files_transaction_prepare(): fix handling of ref lock failure

[thirdparty/git.git] / refs / files-backend.c

#include "../cache.h"
#include "../config.h"
#include "../refs.h"
#include "refs-internal.h"
#include "ref-cache.h"
#include "packed-backend.h"
#include "../iterator.h"
#include "../dir-iterator.h"
#include "../lockfile.h"
#include "../object.h"
#include "../dir.h"

struct ref_lock {
        char *ref_name;
        struct lock_file *lk;
        struct object_id old_oid;
};

/*
 * Future: need to be in "struct repository"
 * when doing a full libification.
 */
struct files_ref_store {
        struct ref_store base;
        unsigned int store_flags;

        char *gitdir;
        char *gitcommondir;

        struct ref_cache *loose;

        struct ref_store *packed_ref_store;
};

static void clear_loose_ref_cache(struct files_ref_store *refs)
{
        if (refs->loose) {
                free_ref_cache(refs->loose);
                refs->loose = NULL;
        }
}

/*
 * Create a new submodule ref cache and add it to the internal
 * set of caches.
 */
static struct ref_store *files_ref_store_create(const char *gitdir,
                                                unsigned int flags)
{
        struct files_ref_store *refs = xcalloc(1, sizeof(*refs));
        struct ref_store *ref_store = (struct ref_store *)refs;
        struct strbuf sb = STRBUF_INIT;

        base_ref_store_init(ref_store, &refs_be_files);
        refs->store_flags = flags;

        refs->gitdir = xstrdup(gitdir);
        get_common_dir_noenv(&sb, gitdir);
        refs->gitcommondir = strbuf_detach(&sb, NULL);
        strbuf_addf(&sb, "%s/packed-refs", refs->gitcommondir);
        refs->packed_ref_store = packed_ref_store_create(sb.buf, flags);
        strbuf_release(&sb);

        return ref_store;
}

/*
 * Die if refs is not the main ref store. caller is used in any
 * necessary error messages.
 */
static void files_assert_main_repository(struct files_ref_store *refs,
                                         const char *caller)
{
        if (refs->store_flags & REF_STORE_MAIN)
                return;

        die("BUG: operation %s only allowed for main ref store", caller);
}

/*
 * Downcast ref_store to files_ref_store. Die if ref_store is not a
 * files_ref_store. required_flags is compared with ref_store's
 * store_flags to ensure the ref_store has all required capabilities.
 * "caller" is used in any necessary error messages.
 */
static struct files_ref_store *files_downcast(struct ref_store *ref_store,
                                              unsigned int required_flags,
                                              const char *caller)
{
        struct files_ref_store *refs;

        if (ref_store->be != &refs_be_files)
                die("BUG: ref_store is type \"%s\" not \"files\" in %s",
                    ref_store->be->name, caller);

        refs = (struct files_ref_store *)ref_store;

        if ((refs->store_flags & required_flags) != required_flags)
                die("BUG: operation %s requires abilities 0x%x, but only have 0x%x",
                    caller, required_flags, refs->store_flags);

        return refs;
}

static void files_reflog_path(struct files_ref_store *refs,
                              struct strbuf *sb,
                              const char *refname)
{
        if (!refname) {
                /*
                 * FIXME: of course this is wrong in multi worktree
                 * setting. To be fixed real soon.
                 */
                strbuf_addf(sb, "%s/logs", refs->gitcommondir);
                return;
        }

        switch (ref_type(refname)) {
        case REF_TYPE_PER_WORKTREE:
        case REF_TYPE_PSEUDOREF:
                strbuf_addf(sb, "%s/logs/%s", refs->gitdir, refname);
                break;
        case REF_TYPE_NORMAL:
                strbuf_addf(sb, "%s/logs/%s", refs->gitcommondir, refname);
                break;
        default:
                die("BUG: unknown ref type %d of ref %s",
                    ref_type(refname), refname);
        }
}

static void files_ref_path(struct files_ref_store *refs,
                           struct strbuf *sb,
                           const char *refname)
{
        switch (ref_type(refname)) {
        case REF_TYPE_PER_WORKTREE:
        case REF_TYPE_PSEUDOREF:
                strbuf_addf(sb, "%s/%s", refs->gitdir, refname);
                break;
        case REF_TYPE_NORMAL:
                strbuf_addf(sb, "%s/%s", refs->gitcommondir, refname);
                break;
        default:
                die("BUG: unknown ref type %d of ref %s",
                    ref_type(refname), refname);
        }
}

/*
 * Read the loose references from the namespace dirname into dir
 * (without recursing).  dirname must end with '/'.  dir must be the
 * directory entry corresponding to dirname.
 */
static void loose_fill_ref_dir(struct ref_store *ref_store,
                               struct ref_dir *dir, const char *dirname)
{
        struct files_ref_store *refs =
                files_downcast(ref_store, REF_STORE_READ, "fill_ref_dir");
        DIR *d;
        struct dirent *de;
        int dirnamelen = strlen(dirname);
        struct strbuf refname;
        struct strbuf path = STRBUF_INIT;
        size_t path_baselen;

        files_ref_path(refs, &path, dirname);
        path_baselen = path.len;

        d = opendir(path.buf);
        if (!d) {
                strbuf_release(&path);
                return;
        }

        strbuf_init(&refname, dirnamelen + 257);
        strbuf_add(&refname, dirname, dirnamelen);

        while ((de = readdir(d)) != NULL) {
                struct object_id oid;
                struct stat st;
                int flag;

                if (de->d_name[0] == '.')
                        continue;
                if (ends_with(de->d_name, ".lock"))
                        continue;
                strbuf_addstr(&refname, de->d_name);
                strbuf_addstr(&path, de->d_name);
                if (stat(path.buf, &st) < 0) {
                        ; /* silently ignore */
                } else if (S_ISDIR(st.st_mode)) {
                        strbuf_addch(&refname, '/');
                        add_entry_to_dir(dir,
                                         create_dir_entry(dir->cache, refname.buf,
                                                          refname.len, 1));
                } else {
                        if (!refs_resolve_ref_unsafe(&refs->base,
                                                     refname.buf,
                                                     RESOLVE_REF_READING,
                                                     oid.hash, &flag)) {
                                oidclr(&oid);
                                flag |= REF_ISBROKEN;
                        } else if (is_null_oid(&oid)) {
                                /*
                                 * It is so astronomically unlikely
                                 * that NULL_SHA1 is the SHA-1 of an
                                 * actual object that we consider its
                                 * appearance in a loose reference
                                 * file to be repo corruption
                                 * (probably due to a software bug).
                                 */
                                flag |= REF_ISBROKEN;
                        }

                        if (check_refname_format(refname.buf,
                                                 REFNAME_ALLOW_ONELEVEL)) {
                                if (!refname_is_safe(refname.buf))
                                        die("loose refname is dangerous: %s", refname.buf);
                                oidclr(&oid);
                                flag |= REF_BAD_NAME | REF_ISBROKEN;
                        }
                        add_entry_to_dir(dir,
                                         create_ref_entry(refname.buf, &oid, flag));
                }
                strbuf_setlen(&refname, dirnamelen);
                strbuf_setlen(&path, path_baselen);
        }
        strbuf_release(&refname);
        strbuf_release(&path);
        closedir(d);

        /*
         * Manually add refs/bisect, which, being per-worktree, might
         * not appear in the directory listing for refs/ in the main
         * repo.
         */
        if (!strcmp(dirname, "refs/")) {
                int pos = search_ref_dir(dir, "refs/bisect/", 12);

                if (pos < 0) {
                        struct ref_entry *child_entry = create_dir_entry(
                                        dir->cache, "refs/bisect/", 12, 1);
                        add_entry_to_dir(dir, child_entry);
                }
        }
}

static struct ref_cache *get_loose_ref_cache(struct files_ref_store *refs)
{
        if (!refs->loose) {
                /*
                 * Mark the top-level directory complete because we
                 * are about to read the only subdirectory that can
                 * hold references:
                 */
                refs->loose = create_ref_cache(&refs->base, loose_fill_ref_dir);

                /* We're going to fill the top level ourselves: */
                refs->loose->root->flag &= ~REF_INCOMPLETE;

                /*
                 * Add an incomplete entry for "refs/" (to be filled
                 * lazily):
                 */
                add_entry_to_dir(get_ref_dir(refs->loose->root),
                                 create_dir_entry(refs->loose, "refs/", 5, 1));
        }
        return refs->loose;
}

static int files_read_raw_ref(struct ref_store *ref_store,
                              const char *refname, unsigned char *sha1,
                              struct strbuf *referent, unsigned int *type)
{
        struct files_ref_store *refs =
                files_downcast(ref_store, REF_STORE_READ, "read_raw_ref");
        struct strbuf sb_contents = STRBUF_INIT;
        struct strbuf sb_path = STRBUF_INIT;
        const char *path;
        const char *buf;
        struct stat st;
        int fd;
        int ret = -1;
        int save_errno;
        int remaining_retries = 3;

        *type = 0;
        strbuf_reset(&sb_path);

        files_ref_path(refs, &sb_path, refname);

        path = sb_path.buf;

stat_ref:
        /*
         * We might have to loop back here to avoid a race
         * condition: first we lstat() the file, then we try
         * to read it as a link or as a file.  But if somebody
         * changes the type of the file (file <-> directory
         * <-> symlink) between the lstat() and reading, then
         * we don't want to report that as an error but rather
         * try again starting with the lstat().
         *
         * We'll keep a count of the retries, though, just to avoid
         * any confusing situation sending us into an infinite loop.
         */

        if (remaining_retries-- <= 0)
                goto out;

        if (lstat(path, &st) < 0) {
                if (errno != ENOENT)
                        goto out;
                if (refs_read_raw_ref(refs->packed_ref_store, refname,
                                      sha1, referent, type)) {
                        errno = ENOENT;
                        goto out;
                }
                ret = 0;
                goto out;
        }

        /* Follow "normalized" - ie "refs/.." symlinks by hand */
        if (S_ISLNK(st.st_mode)) {
                strbuf_reset(&sb_contents);
                if (strbuf_readlink(&sb_contents, path, 0) < 0) {
                        if (errno == ENOENT || errno == EINVAL)
                                /* inconsistent with lstat; retry */
                                goto stat_ref;
                        else
                                goto out;
                }
                if (starts_with(sb_contents.buf, "refs/") &&
                    !check_refname_format(sb_contents.buf, 0)) {
                        strbuf_swap(&sb_contents, referent);
                        *type |= REF_ISSYMREF;
                        ret = 0;
                        goto out;
                }
                /*
                 * It doesn't look like a refname; fall through to just
                 * treating it like a non-symlink, and reading whatever it
                 * points to.
                 */
        }

        /* Is it a directory? */
        if (S_ISDIR(st.st_mode)) {
                /*
                 * Even though there is a directory where the loose
                 * ref is supposed to be, there could still be a
                 * packed ref:
                 */
                if (refs_read_raw_ref(refs->packed_ref_store, refname,
                                      sha1, referent, type)) {
                        errno = EISDIR;
                        goto out;
                }
                ret = 0;
                goto out;
        }

        /*
         * Anything else, just open it and try to use it as
         * a ref
         */
        fd = open(path, O_RDONLY);
        if (fd < 0) {
                if (errno == ENOENT && !S_ISLNK(st.st_mode))
                        /* inconsistent with lstat; retry */
                        goto stat_ref;
                else
                        goto out;
        }
        strbuf_reset(&sb_contents);
        if (strbuf_read(&sb_contents, fd, 256) < 0) {
                int save_errno = errno;
                close(fd);
                errno = save_errno;
                goto out;
        }
        close(fd);
        strbuf_rtrim(&sb_contents);
        buf = sb_contents.buf;
        if (starts_with(buf, "ref:")) {
                buf += 4;
                while (isspace(*buf))
                        buf++;

                strbuf_reset(referent);
                strbuf_addstr(referent, buf);
                *type |= REF_ISSYMREF;
                ret = 0;
                goto out;
        }

        /*
         * Please note that FETCH_HEAD has additional
         * data after the sha.
         */
        if (get_sha1_hex(buf, sha1) ||
            (buf[40] != '\0' && !isspace(buf[40]))) {
                *type |= REF_ISBROKEN;
                errno = EINVAL;
                goto out;
        }

        ret = 0;

out:
        save_errno = errno;
        strbuf_release(&sb_path);
        strbuf_release(&sb_contents);
        errno = save_errno;
        return ret;
}

static void unlock_ref(struct ref_lock *lock)
{
        /* Do not free lock->lk -- atexit() still looks at them */
        if (lock->lk)
                rollback_lock_file(lock->lk);
        free(lock->ref_name);
        free(lock);
}

/*
 * Lock refname, without following symrefs, and set *lock_p to point
 * at a newly-allocated lock object. Fill in lock->old_oid, referent,
 * and type similarly to read_raw_ref().
 *
 * The caller must verify that refname is a "safe" reference name (in
 * the sense of refname_is_safe()) before calling this function.
 *
 * If the reference doesn't already exist, verify that refname doesn't
 * have a D/F conflict with any existing references. extras and skip
 * are passed to refs_verify_refname_available() for this check.
 *
 * If mustexist is not set and the reference is not found or is
 * broken, lock the reference anyway but clear sha1.
 *
 * Return 0 on success. On failure, write an error message to err and
 * return TRANSACTION_NAME_CONFLICT or TRANSACTION_GENERIC_ERROR.
 *
 * Implementation note: This function is basically
 *
 *     lock reference
 *     read_raw_ref()
 *
 * but it includes a lot more code to
 * - Deal with possible races with other processes
 * - Avoid calling refs_verify_refname_available() when it can be
 *   avoided, namely if we were successfully able to read the ref
 * - Generate informative error messages in the case of failure
 */
static int lock_raw_ref(struct files_ref_store *refs,
                        const char *refname, int mustexist,
                        const struct string_list *extras,
                        const struct string_list *skip,
                        struct ref_lock **lock_p,
                        struct strbuf *referent,
                        unsigned int *type,
                        struct strbuf *err)
{
        struct ref_lock *lock;
        struct strbuf ref_file = STRBUF_INIT;
        int attempts_remaining = 3;
        int ret = TRANSACTION_GENERIC_ERROR;

        assert(err);
        files_assert_main_repository(refs, "lock_raw_ref");

        *type = 0;

        /* First lock the file so it can't change out from under us. */

        *lock_p = lock = xcalloc(1, sizeof(*lock));

        lock->ref_name = xstrdup(refname);
        files_ref_path(refs, &ref_file, refname);

retry:
        switch (safe_create_leading_directories(ref_file.buf)) {
        case SCLD_OK:
                break; /* success */
        case SCLD_EXISTS:
                /*
                 * Suppose refname is "refs/foo/bar". We just failed
                 * to create the containing directory, "refs/foo",
                 * because there was a non-directory in the way. This
                 * indicates a D/F conflict, probably because of
                 * another reference such as "refs/foo". There is no
                 * reason to expect this error to be transitory.
                 */
                if (refs_verify_refname_available(&refs->base, refname,
                                                  extras, skip, err)) {
                        if (mustexist) {
                                /*
                                 * To the user the relevant error is
                                 * that the "mustexist" reference is
                                 * missing:
                                 */
                                strbuf_reset(err);
                                strbuf_addf(err, "unable to resolve reference '%s'",
                                            refname);
                        } else {
                                /*
                                 * The error message set by
                                 * refs_verify_refname_available() is
                                 * OK.
                                 */
                                ret = TRANSACTION_NAME_CONFLICT;
                        }
                } else {
                        /*
                         * The file that is in the way isn't a loose
                         * reference. Report it as a low-level
                         * failure.
                         */
                        strbuf_addf(err, "unable to create lock file %s.lock; "
                                    "non-directory in the way",
                                    ref_file.buf);
                }
                goto error_return;
        case SCLD_VANISHED:
                /* Maybe another process was tidying up. Try again. */
                if (--attempts_remaining > 0)
                        goto retry;
                /* fall through */
        default:
                strbuf_addf(err, "unable to create directory for %s",
                            ref_file.buf);
                goto error_return;
        }

        if (!lock->lk)
                lock->lk = xcalloc(1, sizeof(struct lock_file));

        if (hold_lock_file_for_update_timeout(
                            lock->lk, ref_file.buf, LOCK_NO_DEREF,
                            get_files_ref_lock_timeout_ms()) < 0) {
                if (errno == ENOENT && --attempts_remaining > 0) {
                        /*
                         * Maybe somebody just deleted one of the
                         * directories leading to ref_file.  Try
                         * again:
                         */
                        goto retry;
                } else {
                        unable_to_lock_message(ref_file.buf, errno, err);
                        goto error_return;
                }
        }

        /*
         * Now we hold the lock and can read the reference without
         * fear that its value will change.
         */

        if (files_read_raw_ref(&refs->base, refname,
                               lock->old_oid.hash, referent, type)) {
                if (errno == ENOENT) {
                        if (mustexist) {
                                /* Garden variety missing reference. */
                                strbuf_addf(err, "unable to resolve reference '%s'",
                                            refname);
                                goto error_return;
                        } else {
                                /*
                                 * Reference is missing, but that's OK. We
                                 * know that there is not a conflict with
                                 * another loose reference because
                                 * (supposing that we are trying to lock
                                 * reference "refs/foo/bar"):
                                 *
                                 * - We were successfully able to create
                                 *   the lockfile refs/foo/bar.lock, so we
                                 *   know there cannot be a loose reference
                                 *   named "refs/foo".
                                 *
                                 * - We got ENOENT and not EISDIR, so we
                                 *   know that there cannot be a loose
                                 *   reference named "refs/foo/bar/baz".
                                 */
                        }
                } else if (errno == EISDIR) {
                        /*
                         * There is a directory in the way. It might have
                         * contained references that have been deleted. If
                         * we don't require that the reference already
                         * exists, try to remove the directory so that it
                         * doesn't cause trouble when we want to rename the
                         * lockfile into place later.
                         */
                        if (mustexist) {
                                /* Garden variety missing reference. */
                                strbuf_addf(err, "unable to resolve reference '%s'",
                                            refname);
                                goto error_return;
                        } else if (remove_dir_recursively(&ref_file,
                                                          REMOVE_DIR_EMPTY_ONLY)) {
                                if (refs_verify_refname_available(
                                                    &refs->base, refname,
                                                    extras, skip, err)) {
                                        /*
                                         * The error message set by
                                         * verify_refname_available() is OK.
                                         */
                                        ret = TRANSACTION_NAME_CONFLICT;
                                        goto error_return;
                                } else {
                                        /*
                                         * We can't delete the directory,
                                         * but we also don't know of any
                                         * references that it should
                                         * contain.
                                         */
                                        strbuf_addf(err, "there is a non-empty directory '%s' "
                                                    "blocking reference '%s'",
                                                    ref_file.buf, refname);
                                        goto error_return;
                                }
                        }
                } else if (errno == EINVAL && (*type & REF_ISBROKEN)) {
                        strbuf_addf(err, "unable to resolve reference '%s': "
                                    "reference broken", refname);
                        goto error_return;
                } else {
                        strbuf_addf(err, "unable to resolve reference '%s': %s",
                                    refname, strerror(errno));
                        goto error_return;
                }

                /*
                 * If the ref did not exist and we are creating it,
                 * make sure there is no existing packed ref that
                 * conflicts with refname:
                 */
                if (refs_verify_refname_available(
                                    refs->packed_ref_store, refname,
                                    extras, skip, err))
                        goto error_return;
        }

        ret = 0;
        goto out;

error_return:
        unlock_ref(lock);
        *lock_p = NULL;

out:
        strbuf_release(&ref_file);
        return ret;
}

static int files_peel_ref(struct ref_store *ref_store,
                          const char *refname, unsigned char *sha1)
{
        struct files_ref_store *refs =
                files_downcast(ref_store, REF_STORE_READ | REF_STORE_ODB,
                               "peel_ref");
        int flag;
        unsigned char base[20];

        if (current_ref_iter && current_ref_iter->refname == refname) {
                struct object_id peeled;

                if (ref_iterator_peel(current_ref_iter, &peeled))
                        return -1;
                hashcpy(sha1, peeled.hash);
                return 0;
        }

        if (refs_read_ref_full(ref_store, refname,
                               RESOLVE_REF_READING, base, &flag))
                return -1;

        /*
         * If the reference is packed, read its ref_entry from the
         * cache in the hope that we already know its peeled value.
         * We only try this optimization on packed references because
         * (a) forcing the filling of the loose reference cache could
         * be expensive and (b) loose references anyway usually do not
         * have REF_KNOWS_PEELED.
         */
        if (flag & REF_ISPACKED &&
            !refs_peel_ref(refs->packed_ref_store, refname, sha1))
                return 0;

        return peel_object(base, sha1);
}

struct files_ref_iterator {
        struct ref_iterator base;

        struct ref_iterator *iter0;
        unsigned int flags;
};

static int files_ref_iterator_advance(struct ref_iterator *ref_iterator)
{
        struct files_ref_iterator *iter =
                (struct files_ref_iterator *)ref_iterator;
        int ok;

        while ((ok = ref_iterator_advance(iter->iter0)) == ITER_OK) {
                if (iter->flags & DO_FOR_EACH_PER_WORKTREE_ONLY &&
                    ref_type(iter->iter0->refname) != REF_TYPE_PER_WORKTREE)
                        continue;

                if (!(iter->flags & DO_FOR_EACH_INCLUDE_BROKEN) &&
                    !ref_resolves_to_object(iter->iter0->refname,
                                            iter->iter0->oid,
                                            iter->iter0->flags))
                        continue;

                iter->base.refname = iter->iter0->refname;
                iter->base.oid = iter->iter0->oid;
                iter->base.flags = iter->iter0->flags;
                return ITER_OK;
        }

        iter->iter0 = NULL;
        if (ref_iterator_abort(ref_iterator) != ITER_DONE)
                ok = ITER_ERROR;

        return ok;
}

static int files_ref_iterator_peel(struct ref_iterator *ref_iterator,
                                   struct object_id *peeled)
{
        struct files_ref_iterator *iter =
                (struct files_ref_iterator *)ref_iterator;

        return ref_iterator_peel(iter->iter0, peeled);
}

static int files_ref_iterator_abort(struct ref_iterator *ref_iterator)
{
        struct files_ref_iterator *iter =
                (struct files_ref_iterator *)ref_iterator;
        int ok = ITER_DONE;

        if (iter->iter0)
                ok = ref_iterator_abort(iter->iter0);

        base_ref_iterator_free(ref_iterator);
        return ok;
}

static struct ref_iterator_vtable files_ref_iterator_vtable = {
        files_ref_iterator_advance,
        files_ref_iterator_peel,
        files_ref_iterator_abort
};

static struct ref_iterator *files_ref_iterator_begin(
                struct ref_store *ref_store,
                const char *prefix, unsigned int flags)
{
        struct files_ref_store *refs;
        struct ref_iterator *loose_iter, *packed_iter;
        struct files_ref_iterator *iter;
        struct ref_iterator *ref_iterator;
        unsigned int required_flags = REF_STORE_READ;

        if (!(flags & DO_FOR_EACH_INCLUDE_BROKEN))
                required_flags |= REF_STORE_ODB;

        refs = files_downcast(ref_store, required_flags, "ref_iterator_begin");

        iter = xcalloc(1, sizeof(*iter));
        ref_iterator = &iter->base;
        base_ref_iterator_init(ref_iterator, &files_ref_iterator_vtable);

        /*
         * We must make sure that all loose refs are read before
         * accessing the packed-refs file; this avoids a race
         * condition if loose refs are migrated to the packed-refs
         * file by a simultaneous process, but our in-memory view is
         * from before the migration. We ensure this as follows:
         * First, we call start the loose refs iteration with its
         * `prime_ref` argument set to true. This causes the loose
         * references in the subtree to be pre-read into the cache.
         * (If they've already been read, that's OK; we only need to
         * guarantee that they're read before the packed refs, not
         * *how much* before.) After that, we call
         * packed_ref_iterator_begin(), which internally checks
         * whether the packed-ref cache is up to date with what is on
         * disk, and re-reads it if not.
         */

        loose_iter = cache_ref_iterator_begin(get_loose_ref_cache(refs),
                                              prefix, 1);

        /*
         * The packed-refs file might contain broken references, for
         * example an old version of a reference that points at an
         * object that has since been garbage-collected. This is OK as
         * long as there is a corresponding loose reference that
         * overrides it, and we don't want to emit an error message in
         * this case. So ask the packed_ref_store for all of its
         * references, and (if needed) do our own check for broken
         * ones in files_ref_iterator_advance(), after we have merged
         * the packed and loose references.
         */
        packed_iter = refs_ref_iterator_begin(
                        refs->packed_ref_store, prefix, 0,
                        DO_FOR_EACH_INCLUDE_BROKEN);

        iter->iter0 = overlay_ref_iterator_begin(loose_iter, packed_iter);
        iter->flags = flags;

        return ref_iterator;
}

/*
 * Verify that the reference locked by lock has the value old_sha1.
 * Fail if the reference doesn't exist and mustexist is set. Return 0
 * on success. On error, write an error message to err, set errno, and
 * return a negative value.
 */
static int verify_lock(struct ref_store *ref_store, struct ref_lock *lock,
                       const unsigned char *old_sha1, int mustexist,
                       struct strbuf *err)
{
        assert(err);

        if (refs_read_ref_full(ref_store, lock->ref_name,
                               mustexist ? RESOLVE_REF_READING : 0,
                               lock->old_oid.hash, NULL)) {
                if (old_sha1) {
                        int save_errno = errno;
                        strbuf_addf(err, "can't verify ref '%s'", lock->ref_name);
                        errno = save_errno;
                        return -1;
                } else {
                        oidclr(&lock->old_oid);
                        return 0;
                }
        }
        if (old_sha1 && hashcmp(lock->old_oid.hash, old_sha1)) {
                strbuf_addf(err, "ref '%s' is at %s but expected %s",
                            lock->ref_name,
                            oid_to_hex(&lock->old_oid),
                            sha1_to_hex(old_sha1));
                errno = EBUSY;
                return -1;
        }
        return 0;
}

static int remove_empty_directories(struct strbuf *path)
{
        /*
         * we want to create a file but there is a directory there;
         * if that is an empty directory (or a directory that contains
         * only empty directories), remove them.
         */
        return remove_dir_recursively(path, REMOVE_DIR_EMPTY_ONLY);
}

static int create_reflock(const char *path, void *cb)
{
        struct lock_file *lk = cb;

        return hold_lock_file_for_update_timeout(
                        lk, path, LOCK_NO_DEREF,
                        get_files_ref_lock_timeout_ms()) < 0 ? -1 : 0;
}

/*
 * Locks a ref returning the lock on success and NULL on failure.
 * On failure errno is set to something meaningful.
 */
static struct ref_lock *lock_ref_sha1_basic(struct files_ref_store *refs,
                                            const char *refname,
                                            const unsigned char *old_sha1,
                                            const struct string_list *extras,
                                            const struct string_list *skip,
                                            unsigned int flags, int *type,
                                            struct strbuf *err)
{
        struct strbuf ref_file = STRBUF_INIT;
        struct ref_lock *lock;
        int last_errno = 0;
        int mustexist = (old_sha1 && !is_null_sha1(old_sha1));
        int resolve_flags = RESOLVE_REF_NO_RECURSE;
        int resolved;

        files_assert_main_repository(refs, "lock_ref_sha1_basic");
        assert(err);

        lock = xcalloc(1, sizeof(struct ref_lock));

        if (mustexist)
                resolve_flags |= RESOLVE_REF_READING;
        if (flags & REF_DELETING)
                resolve_flags |= RESOLVE_REF_ALLOW_BAD_NAME;

        files_ref_path(refs, &ref_file, refname);
        resolved = !!refs_resolve_ref_unsafe(&refs->base,
                                             refname, resolve_flags,
                                             lock->old_oid.hash, type);
        if (!resolved && errno == EISDIR) {
                /*
                 * we are trying to lock foo but we used to
                 * have foo/bar which now does not exist;
                 * it is normal for the empty directory 'foo'
                 * to remain.
                 */
                if (remove_empty_directories(&ref_file)) {
                        last_errno = errno;
                        if (!refs_verify_refname_available(
                                            &refs->base,
                                            refname, extras, skip, err))
                                strbuf_addf(err, "there are still refs under '%s'",
                                            refname);
                        goto error_return;
                }
                resolved = !!refs_resolve_ref_unsafe(&refs->base,
                                                     refname, resolve_flags,
                                                     lock->old_oid.hash, type);
        }
        if (!resolved) {
                last_errno = errno;
                if (last_errno != ENOTDIR ||
                    !refs_verify_refname_available(&refs->base, refname,
                                                   extras, skip, err))
                        strbuf_addf(err, "unable to resolve reference '%s': %s",
                                    refname, strerror(last_errno));

                goto error_return;
        }

        /*
         * If the ref did not exist and we are creating it, make sure
         * there is no existing packed ref whose name begins with our
         * refname, nor a packed ref whose name is a proper prefix of
         * our refname.
         */
        if (is_null_oid(&lock->old_oid) &&
            refs_verify_refname_available(refs->packed_ref_store, refname,
                                          extras, skip, err)) {
                last_errno = ENOTDIR;
                goto error_return;
        }

        lock->lk = xcalloc(1, sizeof(struct lock_file));

        lock->ref_name = xstrdup(refname);

        if (raceproof_create_file(ref_file.buf, create_reflock, lock->lk)) {
                last_errno = errno;
                unable_to_lock_message(ref_file.buf, errno, err);
                goto error_return;
        }

        if (verify_lock(&refs->base, lock, old_sha1, mustexist, err)) {
                last_errno = errno;
                goto error_return;
        }
        goto out;

 error_return:
        unlock_ref(lock);
        lock = NULL;

 out:
        strbuf_release(&ref_file);
        errno = last_errno;
        return lock;
}

struct ref_to_prune {
        struct ref_to_prune *next;
        unsigned char sha1[20];
        char name[FLEX_ARRAY];
};

enum {
        REMOVE_EMPTY_PARENTS_REF = 0x01,
        REMOVE_EMPTY_PARENTS_REFLOG = 0x02
};

/*
 * Remove empty parent directories associated with the specified
 * reference and/or its reflog, but spare [logs/]refs/ and immediate
 * subdirs. flags is a combination of REMOVE_EMPTY_PARENTS_REF and/or
 * REMOVE_EMPTY_PARENTS_REFLOG.
 */
static void try_remove_empty_parents(struct files_ref_store *refs,
                                     const char *refname,
                                     unsigned int flags)
{
        struct strbuf buf = STRBUF_INIT;
        struct strbuf sb = STRBUF_INIT;
        char *p, *q;
        int i;

        strbuf_addstr(&buf, refname);
        p = buf.buf;
        for (i = 0; i < 2; i++) { /* refs/{heads,tags,...}/ */
                while (*p && *p != '/')
                        p++;
                /* tolerate duplicate slashes; see check_refname_format() */
                while (*p == '/')
                        p++;
        }
        q = buf.buf + buf.len;
        while (flags & (REMOVE_EMPTY_PARENTS_REF | REMOVE_EMPTY_PARENTS_REFLOG)) {
                while (q > p && *q != '/')
                        q--;
                while (q > p && *(q-1) == '/')
                        q--;
                if (q == p)
                        break;
                strbuf_setlen(&buf, q - buf.buf);

                strbuf_reset(&sb);
                files_ref_path(refs, &sb, buf.buf);
                if ((flags & REMOVE_EMPTY_PARENTS_REF) && rmdir(sb.buf))
                        flags &= ~REMOVE_EMPTY_PARENTS_REF;

                strbuf_reset(&sb);
                files_reflog_path(refs, &sb, buf.buf);
                if ((flags & REMOVE_EMPTY_PARENTS_REFLOG) && rmdir(sb.buf))
                        flags &= ~REMOVE_EMPTY_PARENTS_REFLOG;
        }
        strbuf_release(&buf);
        strbuf_release(&sb);
}

/* make sure nobody touched the ref, and unlink */
static void prune_ref(struct files_ref_store *refs, struct ref_to_prune *r)
{
        struct ref_transaction *transaction;
        struct strbuf err = STRBUF_INIT;

        if (check_refname_format(r->name, 0))
                return;

        transaction = ref_store_transaction_begin(&refs->base, &err);
        if (!transaction ||
            ref_transaction_delete(transaction, r->name, r->sha1,
                                   REF_ISPRUNING | REF_NODEREF, NULL, &err) ||
            ref_transaction_commit(transaction, &err)) {
                ref_transaction_free(transaction);
                error("%s", err.buf);
                strbuf_release(&err);
                return;
        }
        ref_transaction_free(transaction);
        strbuf_release(&err);
}

/*
 * Prune the loose versions of the references in the linked list
 * `*refs_to_prune`, freeing the entries in the list as we go.
 */
static void prune_refs(struct files_ref_store *refs, struct ref_to_prune **refs_to_prune)
{
        while (*refs_to_prune) {
                struct ref_to_prune *r = *refs_to_prune;
                *refs_to_prune = r->next;
                prune_ref(refs, r);
                free(r);
        }
}

/*
 * Return true if the specified reference should be packed.
 */
static int should_pack_ref(const char *refname,
                           const struct object_id *oid, unsigned int ref_flags,
                           unsigned int pack_flags)
{
        /* Do not pack per-worktree refs: */
        if (ref_type(refname) != REF_TYPE_NORMAL)
                return 0;

        /* Do not pack non-tags unless PACK_REFS_ALL is set: */
        if (!(pack_flags & PACK_REFS_ALL) && !starts_with(refname, "refs/tags/"))
                return 0;

        /* Do not pack symbolic refs: */
        if (ref_flags & REF_ISSYMREF)
                return 0;

        /* Do not pack broken refs: */
        if (!ref_resolves_to_object(refname, oid, ref_flags))
                return 0;

        return 1;
}

static int files_pack_refs(struct ref_store *ref_store, unsigned int flags)
{
        struct files_ref_store *refs =
                files_downcast(ref_store, REF_STORE_WRITE | REF_STORE_ODB,
                               "pack_refs");
        struct ref_iterator *iter;
        int ok;
        struct ref_to_prune *refs_to_prune = NULL;
        struct strbuf err = STRBUF_INIT;
        struct ref_transaction *transaction;

        transaction = ref_store_transaction_begin(refs->packed_ref_store, &err);
        if (!transaction)
                return -1;

        packed_refs_lock(refs->packed_ref_store, LOCK_DIE_ON_ERROR, &err);

        iter = cache_ref_iterator_begin(get_loose_ref_cache(refs), NULL, 0);
        while ((ok = ref_iterator_advance(iter)) == ITER_OK) {
                /*
                 * If the loose reference can be packed, add an entry
                 * in the packed ref cache. If the reference should be
                 * pruned, also add it to refs_to_prune.
                 */
                if (!should_pack_ref(iter->refname, iter->oid, iter->flags,
                                     flags))
                        continue;

                /*
                 * Add a reference creation for this reference to the
                 * packed-refs transaction:
                 */
                if (ref_transaction_update(transaction, iter->refname,
                                           iter->oid->hash, NULL,
                                           REF_NODEREF, NULL, &err))
                        die("failure preparing to create packed reference %s: %s",
                            iter->refname, err.buf);

                /* Schedule the loose reference for pruning if requested. */
                if ((flags & PACK_REFS_PRUNE)) {
                        struct ref_to_prune *n;
                        FLEX_ALLOC_STR(n, name, iter->refname);
                        hashcpy(n->sha1, iter->oid->hash);
                        n->next = refs_to_prune;
                        refs_to_prune = n;
                }
        }
        if (ok != ITER_DONE)
                die("error while iterating over references");

        if (ref_transaction_commit(transaction, &err))
                die("unable to write new packed-refs: %s", err.buf);

        ref_transaction_free(transaction);

        packed_refs_unlock(refs->packed_ref_store);

        prune_refs(refs, &refs_to_prune);
        strbuf_release(&err);
        return 0;
}

static int files_delete_refs(struct ref_store *ref_store, const char *msg,
                             struct string_list *refnames, unsigned int flags)
{
        struct files_ref_store *refs =
                files_downcast(ref_store, REF_STORE_WRITE, "delete_refs");
        struct strbuf err = STRBUF_INIT;
        int i, result = 0;

        if (!refnames->nr)
                return 0;

        if (packed_refs_lock(refs->packed_ref_store, 0, &err))
                goto error;

        if (refs_delete_refs(refs->packed_ref_store, msg, refnames, flags)) {
                packed_refs_unlock(refs->packed_ref_store);
                goto error;
        }

        packed_refs_unlock(refs->packed_ref_store);

        for (i = 0; i < refnames->nr; i++) {
                const char *refname = refnames->items[i].string;

                if (refs_delete_ref(&refs->base, msg, refname, NULL, flags))
                        result |= error(_("could not remove reference %s"), refname);
        }

        strbuf_release(&err);
        return result;

error:
        /*
         * If we failed to rewrite the packed-refs file, then it is
         * unsafe to try to remove loose refs, because doing so might
         * expose an obsolete packed value for a reference that might
         * even point at an object that has been garbage collected.
         */
        if (refnames->nr == 1)
                error(_("could not delete reference %s: %s"),
                      refnames->items[0].string, err.buf);
        else
                error(_("could not delete references: %s"), err.buf);

        strbuf_release(&err);
        return -1;
}

/*
 * People using contrib's git-new-workdir have .git/logs/refs ->
 * /some/other/path/.git/logs/refs, and that may live on another device.
 *
 * IOW, to avoid cross device rename errors, the temporary renamed log must
 * live into logs/refs.
 */
#define TMP_RENAMED_LOG  "refs/.tmp-renamed-log"

struct rename_cb {
        const char *tmp_renamed_log;
        int true_errno;
};

static int rename_tmp_log_callback(const char *path, void *cb_data)
{
        struct rename_cb *cb = cb_data;

        if (rename(cb->tmp_renamed_log, path)) {
                /*
                 * rename(a, b) when b is an existing directory ought
                 * to result in ISDIR, but Solaris 5.8 gives ENOTDIR.
                 * Sheesh. Record the true errno for error reporting,
                 * but report EISDIR to raceproof_create_file() so
                 * that it knows to retry.
                 */
                cb->true_errno = errno;
                if (errno == ENOTDIR)
                        errno = EISDIR;
                return -1;
        } else {
                return 0;
        }
}

static int rename_tmp_log(struct files_ref_store *refs, const char *newrefname)
{
        struct strbuf path = STRBUF_INIT;
        struct strbuf tmp = STRBUF_INIT;
        struct rename_cb cb;
        int ret;

        files_reflog_path(refs, &path, newrefname);
        files_reflog_path(refs, &tmp, TMP_RENAMED_LOG);
        cb.tmp_renamed_log = tmp.buf;
        ret = raceproof_create_file(path.buf, rename_tmp_log_callback, &cb);
        if (ret) {
                if (errno == EISDIR)
                        error("directory not empty: %s", path.buf);
                else
                        error("unable to move logfile %s to %s: %s",
                              tmp.buf, path.buf,
                              strerror(cb.true_errno));
        }

        strbuf_release(&path);
        strbuf_release(&tmp);
        return ret;
}

static int write_ref_to_lockfile(struct ref_lock *lock,
                                 const struct object_id *oid, struct strbuf *err);
static int commit_ref_update(struct files_ref_store *refs,
                             struct ref_lock *lock,
                             const struct object_id *oid, const char *logmsg,
                             struct strbuf *err);

static int files_rename_ref(struct ref_store *ref_store,
                            const char *oldrefname, const char *newrefname,
                            const char *logmsg)
{
        struct files_ref_store *refs =
                files_downcast(ref_store, REF_STORE_WRITE, "rename_ref");
        struct object_id oid, orig_oid;
        int flag = 0, logmoved = 0;
        struct ref_lock *lock;
        struct stat loginfo;
        struct strbuf sb_oldref = STRBUF_INIT;
        struct strbuf sb_newref = STRBUF_INIT;
        struct strbuf tmp_renamed_log = STRBUF_INIT;
        int log, ret;
        struct strbuf err = STRBUF_INIT;

        files_reflog_path(refs, &sb_oldref, oldrefname);
        files_reflog_path(refs, &sb_newref, newrefname);
        files_reflog_path(refs, &tmp_renamed_log, TMP_RENAMED_LOG);

        log = !lstat(sb_oldref.buf, &loginfo);
        if (log && S_ISLNK(loginfo.st_mode)) {
                ret = error("reflog for %s is a symlink", oldrefname);
                goto out;
        }

        if (!refs_resolve_ref_unsafe(&refs->base, oldrefname,
                                     RESOLVE_REF_READING | RESOLVE_REF_NO_RECURSE,
                                orig_oid.hash, &flag)) {
                ret = error("refname %s not found", oldrefname);
                goto out;
        }

        if (flag & REF_ISSYMREF) {
                ret = error("refname %s is a symbolic ref, renaming it is not supported",
                            oldrefname);
                goto out;
        }
        if (!refs_rename_ref_available(&refs->base, oldrefname, newrefname)) {
                ret = 1;
                goto out;
        }

        if (log && rename(sb_oldref.buf, tmp_renamed_log.buf)) {
                ret = error("unable to move logfile logs/%s to logs/"TMP_RENAMED_LOG": %s",
                            oldrefname, strerror(errno));
                goto out;
        }

        if (refs_delete_ref(&refs->base, logmsg, oldrefname,
                            orig_oid.hash, REF_NODEREF)) {
                error("unable to delete old %s", oldrefname);
                goto rollback;
        }

        /*
         * Since we are doing a shallow lookup, oid is not the
         * correct value to pass to delete_ref as old_oid. But that
         * doesn't matter, because an old_oid check wouldn't add to
         * the safety anyway; we want to delete the reference whatever
         * its current value.
         */
        if (!refs_read_ref_full(&refs->base, newrefname,
                                RESOLVE_REF_READING | RESOLVE_REF_NO_RECURSE,
                                oid.hash, NULL) &&
            refs_delete_ref(&refs->base, NULL, newrefname,
                            NULL, REF_NODEREF)) {
                if (errno == EISDIR) {
                        struct strbuf path = STRBUF_INIT;
                        int result;

                        files_ref_path(refs, &path, newrefname);
                        result = remove_empty_directories(&path);
                        strbuf_release(&path);

                        if (result) {
                                error("Directory not empty: %s", newrefname);
                                goto rollback;
                        }
                } else {
                        error("unable to delete existing %s", newrefname);
                        goto rollback;
                }
        }

        if (log && rename_tmp_log(refs, newrefname))
                goto rollback;

        logmoved = log;

        lock = lock_ref_sha1_basic(refs, newrefname, NULL, NULL, NULL,
                                   REF_NODEREF, NULL, &err);
        if (!lock) {
                error("unable to rename '%s' to '%s': %s", oldrefname, newrefname, err.buf);
                strbuf_release(&err);
                goto rollback;
        }
        oidcpy(&lock->old_oid, &orig_oid);

        if (write_ref_to_lockfile(lock, &orig_oid, &err) ||
            commit_ref_update(refs, lock, &orig_oid, logmsg, &err)) {
                error("unable to write current sha1 into %s: %s", newrefname, err.buf);
                strbuf_release(&err);
                goto rollback;
        }

        ret = 0;
        goto out;

 rollback:
        lock = lock_ref_sha1_basic(refs, oldrefname, NULL, NULL, NULL,
                                   REF_NODEREF, NULL, &err);
        if (!lock) {
                error("unable to lock %s for rollback: %s", oldrefname, err.buf);
                strbuf_release(&err);
                goto rollbacklog;
        }

        flag = log_all_ref_updates;
        log_all_ref_updates = LOG_REFS_NONE;
        if (write_ref_to_lockfile(lock, &orig_oid, &err) ||
            commit_ref_update(refs, lock, &orig_oid, NULL, &err)) {
                error("unable to write current sha1 into %s: %s", oldrefname, err.buf);
                strbuf_release(&err);
        }
        log_all_ref_updates = flag;

 rollbacklog:
        if (logmoved && rename(sb_newref.buf, sb_oldref.buf))
                error("unable to restore logfile %s from %s: %s",
                        oldrefname, newrefname, strerror(errno));
        if (!logmoved && log &&
            rename(tmp_renamed_log.buf, sb_oldref.buf))
                error("unable to restore logfile %s from logs/"TMP_RENAMED_LOG": %s",
                        oldrefname, strerror(errno));
        ret = 1;
 out:
        strbuf_release(&sb_newref);
        strbuf_release(&sb_oldref);
        strbuf_release(&tmp_renamed_log);

        return ret;
}

static int close_ref(struct ref_lock *lock)
{
        if (close_lock_file(lock->lk))
                return -1;
        return 0;
}

static int commit_ref(struct ref_lock *lock)
{
        char *path = get_locked_file_path(lock->lk);
        struct stat st;

        if (!lstat(path, &st) && S_ISDIR(st.st_mode)) {
                /*
                 * There is a directory at the path we want to rename
                 * the lockfile to. Hopefully it is empty; try to
                 * delete it.
                 */
                size_t len = strlen(path);
                struct strbuf sb_path = STRBUF_INIT;

                strbuf_attach(&sb_path, path, len, len);

                /*
                 * If this fails, commit_lock_file() will also fail
                 * and will report the problem.
                 */
                remove_empty_directories(&sb_path);
                strbuf_release(&sb_path);
        } else {
                free(path);
        }

        if (commit_lock_file(lock->lk))
                return -1;
        return 0;
}

static int open_or_create_logfile(const char *path, void *cb)
{
        int *fd = cb;

        *fd = open(path, O_APPEND | O_WRONLY | O_CREAT, 0666);
        return (*fd < 0) ? -1 : 0;
}

/*
 * Create a reflog for a ref. If force_create = 0, only create the
 * reflog for certain refs (those for which should_autocreate_reflog
 * returns non-zero). Otherwise, create it regardless of the reference
 * name. If the logfile already existed or was created, return 0 and
 * set *logfd to the file descriptor opened for appending to the file.
 * If no logfile exists and we decided not to create one, return 0 and
 * set *logfd to -1. On failure, fill in *err, set *logfd to -1, and
 * return -1.
 */
static int log_ref_setup(struct files_ref_store *refs,
                         const char *refname, int force_create,
                         int *logfd, struct strbuf *err)
{
        struct strbuf logfile_sb = STRBUF_INIT;
        char *logfile;

        files_reflog_path(refs, &logfile_sb, refname);
        logfile = strbuf_detach(&logfile_sb, NULL);

        if (force_create || should_autocreate_reflog(refname)) {
                if (raceproof_create_file(logfile, open_or_create_logfile, logfd)) {
                        if (errno == ENOENT)
                                strbuf_addf(err, "unable to create directory for '%s': "
                                            "%s", logfile, strerror(errno));
                        else if (errno == EISDIR)
                                strbuf_addf(err, "there are still logs under '%s'",
                                            logfile);
                        else
                                strbuf_addf(err, "unable to append to '%s': %s",
                                            logfile, strerror(errno));

                        goto error;
                }
        } else {
                *logfd = open(logfile, O_APPEND | O_WRONLY, 0666);
                if (*logfd < 0) {
                        if (errno == ENOENT || errno == EISDIR) {
                                /*
                                 * The logfile doesn't already exist,
                                 * but that is not an error; it only
                                 * means that we won't write log
                                 * entries to it.
                                 */
                                ;
                        } else {
                                strbuf_addf(err, "unable to append to '%s': %s",
                                            logfile, strerror(errno));
                                goto error;
                        }
                }
        }

        if (*logfd >= 0)
                adjust_shared_perm(logfile);

        free(logfile);
        return 0;

error:
        free(logfile);
        return -1;
}

static int files_create_reflog(struct ref_store *ref_store,
                               const char *refname, int force_create,
                               struct strbuf *err)
{
        struct files_ref_store *refs =
                files_downcast(ref_store, REF_STORE_WRITE, "create_reflog");
        int fd;

        if (log_ref_setup(refs, refname, force_create, &fd, err))
                return -1;

        if (fd >= 0)
                close(fd);

        return 0;
}

static int log_ref_write_fd(int fd, const struct object_id *old_oid,
                            const struct object_id *new_oid,
                            const char *committer, const char *msg)
{
        int msglen, written;
        unsigned maxlen, len;
        char *logrec;

        msglen = msg ? strlen(msg) : 0;
        maxlen = strlen(committer) + msglen + 100;
        logrec = xmalloc(maxlen);
        len = xsnprintf(logrec, maxlen, "%s %s %s\n",
                        oid_to_hex(old_oid),
                        oid_to_hex(new_oid),
                        committer);
        if (msglen)
                len += copy_reflog_msg(logrec + len - 1, msg) - 1;

        written = len <= maxlen ? write_in_full(fd, logrec, len) : -1;
        free(logrec);
        if (written != len)
                return -1;

        return 0;
}

static int files_log_ref_write(struct files_ref_store *refs,
                               const char *refname, const struct object_id *old_oid,
                               const struct object_id *new_oid, const char *msg,
                               int flags, struct strbuf *err)
{
        int logfd, result;

        if (log_all_ref_updates == LOG_REFS_UNSET)
                log_all_ref_updates = is_bare_repository() ? LOG_REFS_NONE : LOG_REFS_NORMAL;

        result = log_ref_setup(refs, refname,
                               flags & REF_FORCE_CREATE_REFLOG,
                               &logfd, err);

        if (result)
                return result;

        if (logfd < 0)
                return 0;
        result = log_ref_write_fd(logfd, old_oid, new_oid,
                                  git_committer_info(0), msg);
        if (result) {
                struct strbuf sb = STRBUF_INIT;
                int save_errno = errno;

                files_reflog_path(refs, &sb, refname);
                strbuf_addf(err, "unable to append to '%s': %s",
                            sb.buf, strerror(save_errno));
                strbuf_release(&sb);
                close(logfd);
                return -1;
        }
        if (close(logfd)) {
                struct strbuf sb = STRBUF_INIT;
                int save_errno = errno;

                files_reflog_path(refs, &sb, refname);
                strbuf_addf(err, "unable to append to '%s': %s",
                            sb.buf, strerror(save_errno));
                strbuf_release(&sb);
                return -1;
        }
        return 0;
}

/*
 * Write sha1 into the open lockfile, then close the lockfile. On
 * errors, rollback the lockfile, fill in *err and
 * return -1.
 */
static int write_ref_to_lockfile(struct ref_lock *lock,
                                 const struct object_id *oid, struct strbuf *err)
{
        static char term = '\n';
        struct object *o;
        int fd;

        o = parse_object(oid);
        if (!o) {
                strbuf_addf(err,
                            "trying to write ref '%s' with nonexistent object %s",
                            lock->ref_name, oid_to_hex(oid));
                unlock_ref(lock);
                return -1;
        }
        if (o->type != OBJ_COMMIT && is_branch(lock->ref_name)) {
                strbuf_addf(err,
                            "trying to write non-commit object %s to branch '%s'",
                            oid_to_hex(oid), lock->ref_name);
                unlock_ref(lock);
                return -1;
        }
        fd = get_lock_file_fd(lock->lk);
        if (write_in_full(fd, oid_to_hex(oid), GIT_SHA1_HEXSZ) != GIT_SHA1_HEXSZ ||
            write_in_full(fd, &term, 1) != 1 ||
            close_ref(lock) < 0) {
                strbuf_addf(err,
                            "couldn't write '%s'", get_lock_file_path(lock->lk));
                unlock_ref(lock);
                return -1;
        }
        return 0;
}

/*
 * Commit a change to a loose reference that has already been written
 * to the loose reference lockfile. Also update the reflogs if
 * necessary, using the specified lockmsg (which can be NULL).
 */
static int commit_ref_update(struct files_ref_store *refs,
                             struct ref_lock *lock,
                             const struct object_id *oid, const char *logmsg,
                             struct strbuf *err)
{
        files_assert_main_repository(refs, "commit_ref_update");

        clear_loose_ref_cache(refs);
        if (files_log_ref_write(refs, lock->ref_name,
                                &lock->old_oid, oid,
                                logmsg, 0, err)) {
                char *old_msg = strbuf_detach(err, NULL);
                strbuf_addf(err, "cannot update the ref '%s': %s",
                            lock->ref_name, old_msg);
                free(old_msg);
                unlock_ref(lock);
                return -1;
        }

        if (strcmp(lock->ref_name, "HEAD") != 0) {
                /*
                 * Special hack: If a branch is updated directly and HEAD
                 * points to it (may happen on the remote side of a push
                 * for example) then logically the HEAD reflog should be
                 * updated too.
                 * A generic solution implies reverse symref information,
                 * but finding all symrefs pointing to the given branch
                 * would be rather costly for this rare event (the direct
                 * update of a branch) to be worth it.  So let's cheat and
                 * check with HEAD only which should cover 99% of all usage
                 * scenarios (even 100% of the default ones).
                 */
                struct object_id head_oid;
                int head_flag;
                const char *head_ref;

                head_ref = refs_resolve_ref_unsafe(&refs->base, "HEAD",
                                                   RESOLVE_REF_READING,
                                                   head_oid.hash, &head_flag);
                if (head_ref && (head_flag & REF_ISSYMREF) &&
                    !strcmp(head_ref, lock->ref_name)) {
                        struct strbuf log_err = STRBUF_INIT;
                        if (files_log_ref_write(refs, "HEAD",
                                                &lock->old_oid, oid,
                                                logmsg, 0, &log_err)) {
                                error("%s", log_err.buf);
                                strbuf_release(&log_err);
                        }
                }
        }

        if (commit_ref(lock)) {
                strbuf_addf(err, "couldn't set '%s'", lock->ref_name);
                unlock_ref(lock);
                return -1;
        }

        unlock_ref(lock);
        return 0;
}

static int create_ref_symlink(struct ref_lock *lock, const char *target)
{
        int ret = -1;
#ifndef NO_SYMLINK_HEAD
        char *ref_path = get_locked_file_path(lock->lk);
        unlink(ref_path);
        ret = symlink(target, ref_path);
        free(ref_path);

        if (ret)
                fprintf(stderr, "no symlink - falling back to symbolic ref\n");
#endif
        return ret;
}

static void update_symref_reflog(struct files_ref_store *refs,
                                 struct ref_lock *lock, const char *refname,
                                 const char *target, const char *logmsg)
{
        struct strbuf err = STRBUF_INIT;
        struct object_id new_oid;
        if (logmsg &&
            !refs_read_ref_full(&refs->base, target,
                                RESOLVE_REF_READING, new_oid.hash, NULL) &&
            files_log_ref_write(refs, refname, &lock->old_oid,
                                &new_oid, logmsg, 0, &err)) {
                error("%s", err.buf);
                strbuf_release(&err);
        }
}

static int create_symref_locked(struct files_ref_store *refs,
                                struct ref_lock *lock, const char *refname,
                                const char *target, const char *logmsg)
{
        if (prefer_symlink_refs && !create_ref_symlink(lock, target)) {
                update_symref_reflog(refs, lock, refname, target, logmsg);
                return 0;
        }

        if (!fdopen_lock_file(lock->lk, "w"))
                return error("unable to fdopen %s: %s",
                             lock->lk->tempfile.filename.buf, strerror(errno));

        update_symref_reflog(refs, lock, refname, target, logmsg);

        /* no error check; commit_ref will check ferror */
        fprintf(lock->lk->tempfile.fp, "ref: %s\n", target);
        if (commit_ref(lock) < 0)
                return error("unable to write symref for %s: %s", refname,
                             strerror(errno));
        return 0;
}

static int files_create_symref(struct ref_store *ref_store,
                               const char *refname, const char *target,
                               const char *logmsg)
{
        struct files_ref_store *refs =
                files_downcast(ref_store, REF_STORE_WRITE, "create_symref");
        struct strbuf err = STRBUF_INIT;
        struct ref_lock *lock;
        int ret;

        lock = lock_ref_sha1_basic(refs, refname, NULL,
                                   NULL, NULL, REF_NODEREF, NULL,
                                   &err);
        if (!lock) {
                error("%s", err.buf);
                strbuf_release(&err);
                return -1;
        }

        ret = create_symref_locked(refs, lock, refname, target, logmsg);
        unlock_ref(lock);
        return ret;
}

static int files_reflog_exists(struct ref_store *ref_store,
                               const char *refname)
{
        struct files_ref_store *refs =
                files_downcast(ref_store, REF_STORE_READ, "reflog_exists");
        struct strbuf sb = STRBUF_INIT;
        struct stat st;
        int ret;

        files_reflog_path(refs, &sb, refname);
        ret = !lstat(sb.buf, &st) && S_ISREG(st.st_mode);
        strbuf_release(&sb);
        return ret;
}

static int files_delete_reflog(struct ref_store *ref_store,
                               const char *refname)
{
        struct files_ref_store *refs =
                files_downcast(ref_store, REF_STORE_WRITE, "delete_reflog");
        struct strbuf sb = STRBUF_INIT;
        int ret;

        files_reflog_path(refs, &sb, refname);
        ret = remove_path(sb.buf);
        strbuf_release(&sb);
        return ret;
}

static int show_one_reflog_ent(struct strbuf *sb, each_reflog_ent_fn fn, void *cb_data)
{
        struct object_id ooid, noid;
        char *email_end, *message;
        timestamp_t timestamp;
        int tz;
        const char *p = sb->buf;

        /* old SP new SP name <email> SP time TAB msg LF */
        if (!sb->len || sb->buf[sb->len - 1] != '\n' ||
            parse_oid_hex(p, &ooid, &p) || *p++ != ' ' ||
            parse_oid_hex(p, &noid, &p) || *p++ != ' ' ||
            !(email_end = strchr(p, '>')) ||
            email_end[1] != ' ' ||
            !(timestamp = parse_timestamp(email_end + 2, &message, 10)) ||
            !message || message[0] != ' ' ||
            (message[1] != '+' && message[1] != '-') ||
            !isdigit(message[2]) || !isdigit(message[3]) ||
            !isdigit(message[4]) || !isdigit(message[5]))
                return 0; /* corrupt? */
        email_end[1] = '\0';
        tz = strtol(message + 1, NULL, 10);
        if (message[6] != '\t')
                message += 6;
        else
                message += 7;
        return fn(&ooid, &noid, p, timestamp, tz, message, cb_data);
}

static char *find_beginning_of_line(char *bob, char *scan)
{
        while (bob < scan && *(--scan) != '\n')
                ; /* keep scanning backwards */
        /*
         * Return either beginning of the buffer, or LF at the end of
         * the previous line.
         */
        return scan;
}

static int files_for_each_reflog_ent_reverse(struct ref_store *ref_store,
                                             const char *refname,
                                             each_reflog_ent_fn fn,
                                             void *cb_data)
{
        struct files_ref_store *refs =
                files_downcast(ref_store, REF_STORE_READ,
                               "for_each_reflog_ent_reverse");
        struct strbuf sb = STRBUF_INIT;
        FILE *logfp;
        long pos;
        int ret = 0, at_tail = 1;

        files_reflog_path(refs, &sb, refname);
        logfp = fopen(sb.buf, "r");
        strbuf_release(&sb);
        if (!logfp)
                return -1;

        /* Jump to the end */
        if (fseek(logfp, 0, SEEK_END) < 0)
                ret = error("cannot seek back reflog for %s: %s",
                            refname, strerror(errno));
        pos = ftell(logfp);
        while (!ret && 0 < pos) {
                int cnt;
                size_t nread;
                char buf[BUFSIZ];
                char *endp, *scanp;

                /* Fill next block from the end */
                cnt = (sizeof(buf) < pos) ? sizeof(buf) : pos;
                if (fseek(logfp, pos - cnt, SEEK_SET)) {
                        ret = error("cannot seek back reflog for %s: %s",
                                    refname, strerror(errno));
                        break;
                }
                nread = fread(buf, cnt, 1, logfp);
                if (nread != 1) {
                        ret = error("cannot read %d bytes from reflog for %s: %s",
                                    cnt, refname, strerror(errno));
                        break;
                }
                pos -= cnt;

                scanp = endp = buf + cnt;
                if (at_tail && scanp[-1] == '\n')
                        /* Looking at the final LF at the end of the file */
                        scanp--;
                at_tail = 0;

                while (buf < scanp) {
                        /*
                         * terminating LF of the previous line, or the beginning
                         * of the buffer.
                         */
                        char *bp;

                        bp = find_beginning_of_line(buf, scanp);

                        if (*bp == '\n') {
                                /*
                                 * The newline is the end of the previous line,
                                 * so we know we have complete line starting
                                 * at (bp + 1). Prefix it onto any prior data
                                 * we collected for the line and process it.
                                 */
                                strbuf_splice(&sb, 0, 0, bp + 1, endp - (bp + 1));
                                scanp = bp;
                                endp = bp + 1;
                                ret = show_one_reflog_ent(&sb, fn, cb_data);
                                strbuf_reset(&sb);
                                if (ret)
                                        break;
                        } else if (!pos) {
                                /*
                                 * We are at the start of the buffer, and the
                                 * start of the file; there is no previous
                                 * line, and we have everything for this one.
                                 * Process it, and we can end the loop.
                                 */
                                strbuf_splice(&sb, 0, 0, buf, endp - buf);
                                ret = show_one_reflog_ent(&sb, fn, cb_data);
                                strbuf_reset(&sb);
                                break;
                        }

                        if (bp == buf) {
                                /*
                                 * We are at the start of the buffer, and there
                                 * is more file to read backwards. Which means
                                 * we are in the middle of a line. Note that we
                                 * may get here even if *bp was a newline; that
                                 * just means we are at the exact end of the
                                 * previous line, rather than some spot in the
                                 * middle.
                                 *
                                 * Save away what we have to be combined with
                                 * the data from the next read.
                                 */
                                strbuf_splice(&sb, 0, 0, buf, endp - buf);
                                break;
                        }
                }

        }
        if (!ret && sb.len)
                die("BUG: reverse reflog parser had leftover data");

        fclose(logfp);
        strbuf_release(&sb);
        return ret;
}

static int files_for_each_reflog_ent(struct ref_store *ref_store,
                                     const char *refname,
                                     each_reflog_ent_fn fn, void *cb_data)
{
        struct files_ref_store *refs =
                files_downcast(ref_store, REF_STORE_READ,
                               "for_each_reflog_ent");
        FILE *logfp;
        struct strbuf sb = STRBUF_INIT;
        int ret = 0;

        files_reflog_path(refs, &sb, refname);
        logfp = fopen(sb.buf, "r");
        strbuf_release(&sb);
        if (!logfp)
                return -1;

        while (!ret && !strbuf_getwholeline(&sb, logfp, '\n'))
                ret = show_one_reflog_ent(&sb, fn, cb_data);
        fclose(logfp);
        strbuf_release(&sb);
        return ret;
}

struct files_reflog_iterator {
        struct ref_iterator base;

        struct ref_store *ref_store;
        struct dir_iterator *dir_iterator;
        struct object_id oid;
};

static int files_reflog_iterator_advance(struct ref_iterator *ref_iterator)
{
        struct files_reflog_iterator *iter =
                (struct files_reflog_iterator *)ref_iterator;
        struct dir_iterator *diter = iter->dir_iterator;
        int ok;

        while ((ok = dir_iterator_advance(diter)) == ITER_OK) {
                int flags;

                if (!S_ISREG(diter->st.st_mode))
                        continue;
                if (diter->basename[0] == '.')
                        continue;
                if (ends_with(diter->basename, ".lock"))
                        continue;

                if (refs_read_ref_full(iter->ref_store,
                                       diter->relative_path, 0,
                                       iter->oid.hash, &flags)) {
                        error("bad ref for %s", diter->path.buf);
                        continue;
                }

                iter->base.refname = diter->relative_path;
                iter->base.oid = &iter->oid;
                iter->base.flags = flags;
                return ITER_OK;
        }

        iter->dir_iterator = NULL;
        if (ref_iterator_abort(ref_iterator) == ITER_ERROR)
                ok = ITER_ERROR;
        return ok;
}

static int files_reflog_iterator_peel(struct ref_iterator *ref_iterator,
                                   struct object_id *peeled)
{
        die("BUG: ref_iterator_peel() called for reflog_iterator");
}

static int files_reflog_iterator_abort(struct ref_iterator *ref_iterator)
{
        struct files_reflog_iterator *iter =
                (struct files_reflog_iterator *)ref_iterator;
        int ok = ITER_DONE;

        if (iter->dir_iterator)
                ok = dir_iterator_abort(iter->dir_iterator);

        base_ref_iterator_free(ref_iterator);
        return ok;
}

static struct ref_iterator_vtable files_reflog_iterator_vtable = {
        files_reflog_iterator_advance,
        files_reflog_iterator_peel,
        files_reflog_iterator_abort
};

static struct ref_iterator *files_reflog_iterator_begin(struct ref_store *ref_store)
{
        struct files_ref_store *refs =
                files_downcast(ref_store, REF_STORE_READ,
                               "reflog_iterator_begin");
        struct files_reflog_iterator *iter = xcalloc(1, sizeof(*iter));
        struct ref_iterator *ref_iterator = &iter->base;
        struct strbuf sb = STRBUF_INIT;

        base_ref_iterator_init(ref_iterator, &files_reflog_iterator_vtable);
        files_reflog_path(refs, &sb, NULL);
        iter->dir_iterator = dir_iterator_begin(sb.buf);
        iter->ref_store = ref_store;
        strbuf_release(&sb);
        return ref_iterator;
}

/*
 * If update is a direct update of head_ref (the reference pointed to
 * by HEAD), then add an extra REF_LOG_ONLY update for HEAD.
 */
static int split_head_update(struct ref_update *update,
                             struct ref_transaction *transaction,
                             const char *head_ref,
                             struct string_list *affected_refnames,
                             struct strbuf *err)
{
        struct string_list_item *item;
        struct ref_update *new_update;

        if ((update->flags & REF_LOG_ONLY) ||
            (update->flags & REF_ISPRUNING) ||
            (update->flags & REF_UPDATE_VIA_HEAD))
                return 0;

        if (strcmp(update->refname, head_ref))
                return 0;

        /*
         * First make sure that HEAD is not already in the
         * transaction. This insertion is O(N) in the transaction
         * size, but it happens at most once per transaction.
         */
        item = string_list_insert(affected_refnames, "HEAD");
        if (item->util) {
                /* An entry already existed */
                strbuf_addf(err,
                            "multiple updates for 'HEAD' (including one "
                            "via its referent '%s') are not allowed",
                            update->refname);
                return TRANSACTION_NAME_CONFLICT;
        }

        new_update = ref_transaction_add_update(
                        transaction, "HEAD",
                        update->flags | REF_LOG_ONLY | REF_NODEREF,
                        update->new_oid.hash, update->old_oid.hash,
                        update->msg);

        item->util = new_update;

        return 0;
}

/*
 * update is for a symref that points at referent and doesn't have
 * REF_NODEREF set. Split it into two updates:
 * - The original update, but with REF_LOG_ONLY and REF_NODEREF set
 * - A new, separate update for the referent reference
 * Note that the new update will itself be subject to splitting when
 * the iteration gets to it.
 */
static int split_symref_update(struct files_ref_store *refs,
                               struct ref_update *update,
                               const char *referent,
                               struct ref_transaction *transaction,
                               struct string_list *affected_refnames,
                               struct strbuf *err)
{
        struct string_list_item *item;
        struct ref_update *new_update;
        unsigned int new_flags;

        /*
         * First make sure that referent is not already in the
         * transaction. This insertion is O(N) in the transaction
         * size, but it happens at most once per symref in a
         * transaction.
         */
        item = string_list_insert(affected_refnames, referent);
        if (item->util) {
                /* An entry already existed */
                strbuf_addf(err,
                            "multiple updates for '%s' (including one "
                            "via symref '%s') are not allowed",
                            referent, update->refname);
                return TRANSACTION_NAME_CONFLICT;
        }

        new_flags = update->flags;
        if (!strcmp(update->refname, "HEAD")) {
                /*
                 * Record that the new update came via HEAD, so that
                 * when we process it, split_head_update() doesn't try
                 * to add another reflog update for HEAD. Note that
                 * this bit will be propagated if the new_update
                 * itself needs to be split.
                 */
                new_flags |= REF_UPDATE_VIA_HEAD;
        }

        new_update = ref_transaction_add_update(
                        transaction, referent, new_flags,
                        update->new_oid.hash, update->old_oid.hash,
                        update->msg);

        new_update->parent_update = update;

        /*
         * Change the symbolic ref update to log only. Also, it
         * doesn't need to check its old SHA-1 value, as that will be
         * done when new_update is processed.
         */
        update->flags |= REF_LOG_ONLY | REF_NODEREF;
        update->flags &= ~REF_HAVE_OLD;

        item->util = new_update;

        return 0;
}

/*
 * Return the refname under which update was originally requested.
 */
static const char *original_update_refname(struct ref_update *update)
{
        while (update->parent_update)
                update = update->parent_update;

        return update->refname;
}

/*
 * Check whether the REF_HAVE_OLD and old_oid values stored in update
 * are consistent with oid, which is the reference's current value. If
 * everything is OK, return 0; otherwise, write an error message to
 * err and return -1.
 */
static int check_old_oid(struct ref_update *update, struct object_id *oid,
                         struct strbuf *err)
{
        if (!(update->flags & REF_HAVE_OLD) ||
                   !oidcmp(oid, &update->old_oid))
                return 0;

        if (is_null_oid(&update->old_oid))
                strbuf_addf(err, "cannot lock ref '%s': "
                            "reference already exists",
                            original_update_refname(update));
        else if (is_null_oid(oid))
                strbuf_addf(err, "cannot lock ref '%s': "
                            "reference is missing but expected %s",
                            original_update_refname(update),
                            oid_to_hex(&update->old_oid));
        else
                strbuf_addf(err, "cannot lock ref '%s': "
                            "is at %s but expected %s",
                            original_update_refname(update),
                            oid_to_hex(oid),
                            oid_to_hex(&update->old_oid));

        return -1;
}

/*
 * Prepare for carrying out update:
 * - Lock the reference referred to by update.
 * - Read the reference under lock.
 * - Check that its old SHA-1 value (if specified) is correct, and in
 *   any case record it in update->lock->old_oid for later use when
 *   writing the reflog.
 * - If it is a symref update without REF_NODEREF, split it up into a
 *   REF_LOG_ONLY update of the symref and add a separate update for
 *   the referent to transaction.
 * - If it is an update of head_ref, add a corresponding REF_LOG_ONLY
 *   update of HEAD.
 */
static int lock_ref_for_update(struct files_ref_store *refs,
                               struct ref_update *update,
                               struct ref_transaction *transaction,
                               const char *head_ref,
                               struct string_list *affected_refnames,
                               struct strbuf *err)
{
        struct strbuf referent = STRBUF_INIT;
        int mustexist = (update->flags & REF_HAVE_OLD) &&
                !is_null_oid(&update->old_oid);
        int ret;
        struct ref_lock *lock;

        files_assert_main_repository(refs, "lock_ref_for_update");

        if ((update->flags & REF_HAVE_NEW) && is_null_oid(&update->new_oid))
                update->flags |= REF_DELETING;

        if (head_ref) {
                ret = split_head_update(update, transaction, head_ref,
                                        affected_refnames, err);
                if (ret)
                        return ret;
        }

        ret = lock_raw_ref(refs, update->refname, mustexist,
                           affected_refnames, NULL,
                           &lock, &referent,
                           &update->type, err);
        if (ret) {
                char *reason;

                reason = strbuf_detach(err, NULL);
                strbuf_addf(err, "cannot lock ref '%s': %s",
                            original_update_refname(update), reason);
                free(reason);
                return ret;
        }

        update->backend_data = lock;

        if (update->type & REF_ISSYMREF) {
                if (update->flags & REF_NODEREF) {
                        /*
                         * We won't be reading the referent as part of
                         * the transaction, so we have to read it here
                         * to record and possibly check old_sha1:
                         */
                        if (refs_read_ref_full(&refs->base,
                                               referent.buf, 0,
                                               lock->old_oid.hash, NULL)) {
                                if (update->flags & REF_HAVE_OLD) {
                                        strbuf_addf(err, "cannot lock ref '%s': "
                                                    "error reading reference",
                                                    original_update_refname(update));
                                        return -1;
                                }
                        } else if (check_old_oid(update, &lock->old_oid, err)) {
                                return TRANSACTION_GENERIC_ERROR;
                        }
                } else {
                        /*
                         * Create a new update for the reference this
                         * symref is pointing at. Also, we will record
                         * and verify old_sha1 for this update as part
                         * of processing the split-off update, so we
                         * don't have to do it here.
                         */
                        ret = split_symref_update(refs, update,
                                                  referent.buf, transaction,
                                                  affected_refnames, err);
                        if (ret)
                                return ret;
                }
        } else {
                struct ref_update *parent_update;

                if (check_old_oid(update, &lock->old_oid, err))
                        return TRANSACTION_GENERIC_ERROR;

                /*
                 * If this update is happening indirectly because of a
                 * symref update, record the old SHA-1 in the parent
                 * update:
                 */
                for (parent_update = update->parent_update;
                     parent_update;
                     parent_update = parent_update->parent_update) {
                        struct ref_lock *parent_lock = parent_update->backend_data;
                        oidcpy(&parent_lock->old_oid, &lock->old_oid);
                }
        }

        if ((update->flags & REF_HAVE_NEW) &&
            !(update->flags & REF_DELETING) &&
            !(update->flags & REF_LOG_ONLY)) {
                if (!(update->type & REF_ISSYMREF) &&
                    !oidcmp(&lock->old_oid, &update->new_oid)) {
                        /*
                         * The reference already has the desired
                         * value, so we don't need to write it.
                         */
                } else if (write_ref_to_lockfile(lock, &update->new_oid,
                                                 err)) {
                        char *write_err = strbuf_detach(err, NULL);

                        /*
                         * The lock was freed upon failure of
                         * write_ref_to_lockfile():
                         */
                        update->backend_data = NULL;
                        strbuf_addf(err,
                                    "cannot update ref '%s': %s",
                                    update->refname, write_err);
                        free(write_err);
                        return TRANSACTION_GENERIC_ERROR;
                } else {
                        update->flags |= REF_NEEDS_COMMIT;
                }
        }
        if (!(update->flags & REF_NEEDS_COMMIT)) {
                /*
                 * We didn't call write_ref_to_lockfile(), so
                 * the lockfile is still open. Close it to
                 * free up the file descriptor:
                 */
                if (close_ref(lock)) {
                        strbuf_addf(err, "couldn't close '%s.lock'",
                                    update->refname);
                        return TRANSACTION_GENERIC_ERROR;
                }
        }
        return 0;
}

struct files_transaction_backend_data {
        struct ref_transaction *packed_transaction;
        int packed_refs_locked;
};

/*
 * Unlock any references in `transaction` that are still locked, and
 * mark the transaction closed.
 */
static void files_transaction_cleanup(struct files_ref_store *refs,
                                      struct ref_transaction *transaction)
{
        size_t i;
        struct files_transaction_backend_data *backend_data =
                transaction->backend_data;
        struct strbuf err = STRBUF_INIT;

        for (i = 0; i < transaction->nr; i++) {
                struct ref_update *update = transaction->updates[i];
                struct ref_lock *lock = update->backend_data;

                if (lock) {
                        unlock_ref(lock);
                        update->backend_data = NULL;
                }
        }

        if (backend_data->packed_transaction &&
            ref_transaction_abort(backend_data->packed_transaction, &err)) {
                error("error aborting transaction: %s", err.buf);
                strbuf_release(&err);
        }

        if (backend_data->packed_refs_locked)
                packed_refs_unlock(refs->packed_ref_store);

        free(backend_data);

        transaction->state = REF_TRANSACTION_CLOSED;
}

static int files_transaction_prepare(struct ref_store *ref_store,
                                     struct ref_transaction *transaction,
                                     struct strbuf *err)
{
        struct files_ref_store *refs =
                files_downcast(ref_store, REF_STORE_WRITE,
                               "ref_transaction_prepare");
        size_t i;
        int ret = 0;
        struct string_list affected_refnames = STRING_LIST_INIT_NODUP;
        char *head_ref = NULL;
        int head_type;
        struct object_id head_oid;
        struct files_transaction_backend_data *backend_data;
        struct ref_transaction *packed_transaction = NULL;

        assert(err);

        if (!transaction->nr)
                goto cleanup;

        backend_data = xcalloc(1, sizeof(*backend_data));
        transaction->backend_data = backend_data;

        /*
         * Fail if a refname appears more than once in the
         * transaction. (If we end up splitting up any updates using
         * split_symref_update() or split_head_update(), those
         * functions will check that the new updates don't have the
         * same refname as any existing ones.)
         */
        for (i = 0; i < transaction->nr; i++) {
                struct ref_update *update = transaction->updates[i];
                struct string_list_item *item =
                        string_list_append(&affected_refnames, update->refname);

                /*
                 * We store a pointer to update in item->util, but at
                 * the moment we never use the value of this field
                 * except to check whether it is non-NULL.
                 */
                item->util = update;
        }
        string_list_sort(&affected_refnames);
        if (ref_update_reject_duplicates(&affected_refnames, err)) {
                ret = TRANSACTION_GENERIC_ERROR;
                goto cleanup;
        }

        /*
         * Special hack: If a branch is updated directly and HEAD
         * points to it (may happen on the remote side of a push
         * for example) then logically the HEAD reflog should be
         * updated too.
         *
         * A generic solution would require reverse symref lookups,
         * but finding all symrefs pointing to a given branch would be
         * rather costly for this rare event (the direct update of a
         * branch) to be worth it. So let's cheat and check with HEAD
         * only, which should cover 99% of all usage scenarios (even
         * 100% of the default ones).
         *
         * So if HEAD is a symbolic reference, then record the name of
         * the reference that it points to. If we see an update of
         * head_ref within the transaction, then split_head_update()
         * arranges for the reflog of HEAD to be updated, too.
         */
        head_ref = refs_resolve_refdup(ref_store, "HEAD",
                                       RESOLVE_REF_NO_RECURSE,
                                       head_oid.hash, &head_type);

        if (head_ref && !(head_type & REF_ISSYMREF)) {
                FREE_AND_NULL(head_ref);
        }

        /*
         * Acquire all locks, verify old values if provided, check
         * that new values are valid, and write new values to the
         * lockfiles, ready to be activated. Only keep one lockfile
         * open at a time to avoid running out of file descriptors.
         * Note that lock_ref_for_update() might append more updates
         * to the transaction.
         */
        for (i = 0; i < transaction->nr; i++) {
                struct ref_update *update = transaction->updates[i];

                ret = lock_ref_for_update(refs, update, transaction,
                                          head_ref, &affected_refnames, err);
                if (ret)
                        goto cleanup;

                if (update->flags & REF_DELETING &&
                    !(update->flags & REF_LOG_ONLY) &&
                    !(update->flags & REF_ISPRUNING)) {
                        /*
                         * This reference has to be deleted from
                         * packed-refs if it exists there.
                         */
                        if (!packed_transaction) {
                                packed_transaction = ref_store_transaction_begin(
                                                refs->packed_ref_store, err);
                                if (!packed_transaction) {
                                        ret = TRANSACTION_GENERIC_ERROR;
                                        goto cleanup;
                                }

                                backend_data->packed_transaction =
                                        packed_transaction;
                        }

                        ref_transaction_add_update(
                                        packed_transaction, update->refname,
                                        update->flags & ~REF_HAVE_OLD,
                                        update->new_oid.hash, update->old_oid.hash,
                                        NULL);
                }
        }

        if (packed_transaction) {
                if (packed_refs_lock(refs->packed_ref_store, 0, err)) {
                        ret = TRANSACTION_GENERIC_ERROR;
                        goto cleanup;
                }
                backend_data->packed_refs_locked = 1;
                ret = ref_transaction_prepare(packed_transaction, err);
        }

cleanup:
        free(head_ref);
        string_list_clear(&affected_refnames, 0);

        if (ret)
                files_transaction_cleanup(refs, transaction);
        else
                transaction->state = REF_TRANSACTION_PREPARED;

        return ret;
}

static int files_transaction_finish(struct ref_store *ref_store,
                                    struct ref_transaction *transaction,
                                    struct strbuf *err)
{
        struct files_ref_store *refs =
                files_downcast(ref_store, 0, "ref_transaction_finish");
        size_t i;
        int ret = 0;
        struct strbuf sb = STRBUF_INIT;
        struct files_transaction_backend_data *backend_data;
        struct ref_transaction *packed_transaction;


        assert(err);

        if (!transaction->nr) {
                transaction->state = REF_TRANSACTION_CLOSED;
                return 0;
        }

        backend_data = transaction->backend_data;
        packed_transaction = backend_data->packed_transaction;

        /* Perform updates first so live commits remain referenced */
        for (i = 0; i < transaction->nr; i++) {
                struct ref_update *update = transaction->updates[i];
                struct ref_lock *lock = update->backend_data;

                if (update->flags & REF_NEEDS_COMMIT ||
                    update->flags & REF_LOG_ONLY) {
                        if (files_log_ref_write(refs,
                                                lock->ref_name,
                                                &lock->old_oid,
                                                &update->new_oid,
                                                update->msg, update->flags,
                                                err)) {
                                char *old_msg = strbuf_detach(err, NULL);

                                strbuf_addf(err, "cannot update the ref '%s': %s",
                                            lock->ref_name, old_msg);
                                free(old_msg);
                                unlock_ref(lock);
                                update->backend_data = NULL;
                                ret = TRANSACTION_GENERIC_ERROR;
                                goto cleanup;
                        }
                }
                if (update->flags & REF_NEEDS_COMMIT) {
                        clear_loose_ref_cache(refs);
                        if (commit_ref(lock)) {
                                strbuf_addf(err, "couldn't set '%s'", lock->ref_name);
                                unlock_ref(lock);
                                update->backend_data = NULL;
                                ret = TRANSACTION_GENERIC_ERROR;
                                goto cleanup;
                        }
                }
        }

        /*
         * Now that updates are safely completed, we can perform
         * deletes. First delete the reflogs of any references that
         * will be deleted, since (in the unexpected event of an
         * error) leaving a reference without a reflog is less bad
         * than leaving a reflog without a reference (the latter is a
         * mildly invalid repository state):
         */
        for (i = 0; i < transaction->nr; i++) {
                struct ref_update *update = transaction->updates[i];
                if (update->flags & REF_DELETING &&
                    !(update->flags & REF_LOG_ONLY) &&
                    !(update->flags & REF_ISPRUNING)) {
                        strbuf_reset(&sb);
                        files_reflog_path(refs, &sb, update->refname);
                        if (!unlink_or_warn(sb.buf))
                                try_remove_empty_parents(refs, update->refname,
                                                         REMOVE_EMPTY_PARENTS_REFLOG);
                }
        }

        /*
         * Perform deletes now that updates are safely completed.
         *
         * First delete any packed versions of the references, while
         * retaining the packed-refs lock:
         */
        if (packed_transaction) {
                ret = ref_transaction_commit(packed_transaction, err);
                ref_transaction_free(packed_transaction);
                packed_transaction = NULL;
                backend_data->packed_transaction = NULL;
                if (ret)
                        goto cleanup;
        }

        /* Now delete the loose versions of the references: */
        for (i = 0; i < transaction->nr; i++) {
                struct ref_update *update = transaction->updates[i];
                struct ref_lock *lock = update->backend_data;

                if (update->flags & REF_DELETING &&
                    !(update->flags & REF_LOG_ONLY)) {
                        if (!(update->type & REF_ISPACKED) ||
                            update->type & REF_ISSYMREF) {
                                /* It is a loose reference. */
                                strbuf_reset(&sb);
                                files_ref_path(refs, &sb, lock->ref_name);
                                if (unlink_or_msg(sb.buf, err)) {
                                        ret = TRANSACTION_GENERIC_ERROR;
                                        goto cleanup;
                                }
                                update->flags |= REF_DELETED_LOOSE;
                        }
                }
        }

        clear_loose_ref_cache(refs);

cleanup:
        files_transaction_cleanup(refs, transaction);

        for (i = 0; i < transaction->nr; i++) {
                struct ref_update *update = transaction->updates[i];

                if (update->flags & REF_DELETED_LOOSE) {
                        /*
                         * The loose reference was deleted. Delete any
                         * empty parent directories. (Note that this
                         * can only work because we have already
                         * removed the lockfile.)
                         */
                        try_remove_empty_parents(refs, update->refname,
                                                 REMOVE_EMPTY_PARENTS_REF);
                }
        }

        strbuf_release(&sb);
        return ret;
}

static int files_transaction_abort(struct ref_store *ref_store,
                                   struct ref_transaction *transaction,
                                   struct strbuf *err)
{
        struct files_ref_store *refs =
                files_downcast(ref_store, 0, "ref_transaction_abort");

        files_transaction_cleanup(refs, transaction);
        return 0;
}

static int ref_present(const char *refname,
                       const struct object_id *oid, int flags, void *cb_data)
{
        struct string_list *affected_refnames = cb_data;

        return string_list_has_string(affected_refnames, refname);
}

static int files_initial_transaction_commit(struct ref_store *ref_store,
                                            struct ref_transaction *transaction,
                                            struct strbuf *err)
{
        struct files_ref_store *refs =
                files_downcast(ref_store, REF_STORE_WRITE,
                               "initial_ref_transaction_commit");
        size_t i;
        int ret = 0;
        struct string_list affected_refnames = STRING_LIST_INIT_NODUP;
        struct ref_transaction *packed_transaction = NULL;

        assert(err);

        if (transaction->state != REF_TRANSACTION_OPEN)
                die("BUG: commit called for transaction that is not open");

        /* Fail if a refname appears more than once in the transaction: */
        for (i = 0; i < transaction->nr; i++)
                string_list_append(&affected_refnames,
                                   transaction->updates[i]->refname);
        string_list_sort(&affected_refnames);
        if (ref_update_reject_duplicates(&affected_refnames, err)) {
                ret = TRANSACTION_GENERIC_ERROR;
                goto cleanup;
        }

        /*
         * It's really undefined to call this function in an active
         * repository or when there are existing references: we are
         * only locking and changing packed-refs, so (1) any
         * simultaneous processes might try to change a reference at
         * the same time we do, and (2) any existing loose versions of
         * the references that we are setting would have precedence
         * over our values. But some remote helpers create the remote
         * "HEAD" and "master" branches before calling this function,
         * so here we really only check that none of the references
         * that we are creating already exists.
         */
        if (refs_for_each_rawref(&refs->base, ref_present,
                                 &affected_refnames))
                die("BUG: initial ref transaction called with existing refs");

        packed_transaction = ref_store_transaction_begin(refs->packed_ref_store, err);
        if (!packed_transaction) {
                ret = TRANSACTION_GENERIC_ERROR;
                goto cleanup;
        }

        for (i = 0; i < transaction->nr; i++) {
                struct ref_update *update = transaction->updates[i];

                if ((update->flags & REF_HAVE_OLD) &&
                    !is_null_oid(&update->old_oid))
                        die("BUG: initial ref transaction with old_sha1 set");
                if (refs_verify_refname_available(&refs->base, update->refname,
                                                  &affected_refnames, NULL,
                                                  err)) {
                        ret = TRANSACTION_NAME_CONFLICT;
                        goto cleanup;
                }

                /*
                 * Add a reference creation for this reference to the
                 * packed-refs transaction:
                 */
                ref_transaction_add_update(packed_transaction, update->refname,
                                           update->flags & ~REF_HAVE_OLD,
                                           update->new_oid.hash, update->old_oid.hash,
                                           NULL);
        }

        if (packed_refs_lock(refs->packed_ref_store, 0, err)) {
                ret = TRANSACTION_GENERIC_ERROR;
                goto cleanup;
        }

        if (initial_ref_transaction_commit(packed_transaction, err)) {
                ret = TRANSACTION_GENERIC_ERROR;
                goto cleanup;
        }

cleanup:
        if (packed_transaction)
                ref_transaction_free(packed_transaction);
        packed_refs_unlock(refs->packed_ref_store);
        transaction->state = REF_TRANSACTION_CLOSED;
        string_list_clear(&affected_refnames, 0);
        return ret;
}

struct expire_reflog_cb {
        unsigned int flags;
        reflog_expiry_should_prune_fn *should_prune_fn;
        void *policy_cb;
        FILE *newlog;
        struct object_id last_kept_oid;
};

static int expire_reflog_ent(struct object_id *ooid, struct object_id *noid,
                             const char *email, timestamp_t timestamp, int tz,
                             const char *message, void *cb_data)
{
        struct expire_reflog_cb *cb = cb_data;
        struct expire_reflog_policy_cb *policy_cb = cb->policy_cb;

        if (cb->flags & EXPIRE_REFLOGS_REWRITE)
                ooid = &cb->last_kept_oid;

        if ((*cb->should_prune_fn)(ooid, noid, email, timestamp, tz,
                                   message, policy_cb)) {
                if (!cb->newlog)
                        printf("would prune %s", message);
                else if (cb->flags & EXPIRE_REFLOGS_VERBOSE)
                        printf("prune %s", message);
        } else {
                if (cb->newlog) {
                        fprintf(cb->newlog, "%s %s %s %"PRItime" %+05d\t%s",
                                oid_to_hex(ooid), oid_to_hex(noid),
                                email, timestamp, tz, message);
                        oidcpy(&cb->last_kept_oid, noid);
                }
                if (cb->flags & EXPIRE_REFLOGS_VERBOSE)
                        printf("keep %s", message);
        }
        return 0;
}

static int files_reflog_expire(struct ref_store *ref_store,
                               const char *refname, const unsigned char *sha1,
                               unsigned int flags,
                               reflog_expiry_prepare_fn prepare_fn,
                               reflog_expiry_should_prune_fn should_prune_fn,
                               reflog_expiry_cleanup_fn cleanup_fn,
                               void *policy_cb_data)
{
        struct files_ref_store *refs =
                files_downcast(ref_store, REF_STORE_WRITE, "reflog_expire");
        static struct lock_file reflog_lock;
        struct expire_reflog_cb cb;
        struct ref_lock *lock;
        struct strbuf log_file_sb = STRBUF_INIT;
        char *log_file;
        int status = 0;
        int type;
        struct strbuf err = STRBUF_INIT;
        struct object_id oid;

        memset(&cb, 0, sizeof(cb));
        cb.flags = flags;
        cb.policy_cb = policy_cb_data;
        cb.should_prune_fn = should_prune_fn;

        /*
         * The reflog file is locked by holding the lock on the
         * reference itself, plus we might need to update the
         * reference if --updateref was specified:
         */
        lock = lock_ref_sha1_basic(refs, refname, sha1,
                                   NULL, NULL, REF_NODEREF,
                                   &type, &err);
        if (!lock) {
                error("cannot lock ref '%s': %s", refname, err.buf);
                strbuf_release(&err);
                return -1;
        }
        if (!refs_reflog_exists(ref_store, refname)) {
                unlock_ref(lock);
                return 0;
        }

        files_reflog_path(refs, &log_file_sb, refname);
        log_file = strbuf_detach(&log_file_sb, NULL);
        if (!(flags & EXPIRE_REFLOGS_DRY_RUN)) {
                /*
                 * Even though holding $GIT_DIR/logs/$reflog.lock has
                 * no locking implications, we use the lock_file
                 * machinery here anyway because it does a lot of the
                 * work we need, including cleaning up if the program
                 * exits unexpectedly.
                 */
                if (hold_lock_file_for_update(&reflog_lock, log_file, 0) < 0) {
                        struct strbuf err = STRBUF_INIT;
                        unable_to_lock_message(log_file, errno, &err);
                        error("%s", err.buf);
                        strbuf_release(&err);
                        goto failure;
                }
                cb.newlog = fdopen_lock_file(&reflog_lock, "w");
                if (!cb.newlog) {
                        error("cannot fdopen %s (%s)",
                              get_lock_file_path(&reflog_lock), strerror(errno));
                        goto failure;
                }
        }

        hashcpy(oid.hash, sha1);

        (*prepare_fn)(refname, &oid, cb.policy_cb);
        refs_for_each_reflog_ent(ref_store, refname, expire_reflog_ent, &cb);
        (*cleanup_fn)(cb.policy_cb);

        if (!(flags & EXPIRE_REFLOGS_DRY_RUN)) {
                /*
                 * It doesn't make sense to adjust a reference pointed
                 * to by a symbolic ref based on expiring entries in
                 * the symbolic reference's reflog. Nor can we update
                 * a reference if there are no remaining reflog
                 * entries.
                 */
                int update = (flags & EXPIRE_REFLOGS_UPDATE_REF) &&
                        !(type & REF_ISSYMREF) &&
                        !is_null_oid(&cb.last_kept_oid);

                if (close_lock_file(&reflog_lock)) {
                        status |= error("couldn't write %s: %s", log_file,
                                        strerror(errno));
                } else if (update &&
                           (write_in_full(get_lock_file_fd(lock->lk),
                                oid_to_hex(&cb.last_kept_oid), GIT_SHA1_HEXSZ) != GIT_SHA1_HEXSZ ||
                            write_str_in_full(get_lock_file_fd(lock->lk), "\n") != 1 ||
                            close_ref(lock) < 0)) {
                        status |= error("couldn't write %s",
                                        get_lock_file_path(lock->lk));
                        rollback_lock_file(&reflog_lock);
                } else if (commit_lock_file(&reflog_lock)) {
                        status |= error("unable to write reflog '%s' (%s)",
                                        log_file, strerror(errno));
                } else if (update && commit_ref(lock)) {
                        status |= error("couldn't set %s", lock->ref_name);
                }
        }
        free(log_file);
        unlock_ref(lock);
        return status;

 failure:
        rollback_lock_file(&reflog_lock);
        free(log_file);
        unlock_ref(lock);
        return -1;
}

static int files_init_db(struct ref_store *ref_store, struct strbuf *err)
{
        struct files_ref_store *refs =
                files_downcast(ref_store, REF_STORE_WRITE, "init_db");
        struct strbuf sb = STRBUF_INIT;

        /*
         * Create .git/refs/{heads,tags}
         */
        files_ref_path(refs, &sb, "refs/heads");
        safe_create_dir(sb.buf, 1);

        strbuf_reset(&sb);
        files_ref_path(refs, &sb, "refs/tags");
        safe_create_dir(sb.buf, 1);

        strbuf_release(&sb);
        return 0;
}

struct ref_storage_be refs_be_files = {
        NULL,
        "files",
        files_ref_store_create,
        files_init_db,
        files_transaction_prepare,
        files_transaction_finish,
        files_transaction_abort,
        files_initial_transaction_commit,

        files_pack_refs,
        files_peel_ref,
        files_create_symref,
        files_delete_refs,
        files_rename_ref,

        files_ref_iterator_begin,
        files_read_raw_ref,

        files_reflog_iterator_begin,
        files_for_each_reflog_ent,
        files_for_each_reflog_ent_reverse,
        files_reflog_exists,
        files_create_reflog,
        files_delete_reflog,
        files_reflog_expire
};