Use xmemdupz() in many places.

[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 "cache.h"
#include "cache-tree.h"
#include "commit.h"
#include "blob.h"
#include "tree-walk.h"
#include "diff.h"
#include "diffcore.h"
#include "run-command.h"
#include "tag.h"
#include "unpack-trees.h"
#include "path-list.h"
#include "xdiff-interface.h"
#include "interpolate.h"
#include "attr.h"

static int subtree_merge;

static struct tree *shift_tree_object(struct tree *one, struct tree *two)
{
        unsigned char shifted[20];

        /*
         * NEEDSWORK: this limits the recursion depth to hardcoded
         * value '2' to avoid excessive overhead.
         */
        shift_tree(one->object.sha1, two->object.sha1, shifted, 2);
        if (!hashcmp(two->object.sha1, shifted))
                return two;
        return lookup_tree(shifted);
}

/*
 * A virtual commit has
 * - (const char *)commit->util set to the name, and
 * - *(int *)commit->object.sha1 set to the virtual id.
 */

static unsigned commit_list_count(const struct commit_list *l)
{
        unsigned c = 0;
        for (; l; l = l->next )
                c++;
        return c;
}

static struct commit *make_virtual_commit(struct tree *tree, const char *comment)
{
        struct commit *commit = xcalloc(1, sizeof(struct commit));
        static unsigned virtual_id = 1;
        commit->tree = tree;
        commit->util = (void*)comment;
        *(int*)commit->object.sha1 = virtual_id++;
        /* avoid warnings */
        commit->object.parsed = 1;
        return commit;
}

/*
 * Since we use get_tree_entry(), which does not put the read object into
 * the object pool, we cannot rely on a == b.
 */
static int sha_eq(const unsigned char *a, const unsigned char *b)
{
        if (!a && !b)
                return 2;
        return a && b && hashcmp(a, b) == 0;
}

/*
 * Since we want to write the index eventually, we cannot reuse the index
 * for these (temporary) data.
 */
struct stage_data
{
        struct
        {
                unsigned mode;
                unsigned char sha[20];
        } stages[4];
        unsigned processed:1;
};

struct output_buffer
{
        struct output_buffer *next;
        char *str;
};

static struct path_list current_file_set = {NULL, 0, 0, 1};
static struct path_list current_directory_set = {NULL, 0, 0, 1};

static int call_depth = 0;
static int verbosity = 2;
static int buffer_output = 1;
static struct output_buffer *output_list, *output_end;

static int show (int v)
{
        return (!call_depth && verbosity >= v) || verbosity >= 5;
}

static void output(int v, const char *fmt, ...)
{
        va_list args;
        va_start(args, fmt);
        if (buffer_output && show(v)) {
                struct output_buffer *b = xmalloc(sizeof(*b));
                nfvasprintf(&b->str, fmt, args);
                b->next = NULL;
                if (output_end)
                        output_end->next = b;
                else
                        output_list = b;
                output_end = b;
        } else if (show(v)) {
                int i;
                for (i = call_depth; i--;)
                        fputs("  ", stdout);
                vfprintf(stdout, fmt, args);
                fputc('\n', stdout);
        }
        va_end(args);
}

static void flush_output(void)
{
        struct output_buffer *b, *n;
        for (b = output_list; b; b = n) {
                int i;
                for (i = call_depth; i--;)
                        fputs("  ", stdout);
                fputs(b->str, stdout);
                fputc('\n', stdout);
                n = b->next;
                free(b->str);
                free(b);
        }
        output_list = NULL;
        output_end = NULL;
}

static void output_commit_title(struct commit *commit)
{
        int i;
        flush_output();
        for (i = call_depth; i--;)
                fputs("  ", stdout);
        if (commit->util)
                printf("virtual %s\n", (char *)commit->util);
        else {
                printf("%s ", find_unique_abbrev(commit->object.sha1, DEFAULT_ABBREV));
                if (parse_commit(commit) != 0)
                        printf("(bad commit)\n");
                else {
                        const char *s;
                        int len;
                        for (s = commit->buffer; *s; s++)
                                if (*s == '\n' && s[1] == '\n') {
                                        s += 2;
                                        break;
                                }
                        for (len = 0; s[len] && '\n' != s[len]; len++)
                                ; /* do nothing */
                        printf("%.*s\n", len, s);
                }
        }
}

static int add_cacheinfo(unsigned int mode, const unsigned char *sha1,
                const char *path, int stage, int refresh, int options)
{
        struct cache_entry *ce;
        ce = make_cache_entry(mode, sha1 ? sha1 : null_sha1, path, stage, refresh);
        if (!ce)
                return error("addinfo_cache failed for path '%s'", path);
        return add_cache_entry(ce, options);
}

/*
 * This is a global variable which is used in a number of places but
 * only written to in the 'merge' function.
 *
 * index_only == 1    => Don't leave any non-stage 0 entries in the cache and
 *                       don't update the working directory.
 *               0    => Leave unmerged entries in the cache and update
 *                       the working directory.
 */
static int index_only = 0;

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 git_merge_trees(int index_only,
                           struct tree *common,
                           struct tree *head,
                           struct tree *merge)
{
        int rc;
        struct tree_desc t[3];
        struct unpack_trees_options opts;

        memset(&opts, 0, sizeof(opts));
        if (index_only)
                opts.index_only = 1;
        else
                opts.update = 1;
        opts.merge = 1;
        opts.head_idx = 2;
        opts.fn = threeway_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, &opts);
        cache_tree_free(&active_cache_tree);
        return rc;
}

static int unmerged_index(void)
{
        int i;
        for (i = 0; i < active_nr; i++) {
                struct cache_entry *ce = active_cache[i];
                if (ce_stage(ce))
                        return 1;
        }
        return 0;
}

static struct tree *git_write_tree(void)
{
        struct tree *result = NULL;

        if (unmerged_index()) {
                int i;
                output(0, "There are unmerged index entries:");
                for (i = 0; i < active_nr; i++) {
                        struct cache_entry *ce = active_cache[i];
                        if (ce_stage(ce))
                                output(0, "%d %.*s", ce_stage(ce), ce_namelen(ce), ce->name);
                }
                return NULL;
        }

        if (!active_cache_tree)
                active_cache_tree = cache_tree();

        if (!cache_tree_fully_valid(active_cache_tree) &&
            cache_tree_update(active_cache_tree,
                              active_cache, active_nr, 0, 0) < 0)
                die("error building trees");

        result = lookup_tree(active_cache_tree->sha1);

        return result;
}

static int save_files_dirs(const unsigned char *sha1,
                const char *base, int baselen, const char *path,
                unsigned int mode, int stage)
{
        int len = strlen(path);
        char *newpath = xmalloc(baselen + len + 1);
        memcpy(newpath, base, baselen);
        memcpy(newpath + baselen, path, len);
        newpath[baselen + len] = '\0';

        if (S_ISDIR(mode))
                path_list_insert(newpath, &current_directory_set);
        else
                path_list_insert(newpath, &current_file_set);
        free(newpath);

        return READ_TREE_RECURSIVE;
}

static int get_files_dirs(struct tree *tree)
{
        int n;
        if (read_tree_recursive(tree, "", 0, 0, NULL, save_files_dirs) != 0)
                return 0;
        n = current_file_set.nr + current_directory_set.nr;
        return n;
}

/*
 * Returns a 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(const char *path,
                struct tree *o, struct tree *a, struct tree *b,
                struct path_list *entries)
{
        struct path_list_item *item;
        struct stage_data *e = xcalloc(1, sizeof(struct stage_data));
        get_tree_entry(o->object.sha1, path,
                        e->stages[1].sha, &e->stages[1].mode);
        get_tree_entry(a->object.sha1, path,
                        e->stages[2].sha, &e->stages[2].mode);
        get_tree_entry(b->object.sha1, path,
                        e->stages[3].sha, &e->stages[3].mode);
        item = path_list_insert(path, entries);
        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 path_list *get_unmerged(void)
{
        struct path_list *unmerged = xcalloc(1, sizeof(struct path_list));
        int i;

        unmerged->strdup_paths = 1;

        for (i = 0; i < active_nr; i++) {
                struct path_list_item *item;
                struct stage_data *e;
                struct cache_entry *ce = active_cache[i];
                if (!ce_stage(ce))
                        continue;

                item = path_list_lookup(ce->name, unmerged);
                if (!item) {
                        item = path_list_insert(ce->name, unmerged);
                        item->util = xcalloc(1, sizeof(struct stage_data));
                }
                e = item->util;
                e->stages[ce_stage(ce)].mode = ntohl(ce->ce_mode);
                hashcpy(e->stages[ce_stage(ce)].sha, ce->sha1);
        }

        return unmerged;
}

struct rename
{
        struct diff_filepair *pair;
        struct stage_data *src_entry;
        struct stage_data *dst_entry;
        unsigned processed:1;
};

/*
 * Get information of all renames which occurred between 'o_tree' and
 * 'tree'. 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 path_list *get_renames(struct tree *tree,
                                        struct tree *o_tree,
                                        struct tree *a_tree,
                                        struct tree *b_tree,
                                        struct path_list *entries)
{
        int i;
        struct path_list *renames;
        struct diff_options opts;

        renames = xcalloc(1, sizeof(struct path_list));
        diff_setup(&opts);
        opts.recursive = 1;
        opts.detect_rename = DIFF_DETECT_RENAME;
        opts.output_format = DIFF_FORMAT_NO_OUTPUT;
        if (diff_setup_done(&opts) < 0)
                die("diff setup failed");
        diff_tree_sha1(o_tree->object.sha1, tree->object.sha1, "", &opts);
        diffcore_std(&opts);
        for (i = 0; i < diff_queued_diff.nr; ++i) {
                struct path_list_item *item;
                struct rename *re;
                struct diff_filepair *pair = diff_queued_diff.queue[i];
                if (pair->status != 'R') {
                        diff_free_filepair(pair);
                        continue;
                }
                re = xmalloc(sizeof(*re));
                re->processed = 0;
                re->pair = pair;
                item = path_list_lookup(re->pair->one->path, entries);
                if (!item)
                        re->src_entry = insert_stage_data(re->pair->one->path,
                                        o_tree, a_tree, b_tree, entries);
                else
                        re->src_entry = item->util;

                item = path_list_lookup(re->pair->two->path, entries);
                if (!item)
                        re->dst_entry = insert_stage_data(re->pair->two->path,
                                        o_tree, a_tree, b_tree, entries);
                else
                        re->dst_entry = item->util;
                item = path_list_insert(pair->one->path, renames);
                item->util = re;
        }
        opts.output_format = DIFF_FORMAT_NO_OUTPUT;
        diff_queued_diff.nr = 0;
        diff_flush(&opts);
        return renames;
}

static int update_stages(const char *path, struct diff_filespec *o,
                         struct diff_filespec *a, struct diff_filespec *b,
                         int clear)
{
        int options = ADD_CACHE_OK_TO_ADD | ADD_CACHE_OK_TO_REPLACE;
        if (clear)
                if (remove_file_from_cache(path))
                        return -1;
        if (o)
                if (add_cacheinfo(o->mode, o->sha1, path, 1, 0, options))
                        return -1;
        if (a)
                if (add_cacheinfo(a->mode, a->sha1, path, 2, 0, options))
                        return -1;
        if (b)
                if (add_cacheinfo(b->mode, b->sha1, path, 3, 0, options))
                        return -1;
        return 0;
}

static int remove_path(const char *name)
{
        int ret;
        char *slash, *dirs;

        ret = unlink(name);
        if (ret)
                return ret;
        dirs = xstrdup(name);
        while ((slash = strrchr(name, '/'))) {
                *slash = '\0';
                if (rmdir(name) != 0)
                        break;
        }
        free(dirs);
        return ret;
}

static int remove_file(int clean, const char *path, int no_wd)
{
        int update_cache = index_only || clean;
        int update_working_directory = !index_only && !no_wd;

        if (update_cache) {
                if (remove_file_from_cache(path))
                        return -1;
        }
        if (update_working_directory) {
                unlink(path);
                if (errno != ENOENT || errno != EISDIR)
                        return -1;
                remove_path(path);
        }
        return 0;
}

static char *unique_path(const char *path, const char *branch)
{
        char *newpath = xmalloc(strlen(path) + 1 + strlen(branch) + 8 + 1);
        int suffix = 0;
        struct stat st;
        char *p = newpath + strlen(path);
        strcpy(newpath, path);
        *(p++) = '~';
        strcpy(p, branch);
        for (; *p; ++p)
                if ('/' == *p)
                        *p = '_';
        while (path_list_has_path(&current_file_set, newpath) ||
               path_list_has_path(&current_directory_set, newpath) ||
               lstat(newpath, &st) == 0)
                sprintf(p, "_%d", suffix++);

        path_list_insert(newpath, &current_file_set);
        return newpath;
}

static int mkdir_p(const char *path, unsigned long mode)
{
        /* path points to cache entries, so xstrdup before messing with it */
        char *buf = xstrdup(path);
        int result = safe_create_leading_directories(buf);
        free(buf);
        return result;
}

static void flush_buffer(int fd, const char *buf, unsigned long size)
{
        while (size > 0) {
                long ret = write_in_full(fd, buf, size);
                if (ret < 0) {
                        /* Ignore epipe */
                        if (errno == EPIPE)
                                break;
                        die("merge-recursive: %s", strerror(errno));
                } else if (!ret) {
                        die("merge-recursive: disk full?");
                }
                size -= ret;
                buf += ret;
        }
}

static int make_room_for_path(const char *path)
{
        int status;
        const char *msg = "failed to create path '%s'%s";

        status = mkdir_p(path, 0777);
        if (status) {
                if (status == -3) {
                        /* something else exists */
                        error(msg, path, ": perhaps a D/F conflict?");
                        return -1;
                }
                die(msg, 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 error(msg, path, ": perhaps a D/F conflict?");
}

static void update_file_flags(const unsigned char *sha,
                              unsigned mode,
                              const char *path,
                              int update_cache,
                              int update_wd)
{
        if (index_only)
                update_wd = 0;

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

                buf = read_sha1_file(sha, &type, &size);
                if (!buf)
                        die("cannot read object %s '%s'", sha1_to_hex(sha), path);
                if (type != OBJ_BLOB)
                        die("blob expected for %s '%s'", sha1_to_hex(sha), path);

                if (make_room_for_path(path) < 0) {
                        update_wd = 0;
                        goto update_index;
                }
                if (S_ISREG(mode) || (!has_symlinks && S_ISLNK(mode))) {
                        int fd;
                        if (mode & 0100)
                                mode = 0777;
                        else
                                mode = 0666;
                        fd = open(path, O_WRONLY | O_TRUNC | O_CREAT, mode);
                        if (fd < 0)
                                die("failed to open %s: %s", path, strerror(errno));
                        flush_buffer(fd, buf, size);
                        close(fd);
                } else if (S_ISLNK(mode)) {
                        char *lnk = xmemdupz(buf, size);
                        mkdir_p(path, 0777);
                        unlink(path);
                        symlink(lnk, path);
                        free(lnk);
                } else
                        die("do not know what to do with %06o %s '%s'",
                            mode, sha1_to_hex(sha), path);
        }
 update_index:
        if (update_cache)
                add_cacheinfo(mode, sha, path, 0, update_wd, ADD_CACHE_OK_TO_ADD);
}

static void update_file(int clean,
                        const unsigned char *sha,
                        unsigned mode,
                        const char *path)
{
        update_file_flags(sha, mode, path, index_only || clean, !index_only);
}

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

struct merge_file_info
{
        unsigned char sha[20];
        unsigned mode;
        unsigned clean:1,
                 merge:1;
};

static void fill_mm(const unsigned char *sha1, mmfile_t *mm)
{
        unsigned long size;
        enum object_type type;

        if (!hashcmp(sha1, null_sha1)) {
                mm->ptr = xstrdup("");
                mm->size = 0;
                return;
        }

        mm->ptr = read_sha1_file(sha1, &type, &size);
        if (!mm->ptr || type != OBJ_BLOB)
                die("unable to read blob object %s", sha1_to_hex(sha1));
        mm->size = size;
}

/*
 * Customizable low-level merge drivers support.
 */

struct ll_merge_driver;
typedef int (*ll_merge_fn)(const struct ll_merge_driver *,
                           const char *path,
                           mmfile_t *orig,
                           mmfile_t *src1, const char *name1,
                           mmfile_t *src2, const char *name2,
                           mmbuffer_t *result);

struct ll_merge_driver {
        const char *name;
        const char *description;
        ll_merge_fn fn;
        const char *recursive;
        struct ll_merge_driver *next;
        char *cmdline;
};

/*
 * Built-in low-levels
 */
static int ll_binary_merge(const struct ll_merge_driver *drv_unused,
                           const char *path_unused,
                           mmfile_t *orig,
                           mmfile_t *src1, const char *name1,
                           mmfile_t *src2, const char *name2,
                           mmbuffer_t *result)
{
        /*
         * The tentative merge result is "ours" for the final round,
         * or common ancestor for an internal merge.  Still return
         * "conflicted merge" status.
         */
        mmfile_t *stolen = index_only ? orig : src1;

        result->ptr = stolen->ptr;
        result->size = stolen->size;
        stolen->ptr = NULL;
        return 1;
}

static int ll_xdl_merge(const struct ll_merge_driver *drv_unused,
                        const char *path_unused,
                        mmfile_t *orig,
                        mmfile_t *src1, const char *name1,
                        mmfile_t *src2, const char *name2,
                        mmbuffer_t *result)
{
        xpparam_t xpp;

        if (buffer_is_binary(orig->ptr, orig->size) ||
            buffer_is_binary(src1->ptr, src1->size) ||
            buffer_is_binary(src2->ptr, src2->size)) {
                warning("Cannot merge binary files: %s vs. %s\n",
                        name1, name2);
                return ll_binary_merge(drv_unused, path_unused,
                                       orig, src1, name1,
                                       src2, name2,
                                       result);
        }

        memset(&xpp, 0, sizeof(xpp));
        return xdl_merge(orig,
                         src1, name1,
                         src2, name2,
                         &xpp, XDL_MERGE_ZEALOUS,
                         result);
}

static int ll_union_merge(const struct ll_merge_driver *drv_unused,
                          const char *path_unused,
                          mmfile_t *orig,
                          mmfile_t *src1, const char *name1,
                          mmfile_t *src2, const char *name2,
                          mmbuffer_t *result)
{
        char *src, *dst;
        long size;
        const int marker_size = 7;

        int status = ll_xdl_merge(drv_unused, path_unused,
                                  orig, src1, NULL, src2, NULL, result);
        if (status <= 0)
                return status;
        size = result->size;
        src = dst = result->ptr;
        while (size) {
                char ch;
                if ((marker_size < size) &&
                    (*src == '<' || *src == '=' || *src == '>')) {
                        int i;
                        ch = *src;
                        for (i = 0; i < marker_size; i++)
                                if (src[i] != ch)
                                        goto not_a_marker;
                        if (src[marker_size] != '\n')
                                goto not_a_marker;
                        src += marker_size + 1;
                        size -= marker_size + 1;
                        continue;
                }
        not_a_marker:
                do {
                        ch = *src++;
                        *dst++ = ch;
                        size--;
                } while (ch != '\n' && size);
        }
        result->size = dst - result->ptr;
        return 0;
}

#define LL_BINARY_MERGE 0
#define LL_TEXT_MERGE 1
#define LL_UNION_MERGE 2
static struct ll_merge_driver ll_merge_drv[] = {
        { "binary", "built-in binary merge", ll_binary_merge },
        { "text", "built-in 3-way text merge", ll_xdl_merge },
        { "union", "built-in union merge", ll_union_merge },
};

static void create_temp(mmfile_t *src, char *path)
{
        int fd;

        strcpy(path, ".merge_file_XXXXXX");
        fd = xmkstemp(path);
        if (write_in_full(fd, src->ptr, src->size) != src->size)
                die("unable to write temp-file");
        close(fd);
}

/*
 * User defined low-level merge driver support.
 */
static int ll_ext_merge(const struct ll_merge_driver *fn,
                        const char *path,
                        mmfile_t *orig,
                        mmfile_t *src1, const char *name1,
                        mmfile_t *src2, const char *name2,
                        mmbuffer_t *result)
{
        char temp[3][50];
        char cmdbuf[2048];
        struct interp table[] = {
                { "%O" },
                { "%A" },
                { "%B" },
        };
        struct child_process child;
        const char *args[20];
        int status, fd, i;
        struct stat st;

        if (fn->cmdline == NULL)
                die("custom merge driver %s lacks command line.", fn->name);

        result->ptr = NULL;
        result->size = 0;
        create_temp(orig, temp[0]);
        create_temp(src1, temp[1]);
        create_temp(src2, temp[2]);

        interp_set_entry(table, 0, temp[0]);
        interp_set_entry(table, 1, temp[1]);
        interp_set_entry(table, 2, temp[2]);

        output(1, "merging %s using %s", path,
               fn->description ? fn->description : fn->name);

        interpolate(cmdbuf, sizeof(cmdbuf), fn->cmdline, table, 3);

        memset(&child, 0, sizeof(child));
        child.argv = args;
        args[0] = "sh";
        args[1] = "-c";
        args[2] = cmdbuf;
        args[3] = NULL;

        status = run_command(&child);
        if (status < -ERR_RUN_COMMAND_FORK)
                ; /* failure in run-command */
        else
                status = -status;
        fd = open(temp[1], O_RDONLY);
        if (fd < 0)
                goto bad;
        if (fstat(fd, &st))
                goto close_bad;
        result->size = st.st_size;
        result->ptr = xmalloc(result->size + 1);
        if (read_in_full(fd, result->ptr, result->size) != result->size) {
                free(result->ptr);
                result->ptr = NULL;
                result->size = 0;
        }
 close_bad:
        close(fd);
 bad:
        for (i = 0; i < 3; i++)
                unlink(temp[i]);
        return status;
}

/*
 * merge.default and merge.driver configuration items
 */
static struct ll_merge_driver *ll_user_merge, **ll_user_merge_tail;
static const char *default_ll_merge;

static int read_merge_config(const char *var, const char *value)
{
        struct ll_merge_driver *fn;
        const char *ep, *name;
        int namelen;

        if (!strcmp(var, "merge.default")) {
                if (value)
                        default_ll_merge = strdup(value);
                return 0;
        }

        /*
         * We are not interested in anything but "merge.<name>.variable";
         * especially, we do not want to look at variables such as
         * "merge.summary", "merge.tool", and "merge.verbosity".
         */
        if (prefixcmp(var, "merge.") || (ep = strrchr(var, '.')) == var + 5)
                return 0;

        /*
         * Find existing one as we might be processing merge.<name>.var2
         * after seeing merge.<name>.var1.
         */
        name = var + 6;
        namelen = ep - name;
        for (fn = ll_user_merge; fn; fn = fn->next)
                if (!strncmp(fn->name, name, namelen) && !fn->name[namelen])
                        break;
        if (!fn) {
                fn = xcalloc(1, sizeof(struct ll_merge_driver));
                fn->name = xmemdupz(name, namelen);
                fn->fn = ll_ext_merge;
                *ll_user_merge_tail = fn;
                ll_user_merge_tail = &(fn->next);
        }

        ep++;

        if (!strcmp("name", ep)) {
                if (!value)
                        return error("%s: lacks value", var);
                fn->description = strdup(value);
                return 0;
        }

        if (!strcmp("driver", ep)) {
                if (!value)
                        return error("%s: lacks value", var);
                /*
                 * merge.<name>.driver specifies the command line:
                 *
                 *      command-line
                 *
                 * The command-line will be interpolated with the following
                 * tokens and is given to the shell:
                 *
                 *    %O - temporary file name for the merge base.
                 *    %A - temporary file name for our version.
                 *    %B - temporary file name for the other branches' version.
                 *
                 * The external merge driver should write the results in the
                 * file named by %A, and signal that it has done with zero exit
                 * status.
                 */
                fn->cmdline = strdup(value);
                return 0;
        }

        if (!strcmp("recursive", ep)) {
                if (!value)
                        return error("%s: lacks value", var);
                fn->recursive = strdup(value);
                return 0;
        }

        return 0;
}

static void initialize_ll_merge(void)
{
        if (ll_user_merge_tail)
                return;
        ll_user_merge_tail = &ll_user_merge;
        git_config(read_merge_config);
}

static const struct ll_merge_driver *find_ll_merge_driver(const char *merge_attr)
{
        struct ll_merge_driver *fn;
        const char *name;
        int i;

        initialize_ll_merge();

        if (ATTR_TRUE(merge_attr))
                return &ll_merge_drv[LL_TEXT_MERGE];
        else if (ATTR_FALSE(merge_attr))
                return &ll_merge_drv[LL_BINARY_MERGE];
        else if (ATTR_UNSET(merge_attr)) {
                if (!default_ll_merge)
                        return &ll_merge_drv[LL_TEXT_MERGE];
                else
                        name = default_ll_merge;
        }
        else
                name = merge_attr;

        for (fn = ll_user_merge; fn; fn = fn->next)
                if (!strcmp(fn->name, name))
                        return fn;

        for (i = 0; i < ARRAY_SIZE(ll_merge_drv); i++)
                if (!strcmp(ll_merge_drv[i].name, name))
                        return &ll_merge_drv[i];

        /* default to the 3-way */
        return &ll_merge_drv[LL_TEXT_MERGE];
}

static const char *git_path_check_merge(const char *path)
{
        static struct git_attr_check attr_merge_check;

        if (!attr_merge_check.attr)
                attr_merge_check.attr = git_attr("merge", 5);

        if (git_checkattr(path, 1, &attr_merge_check))
                return NULL;
        return attr_merge_check.value;
}

static int ll_merge(mmbuffer_t *result_buf,
                    struct diff_filespec *o,
                    struct diff_filespec *a,
                    struct diff_filespec *b,
                    const char *branch1,
                    const char *branch2)
{
        mmfile_t orig, src1, src2;
        char *name1, *name2;
        int merge_status;
        const char *ll_driver_name;
        const struct ll_merge_driver *driver;

        name1 = xstrdup(mkpath("%s:%s", branch1, a->path));
        name2 = xstrdup(mkpath("%s:%s", branch2, b->path));

        fill_mm(o->sha1, &orig);
        fill_mm(a->sha1, &src1);
        fill_mm(b->sha1, &src2);

        ll_driver_name = git_path_check_merge(a->path);
        driver = find_ll_merge_driver(ll_driver_name);

        if (index_only && driver->recursive)
                driver = find_ll_merge_driver(driver->recursive);
        merge_status = driver->fn(driver, a->path,
                                  &orig, &src1, name1, &src2, name2,
                                  result_buf);

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

static struct merge_file_info merge_file(struct diff_filespec *o,
                struct diff_filespec *a, struct diff_filespec *b,
                const char *branch1, const char *branch2)
{
        struct merge_file_info result;
        result.merge = 0;
        result.clean = 1;

        if ((S_IFMT & a->mode) != (S_IFMT & b->mode)) {
                result.clean = 0;
                if (S_ISREG(a->mode)) {
                        result.mode = a->mode;
                        hashcpy(result.sha, a->sha1);
                } else {
                        result.mode = b->mode;
                        hashcpy(result.sha, b->sha1);
                }
        } else {
                if (!sha_eq(a->sha1, o->sha1) && !sha_eq(b->sha1, o->sha1))
                        result.merge = 1;

                result.mode = a->mode == o->mode ? b->mode: a->mode;

                if (sha_eq(a->sha1, o->sha1))
                        hashcpy(result.sha, b->sha1);
                else if (sha_eq(b->sha1, o->sha1))
                        hashcpy(result.sha, a->sha1);
                else if (S_ISREG(a->mode)) {
                        mmbuffer_t result_buf;
                        int merge_status;

                        merge_status = ll_merge(&result_buf, o, a, b,
                                                branch1, branch2);

                        if ((merge_status < 0) || !result_buf.ptr)
                                die("Failed to execute internal merge");

                        if (write_sha1_file(result_buf.ptr, result_buf.size,
                                            blob_type, result.sha))
                                die("Unable to add %s to database",
                                    a->path);

                        free(result_buf.ptr);
                        result.clean = (merge_status == 0);
                } else {
                        if (!(S_ISLNK(a->mode) || S_ISLNK(b->mode)))
                                die("cannot merge modes?");

                        hashcpy(result.sha, a->sha1);

                        if (!sha_eq(a->sha1, b->sha1))
                                result.clean = 0;
                }
        }

        return result;
}

static void conflict_rename_rename(struct rename *ren1,
                                   const char *branch1,
                                   struct rename *ren2,
                                   const char *branch2)
{
        char *del[2];
        int delp = 0;
        const char *ren1_dst = ren1->pair->two->path;
        const char *ren2_dst = ren2->pair->two->path;
        const char *dst_name1 = ren1_dst;
        const char *dst_name2 = ren2_dst;
        if (path_list_has_path(&current_directory_set, ren1_dst)) {
                dst_name1 = del[delp++] = unique_path(ren1_dst, branch1);
                output(1, "%s is a directory in %s added as %s instead",
                       ren1_dst, branch2, dst_name1);
                remove_file(0, ren1_dst, 0);
        }
        if (path_list_has_path(&current_directory_set, ren2_dst)) {
                dst_name2 = del[delp++] = unique_path(ren2_dst, branch2);
                output(1, "%s is a directory in %s added as %s instead",
                       ren2_dst, branch1, dst_name2);
                remove_file(0, ren2_dst, 0);
        }
        if (index_only) {
                remove_file_from_cache(dst_name1);
                remove_file_from_cache(dst_name2);
                /*
                 * Uncomment to leave the conflicting names in the resulting tree
                 *
                 * update_file(0, ren1->pair->two->sha1, ren1->pair->two->mode, dst_name1);
                 * update_file(0, ren2->pair->two->sha1, ren2->pair->two->mode, dst_name2);
                 */
        } else {
                update_stages(dst_name1, NULL, ren1->pair->two, NULL, 1);
                update_stages(dst_name2, NULL, NULL, ren2->pair->two, 1);
        }
        while (delp--)
                free(del[delp]);
}

static void conflict_rename_dir(struct rename *ren1,
                                const char *branch1)
{
        char *new_path = unique_path(ren1->pair->two->path, branch1);
        output(1, "Renamed %s to %s instead", ren1->pair->one->path, new_path);
        remove_file(0, ren1->pair->two->path, 0);
        update_file(0, ren1->pair->two->sha1, ren1->pair->two->mode, new_path);
        free(new_path);
}

static void conflict_rename_rename_2(struct rename *ren1,
                                     const char *branch1,
                                     struct rename *ren2,
                                     const char *branch2)
{
        char *new_path1 = unique_path(ren1->pair->two->path, branch1);
        char *new_path2 = unique_path(ren2->pair->two->path, branch2);
        output(1, "Renamed %s to %s and %s to %s instead",
               ren1->pair->one->path, new_path1,
               ren2->pair->one->path, new_path2);
        remove_file(0, ren1->pair->two->path, 0);
        update_file(0, ren1->pair->two->sha1, ren1->pair->two->mode, new_path1);
        update_file(0, ren2->pair->two->sha1, ren2->pair->two->mode, new_path2);
        free(new_path2);
        free(new_path1);
}

static int process_renames(struct path_list *a_renames,
                           struct path_list *b_renames,
                           const char *a_branch,
                           const char *b_branch)
{
        int clean_merge = 1, i, j;
        struct path_list a_by_dst = {NULL, 0, 0, 0}, b_by_dst = {NULL, 0, 0, 0};
        const struct rename *sre;

        for (i = 0; i < a_renames->nr; i++) {
                sre = a_renames->items[i].util;
                path_list_insert(sre->pair->two->path, &a_by_dst)->util
                        = sre->dst_entry;
        }
        for (i = 0; i < b_renames->nr; i++) {
                sre = b_renames->items[i].util;
                path_list_insert(sre->pair->two->path, &b_by_dst)->util
                        = sre->dst_entry;
        }

        for (i = 0, j = 0; i < a_renames->nr || j < b_renames->nr;) {
                int compare;
                char *src;
                struct path_list *renames1, *renames2, *renames2Dst;
                struct rename *ren1 = NULL, *ren2 = NULL;
                const char *branch1, *branch2;
                const char *ren1_src, *ren1_dst;

                if (i >= a_renames->nr) {
                        compare = 1;
                        ren2 = b_renames->items[j++].util;
                } else if (j >= b_renames->nr) {
                        compare = -1;
                        ren1 = a_renames->items[i++].util;
                } else {
                        compare = strcmp(a_renames->items[i].path,
                                        b_renames->items[j].path);
                        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;
                        renames2 = b_renames;
                        renames2Dst = &b_by_dst;
                        branch1 = a_branch;
                        branch2 = b_branch;
                } else {
                        struct rename *tmp;
                        renames1 = b_renames;
                        renames2 = a_renames;
                        renames2Dst = &a_by_dst;
                        branch1 = b_branch;
                        branch2 = a_branch;
                        tmp = ren2;
                        ren2 = ren1;
                        ren1 = tmp;
                }
                src = ren1->pair->one->path;

                ren1->dst_entry->processed = 1;
                ren1->src_entry->processed = 1;

                if (ren1->processed)
                        continue;
                ren1->processed = 1;

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

                if (ren2) {
                        const char *ren2_src = ren2->pair->one->path;
                        const char *ren2_dst = ren2->pair->two->path;
                        /* Renamed in 1 and renamed in 2 */
                        if (strcmp(ren1_src, ren2_src) != 0)
                                die("ren1.src != ren2.src");
                        ren2->dst_entry->processed = 1;
                        ren2->processed = 1;
                        if (strcmp(ren1_dst, ren2_dst) != 0) {
                                clean_merge = 0;
                                output(1, "CONFLICT (rename/rename): "
                                       "Rename \"%s\"->\"%s\" in branch \"%s\" "
                                       "rename \"%s\"->\"%s\" in \"%s\"%s",
                                       src, ren1_dst, branch1,
                                       src, ren2_dst, branch2,
                                       index_only ? " (left unresolved)": "");
                                if (index_only) {
                                        remove_file_from_cache(src);
                                        update_file(0, ren1->pair->one->sha1,
                                                    ren1->pair->one->mode, src);
                                }
                                conflict_rename_rename(ren1, branch1, ren2, branch2);
                        } else {
                                struct merge_file_info mfi;
                                remove_file(1, ren1_src, 1);
                                mfi = merge_file(ren1->pair->one,
                                                 ren1->pair->two,
                                                 ren2->pair->two,
                                                 branch1,
                                                 branch2);
                                if (mfi.merge || !mfi.clean)
                                        output(1, "Renamed %s->%s", src, ren1_dst);

                                if (mfi.merge)
                                        output(2, "Auto-merged %s", ren1_dst);

                                if (!mfi.clean) {
                                        output(1, "CONFLICT (content): merge conflict in %s",
                                               ren1_dst);
                                        clean_merge = 0;

                                        if (!index_only)
                                                update_stages(ren1_dst,
                                                              ren1->pair->one,
                                                              ren1->pair->two,
                                                              ren2->pair->two,
                                                              1 /* clear */);
                                }
                                update_file(mfi.clean, mfi.sha, mfi.mode, ren1_dst);
                        }
                } else {
                        /* Renamed in 1, maybe changed in 2 */
                        struct path_list_item *item;
                        /* we only use sha1 and mode of these */
                        struct diff_filespec src_other, dst_other;
                        int try_merge, stage = a_renames == renames1 ? 3: 2;

                        remove_file(1, ren1_src, index_only || stage == 3);

                        hashcpy(src_other.sha1, ren1->src_entry->stages[stage].sha);
                        src_other.mode = ren1->src_entry->stages[stage].mode;
                        hashcpy(dst_other.sha1, ren1->dst_entry->stages[stage].sha);
                        dst_other.mode = ren1->dst_entry->stages[stage].mode;

                        try_merge = 0;

                        if (path_list_has_path(&current_directory_set, ren1_dst)) {
                                clean_merge = 0;
                                output(1, "CONFLICT (rename/directory): Renamed %s->%s in %s "
                                       " directory %s added in %s",
                                       ren1_src, ren1_dst, branch1,
                                       ren1_dst, branch2);
                                conflict_rename_dir(ren1, branch1);
                        } else if (sha_eq(src_other.sha1, null_sha1)) {
                                clean_merge = 0;
                                output(1, "CONFLICT (rename/delete): Renamed %s->%s in %s "
                                       "and deleted in %s",
                                       ren1_src, ren1_dst, branch1,
                                       branch2);
                                update_file(0, ren1->pair->two->sha1, ren1->pair->two->mode, ren1_dst);
                        } else if (!sha_eq(dst_other.sha1, null_sha1)) {
                                const char *new_path;
                                clean_merge = 0;
                                try_merge = 1;
                                output(1, "CONFLICT (rename/add): Renamed %s->%s in %s. "
                                       "%s added in %s",
                                       ren1_src, ren1_dst, branch1,
                                       ren1_dst, branch2);
                                new_path = unique_path(ren1_dst, branch2);
                                output(1, "Added as %s instead", new_path);
                                update_file(0, dst_other.sha1, dst_other.mode, new_path);
                        } else if ((item = path_list_lookup(ren1_dst, renames2Dst))) {
                                ren2 = item->util;
                                clean_merge = 0;
                                ren2->processed = 1;
                                output(1, "CONFLICT (rename/rename): Renamed %s->%s in %s. "
                                       "Renamed %s->%s in %s",
                                       ren1_src, ren1_dst, branch1,
                                       ren2->pair->one->path, ren2->pair->two->path, branch2);
                                conflict_rename_rename_2(ren1, branch1, ren2, branch2);
                        } else
                                try_merge = 1;

                        if (try_merge) {
                                struct diff_filespec *o, *a, *b;
                                struct merge_file_info mfi;
                                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;
                                }
                                mfi = merge_file(o, a, b,
                                                a_branch, b_branch);

                                if (mfi.clean &&
                                    sha_eq(mfi.sha, ren1->pair->two->sha1) &&
                                    mfi.mode == ren1->pair->two->mode)
                                        /*
                                         * This messaged is part of
                                         * t6022 test. If you change
                                         * it update the test too.
                                         */
                                        output(3, "Skipped %s (merged same as existing)", ren1_dst);
                                else {
                                        if (mfi.merge || !mfi.clean)
                                                output(1, "Renamed %s => %s", ren1_src, ren1_dst);
                                        if (mfi.merge)
                                                output(2, "Auto-merged %s", ren1_dst);
                                        if (!mfi.clean) {
                                                output(1, "CONFLICT (rename/modify): Merge conflict in %s",
                                                       ren1_dst);
                                                clean_merge = 0;

                                                if (!index_only)
                                                        update_stages(ren1_dst,
                                                                      o, a, b, 1);
                                        }
                                        update_file(mfi.clean, mfi.sha, mfi.mode, ren1_dst);
                                }
                        }
                }
        }
        path_list_clear(&a_by_dst, 0);
        path_list_clear(&b_by_dst, 0);

        return clean_merge;
}

static unsigned char *stage_sha(const unsigned char *sha, unsigned mode)
{
        return (is_null_sha1(sha) || mode == 0) ? NULL: (unsigned char *)sha;
}

/* Per entry merge function */
static int process_entry(const char *path, struct stage_data *entry,
                         const char *branch1,
                         const char *branch2)
{
        /*
        printf("processing entry, clean cache: %s\n", index_only ? "yes": "no");
        print_index_entry("\tpath: ", entry);
        */
        int clean_merge = 1;
        unsigned o_mode = entry->stages[1].mode;
        unsigned a_mode = entry->stages[2].mode;
        unsigned b_mode = entry->stages[3].mode;
        unsigned char *o_sha = stage_sha(entry->stages[1].sha, o_mode);
        unsigned char *a_sha = stage_sha(entry->stages[2].sha, a_mode);
        unsigned char *b_sha = stage_sha(entry->stages[3].sha, b_mode);

        if (o_sha && (!a_sha || !b_sha)) {
                /* Case A: Deleted in one */
                if ((!a_sha && !b_sha) ||
                    (sha_eq(a_sha, o_sha) && !b_sha) ||
                    (!a_sha && sha_eq(b_sha, o_sha))) {
                        /* Deleted in both or deleted in one and
                         * unchanged in the other */
                        if (a_sha)
                                output(2, "Removed %s", path);
                        /* do not touch working file if it did not exist */
                        remove_file(1, path, !a_sha);
                } else {
                        /* Deleted in one and changed in the other */
                        clean_merge = 0;
                        if (!a_sha) {
                                output(1, "CONFLICT (delete/modify): %s deleted in %s "
                                       "and modified in %s. Version %s of %s left in tree.",
                                       path, branch1,
                                       branch2, branch2, path);
                                update_file(0, b_sha, b_mode, path);
                        } else {
                                output(1, "CONFLICT (delete/modify): %s deleted in %s "
                                       "and modified in %s. Version %s of %s left in tree.",
                                       path, branch2,
                                       branch1, branch1, path);
                                update_file(0, a_sha, a_mode, path);
                        }
                }

        } else if ((!o_sha && a_sha && !b_sha) ||
                   (!o_sha && !a_sha && b_sha)) {
                /* Case B: Added in one. */
                const char *add_branch;
                const char *other_branch;
                unsigned mode;
                const unsigned char *sha;
                const char *conf;

                if (a_sha) {
                        add_branch = branch1;
                        other_branch = branch2;
                        mode = a_mode;
                        sha = a_sha;
                        conf = "file/directory";
                } else {
                        add_branch = branch2;
                        other_branch = branch1;
                        mode = b_mode;
                        sha = b_sha;
                        conf = "directory/file";
                }
                if (path_list_has_path(&current_directory_set, path)) {
                        const char *new_path = unique_path(path, add_branch);
                        clean_merge = 0;
                        output(1, "CONFLICT (%s): There is a directory with name %s in %s. "
                               "Added %s as %s",
                               conf, path, other_branch, path, new_path);
                        remove_file(0, path, 0);
                        update_file(0, sha, mode, new_path);
                } else {
                        output(2, "Added %s", path);
                        update_file(1, sha, mode, path);
                }
        } else if (a_sha && b_sha) {
                /* Case C: Added in both (check for same permissions) and */
                /* case D: Modified in both, but differently. */
                const char *reason = "content";
                struct merge_file_info mfi;
                struct diff_filespec o, a, b;

                if (!o_sha) {
                        reason = "add/add";
                        o_sha = (unsigned char *)null_sha1;
                }
                output(2, "Auto-merged %s", path);
                o.path = a.path = b.path = (char *)path;
                hashcpy(o.sha1, o_sha);
                o.mode = o_mode;
                hashcpy(a.sha1, a_sha);
                a.mode = a_mode;
                hashcpy(b.sha1, b_sha);
                b.mode = b_mode;

                mfi = merge_file(&o, &a, &b,
                                 branch1, branch2);

                if (mfi.clean)
                        update_file(1, mfi.sha, mfi.mode, path);
                else {
                        clean_merge = 0;
                        output(1, "CONFLICT (%s): Merge conflict in %s",
                                        reason, path);

                        if (index_only)
                                update_file(0, mfi.sha, mfi.mode, path);
                        else
                                update_file_flags(mfi.sha, mfi.mode, path,
                                              0 /* update_cache */, 1 /* update_working_directory */);
                }
        } else if (!o_sha && !a_sha && !b_sha) {
                /*
                 * this entry was deleted altogether. a_mode == 0 means
                 * we had that path and want to actively remove it.
                 */
                remove_file(1, path, !a_mode);
        } else
                die("Fatal merge failure, shouldn't happen.");

        return clean_merge;
}

static int merge_trees(struct tree *head,
                       struct tree *merge,
                       struct tree *common,
                       const char *branch1,
                       const char *branch2,
                       struct tree **result)
{
        int code, clean;

        if (subtree_merge) {
                merge = shift_tree_object(head, merge);
                common = shift_tree_object(head, common);
        }

        if (sha_eq(common->object.sha1, merge->object.sha1)) {
                output(0, "Already uptodate!");
                *result = head;
                return 1;
        }

        code = git_merge_trees(index_only, common, head, merge);

        if (code != 0)
                die("merging of trees %s and %s failed",
                    sha1_to_hex(head->object.sha1),
                    sha1_to_hex(merge->object.sha1));

        if (unmerged_index()) {
                struct path_list *entries, *re_head, *re_merge;
                int i;
                path_list_clear(&current_file_set, 1);
                path_list_clear(&current_directory_set, 1);
                get_files_dirs(head);
                get_files_dirs(merge);

                entries = get_unmerged();
                re_head  = get_renames(head, common, head, merge, entries);
                re_merge = get_renames(merge, common, head, merge, entries);
                clean = process_renames(re_head, re_merge,
                                branch1, branch2);
                for (i = 0; i < entries->nr; i++) {
                        const char *path = entries->items[i].path;
                        struct stage_data *e = entries->items[i].util;
                        if (!e->processed
                                && !process_entry(path, e, branch1, branch2))
                                clean = 0;
                }

                path_list_clear(re_merge, 0);
                path_list_clear(re_head, 0);
                path_list_clear(entries, 1);

        }
        else
                clean = 1;

        if (index_only)
                *result = git_write_tree();

        return clean;
}

static struct commit_list *reverse_commit_list(struct commit_list *list)
{
        struct commit_list *next = NULL, *current, *backup;
        for (current = list; current; current = backup) {
                backup = current->next;
                current->next = next;
                next = current;
        }
        return next;
}

/*
 * Merge the commits h1 and h2, return the resulting virtual
 * commit object and a flag indicating the cleanness of the merge.
 */
static int merge(struct commit *h1,
                 struct commit *h2,
                 const char *branch1,
                 const char *branch2,
                 struct commit_list *ca,
                 struct commit **result)
{
        struct commit_list *iter;
        struct commit *merged_common_ancestors;
        struct tree *mrtree;
        int clean;

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

        if (!ca) {
                ca = get_merge_bases(h1, h2, 1);
                ca = reverse_commit_list(ca);
        }

        if (show(5)) {
                output(5, "found %u common ancestor(s):", commit_list_count(ca));
                for (iter = ca; iter; iter = iter->next)
                        output_commit_title(iter->item);
        }

        merged_common_ancestors = pop_commit(&ca);
        if (merged_common_ancestors == NULL) {
                /* if there is no common ancestor, make an empty tree */
                struct tree *tree = xcalloc(1, sizeof(struct tree));

                tree->object.parsed = 1;
                tree->object.type = OBJ_TREE;
                pretend_sha1_file(NULL, 0, OBJ_TREE, tree->object.sha1);
                merged_common_ancestors = make_virtual_commit(tree, "ancestor");
        }

        for (iter = ca; iter; iter = iter->next) {
                call_depth++;
                /*
                 * When the merge fails, the result contains files
                 * with conflict markers. The cleanness flag is
                 * ignored, it was never actually used, as result of
                 * merge_trees has always overwritten it: the committed
                 * "conflicts" were already resolved.
                 */
                discard_cache();
                merge(merged_common_ancestors, iter->item,
                      "Temporary merge branch 1",
                      "Temporary merge branch 2",
                      NULL,
                      &merged_common_ancestors);
                call_depth--;

                if (!merged_common_ancestors)
                        die("merge returned no commit");
        }

        discard_cache();
        if (!call_depth) {
                read_cache();
                index_only = 0;
        } else
                index_only = 1;

        clean = merge_trees(h1->tree, h2->tree, merged_common_ancestors->tree,
                            branch1, branch2, &mrtree);

        if (index_only) {
                *result = make_virtual_commit(mrtree, "merged tree");
                commit_list_insert(h1, &(*result)->parents);
                commit_list_insert(h2, &(*result)->parents->next);
        }
        flush_output();
        return clean;
}

static const char *better_branch_name(const char *branch)
{
        static char githead_env[8 + 40 + 1];
        char *name;

        if (strlen(branch) != 40)
                return branch;
        sprintf(githead_env, "GITHEAD_%s", branch);
        name = getenv(githead_env);
        return name ? name : branch;
}

static struct commit *get_ref(const char *ref)
{
        unsigned char sha1[20];
        struct object *object;

        if (get_sha1(ref, sha1))
                die("Could not resolve ref '%s'", ref);
        object = deref_tag(parse_object(sha1), ref, strlen(ref));
        if (object->type == OBJ_TREE)
                return make_virtual_commit((struct tree*)object,
                        better_branch_name(ref));
        if (object->type != OBJ_COMMIT)
                return NULL;
        if (parse_commit((struct commit *)object))
                die("Could not parse commit '%s'", sha1_to_hex(object->sha1));
        return (struct commit *)object;
}

static int merge_config(const char *var, const char *value)
{
        if (!strcasecmp(var, "merge.verbosity")) {
                verbosity = git_config_int(var, value);
                return 0;
        }
        return git_default_config(var, value);
}

int main(int argc, char *argv[])
{
        static const char *bases[20];
        static unsigned bases_count = 0;
        int i, clean;
        const char *branch1, *branch2;
        struct commit *result, *h1, *h2;
        struct commit_list *ca = NULL;
        struct lock_file *lock = xcalloc(1, sizeof(struct lock_file));
        int index_fd;

        if (argv[0]) {
                int namelen = strlen(argv[0]);
                if (8 < namelen &&
                    !strcmp(argv[0] + namelen - 8, "-subtree"))
                        subtree_merge = 1;
        }

        git_config(merge_config);
        if (getenv("GIT_MERGE_VERBOSITY"))
                verbosity = strtol(getenv("GIT_MERGE_VERBOSITY"), NULL, 10);

        if (argc < 4)
                die("Usage: %s <base>... -- <head> <remote> ...\n", argv[0]);

        for (i = 1; i < argc; ++i) {
                if (!strcmp(argv[i], "--"))
                        break;
                if (bases_count < sizeof(bases)/sizeof(*bases))
                        bases[bases_count++] = argv[i];
        }
        if (argc - i != 3) /* "--" "<head>" "<remote>" */
                die("Not handling anything other than two heads merge.");
        if (verbosity >= 5)
                buffer_output = 0;

        branch1 = argv[++i];
        branch2 = argv[++i];

        h1 = get_ref(branch1);
        h2 = get_ref(branch2);

        branch1 = better_branch_name(branch1);
        branch2 = better_branch_name(branch2);

        if (show(3))
                printf("Merging %s with %s\n", branch1, branch2);

        index_fd = hold_locked_index(lock, 1);

        for (i = 0; i < bases_count; i++) {
                struct commit *ancestor = get_ref(bases[i]);
                ca = commit_list_insert(ancestor, &ca);
        }
        clean = merge(h1, h2, branch1, branch2, ca, &result);

        if (active_cache_changed &&
            (write_cache(index_fd, active_cache, active_nr) ||
             close(index_fd) || commit_locked_index(lock)))
                        die ("unable to write %s", get_index_file());

        return clean ? 0: 1;
}