre;

        /*
         * FIXME: As string-list.h notes, it's O(n^2) to build a sorted
         * string_list one-by-one, but O(n log n) to build it unsorted and
         * then sort it.  Note that as we build the list, we do not need to
         * check if the existing destination path is already in the list,
         * because the structure of diffcore_rename guarantees we won't
         * have duplicates.
         */
        for (i = 0; i < a_renames->nr; i++) {
                sre = a_renames->items[i].util;
                string_list_insert(&a_by_dst, sre->pair->two->path)->util
                        = (void *)sre;
        }
        for (i = 0; i < b_renames->nr; i++) {
                sre = b_renames->items[i].util;
                string_list_insert(&b_by_dst, sre->pair->two->path)->util
                        = (void *)sre;
        }

        for (i = 0, j = 0; i < a_renames->nr || j < b_renames->nr;) {
                struct string_list *renames1, *renames2Dst;
                struct rename *ren1 = NULL, *ren2 = NULL;
                const char *ren1_src, *ren1_dst;
                struct string_list_item *lookup;

                if (i >= a_renames->nr) {
                        ren2 = b_renames->items[j++].util;
                } else if (j >= b_renames->nr) {
                        ren1 = a_renames->items[i++].util;
                } else {
                        int compare = strcmp(a_renames->items[i].string,
                                             b_renames->items[j].string);
                        if (compare <= 0)
                                ren1 = a_renames->items[i++].util;
                        if (compare >= 0)
                                ren2 = b_renames->items[j++].util;
                }

                /* TODO: refactor, so that 1/2 are not needed */
                if (ren1) {
                        renames1 = a_renames;
                        renames2Dst = &b_by_dst;
                } else {
                        renames1 = b_renames;
                        renames2Dst = &a_by_dst;
                        SWAP(ren2, ren1);
                }

                if (ren1->processed)
                        continue;
                ren1->processed = 1;
                ren1->dst_entry->processed = 1;
                /* BUG: We should only mark src_entry as processed if we
                 * are not dealing with a rename + add-source case.
                 */
                ren1->src_entry->processed = 1;

                ren1_src = ren1->pair->one->path;
                ren1_dst = ren1->pair->two->path;

                if (ren2) {
                        /* One file renamed on both sides */
                        const char *ren2_src = ren2->pair->one->path;
                        const char *ren2_dst = ren2->pair->two->path;
                        enum rename_type rename_type;
                        if (strcmp(ren1_src, ren2_src) != 0)
                                BUG("ren1_src != ren2_src");
                        ren2->dst_entry->processed = 1;
                        ren2->processed = 1;
                        if (strcmp(ren1_dst, ren2_dst) != 0) {
                                rename_type = RENAME_ONE_FILE_TO_TWO;
                                clean_merge = 0;
                        } else {
                                rename_type = RENAME_ONE_FILE_TO_ONE;
                                /* BUG: We should only remove ren1_src in
                                 * the base stage (think of rename +
                                 * add-source cases).
                                 */
                                remove_file(opt, 1, ren1_src, 1);
                                update_entry(ren1->dst_entry,
                                             ren1->pair->one,
                                             ren1->pair->two,
                                             ren2->pair->two);
                        }
                        setup_rename_conflict_info(rename_type, opt, ren1, ren2);
                } else if ((lookup = string_list_lookup(renames2Dst, ren1_dst))) {
                        /* Two different files renamed to the same thing */
                        char *ren2_dst;
                        ren2 = lookup->util;
                        ren2_dst = ren2->pair->two->path;
                        if (strcmp(ren1_dst, ren2_dst) != 0)
                                BUG("ren1_dst != ren2_dst");

                        clean_merge = 0;
                        ren2->processed = 1;
                        /*
                         * BUG: We should only mark src_entry as processed
                         * if we are not dealing with a rename + add-source
                         * case.
                         */
                        ren2->src_entry->processed = 1;

                        setup_rename_conflict_info(RENAME_TWO_FILES_TO_ONE,
                                                   opt, ren1, ren2);
                } else {
                        /* Renamed in 1, maybe changed in 2 */
                        /* we only use sha1 and mode of these */
                        struct diff_filespec src_other, dst_other;
                        int try_merge;

                        /*
                         * unpack_trees loads entries from common-commit
                         * into stage 1, from head-commit into stage 2, and
                         * from merge-commit into stage 3.  We keep track
                         * of which side corresponds to the rename.
                         */
                        int renamed_stage = a_renames == renames1 ? 2 : 3;
                        int other_stage =   a_renames == renames1 ? 3 : 2;

                        /*
                         * Directory renames have a funny corner case...
                         */
                        int renamed_to_self = !strcmp(ren1_src, ren1_dst);

                        /* BUG: We should only remove ren1_src in the base
                         * stage and in other_stage (think of rename +
                         * add-source case).
                         */
                        if (!renamed_to_self)
                                remove_file(opt, 1, ren1_src,
                                            renamed_stage == 2 ||
                                            !was_tracked(opt, ren1_src));

                        oidcpy(&src_other.oid,
                               &ren1->src_entry->stages[other_stage].oid);
                        src_other.mode = ren1->src_entry->stages[other_stage].mode;
                        oidcpy(&dst_other.oid,
                               &ren1->dst_entry->stages[other_stage].oid);
                        dst_other.mode = ren1->dst_entry->stages[other_stage].mode;
                        try_merge = 0;

                        if (oideq(&src_other.oid, null_oid()) &&
                            ren1->dir_rename_original_type == 'A') {
                                setup_rename_conflict_info(RENAME_VIA_DIR,
                                                           opt, ren1, NULL);
                        } else if (renamed_to_self) {
                                setup_rename_conflict_info(RENAME_NORMAL,
                                                           opt, ren1, NULL);
                        } else if (oideq(&src_other.oid, null_oid())) {
                                setup_rename_conflict_info(RENAME_DELETE,
                                                           opt, ren1, NULL);
                        } else if ((dst_other.mode == ren1->pair->two->mode) &&
                                   oideq(&dst_other.oid, &ren1->pair->two->oid)) {
                                /*
                                 * Added file on the other side identical to
                                 * the file being renamed: clean merge.
                                 * Also, there is no need to overwrite the
                                 * file already in the working copy, so call
                                 * update_file_flags() instead of
                                 * update_file().
                                 */
                                if (update_file_flags(opt,
                                                      ren1->pair->two,
                                                      ren1_dst,
                                                      1, /* update_cache */
                                                      0  /* update_wd    */))
                                        clean_merge = -1;
                        } else if (!oideq(&dst_other.oid, null_oid())) {
                                /*
                                 * Probably not a clean merge, but it's
                                 * premature to set clean_merge to 0 here,
                                 * because if the rename merges cleanly and
                                 * the merge exactly matches the newly added
                                 * file, then the merge will be clean.
                                 */
                                setup_rename_conflict_info(RENAME_ADD,
                                                           opt, ren1, NULL);
                        } else
                                try_merge = 1;

                        if (clean_merge < 0)
                                goto cleanup_and_return;
                        if (try_merge) {
                                struct diff_filespec *o, *a, *b;
                                src_other.path = (char *)ren1_src;

                                o = ren1->pair->one;
                                if (a_renames == renames1) {
                                        a = ren1->pair->two;
                                        b = &src_other;
                                } else {
                                        b = ren1->pair->two;
                                        a = &src_other;
                                }
                                update_entry(ren1->dst_entry, o, a, b);
                                setup_rename_conflict_info(RENAME_NORMAL,
                                                           opt, ren1, NULL);
                        }
                }
        }
cleanup_and_return:
        string_list_clear(&a_by_dst, 0);
        string_list_clear(&b_by_dst, 0);

        return clean_merge;
}

struct rename_info {
        struct string_list *head_renames;
        struct string_list *merge_renames;
};

static void initial_cleanup_rename(struct diff_queue_struct *pairs,
                                   struct hashmap *dir_renames)
{
        struct hashmap_iter iter;
        struct dir_rename_entry *e;

        hashmap_for_each_entry(dir_renames, &iter, e,
                                ent /* member name */) {
                free(e->dir);
                strbuf_release(&e->new_dir);
                /* possible_new_dirs already cleared in get_directory_renames */
        }
        hashmap_clear_and_free(dir_renames, struct dir_rename_entry, ent);
        free(dir_renames);

        free(pairs->queue);
        free(pairs);
}

static int detect_and_process_renames(struct merge_options *opt,
                                      struct tree *common,
                                      struct tree *head,
                                      struct tree *merge,
                                      struct string_list *entries,
                                      struct rename_info *ri)
{
        struct diff_queue_struct *head_pairs, *merge_pairs;
        struct hashmap *dir_re_head, *dir_re_merge;
        int clean = 1;

        ri->head_renames = NULL;
        ri->merge_renames = NULL;

        if (!merge_detect_rename(opt))
                return 1;

        head_pairs = get_diffpairs(opt, common, head);
        merge_pairs = get_diffpairs(opt, common, merge);

        if ((opt->detect_directory_renames == MERGE_DIRECTORY_RENAMES_TRUE) ||
            (opt->detect_directory_renames == MERGE_DIRECTORY_RENAMES_CONFLICT &&
             !opt->priv->call_depth)) {
                dir_re_head = get_directory_renames(head_pairs);
                dir_re_merge = get_directory_renames(merge_pairs);

                handle_directory_level_conflicts(opt,
                                                 dir_re_head, head,
                                                 dir_re_merge, merge);
        } else {
                dir_re_head  = xmalloc(sizeof(*dir_re_head));
                dir_re_merge = xmalloc(sizeof(*dir_re_merge));
                dir_rename_init(dir_re_head);
                dir_rename_init(dir_re_merge);
        }

        ri->head_renames  = get_renames(opt, opt->branch1, head_pairs,
                                        dir_re_merge, dir_re_head, head,
                                        common, head, merge, entries,
                                        &clean);
        if (clean < 0)
                goto cleanup;
        ri->merge_renames = get_renames(opt, opt->branch2, merge_pairs,
                                        dir_re_head, dir_re_merge, merge,
                                        common, head, merge, entries,
                                        &clean);
        if (clean < 0)
                goto cleanup;
        clean &= process_renames(opt, ri->head_renames, ri->merge_renames);

cleanup:
        /*
         * Some cleanup is deferred until cleanup_renames() because the
         * data structures are still needed and referenced in
         * process_entry().  But there are a few things we can free now.
         */
        initial_cleanup_rename(head_pairs, dir_re_head);
        initial_cleanup_rename(merge_pairs, dir_re_merge);

        return clean;
}

static void final_cleanup_rename(struct string_list *rename)
{
        const struct rename *re;
        int i;

        if (!rename)
                return;

        for (i = 0; i < rename->nr; i++) {
                re = rename->items[i].util;
                diff_free_filepair(re->pair);
        }
        string_list_clear(rename, 1);
        free(rename);
}

static void final_cleanup_renames(struct rename_info *re_info)
{
        final_cleanup_rename(re_info->head_renames);
        final_cleanup_rename(re_info->merge_renames);
}

static int read_oid_strbuf(struct merge_options *opt,
                           const struct object_id *oid,
                           struct strbuf *dst)
{
        void *buf;
        enum object_type type;
        unsigned long size;
        buf = repo_read_object_file(the_repository, oid, &type, &size);
        if (!buf)
                return err(opt, _("cannot read object %s"), oid_to_hex(oid));
        if (type != OBJ_BLOB) {
                free(buf);
                return err(opt, _("object %s is not a blob"), oid_to_hex(oid));
        }
        strbuf_attach(dst, buf, size, size + 1);
        return 0;
}

static int blob_unchanged(struct merge_options *opt,
                          const struct diff_filespec *o,
                          const struct diff_filespec *a,
                          int renormalize, const char *path)
{
        struct strbuf obuf = STRBUF_INIT;
        struct strbuf abuf = STRBUF_INIT;
        int ret = 0; /* assume changed for safety */
        struct index_state *idx = opt->repo->index;

        if (a->mode != o->mode)
                return 0;
        if (oideq(&o->oid, &a->oid))
                return 1;
        if (!renormalize)
                return 0;

        if (read_oid_strbuf(opt, &o->oid, &obuf) ||
            read_oid_strbuf(opt, &a->oid, &abuf))
                goto error_return;
        /*
         * Note: binary | is used so that both renormalizations are
         * performed.  Comparison can be skipped if both files are
         * unchanged since their sha1s have already been compared.
         */
        if (renormalize_buffer(idx, path, obuf.buf, obuf.len, &obuf) |
            renormalize_buffer(idx, path, abuf.buf, abuf.len, &abuf))
                ret = (obuf.len == abuf.len && !memcmp(obuf.buf, abuf.buf, obuf.len));

error_return:
        strbuf_release(&obuf);
        strbuf_release(&abuf);
        return ret;
}

static int handle_modify_delete(struct merge_options *opt,
                                const char *path,
                                const struct diff_filespec *o,
                                const struct diff_filespec *a,
                                const struct diff_filespec *b)
{
        const char *modify_branch, *delete_branch;
        const struct diff_filespec *changed;

        if (is_valid(a)) {
                modify_branch = opt->branch1;
                delete_branch = opt->branch2;
                changed = a;
        } else {
                modify_branch = opt->branch2;
                delete_branch = opt->branch1;
                changed = b;
        }

        return handle_change_delete(opt,
                                    path, NULL,
                                    o, changed,
                                    modify_branch, delete_branch,
                                    _("modify"), _("modified"));
}

static int handle_content_merge(struct merge_file_info *mfi,
                                struct merge_options *opt,
                                const char *path,
                                int is_dirty,
                                const struct diff_filespec *o,
                                const struct diff_filespec *a,
                                const struct diff_filespec *b,
                                struct rename_conflict_info *ci)
{
        const char *reason = _("content");
        unsigned df_conflict_remains = 0;

        if (!is_valid(o))
                reason = _("add/add");

        assert(o->path && a->path && b->path);
        if (ci && dir_in_way(opt->repo->index, path, !opt->priv->call_depth,
                             S_ISGITLINK(ci->ren1->pair->two->mode)))
                df_conflict_remains = 1;

        if (merge_mode_and_contents(opt, o, a, b, path,
                                    opt->branch1, opt->branch2,
                                    opt->priv->call_depth * 2, mfi))
                return -1;

        /*
         * We can skip updating the working tree file iff:
         *   a) The merge is clean
         *   b) The merge matches what was in HEAD (content, mode, pathname)
         *   c) The target path is usable (i.e. not involved in D/F conflict)
         */
        if (mfi->clean && was_tracked_and_matches(opt, path, &mfi->blob) &&
            !df_conflict_remains) {
                int pos;
                struct cache_entry *ce;

                output(opt, 3, _("Skipped %s (merged same as existing)"), path);
                if (add_cacheinfo(opt, &mfi->blob, path,
                                  0, (!opt->priv->call_depth && !is_dirty), 0))
                        return -1;
                /*
                 * However, add_cacheinfo() will delete the old cache entry
                 * and add a new one.  We need to copy over any skip_worktree
                 * flag to avoid making the file appear as if it were
                 * deleted by the user.
                 */
                pos = index_name_pos(&opt->priv->orig_index, path, strlen(path));
                ce = opt->priv->orig_index.cache[pos];
                if (ce_skip_worktree(ce)) {
                        pos = index_name_pos(opt->repo->index, path, strlen(path));
                        ce = opt->repo->index->cache[pos];
                        ce->ce_flags |= CE_SKIP_WORKTREE;
                }
                return mfi->clean;
        }

        if (!mfi->clean) {
                if (S_ISGITLINK(mfi->blob.mode))
                        reason = _("submodule");
                output(opt, 1, _("CONFLICT (%s): Merge conflict in %s"),
                                reason, path);
                if (ci && !df_conflict_remains)
                        if (update_stages(opt, path, o, a, b))
                                return -1;
        }

        if (df_conflict_remains || is_dirty) {
                char *new_path;
                if (opt->priv->call_depth) {
                        remove_file_from_index(opt->repo->index, path);
                } else {
                        if (!mfi->clean) {
                                if (update_stages(opt, path, o, a, b))
                                        return -1;
                        } else {
                                int file_from_stage2 = was_tracked(opt, path);

                                if (update_stages(opt, path, NULL,
                                                  file_from_stage2 ? &mfi->blob : NULL,
                                                  file_from_stage2 ? NULL : &mfi->blob))
                                        return -1;
                        }

                }
                new_path = unique_path(opt, path, ci->ren1->branch);
                if (is_dirty) {
                        output(opt, 1, _("Refusing to lose dirty file at %s"),
                               path);
                }
                output(opt, 1, _("Adding as %s instead"), new_path);
                if (update_file(opt, 0, &mfi->blob, new_path)) {
                        free(new_path);
                        return -1;
                }
                free(new_path);
                mfi->clean = 0;
        } else if (update_file(opt, mfi->clean, &mfi->blob, path))
                return -1;
        return !is_dirty && mfi->clean;
}

static int handle_rename_normal(struct merge_options *opt,
                                const char *path,
                                const struct diff_filespec *o,
                                const struct diff_filespec *a,
                                const struct diff_filespec *b,
                                struct rename_conflict_info *ci)
{
        struct rename *ren = ci->ren1;
        struct merge_file_info mfi;
        int clean;

        /* Merge the content and write it out */
        clean = handle_content_merge(&mfi, opt, path, was_dirty(opt, path),
                                     o, a, b, ci);

        if (clean &&
            opt->detect_directory_renames == MERGE_DIRECTORY_RENAMES_CONFLICT &&
            ren->dir_rename_original_dest) {
                if (update_stages(opt, path,
                                  &mfi.blob, &mfi.blob, &mfi.blob))
                        return -1;
                clean = 0; /* not clean, but conflicted */
        }
        return clean;
}

static void dir_rename_warning(const char *msg,
                               int is_add,
                               int clean,
                               struct merge_options *opt,
                               struct rename *ren)
{
        const char *other_branch;
        other_branch = (ren->branch == opt->branch1 ?
                        opt->branch2 : opt->branch1);
        if (is_add) {
                output(opt, clean ? 2 : 1, msg,
                       ren->pair->one->path, ren->branch,
                       other_branch, ren->pair->two->path);
                return;
        }
        output(opt, clean ? 2 : 1, msg,
               ren->pair->one->path, ren->dir_rename_original_dest, ren->branch,
               other_branch, ren->pair->two->path);
}
static int warn_about_dir_renamed_entries(struct merge_options *opt,
                                          struct rename *ren)
{
        const char *msg;
        int clean = 1, is_add;

        if (!ren)
                return clean;

        /* Return early if ren was not affected/created by a directory rename */
        if (!ren->dir_rename_original_dest)
                return clean;

        /* Sanity checks */
        assert(opt->detect_directory_renames > MERGE_DIRECTORY_RENAMES_NONE);
        assert(ren->dir_rename_original_type == 'A' ||
               ren->dir_rename_original_type == 'R');

        /* Check whether to treat directory renames as a conflict */
        clean = (opt->detect_directory_renames == MERGE_DIRECTORY_RENAMES_TRUE);

        is_add = (ren->dir_rename_original_type == 'A');
        if (ren->dir_rename_original_type == 'A' && clean) {
                msg = _("Path updated: %s added in %s inside a "
                        "directory that was renamed in %s; moving it to %s.");
        } else if (ren->dir_rename_original_type == 'A' && !clean) {
                msg = _("CONFLICT (file location): %s added in %s "
                        "inside a directory that was renamed in %s, "
                        "suggesting it should perhaps be moved to %s.");
        } else if (ren->dir_rename_original_type == 'R' && clean) {
                msg = _("Path updated: %s renamed to %s in %s, inside a "
                        "directory that was renamed in %s; moving it to %s.");
        } else if (ren->dir_rename_original_type == 'R' && !clean) {
                msg = _("CONFLICT (file location): %s renamed to %s in %s, "
                        "inside a directory that was renamed in %s, "
                        "suggesting it should perhaps be moved to %s.");
        } else {
                BUG("Impossible dir_rename_original_type/clean combination");
        }
        dir_rename_warning(msg, is_add, clean, opt, ren);

        return clean;
}

/* Per entry merge function */
static int process_entry(struct merge_options *opt,
                         const char *path, struct stage_data *entry)
{
        int clean_merge = 1;
        int normalize = opt->renormalize;

        struct diff_filespec *o = &entry->stages[1];
        struct diff_filespec *a = &entry->stages[2];
        struct diff_filespec *b = &entry->stages[3];
        int o_valid = is_valid(o);
        int a_valid = is_valid(a);
        int b_valid = is_valid(b);
        o->path = a->path = b->path = (char*)path;

        entry->processed = 1;
        if (entry->rename_conflict_info) {
                struct rename_conflict_info *ci = entry->rename_conflict_info;
                struct diff_filespec *temp;
                int path_clean;

                path_clean = warn_about_dir_renamed_entries(opt, ci->ren1);
                path_clean &= warn_about_dir_renamed_entries(opt, ci->ren2);

                /*
                 * For cases with a single rename, {o,a,b}->path have all been
                 * set to the rename target path; we need to set two of these
                 * back to the rename source.
                 * For rename/rename conflicts, we'll manually fix paths below.
                 */
                temp = (opt->branch1 == ci->ren1->branch) ? b : a;
                o->path = temp->path = ci->ren1->pair->one->path;
                if (ci->ren2) {
                        assert(opt->branch1 == ci->ren1->branch);
                }

                switch (ci->rename_type) {
                case RENAME_NORMAL:
                case RENAME_ONE_FILE_TO_ONE:
                        clean_merge = handle_rename_normal(opt, path, o, a, b,
                                                           ci);
                        break;
                case RENAME_VIA_DIR:
                        clean_merge = handle_rename_via_dir(opt, ci);
                        break;
                case RENAME_ADD:
                        /*
                         * Probably unclean merge, but if the renamed file
                         * merges cleanly and the result can then be
                         * two-way merged cleanly with the added file, I
                         * guess it's a clean merge?
                         */
                        clean_merge = handle_rename_add(opt, ci);
                        break;
                case RENAME_DELETE:
                        clean_merge = 0;
                        if (handle_rename_delete(opt, ci))
                                clean_merge = -1;
                        break;
                case RENAME_ONE_FILE_TO_TWO:
                        /*
                         * Manually fix up paths; note:
                         * ren[12]->pair->one->path are equal.
                         */
                        o->path = ci->ren1->pair->one->path;
                        a->path = ci->ren1->pair->two->path;
                        b->path = ci->ren2->pair->two->path;

                        clean_merge = 0;
                        if (handle_rename_rename_1to2(opt, ci))
                                clean_merge = -1;
                        break;
                case RENAME_TWO_FILES_TO_ONE:
                        /*
                         * Manually fix up paths; note,
                         * ren[12]->pair->two->path are actually equal.
                         */
                        o->path = NULL;
                        a->path = ci->ren1->pair->two->path;
                        b->path = ci->ren2->pair->two->path;

                        /*
                         * Probably unclean merge, but if the two renamed
                         * files merge cleanly and the two resulting files
                         * can then be two-way merged cleanly, I guess it's
                         * a clean merge?
                         */
                        clean_merge = handle_rename_rename_2to1(opt, ci);
                        break;
                default:
                        entry->processed = 0;
                        break;
                }
                if (path_clean < clean_merge)
                        clean_merge = path_clean;
        } else if (o_valid && (!a_valid || !b_valid)) {
                /* Case A: Deleted in one */
                if ((!a_valid && !b_valid) ||
                    (!b_valid && blob_unchanged(opt, o, a, normalize, path)) ||
                    (!a_valid && blob_unchanged(opt, o, b, normalize, path))) {
                        /* Deleted in both or deleted in one and
                         * unchanged in the other */
                        if (a_valid)
                                output(opt, 2, _("Removing %s"), path);
                        /* do not touch working file if it did not exist */
                        remove_file(opt, 1, path, !a_valid);
                } else {
                        /* Modify/delete; deleted side may have put a directory in the way */
                        clean_merge = 0;
                        if (handle_modify_delete(opt, path, o, a, b))
                                clean_merge = -1;
                }
        } else if ((!o_valid && a_valid && !b_valid) ||
                   (!o_valid && !a_valid && b_valid)) {
                /* Case B: Added in one. */
                /* [nothing|directory] -> ([nothing|directory], file) */

                const char *add_branch;
                const char *other_branch;
                const char *conf;
                const struct diff_filespec *contents;

                if (a_valid) {
                        add_branch = opt->branch1;
                        other_branch = opt->branch2;
                        contents = a;
                        conf = _("file/directory");
                } else {
                        add_branch = opt->branch2;
                        other_branch = opt->branch1;
                        contents = b;
                        conf = _("directory/file");
                }
                if (dir_in_way(opt->repo->index, path,
                               !opt->priv->call_depth && !S_ISGITLINK(a->mode),
                               0)) {
                        char *new_path = unique_path(opt, path, add_branch);
                        clean_merge = 0;
                        output(opt, 1, _("CONFLICT (%s): There is a directory with name %s in %s. "
                               "Adding %s as %s"),
                               conf, path, other_branch, path, new_path);
                        if (update_file(opt, 0, contents, new_path))
                                clean_merge = -1;
                        else if (opt->priv->call_depth)
                                remove_file_from_index(opt->repo->index, path);
                        free(new_path);
                } else {
                        output(opt, 2, _("Adding %s"), path);
                        /* do not overwrite file if already present */
                        if (update_file_flags(opt, contents, path, 1, !a_valid))
                                clean_merge = -1;
                }
        } else if (a_valid && b_valid) {
                if (!o_valid) {
                        /* Case C: Added in both (check for same permissions) */
                        output(opt, 1,
                               _("CONFLICT (add/add): Merge conflict in %s"),
                               path);
                        clean_merge = handle_file_collision(opt,
                                                            path, NULL, NULL,
                                                            opt->branch1,
                                                            opt->branch2,
                                                            a, b);
                } else {
                        /* case D: Modified in both, but differently. */
                        struct merge_file_info mfi;
                        int is_dirty = 0; /* unpack_trees would have bailed if dirty */
                        clean_merge = handle_content_merge(&mfi, opt, path,
                                                           is_dirty,
                                                           o, a, b, NULL);
                }
        } else if (!o_valid && !a_valid && !b_valid) {
                /*
                 * this entry was deleted altogether. a_mode == 0 means
                 * we had that path and want to actively remove it.
                 */
                remove_file(opt, 1, path, !a->mode);
        } else
                BUG("fatal merge failure, shouldn't happen.");

        return clean_merge;
}

static int merge_trees_internal(struct merge_options *opt,
                                struct tree *head,
                                struct tree *merge,
                                struct tree *merge_base,
                                struct tree **result)
{
        struct index_state *istate = opt->repo->index;
        int code, clean;

        if (opt->subtree_shift) {
                merge = shift_tree_object(opt->repo, head, merge,
                                          opt->subtree_shift);
                merge_base = shift_tree_object(opt->repo, head, merge_base,
                                               opt->subtree_shift);
        }

        if (oideq(&merge_base->object.oid, &merge->object.oid)) {
                output(opt, 0, _("Already up to date."));
                *result = head;
                return 1;
        }

        code = unpack_trees_start(opt, merge_base, head, merge);

        if (code != 0) {
                if (show(opt, 4) || opt->priv->call_depth)
                        err(opt, _("merging of trees %s and %s failed"),
                            oid_to_hex(&head->object.oid),
                            oid_to_hex(&merge->object.oid));
                unpack_trees_finish(opt);
                return -1;
        }

        if (unmerged_index(istate)) {
                struct string_list *entries;
                struct rename_info re_info;
                int i;
                /*
                 * Only need the hashmap while processing entries, so
                 * initialize it here and free it when we are done running
                 * through the entries. Keeping it in the merge_options as
                 * opposed to decaring a local hashmap is for convenience
                 * so that we don't have to pass it to around.
                 */
                hashmap_init(&opt->priv->current_file_dir_set, path_hashmap_cmp,
                             NULL, 512);
                get_files_dirs(opt, head);
                get_files_dirs(opt, merge);

                entries = get_unmerged(opt->repo->index);
                clean = detect_and_process_renames(opt, merge_base, head, merge,
                                                   entries, &re_info);
                record_df_conflict_files(opt, entries);
                if (clean < 0)
                        goto cleanup;
                for (i = entries->nr-1; 0 <= i; i--) {
                        const char *path = entries->items[i].string;
                        struct stage_data *e = entries->items[i].util;
                        if (!e->processed) {
                                int ret = process_entry(opt, path, e);
                                if (!ret)
                                        clean = 0;
                                else if (ret < 0) {
                                        clean = ret;
                                        goto cleanup;
                                }
                        }
                }
                for (i = 0; i < entries->nr; i++) {
                        struct stage_data *e = entries->items[i].util;
                        if (!e->processed)
                                BUG("unprocessed path??? %s",
                                    entries->items[i].string);
                }

        cleanup:
                final_cleanup_renames(&re_info);

                string_list_clear(entries, 1);
                free(entries);

                hashmap_clear_and_free(&opt->priv->current_file_dir_set,
                                        struct path_hashmap_entry, e);

                if (clean < 0) {
                        unpack_trees_finish(opt);
                        return clean;
                }
        }
        else
                clean = 1;

        unpack_trees_finish(opt);

        if (opt->priv->call_depth &&
            !(*result = write_in_core_index_as_tree(opt->repo)))
                return -1;

        return clean;
}

/*
 * Merge the commits h1 and h2, returning a flag (int) indicating the
 * cleanness of the merge.  Also, if opt->priv->call_depth, create a
 * virtual commit and write its location to *result.
 */
static int merge_recursive_internal(struct merge_options *opt,
                                    struct commit *h1,
                                    struct commit *h2,
                                    struct commit_list *merge_bases,
                                    struct commit **result)
{
        struct commit_list *iter;
        struct commit *merged_merge_bases;
        struct tree *result_tree;
        int clean;
        const char *ancestor_name;
        struct strbuf merge_base_abbrev = STRBUF_INIT;

        if (show(opt, 4)) {
                output(opt, 4, _("Merging:"));
                output_commit_title(opt, h1);
                output_commit_title(opt, h2);
        }

        if (!merge_bases) {
                merge_bases = repo_get_merge_bases(the_repository, h1, h2);
                merge_bases = reverse_commit_list(merge_bases);
        }

        if (show(opt, 5)) {
                unsigned cnt = commit_list_count(merge_bases);

                output(opt, 5, Q_("found %u common ancestor:",
                                "found %u common ancestors:", cnt), cnt);
                for (iter = merge_bases; iter; iter = iter->next)
                        output_commit_title(opt, iter->item);
        }

        merged_merge_bases = pop_commit(&merge_bases);
        if (!merged_merge_bases) {
                /* if there is no common ancestor, use an empty tree */
                struct tree *tree;

                tree = lookup_tree(opt->repo, opt->repo->hash_algo->empty_tree);
                merged_merge_bases = make_virtual_commit(opt->repo, tree,
                                                         "ancestor");
                ancestor_name = "empty tree";
        } else if (opt->ancestor && !opt->priv->call_depth) {
                ancestor_name = opt->ancestor;
        } else if (merge_bases) {
                ancestor_name = "merged common ancestors";
        } else {
                strbuf_add_unique_abbrev(&merge_base_abbrev,
                                         &merged_merge_bases->object.oid,
                                         DEFAULT_ABBREV);
                ancestor_name = merge_base_abbrev.buf;
        }

        for (iter = merge_bases; iter; iter = iter->next) {
                const char *saved_b1, *saved_b2;
                opt->priv->call_depth++;
                /*
                 * When the merge fails, the result contains files
                 * with conflict markers. The cleanness flag is
                 * ignored (unless indicating an error), it was never
                 * actually used, as result of merge_trees has always
                 * overwritten it: the committed "conflicts" were
                 * already resolved.
                 */
                discard_index(opt->repo->index);
                saved_b1 = opt->branch1;
                saved_b2 = opt->branch2;
                opt->branch1 = "Temporary merge branch 1";
                opt->branch2 = "Temporary merge branch 2";
                if (merge_recursive_internal(opt, merged_merge_bases, iter->item,
                                             NULL, &merged_merge_bases) < 0)
                        return -1;
                opt->branch1 = saved_b1;
                opt->branch2 = saved_b2;
                opt->priv->call_depth--;

                if (!merged_merge_bases)
                        return err(opt, _("merge returned no commit"));
        }

        /*
         * FIXME: Since merge_recursive_internal() is only ever called by
         * places that ensure the index is loaded first
         * (e.g. builtin/merge.c, rebase/sequencer, etc.), in the common
         * case where the merge base was unique that means when we get here
         * we immediately discard the index and re-read it, which is a
         * complete waste of time.  We should only be discarding and
         * re-reading if we were forced to recurse.
         */
        discard_index(opt->repo->index);
        if (!opt->priv->call_depth)
                repo_read_index(opt->repo);

        opt->ancestor = ancestor_name;
        clean = merge_trees_internal(opt,
                                     repo_get_commit_tree(opt->repo, h1),
                                     repo_get_commit_tree(opt->repo, h2),
                                     repo_get_commit_tree(opt->repo,
                                                          merged_merge_bases),
                                     &result_tree);
        strbuf_release(&merge_base_abbrev);
        opt->ancestor = NULL;  /* avoid accidental re-use of opt->ancestor */
        if (clean < 0) {
                flush_output(opt);
                return clean;
        }

        if (opt->priv->call_depth) {
                *result = make_virtual_commit(opt->repo, result_tree,
                                              "merged tree");
                commit_list_insert(h1, &(*result)->parents);
                commit_list_insert(h2, &(*result)->parents->next);
        }
        return clean;
}

static int merge_start(struct merge_options *opt, struct tree *head)
{
        struct strbuf sb = STRBUF_INIT;

        /* Sanity checks on opt */
        assert(opt->repo);

        assert(opt->branch1 && opt->branch2);

        assert(opt->detect_renames >= -1 &&
               opt->detect_renames <= DIFF_DETECT_COPY);
        assert(opt->detect_directory_renames >= MERGE_DIRECTORY_RENAMES_NONE &&
               opt->detect_directory_renames <= MERGE_DIRECTORY_RENAMES_TRUE);
        assert(opt->rename_limit >= -1);
        assert(opt->rename_score >= 0 && opt->rename_score <= MAX_SCORE);
        assert(opt->show_rename_progress >= 0 && opt->show_rename_progress <= 1);

        assert(opt->xdl_opts >= 0);
        assert(opt->recursive_variant >= MERGE_VARIANT_NORMAL &&
               opt->recursive_variant <= MERGE_VARIANT_THEIRS);

        assert(opt->verbosity >= 0 && opt->verbosity <= 5);
        assert(opt->buffer_output <= 2);
        assert(opt->obuf.len == 0);

        assert(opt->priv == NULL);

        /* Not supported; option specific to merge-ort */
        assert(!opt->record_conflict_msgs_as_headers);
        assert(!opt->msg_header_prefix);

        /* Sanity check on repo state; index must match head */
        if (repo_index_has_changes(opt->repo, head, &sb)) {
                err(opt, _("Your local changes to the following files would be overwritten by merge:\n  %s"),
                    sb.buf);
                strbuf_release(&sb);
                return -1;
        }

        CALLOC_ARRAY(opt->priv, 1);
        string_list_init_dup(&opt->priv->df_conflict_file_set);
        return 0;
}

static void merge_finalize(struct merge_options *opt)
{
        flush_output(opt);
        if (!opt->priv->call_depth && opt->buffer_output < 2)
                strbuf_release(&opt->obuf);
        if (show(opt, 2))
                diff_warn_rename_limit("merge.renamelimit",
                                       opt->priv->needed_rename_limit, 0);
        FREE_AND_NULL(opt->priv);
}

int merge_trees(struct merge_options *opt,
                struct tree *head,
                struct tree *merge,
                struct tree *merge_base)
{
        int clean;
        struct tree *ignored;

        assert(opt->ancestor != NULL);

        if (merge_start(opt, head))
                return -1;
        clean = merge_trees_internal(opt, head, merge, merge_base, &ignored);
        merge_finalize(opt);

        return clean;
}

int merge_recursive(struct merge_options *opt,
                    struct commit *h1,
                    struct commit *h2,
                    struct commit_list *merge_bases,
                    struct commit **result)
{
        int clean;

        assert(opt->ancestor == NULL ||
               !strcmp(opt->ancestor, "constructed merge base"));

        prepare_repo_settings(opt->repo);
        opt->repo->settings.command_requires_full_index = 1;

        if (merge_start(opt, repo_get_commit_tree(opt->repo, h1)))
                return -1;
        clean = merge_recursive_internal(opt, h1, h2, merge_bases, result);
        merge_finalize(opt);

        return clean;
}

static struct commit *get_ref(struct repository *repo,
                              const struct object_id *oid,
                              const char *name)
{
        struct object *object;

        object = deref_tag(repo, parse_object(repo, oid),
                           name, strlen(name));
        if (!object)
                return NULL;
        if (object->type == OBJ_TREE)
                return make_virtual_commit(repo, (struct tree*)object, name);
        if (object->type != OBJ_COMMIT)
                return NULL;
        if (repo_parse_commit(repo, (struct commit *)object))
                return NULL;
        return (struct commit *)object;
}

int merge_recursive_generic(struct merge_options *opt,
                            const struct object_id *head,
                            const struct object_id *merge,
                            int num_merge_bases,
                            const struct object_id **merge_bases,
                            struct commit **result)
{
        int clean;
        struct lock_file lock = LOCK_INIT;
        struct commit *head_commit = get_ref(opt->repo, head, opt->branch1);
        struct commit *next_commit = get_ref(opt->repo, merge, opt->branch2);
        struct commit_list *ca = NULL;

        if (merge_bases) {
                int i;
                for (i = 0; i < num_merge_bases; ++i) {
                        struct commit *base;
                        if (!(base = get_ref(opt->repo, merge_bases[i],
                                             oid_to_hex(merge_bases[i]))))
                                return err(opt, _("Could not parse object '%s'"),
                                           oid_to_hex(merge_bases[i]));
                        commit_list_insert(base, &ca);
                }
                if (num_merge_bases == 1)
                        opt->ancestor = "constructed merge base";
        }

        repo_hold_locked_index(opt->repo, &lock, LOCK_DIE_ON_ERROR);
        clean = merge_recursive(opt, head_commit, next_commit, ca,
                                result);
        if (clean < 0) {
                rollback_lock_file(&lock);
                return clean;
        }

        if (write_locked_index(opt->repo->index, &lock,
                               COMMIT_LOCK | SKIP_IF_UNCHANGED))
                return err(opt, _("Unable to write index."));

        return clean ? 0 : 1;
}

static void merge_recursive_config(struct merge_options *opt)
{
        char *value = NULL;
        int renormalize = 0;
        git_config_get_int("merge.verbosity", &opt->verbosity);
        git_config_get_int("diff.renamelimit", &opt->rename_limit);
        git_config_get_int("merge.renamelimit", &opt->rename_limit);
        git_config_get_bool("merge.renormalize", &renormalize);
        opt->renormalize = renormalize;
        if (!git_config_get_string("diff.renames", &value)) {
                opt->detect_renames = git_config_rename("diff.renames", value);
                free(value);
        }
        if (!git_config_get_string("merge.renames", &value)) {
                opt->detect_renames = git_config_rename("merge.renames", value);
                free(value);
        }
        if (!git_config_get_string("merge.directoryrenames", &value)) {
                int boolval = git_parse_maybe_bool(value);
                if (0 <= boolval) {
                        opt->detect_directory_renames = boolval ?
                                MERGE_DIRECTORY_RENAMES_TRUE :
                                MERGE_DIRECTORY_RENAMES_NONE;
                } else if (!strcasecmp(value, "conflict")) {
                        opt->detect_directory_renames =
                                MERGE_DIRECTORY_RENAMES_CONFLICT;
                } /* avoid erroring on values from future versions of git */
                free(value);
        }
        git_config(git_xmerge_config, NULL);
}

void init_merge_options(struct merge_options *opt,
                        struct repository *repo)
{
        const char *merge_verbosity;
        memset(opt, 0, sizeof(struct merge_options));

        opt->repo = repo;

        opt->detect_renames = -1;
        opt->detect_directory_renames = MERGE_DIRECTORY_RENAMES_CONFLICT;
        opt->rename_limit = -1;

        opt->verbosity = 2;
        opt->buffer_output = 1;
        strbuf_init(&opt->obuf, 0);

        opt->renormalize = 0;

        merge_recursive_config(opt);
        merge_verbosity = getenv("GIT_MERGE_VERBOSITY");
        if (merge_verbosity)
                opt->verbosity = strtol(merge_verbosity, NULL, 10);
        if (opt->verbosity >= 5)
                opt->buffer_output = 0;
}

/*
 * For now, members of merge_options do not need deep copying, but
 * it may change in the future, in which case we would need to update
 * this, and also make a matching change to clear_merge_options() to
 * release the resources held by a copied instance.
 */
void copy_merge_options(struct merge_options *dst, struct merge_options *src)
{
        *dst = *src;
}

void clear_merge_options(struct merge_options *opt UNUSED)
{
        ; /* no-op as our copy is shallow right now */
}

int parse_merge_opt(struct merge_options *opt, const char *s)
{
        const char *arg;

        if (!s || !*s)
                return -1;
        if (!strcmp(s, "ours"))
                opt->recursive_variant = MERGE_VARIANT_OURS;
        else if (!strcmp(s, "theirs"))
                opt->recursive_variant = MERGE_VARIANT_THEIRS;
        else if (!strcmp(s, "subtree"))
                opt->subtree_shift = "";
        else if (skip_prefix(s, "subtree=", &arg))
                opt->subtree_shift = arg;
        else if (!strcmp(s, "patience"))
                opt->xdl_opts = DIFF_WITH_ALG(opt, PATIENCE_DIFF);
        else if (!strcmp(s, "histogram"))
                opt->xdl_opts = DIFF_WITH_ALG(opt, HISTOGRAM_DIFF);
        else if (skip_prefix(s, "diff-algorithm=", &arg)) {
                long value = parse_algorithm_value(arg);
                if (value < 0)
                        return -1;
                /* clear out previous settings */
                DIFF_XDL_CLR(opt, NEED_MINIMAL);
                opt->xdl_opts &= ~XDF_DIFF_ALGORITHM_MASK;
                opt->xdl_opts |= value;
        }
        else if (!strcmp(s, "ignore-space-change"))
                DIFF_XDL_SET(opt, IGNORE_WHITESPACE_CHANGE);
        else if (!strcmp(s, "ignore-all-space"))
                DIFF_XDL_SET(opt, IGNORE_WHITESPACE);
        else if (!strcmp(s, "ignore-space-at-eol"))
                DIFF_XDL_SET(opt, IGNORE_WHITESPACE_AT_EOL);
        else if (!strcmp(s, "ignore-cr-at-eol"))
                DIFF_XDL_SET(opt, IGNORE_CR_AT_EOL);
        else if (!strcmp(s, "renormalize"))
                opt->renormalize = 1;
        else if (!strcmp(s, "no-renormalize"))
                opt->renormalize = 0;
        else if (!strcmp(s, "no-renames"))
                opt->detect_renames = 0;
        else if (!strcmp(s, "find-renames")) {
                opt->detect_renames = 1;
                opt->rename_score = 0;
        }
        else if (skip_prefix(s, "find-renames=", &arg) ||
                 skip_prefix(s, "rename-threshold=", &arg)) {
                if ((opt->rename_score = parse_rename_score(&arg)) == -1 || *arg != 0)
                        return -1;
                opt->detect_renames = 1;
        }
        /*
         * Please update $__git_merge_strategy_options in
         * git-completion.bash when you add new options
         */
        else
                return -1;
        return 0;
}