refs: introduce an iterator interface

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

#include "../cache.h"
#include "../refs.h"
#include "refs-internal.h"
#include "../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;
};

struct ref_entry;

/*
 * Information used (along with the information in ref_entry) to
 * describe a single cached reference.  This data structure only
 * occurs embedded in a union in struct ref_entry, and only when
 * (ref_entry->flag & REF_DIR) is zero.
 */
struct ref_value {
        /*
         * The name of the object to which this reference resolves
         * (which may be a tag object).  If REF_ISBROKEN, this is
         * null.  If REF_ISSYMREF, then this is the name of the object
         * referred to by the last reference in the symlink chain.
         */
        struct object_id oid;

        /*
         * If REF_KNOWS_PEELED, then this field holds the peeled value
         * of this reference, or null if the reference is known not to
         * be peelable.  See the documentation for peel_ref() for an
         * exact definition of "peelable".
         */
        struct object_id peeled;
};

struct ref_cache;

/*
 * Information used (along with the information in ref_entry) to
 * describe a level in the hierarchy of references.  This data
 * structure only occurs embedded in a union in struct ref_entry, and
 * only when (ref_entry.flag & REF_DIR) is set.  In that case,
 * (ref_entry.flag & REF_INCOMPLETE) determines whether the references
 * in the directory have already been read:
 *
 *     (ref_entry.flag & REF_INCOMPLETE) unset -- a directory of loose
 *         or packed references, already read.
 *
 *     (ref_entry.flag & REF_INCOMPLETE) set -- a directory of loose
 *         references that hasn't been read yet (nor has any of its
 *         subdirectories).
 *
 * Entries within a directory are stored within a growable array of
 * pointers to ref_entries (entries, nr, alloc).  Entries 0 <= i <
 * sorted are sorted by their component name in strcmp() order and the
 * remaining entries are unsorted.
 *
 * Loose references are read lazily, one directory at a time.  When a
 * directory of loose references is read, then all of the references
 * in that directory are stored, and REF_INCOMPLETE stubs are created
 * for any subdirectories, but the subdirectories themselves are not
 * read.  The reading is triggered by get_ref_dir().
 */
struct ref_dir {
        int nr, alloc;

        /*
         * Entries with index 0 <= i < sorted are sorted by name.  New
         * entries are appended to the list unsorted, and are sorted
         * only when required; thus we avoid the need to sort the list
         * after the addition of every reference.
         */
        int sorted;

        /* A pointer to the ref_cache that contains this ref_dir. */
        struct ref_cache *ref_cache;

        struct ref_entry **entries;
};

/*
 * Bit values for ref_entry::flag.  REF_ISSYMREF=0x01,
 * REF_ISPACKED=0x02, REF_ISBROKEN=0x04 and REF_BAD_NAME=0x08 are
 * public values; see refs.h.
 */

/*
 * The field ref_entry->u.value.peeled of this value entry contains
 * the correct peeled value for the reference, which might be
 * null_sha1 if the reference is not a tag or if it is broken.
 */
#define REF_KNOWS_PEELED 0x10

/* ref_entry represents a directory of references */
#define REF_DIR 0x20

/*
 * Entry has not yet been read from disk (used only for REF_DIR
 * entries representing loose references)
 */
#define REF_INCOMPLETE 0x40

/*
 * A ref_entry represents either a reference or a "subdirectory" of
 * references.
 *
 * Each directory in the reference namespace is represented by a
 * ref_entry with (flags & REF_DIR) set and containing a subdir member
 * that holds the entries in that directory that have been read so
 * far.  If (flags & REF_INCOMPLETE) is set, then the directory and
 * its subdirectories haven't been read yet.  REF_INCOMPLETE is only
 * used for loose reference directories.
 *
 * References are represented by a ref_entry with (flags & REF_DIR)
 * unset and a value member that describes the reference's value.  The
 * flag member is at the ref_entry level, but it is also needed to
 * interpret the contents of the value field (in other words, a
 * ref_value object is not very much use without the enclosing
 * ref_entry).
 *
 * Reference names cannot end with slash and directories' names are
 * always stored with a trailing slash (except for the top-level
 * directory, which is always denoted by "").  This has two nice
 * consequences: (1) when the entries in each subdir are sorted
 * lexicographically by name (as they usually are), the references in
 * a whole tree can be generated in lexicographic order by traversing
 * the tree in left-to-right, depth-first order; (2) the names of
 * references and subdirectories cannot conflict, and therefore the
 * presence of an empty subdirectory does not block the creation of a
 * similarly-named reference.  (The fact that reference names with the
 * same leading components can conflict *with each other* is a
 * separate issue that is regulated by verify_refname_available().)
 *
 * Please note that the name field contains the fully-qualified
 * reference (or subdirectory) name.  Space could be saved by only
 * storing the relative names.  But that would require the full names
 * to be generated on the fly when iterating in do_for_each_ref(), and
 * would break callback functions, who have always been able to assume
 * that the name strings that they are passed will not be freed during
 * the iteration.
 */
struct ref_entry {
        unsigned char flag; /* ISSYMREF? ISPACKED? */
        union {
                struct ref_value value; /* if not (flags&REF_DIR) */
                struct ref_dir subdir; /* if (flags&REF_DIR) */
        } u;
        /*
         * The full name of the reference (e.g., "refs/heads/master")
         * or the full name of the directory with a trailing slash
         * (e.g., "refs/heads/"):
         */
        char name[FLEX_ARRAY];
};

static void read_loose_refs(const char *dirname, struct ref_dir *dir);
static int search_ref_dir(struct ref_dir *dir, const char *refname, size_t len);
static struct ref_entry *create_dir_entry(struct ref_cache *ref_cache,
                                          const char *dirname, size_t len,
                                          int incomplete);
static void add_entry_to_dir(struct ref_dir *dir, struct ref_entry *entry);

static struct ref_dir *get_ref_dir(struct ref_entry *entry)
{
        struct ref_dir *dir;
        assert(entry->flag & REF_DIR);
        dir = &entry->u.subdir;
        if (entry->flag & REF_INCOMPLETE) {
                read_loose_refs(entry->name, dir);

                /*
                 * Manually add refs/bisect, which, being
                 * per-worktree, might not appear in the directory
                 * listing for refs/ in the main repo.
                 */
                if (!strcmp(entry->name, "refs/")) {
                        int pos = search_ref_dir(dir, "refs/bisect/", 12);
                        if (pos < 0) {
                                struct ref_entry *child_entry;
                                child_entry = create_dir_entry(dir->ref_cache,
                                                               "refs/bisect/",
                                                               12, 1);
                                add_entry_to_dir(dir, child_entry);
                                read_loose_refs("refs/bisect",
                                                &child_entry->u.subdir);
                        }
                }
                entry->flag &= ~REF_INCOMPLETE;
        }
        return dir;
}

static struct ref_entry *create_ref_entry(const char *refname,
                                          const unsigned char *sha1, int flag,
                                          int check_name)
{
        struct ref_entry *ref;

        if (check_name &&
            check_refname_format(refname, REFNAME_ALLOW_ONELEVEL))
                die("Reference has invalid format: '%s'", refname);
        FLEX_ALLOC_STR(ref, name, refname);
        hashcpy(ref->u.value.oid.hash, sha1);
        oidclr(&ref->u.value.peeled);
        ref->flag = flag;
        return ref;
}

static void clear_ref_dir(struct ref_dir *dir);

static void free_ref_entry(struct ref_entry *entry)
{
        if (entry->flag & REF_DIR) {
                /*
                 * Do not use get_ref_dir() here, as that might
                 * trigger the reading of loose refs.
                 */
                clear_ref_dir(&entry->u.subdir);
        }
        free(entry);
}

/*
 * Add a ref_entry to the end of dir (unsorted).  Entry is always
 * stored directly in dir; no recursion into subdirectories is
 * done.
 */
static void add_entry_to_dir(struct ref_dir *dir, struct ref_entry *entry)
{
        ALLOC_GROW(dir->entries, dir->nr + 1, dir->alloc);
        dir->entries[dir->nr++] = entry;
        /* optimize for the case that entries are added in order */
        if (dir->nr == 1 ||
            (dir->nr == dir->sorted + 1 &&
             strcmp(dir->entries[dir->nr - 2]->name,
                    dir->entries[dir->nr - 1]->name) < 0))
                dir->sorted = dir->nr;
}

/*
 * Clear and free all entries in dir, recursively.
 */
static void clear_ref_dir(struct ref_dir *dir)
{
        int i;
        for (i = 0; i < dir->nr; i++)
                free_ref_entry(dir->entries[i]);
        free(dir->entries);
        dir->sorted = dir->nr = dir->alloc = 0;
        dir->entries = NULL;
}

/*
 * Create a struct ref_entry object for the specified dirname.
 * dirname is the name of the directory with a trailing slash (e.g.,
 * "refs/heads/") or "" for the top-level directory.
 */
static struct ref_entry *create_dir_entry(struct ref_cache *ref_cache,
                                          const char *dirname, size_t len,
                                          int incomplete)
{
        struct ref_entry *direntry;
        FLEX_ALLOC_MEM(direntry, name, dirname, len);
        direntry->u.subdir.ref_cache = ref_cache;
        direntry->flag = REF_DIR | (incomplete ? REF_INCOMPLETE : 0);
        return direntry;
}

static int ref_entry_cmp(const void *a, const void *b)
{
        struct ref_entry *one = *(struct ref_entry **)a;
        struct ref_entry *two = *(struct ref_entry **)b;
        return strcmp(one->name, two->name);
}

static void sort_ref_dir(struct ref_dir *dir);

struct string_slice {
        size_t len;
        const char *str;
};

static int ref_entry_cmp_sslice(const void *key_, const void *ent_)
{
        const struct string_slice *key = key_;
        const struct ref_entry *ent = *(const struct ref_entry * const *)ent_;
        int cmp = strncmp(key->str, ent->name, key->len);
        if (cmp)
                return cmp;
        return '\0' - (unsigned char)ent->name[key->len];
}

/*
 * Return the index of the entry with the given refname from the
 * ref_dir (non-recursively), sorting dir if necessary.  Return -1 if
 * no such entry is found.  dir must already be complete.
 */
static int search_ref_dir(struct ref_dir *dir, const char *refname, size_t len)
{
        struct ref_entry **r;
        struct string_slice key;

        if (refname == NULL || !dir->nr)
                return -1;

        sort_ref_dir(dir);
        key.len = len;
        key.str = refname;
        r = bsearch(&key, dir->entries, dir->nr, sizeof(*dir->entries),
                    ref_entry_cmp_sslice);

        if (r == NULL)
                return -1;

        return r - dir->entries;
}

/*
 * Search for a directory entry directly within dir (without
 * recursing).  Sort dir if necessary.  subdirname must be a directory
 * name (i.e., end in '/').  If mkdir is set, then create the
 * directory if it is missing; otherwise, return NULL if the desired
 * directory cannot be found.  dir must already be complete.
 */
static struct ref_dir *search_for_subdir(struct ref_dir *dir,
                                         const char *subdirname, size_t len,
                                         int mkdir)
{
        int entry_index = search_ref_dir(dir, subdirname, len);
        struct ref_entry *entry;
        if (entry_index == -1) {
                if (!mkdir)
                        return NULL;
                /*
                 * Since dir is complete, the absence of a subdir
                 * means that the subdir really doesn't exist;
                 * therefore, create an empty record for it but mark
                 * the record complete.
                 */
                entry = create_dir_entry(dir->ref_cache, subdirname, len, 0);
                add_entry_to_dir(dir, entry);
        } else {
                entry = dir->entries[entry_index];
        }
        return get_ref_dir(entry);
}

/*
 * If refname is a reference name, find the ref_dir within the dir
 * tree that should hold refname.  If refname is a directory name
 * (i.e., ends in '/'), then return that ref_dir itself.  dir must
 * represent the top-level directory and must already be complete.
 * Sort ref_dirs and recurse into subdirectories as necessary.  If
 * mkdir is set, then create any missing directories; otherwise,
 * return NULL if the desired directory cannot be found.
 */
static struct ref_dir *find_containing_dir(struct ref_dir *dir,
                                           const char *refname, int mkdir)
{
        const char *slash;
        for (slash = strchr(refname, '/'); slash; slash = strchr(slash + 1, '/')) {
                size_t dirnamelen = slash - refname + 1;
                struct ref_dir *subdir;
                subdir = search_for_subdir(dir, refname, dirnamelen, mkdir);
                if (!subdir) {
                        dir = NULL;
                        break;
                }
                dir = subdir;
        }

        return dir;
}

/*
 * Find the value entry with the given name in dir, sorting ref_dirs
 * and recursing into subdirectories as necessary.  If the name is not
 * found or it corresponds to a directory entry, return NULL.
 */
static struct ref_entry *find_ref(struct ref_dir *dir, const char *refname)
{
        int entry_index;
        struct ref_entry *entry;
        dir = find_containing_dir(dir, refname, 0);
        if (!dir)
                return NULL;
        entry_index = search_ref_dir(dir, refname, strlen(refname));
        if (entry_index == -1)
                return NULL;
        entry = dir->entries[entry_index];
        return (entry->flag & REF_DIR) ? NULL : entry;
}

/*
 * Remove the entry with the given name from dir, recursing into
 * subdirectories as necessary.  If refname is the name of a directory
 * (i.e., ends with '/'), then remove the directory and its contents.
 * If the removal was successful, return the number of entries
 * remaining in the directory entry that contained the deleted entry.
 * If the name was not found, return -1.  Please note that this
 * function only deletes the entry from the cache; it does not delete
 * it from the filesystem or ensure that other cache entries (which
 * might be symbolic references to the removed entry) are updated.
 * Nor does it remove any containing dir entries that might be made
 * empty by the removal.  dir must represent the top-level directory
 * and must already be complete.
 */
static int remove_entry(struct ref_dir *dir, const char *refname)
{
        int refname_len = strlen(refname);
        int entry_index;
        struct ref_entry *entry;
        int is_dir = refname[refname_len - 1] == '/';
        if (is_dir) {
                /*
                 * refname represents a reference directory.  Remove
                 * the trailing slash; otherwise we will get the
                 * directory *representing* refname rather than the
                 * one *containing* it.
                 */
                char *dirname = xmemdupz(refname, refname_len - 1);
                dir = find_containing_dir(dir, dirname, 0);
                free(dirname);
        } else {
                dir = find_containing_dir(dir, refname, 0);
        }
        if (!dir)
                return -1;
        entry_index = search_ref_dir(dir, refname, refname_len);
        if (entry_index == -1)
                return -1;
        entry = dir->entries[entry_index];

        memmove(&dir->entries[entry_index],
                &dir->entries[entry_index + 1],
                (dir->nr - entry_index - 1) * sizeof(*dir->entries)
                );
        dir->nr--;
        if (dir->sorted > entry_index)
                dir->sorted--;
        free_ref_entry(entry);
        return dir->nr;
}

/*
 * Add a ref_entry to the ref_dir (unsorted), recursing into
 * subdirectories as necessary.  dir must represent the top-level
 * directory.  Return 0 on success.
 */
static int add_ref(struct ref_dir *dir, struct ref_entry *ref)
{
        dir = find_containing_dir(dir, ref->name, 1);
        if (!dir)
                return -1;
        add_entry_to_dir(dir, ref);
        return 0;
}

/*
 * Emit a warning and return true iff ref1 and ref2 have the same name
 * and the same sha1.  Die if they have the same name but different
 * sha1s.
 */
static int is_dup_ref(const struct ref_entry *ref1, const struct ref_entry *ref2)
{
        if (strcmp(ref1->name, ref2->name))
                return 0;

        /* Duplicate name; make sure that they don't conflict: */

        if ((ref1->flag & REF_DIR) || (ref2->flag & REF_DIR))
                /* This is impossible by construction */
                die("Reference directory conflict: %s", ref1->name);

        if (oidcmp(&ref1->u.value.oid, &ref2->u.value.oid))
                die("Duplicated ref, and SHA1s don't match: %s", ref1->name);

        warning("Duplicated ref: %s", ref1->name);
        return 1;
}

/*
 * Sort the entries in dir non-recursively (if they are not already
 * sorted) and remove any duplicate entries.
 */
static void sort_ref_dir(struct ref_dir *dir)
{
        int i, j;
        struct ref_entry *last = NULL;

        /*
         * This check also prevents passing a zero-length array to qsort(),
         * which is a problem on some platforms.
         */
        if (dir->sorted == dir->nr)
                return;

        qsort(dir->entries, dir->nr, sizeof(*dir->entries), ref_entry_cmp);

        /* Remove any duplicates: */
        for (i = 0, j = 0; j < dir->nr; j++) {
                struct ref_entry *entry = dir->entries[j];
                if (last && is_dup_ref(last, entry))
                        free_ref_entry(entry);
                else
                        last = dir->entries[i++] = entry;
        }
        dir->sorted = dir->nr = i;
}

/*
 * Return true if refname, which has the specified oid and flags, can
 * be resolved to an object in the database. If the referred-to object
 * does not exist, emit a warning and return false.
 */
static int ref_resolves_to_object(const char *refname,
                                  const struct object_id *oid,
                                  unsigned int flags)
{
        if (flags & REF_ISBROKEN)
                return 0;
        if (!has_sha1_file(oid->hash)) {
                error("%s does not point to a valid object!", refname);
                return 0;
        }
        return 1;
}

/*
 * Return true if the reference described by entry can be resolved to
 * an object in the database; otherwise, emit a warning and return
 * false.
 */
static int entry_resolves_to_object(struct ref_entry *entry)
{
        return ref_resolves_to_object(entry->name,
                                      &entry->u.value.oid, entry->flag);
}

/*
 * current_ref is a performance hack: when iterating over references
 * using the for_each_ref*() functions, current_ref is set to the
 * current reference's entry before calling the callback function.  If
 * the callback function calls peel_ref(), then peel_ref() first
 * checks whether the reference to be peeled is the current reference
 * (it usually is) and if so, returns that reference's peeled version
 * if it is available.  This avoids a refname lookup in a common case.
 */
static struct ref_entry *current_ref;

typedef int each_ref_entry_fn(struct ref_entry *entry, void *cb_data);

struct ref_entry_cb {
        const char *prefix;
        int trim;
        int flags;
        each_ref_fn *fn;
        void *cb_data;
};

/*
 * Handle one reference in a do_for_each_ref*()-style iteration,
 * calling an each_ref_fn for each entry.
 */
static int do_one_ref(struct ref_entry *entry, void *cb_data)
{
        struct ref_entry_cb *data = cb_data;
        struct ref_entry *old_current_ref;
        int retval;

        if (!starts_with(entry->name, data->prefix))
                return 0;

        if (!(data->flags & DO_FOR_EACH_INCLUDE_BROKEN) &&
              !entry_resolves_to_object(entry))
                return 0;

        /* Store the old value, in case this is a recursive call: */
        old_current_ref = current_ref;
        current_ref = entry;
        retval = data->fn(entry->name + data->trim, &entry->u.value.oid,
                          entry->flag, data->cb_data);
        current_ref = old_current_ref;
        return retval;
}

/*
 * Call fn for each reference in dir that has index in the range
 * offset <= index < dir->nr.  Recurse into subdirectories that are in
 * that index range, sorting them before iterating.  This function
 * does not sort dir itself; it should be sorted beforehand.  fn is
 * called for all references, including broken ones.
 */
static int do_for_each_entry_in_dir(struct ref_dir *dir, int offset,
                                    each_ref_entry_fn fn, void *cb_data)
{
        int i;
        assert(dir->sorted == dir->nr);
        for (i = offset; i < dir->nr; i++) {
                struct ref_entry *entry = dir->entries[i];
                int retval;
                if (entry->flag & REF_DIR) {
                        struct ref_dir *subdir = get_ref_dir(entry);
                        sort_ref_dir(subdir);
                        retval = do_for_each_entry_in_dir(subdir, 0, fn, cb_data);
                } else {
                        retval = fn(entry, cb_data);
                }
                if (retval)
                        return retval;
        }
        return 0;
}

/*
 * Call fn for each reference in the union of dir1 and dir2, in order
 * by refname.  Recurse into subdirectories.  If a value entry appears
 * in both dir1 and dir2, then only process the version that is in
 * dir2.  The input dirs must already be sorted, but subdirs will be
 * sorted as needed.  fn is called for all references, including
 * broken ones.
 */
static int do_for_each_entry_in_dirs(struct ref_dir *dir1,
                                     struct ref_dir *dir2,
                                     each_ref_entry_fn fn, void *cb_data)
{
        int retval;
        int i1 = 0, i2 = 0;

        assert(dir1->sorted == dir1->nr);
        assert(dir2->sorted == dir2->nr);
        while (1) {
                struct ref_entry *e1, *e2;
                int cmp;
                if (i1 == dir1->nr) {
                        return do_for_each_entry_in_dir(dir2, i2, fn, cb_data);
                }
                if (i2 == dir2->nr) {
                        return do_for_each_entry_in_dir(dir1, i1, fn, cb_data);
                }
                e1 = dir1->entries[i1];
                e2 = dir2->entries[i2];
                cmp = strcmp(e1->name, e2->name);
                if (cmp == 0) {
                        if ((e1->flag & REF_DIR) && (e2->flag & REF_DIR)) {
                                /* Both are directories; descend them in parallel. */
                                struct ref_dir *subdir1 = get_ref_dir(e1);
                                struct ref_dir *subdir2 = get_ref_dir(e2);
                                sort_ref_dir(subdir1);
                                sort_ref_dir(subdir2);
                                retval = do_for_each_entry_in_dirs(
                                                subdir1, subdir2, fn, cb_data);
                                i1++;
                                i2++;
                        } else if (!(e1->flag & REF_DIR) && !(e2->flag & REF_DIR)) {
                                /* Both are references; ignore the one from dir1. */
                                retval = fn(e2, cb_data);
                                i1++;
                                i2++;
                        } else {
                                die("conflict between reference and directory: %s",
                                    e1->name);
                        }
                } else {
                        struct ref_entry *e;
                        if (cmp < 0) {
                                e = e1;
                                i1++;
                        } else {
                                e = e2;
                                i2++;
                        }
                        if (e->flag & REF_DIR) {
                                struct ref_dir *subdir = get_ref_dir(e);
                                sort_ref_dir(subdir);
                                retval = do_for_each_entry_in_dir(
                                                subdir, 0, fn, cb_data);
                        } else {
                                retval = fn(e, cb_data);
                        }
                }
                if (retval)
                        return retval;
        }
}

/*
 * Load all of the refs from the dir into our in-memory cache. The hard work
 * of loading loose refs is done by get_ref_dir(), so we just need to recurse
 * through all of the sub-directories. We do not even need to care about
 * sorting, as traversal order does not matter to us.
 */
static void prime_ref_dir(struct ref_dir *dir)
{
        int i;
        for (i = 0; i < dir->nr; i++) {
                struct ref_entry *entry = dir->entries[i];
                if (entry->flag & REF_DIR)
                        prime_ref_dir(get_ref_dir(entry));
        }
}

/*
 * A level in the reference hierarchy that is currently being iterated
 * through.
 */
struct cache_ref_iterator_level {
        /*
         * The ref_dir being iterated over at this level. The ref_dir
         * is sorted before being stored here.
         */
        struct ref_dir *dir;

        /*
         * The index of the current entry within dir (which might
         * itself be a directory). If index == -1, then the iteration
         * hasn't yet begun. If index == dir->nr, then the iteration
         * through this level is over.
         */
        int index;
};

/*
 * Represent an iteration through a ref_dir in the memory cache. The
 * iteration recurses through subdirectories.
 */
struct cache_ref_iterator {
        struct ref_iterator base;

        /*
         * The number of levels currently on the stack. This is always
         * at least 1, because when it becomes zero the iteration is
         * ended and this struct is freed.
         */
        size_t levels_nr;

        /* The number of levels that have been allocated on the stack */
        size_t levels_alloc;

        /*
         * A stack of levels. levels[0] is the uppermost level that is
         * being iterated over in this iteration. (This is not
         * necessary the top level in the references hierarchy. If we
         * are iterating through a subtree, then levels[0] will hold
         * the ref_dir for that subtree, and subsequent levels will go
         * on from there.)
         */
        struct cache_ref_iterator_level *levels;
};

static int cache_ref_iterator_advance(struct ref_iterator *ref_iterator)
{
        struct cache_ref_iterator *iter =
                (struct cache_ref_iterator *)ref_iterator;

        while (1) {
                struct cache_ref_iterator_level *level =
                        &iter->levels[iter->levels_nr - 1];
                struct ref_dir *dir = level->dir;
                struct ref_entry *entry;

                if (level->index == -1)
                        sort_ref_dir(dir);

                if (++level->index == level->dir->nr) {
                        /* This level is exhausted; pop up a level */
                        if (--iter->levels_nr == 0)
                                return ref_iterator_abort(ref_iterator);

                        continue;
                }

                entry = dir->entries[level->index];

                if (entry->flag & REF_DIR) {
                        /* push down a level */
                        ALLOC_GROW(iter->levels, iter->levels_nr + 1,
                                   iter->levels_alloc);

                        level = &iter->levels[iter->levels_nr++];
                        level->dir = get_ref_dir(entry);
                        level->index = -1;
                } else {
                        iter->base.refname = entry->name;
                        iter->base.oid = &entry->u.value.oid;
                        iter->base.flags = entry->flag;
                        return ITER_OK;
                }
        }
}

static enum peel_status peel_entry(struct ref_entry *entry, int repeel);

static int cache_ref_iterator_peel(struct ref_iterator *ref_iterator,
                                   struct object_id *peeled)
{
        struct cache_ref_iterator *iter =
                (struct cache_ref_iterator *)ref_iterator;
        struct cache_ref_iterator_level *level;
        struct ref_entry *entry;

        level = &iter->levels[iter->levels_nr - 1];

        if (level->index == -1)
                die("BUG: peel called before advance for cache iterator");

        entry = level->dir->entries[level->index];

        if (peel_entry(entry, 0))
                return -1;
        hashcpy(peeled->hash, entry->u.value.peeled.hash);
        return 0;
}

static int cache_ref_iterator_abort(struct ref_iterator *ref_iterator)
{
        struct cache_ref_iterator *iter =
                (struct cache_ref_iterator *)ref_iterator;

        free(iter->levels);
        base_ref_iterator_free(ref_iterator);
        return ITER_DONE;
}

static struct ref_iterator_vtable cache_ref_iterator_vtable = {
        cache_ref_iterator_advance,
        cache_ref_iterator_peel,
        cache_ref_iterator_abort
};

static struct ref_iterator *cache_ref_iterator_begin(struct ref_dir *dir)
{
        struct cache_ref_iterator *iter;
        struct ref_iterator *ref_iterator;
        struct cache_ref_iterator_level *level;

        iter = xcalloc(1, sizeof(*iter));
        ref_iterator = &iter->base;
        base_ref_iterator_init(ref_iterator, &cache_ref_iterator_vtable);
        ALLOC_GROW(iter->levels, 10, iter->levels_alloc);

        iter->levels_nr = 1;
        level = &iter->levels[0];
        level->index = -1;
        level->dir = dir;

        return ref_iterator;
}

struct nonmatching_ref_data {
        const struct string_list *skip;
        const char *conflicting_refname;
};

static int nonmatching_ref_fn(struct ref_entry *entry, void *vdata)
{
        struct nonmatching_ref_data *data = vdata;

        if (data->skip && string_list_has_string(data->skip, entry->name))
                return 0;

        data->conflicting_refname = entry->name;
        return 1;
}

/*
 * Return 0 if a reference named refname could be created without
 * conflicting with the name of an existing reference in dir.
 * See verify_refname_available for more information.
 */
static int verify_refname_available_dir(const char *refname,
                                        const struct string_list *extras,
                                        const struct string_list *skip,
                                        struct ref_dir *dir,
                                        struct strbuf *err)
{
        const char *slash;
        const char *extra_refname;
        int pos;
        struct strbuf dirname = STRBUF_INIT;
        int ret = -1;

        /*
         * For the sake of comments in this function, suppose that
         * refname is "refs/foo/bar".
         */

        assert(err);

        strbuf_grow(&dirname, strlen(refname) + 1);
        for (slash = strchr(refname, '/'); slash; slash = strchr(slash + 1, '/')) {
                /* Expand dirname to the new prefix, not including the trailing slash: */
                strbuf_add(&dirname, refname + dirname.len, slash - refname - dirname.len);

                /*
                 * We are still at a leading dir of the refname (e.g.,
                 * "refs/foo"; if there is a reference with that name,
                 * it is a conflict, *unless* it is in skip.
                 */
                if (dir) {
                        pos = search_ref_dir(dir, dirname.buf, dirname.len);
                        if (pos >= 0 &&
                            (!skip || !string_list_has_string(skip, dirname.buf))) {
                                /*
                                 * We found a reference whose name is
                                 * a proper prefix of refname; e.g.,
                                 * "refs/foo", and is not in skip.
                                 */
                                strbuf_addf(err, "'%s' exists; cannot create '%s'",
                                            dirname.buf, refname);
                                goto cleanup;
                        }
                }

                if (extras && string_list_has_string(extras, dirname.buf) &&
                    (!skip || !string_list_has_string(skip, dirname.buf))) {
                        strbuf_addf(err, "cannot process '%s' and '%s' at the same time",
                                    refname, dirname.buf);
                        goto cleanup;
                }

                /*
                 * Otherwise, we can try to continue our search with
                 * the next component. So try to look up the
                 * directory, e.g., "refs/foo/". If we come up empty,
                 * we know there is nothing under this whole prefix,
                 * but even in that case we still have to continue the
                 * search for conflicts with extras.
                 */
                strbuf_addch(&dirname, '/');
                if (dir) {
                        pos = search_ref_dir(dir, dirname.buf, dirname.len);
                        if (pos < 0) {
                                /*
                                 * There was no directory "refs/foo/",
                                 * so there is nothing under this
                                 * whole prefix. So there is no need
                                 * to continue looking for conflicting
                                 * references. But we need to continue
                                 * looking for conflicting extras.
                                 */
                                dir = NULL;
                        } else {
                                dir = get_ref_dir(dir->entries[pos]);
                        }
                }
        }

        /*
         * We are at the leaf of our refname (e.g., "refs/foo/bar").
         * There is no point in searching for a reference with that
         * name, because a refname isn't considered to conflict with
         * itself. But we still need to check for references whose
         * names are in the "refs/foo/bar/" namespace, because they
         * *do* conflict.
         */
        strbuf_addstr(&dirname, refname + dirname.len);
        strbuf_addch(&dirname, '/');

        if (dir) {
                pos = search_ref_dir(dir, dirname.buf, dirname.len);

                if (pos >= 0) {
                        /*
                         * We found a directory named "$refname/"
                         * (e.g., "refs/foo/bar/"). It is a problem
                         * iff it contains any ref that is not in
                         * "skip".
                         */
                        struct nonmatching_ref_data data;

                        data.skip = skip;
                        data.conflicting_refname = NULL;
                        dir = get_ref_dir(dir->entries[pos]);
                        sort_ref_dir(dir);
                        if (do_for_each_entry_in_dir(dir, 0, nonmatching_ref_fn, &data)) {
                                strbuf_addf(err, "'%s' exists; cannot create '%s'",
                                            data.conflicting_refname, refname);
                                goto cleanup;
                        }
                }
        }

        extra_refname = find_descendant_ref(dirname.buf, extras, skip);
        if (extra_refname)
                strbuf_addf(err, "cannot process '%s' and '%s' at the same time",
                            refname, extra_refname);
        else
                ret = 0;

cleanup:
        strbuf_release(&dirname);
        return ret;
}

struct packed_ref_cache {
        struct ref_entry *root;

        /*
         * Count of references to the data structure in this instance,
         * including the pointer from ref_cache::packed if any.  The
         * data will not be freed as long as the reference count is
         * nonzero.
         */
        unsigned int referrers;

        /*
         * Iff the packed-refs file associated with this instance is
         * currently locked for writing, this points at the associated
         * lock (which is owned by somebody else).  The referrer count
         * is also incremented when the file is locked and decremented
         * when it is unlocked.
         */
        struct lock_file *lock;

        /* The metadata from when this packed-refs cache was read */
        struct stat_validity validity;
};

/*
 * Future: need to be in "struct repository"
 * when doing a full libification.
 */
static struct ref_cache {
        struct ref_cache *next;
        struct ref_entry *loose;
        struct packed_ref_cache *packed;
        /*
         * The submodule name, or "" for the main repo.  We allocate
         * length 1 rather than FLEX_ARRAY so that the main ref_cache
         * is initialized correctly.
         */
        char name[1];
} ref_cache, *submodule_ref_caches;

/* Lock used for the main packed-refs file: */
static struct lock_file packlock;

/*
 * Increment the reference count of *packed_refs.
 */
static void acquire_packed_ref_cache(struct packed_ref_cache *packed_refs)
{
        packed_refs->referrers++;
}

/*
 * Decrease the reference count of *packed_refs.  If it goes to zero,
 * free *packed_refs and return true; otherwise return false.
 */
static int release_packed_ref_cache(struct packed_ref_cache *packed_refs)
{
        if (!--packed_refs->referrers) {
                free_ref_entry(packed_refs->root);
                stat_validity_clear(&packed_refs->validity);
                free(packed_refs);
                return 1;
        } else {
                return 0;
        }
}

static void clear_packed_ref_cache(struct ref_cache *refs)
{
        if (refs->packed) {
                struct packed_ref_cache *packed_refs = refs->packed;

                if (packed_refs->lock)
                        die("internal error: packed-ref cache cleared while locked");
                refs->packed = NULL;
                release_packed_ref_cache(packed_refs);
        }
}

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

/*
 * Create a new submodule ref cache and add it to the internal
 * set of caches.
 */
static struct ref_cache *create_ref_cache(const char *submodule)
{
        struct ref_cache *refs;
        if (!submodule)
                submodule = "";
        FLEX_ALLOC_STR(refs, name, submodule);
        refs->next = submodule_ref_caches;
        submodule_ref_caches = refs;
        return refs;
}

static struct ref_cache *lookup_ref_cache(const char *submodule)
{
        struct ref_cache *refs;

        if (!submodule || !*submodule)
                return &ref_cache;

        for (refs = submodule_ref_caches; refs; refs = refs->next)
                if (!strcmp(submodule, refs->name))
                        return refs;
        return NULL;
}

/*
 * Return a pointer to a ref_cache for the specified submodule. For
 * the main repository, use submodule==NULL; such a call cannot fail.
 * For a submodule, the submodule must exist and be a nonbare
 * repository, otherwise return NULL.
 *
 * The returned structure will be allocated and initialized but not
 * necessarily populated; it should not be freed.
 */
static struct ref_cache *get_ref_cache(const char *submodule)
{
        struct ref_cache *refs = lookup_ref_cache(submodule);

        if (!refs) {
                struct strbuf submodule_sb = STRBUF_INIT;

                strbuf_addstr(&submodule_sb, submodule);
                if (is_nonbare_repository_dir(&submodule_sb))
                        refs = create_ref_cache(submodule);
                strbuf_release(&submodule_sb);
        }

        return refs;
}

/* The length of a peeled reference line in packed-refs, including EOL: */
#define PEELED_LINE_LENGTH 42

/*
 * The packed-refs header line that we write out.  Perhaps other
 * traits will be added later.  The trailing space is required.
 */
static const char PACKED_REFS_HEADER[] =
        "# pack-refs with: peeled fully-peeled \n";

/*
 * Parse one line from a packed-refs file.  Write the SHA1 to sha1.
 * Return a pointer to the refname within the line (null-terminated),
 * or NULL if there was a problem.
 */
static const char *parse_ref_line(struct strbuf *line, unsigned char *sha1)
{
        const char *ref;

        /*
         * 42: the answer to everything.
         *
         * In this case, it happens to be the answer to
         *  40 (length of sha1 hex representation)
         *  +1 (space in between hex and name)
         *  +1 (newline at the end of the line)
         */
        if (line->len <= 42)
                return NULL;

        if (get_sha1_hex(line->buf, sha1) < 0)
                return NULL;
        if (!isspace(line->buf[40]))
                return NULL;

        ref = line->buf + 41;
        if (isspace(*ref))
                return NULL;

        if (line->buf[line->len - 1] != '\n')
                return NULL;
        line->buf[--line->len] = 0;

        return ref;
}

/*
 * Read f, which is a packed-refs file, into dir.
 *
 * A comment line of the form "# pack-refs with: " may contain zero or
 * more traits. We interpret the traits as follows:
 *
 *   No traits:
 *
 *      Probably no references are peeled. But if the file contains a
 *      peeled value for a reference, we will use it.
 *
 *   peeled:
 *
 *      References under "refs/tags/", if they *can* be peeled, *are*
 *      peeled in this file. References outside of "refs/tags/" are
 *      probably not peeled even if they could have been, but if we find
 *      a peeled value for such a reference we will use it.
 *
 *   fully-peeled:
 *
 *      All references in the file that can be peeled are peeled.
 *      Inversely (and this is more important), any references in the
 *      file for which no peeled value is recorded is not peelable. This
 *      trait should typically be written alongside "peeled" for
 *      compatibility with older clients, but we do not require it
 *      (i.e., "peeled" is a no-op if "fully-peeled" is set).
 */
static void read_packed_refs(FILE *f, struct ref_dir *dir)
{
        struct ref_entry *last = NULL;
        struct strbuf line = STRBUF_INIT;
        enum { PEELED_NONE, PEELED_TAGS, PEELED_FULLY } peeled = PEELED_NONE;

        while (strbuf_getwholeline(&line, f, '\n') != EOF) {
                unsigned char sha1[20];
                const char *refname;
                const char *traits;

                if (skip_prefix(line.buf, "# pack-refs with:", &traits)) {
                        if (strstr(traits, " fully-peeled "))
                                peeled = PEELED_FULLY;
                        else if (strstr(traits, " peeled "))
                                peeled = PEELED_TAGS;
                        /* perhaps other traits later as well */
                        continue;
                }

                refname = parse_ref_line(&line, sha1);
                if (refname) {
                        int flag = REF_ISPACKED;

                        if (check_refname_format(refname, REFNAME_ALLOW_ONELEVEL)) {
                                if (!refname_is_safe(refname))
                                        die("packed refname is dangerous: %s", refname);
                                hashclr(sha1);
                                flag |= REF_BAD_NAME | REF_ISBROKEN;
                        }
                        last = create_ref_entry(refname, sha1, flag, 0);
                        if (peeled == PEELED_FULLY ||
                            (peeled == PEELED_TAGS && starts_with(refname, "refs/tags/")))
                                last->flag |= REF_KNOWS_PEELED;
                        add_ref(dir, last);
                        continue;
                }
                if (last &&
                    line.buf[0] == '^' &&
                    line.len == PEELED_LINE_LENGTH &&
                    line.buf[PEELED_LINE_LENGTH - 1] == '\n' &&
                    !get_sha1_hex(line.buf + 1, sha1)) {
                        hashcpy(last->u.value.peeled.hash, sha1);
                        /*
                         * Regardless of what the file header said,
                         * we definitely know the value of *this*
                         * reference:
                         */
                        last->flag |= REF_KNOWS_PEELED;
                }
        }

        strbuf_release(&line);
}

/*
 * Get the packed_ref_cache for the specified ref_cache, creating it
 * if necessary.
 */
static struct packed_ref_cache *get_packed_ref_cache(struct ref_cache *refs)
{
        char *packed_refs_file;

        if (*refs->name)
                packed_refs_file = git_pathdup_submodule(refs->name, "packed-refs");
        else
                packed_refs_file = git_pathdup("packed-refs");

        if (refs->packed &&
            !stat_validity_check(&refs->packed->validity, packed_refs_file))
                clear_packed_ref_cache(refs);

        if (!refs->packed) {
                FILE *f;

                refs->packed = xcalloc(1, sizeof(*refs->packed));
                acquire_packed_ref_cache(refs->packed);
                refs->packed->root = create_dir_entry(refs, "", 0, 0);
                f = fopen(packed_refs_file, "r");
                if (f) {
                        stat_validity_update(&refs->packed->validity, fileno(f));
                        read_packed_refs(f, get_ref_dir(refs->packed->root));
                        fclose(f);
                }
        }
        free(packed_refs_file);
        return refs->packed;
}

static struct ref_dir *get_packed_ref_dir(struct packed_ref_cache *packed_ref_cache)
{
        return get_ref_dir(packed_ref_cache->root);
}

static struct ref_dir *get_packed_refs(struct ref_cache *refs)
{
        return get_packed_ref_dir(get_packed_ref_cache(refs));
}

/*
 * Add a reference to the in-memory packed reference cache.  This may
 * only be called while the packed-refs file is locked (see
 * lock_packed_refs()).  To actually write the packed-refs file, call
 * commit_packed_refs().
 */
static void add_packed_ref(const char *refname, const unsigned char *sha1)
{
        struct packed_ref_cache *packed_ref_cache =
                get_packed_ref_cache(&ref_cache);

        if (!packed_ref_cache->lock)
                die("internal error: packed refs not locked");
        add_ref(get_packed_ref_dir(packed_ref_cache),
                create_ref_entry(refname, sha1, REF_ISPACKED, 1));
}

/*
 * 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 read_loose_refs(const char *dirname, struct ref_dir *dir)
{
        struct ref_cache *refs = dir->ref_cache;
        DIR *d;
        struct dirent *de;
        int dirnamelen = strlen(dirname);
        struct strbuf refname;
        struct strbuf path = STRBUF_INIT;
        size_t path_baselen;

        if (*refs->name)
                strbuf_git_path_submodule(&path, refs->name, "%s", dirname);
        else
                strbuf_git_path(&path, "%s", 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) {
                unsigned char sha1[20];
                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(refs, refname.buf,
                                                          refname.len, 1));
                } else {
                        int read_ok;

                        if (*refs->name) {
                                hashclr(sha1);
                                flag = 0;
                                read_ok = !resolve_gitlink_ref(refs->name,
                                                               refname.buf, sha1);
                        } else {
                                read_ok = !read_ref_full(refname.buf,
                                                         RESOLVE_REF_READING,
                                                         sha1, &flag);
                        }

                        if (!read_ok) {
                                hashclr(sha1);
                                flag |= REF_ISBROKEN;
                        } else if (is_null_sha1(sha1)) {
                                /*
                                 * 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);
                                hashclr(sha1);
                                flag |= REF_BAD_NAME | REF_ISBROKEN;
                        }
                        add_entry_to_dir(dir,
                                         create_ref_entry(refname.buf, sha1, flag, 0));
                }
                strbuf_setlen(&refname, dirnamelen);
                strbuf_setlen(&path, path_baselen);
        }
        strbuf_release(&refname);
        strbuf_release(&path);
        closedir(d);
}

static struct ref_dir *get_loose_refs(struct ref_cache *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_dir_entry(refs, "", 0, 0);
                /*
                 * Create an incomplete entry for "refs/":
                 */
                add_entry_to_dir(get_ref_dir(refs->loose),
                                 create_dir_entry(refs, "refs/", 5, 1));
        }
        return get_ref_dir(refs->loose);
}

#define MAXREFLEN (1024)

/*
 * Called by resolve_gitlink_ref_recursive() after it failed to read
 * from the loose refs in ref_cache refs. Find <refname> in the
 * packed-refs file for the submodule.
 */
static int resolve_gitlink_packed_ref(struct ref_cache *refs,
                                      const char *refname, unsigned char *sha1)
{
        struct ref_entry *ref;
        struct ref_dir *dir = get_packed_refs(refs);

        ref = find_ref(dir, refname);
        if (ref == NULL)
                return -1;

        hashcpy(sha1, ref->u.value.oid.hash);
        return 0;
}

static int resolve_gitlink_ref_recursive(struct ref_cache *refs,
                                         const char *refname, unsigned char *sha1,
                                         int recursion)
{
        int fd, len;
        char buffer[128], *p;
        char *path;

        if (recursion > SYMREF_MAXDEPTH || strlen(refname) > MAXREFLEN)
                return -1;
        path = *refs->name
                ? git_pathdup_submodule(refs->name, "%s", refname)
                : git_pathdup("%s", refname);
        fd = open(path, O_RDONLY);
        free(path);
        if (fd < 0)
                return resolve_gitlink_packed_ref(refs, refname, sha1);

        len = read(fd, buffer, sizeof(buffer)-1);
        close(fd);
        if (len < 0)
                return -1;
        while (len && isspace(buffer[len-1]))
                len--;
        buffer[len] = 0;

        /* Was it a detached head or an old-fashioned symlink? */
        if (!get_sha1_hex(buffer, sha1))
                return 0;

        /* Symref? */
        if (strncmp(buffer, "ref:", 4))
                return -1;
        p = buffer + 4;
        while (isspace(*p))
                p++;

        return resolve_gitlink_ref_recursive(refs, p, sha1, recursion+1);
}

int resolve_gitlink_ref(const char *path, const char *refname, unsigned char *sha1)
{
        int len = strlen(path), retval;
        struct strbuf submodule = STRBUF_INIT;
        struct ref_cache *refs;

        while (len && path[len-1] == '/')
                len--;
        if (!len)
                return -1;

        strbuf_add(&submodule, path, len);
        refs = get_ref_cache(submodule.buf);
        if (!refs) {
                strbuf_release(&submodule);
                return -1;
        }
        strbuf_release(&submodule);

        retval = resolve_gitlink_ref_recursive(refs, refname, sha1, 0);
        return retval;
}

/*
 * Return the ref_entry for the given refname from the packed
 * references.  If it does not exist, return NULL.
 */
static struct ref_entry *get_packed_ref(const char *refname)
{
        return find_ref(get_packed_refs(&ref_cache), refname);
}

/*
 * A loose ref file doesn't exist; check for a packed ref.
 */
static int resolve_missing_loose_ref(const char *refname,
                                     unsigned char *sha1,
                                     unsigned int *flags)
{
        struct ref_entry *entry;

        /*
         * The loose reference file does not exist; check for a packed
         * reference.
         */
        entry = get_packed_ref(refname);
        if (entry) {
                hashcpy(sha1, entry->u.value.oid.hash);
                *flags |= REF_ISPACKED;
                return 0;
        }
        /* refname is not a packed reference. */
        return -1;
}

int read_raw_ref(const char *refname, unsigned char *sha1,
                 struct strbuf *referent, unsigned int *type)
{
        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;

        *type = 0;
        strbuf_reset(&sb_path);
        strbuf_git_path(&sb_path, "%s", 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().
         */

        if (lstat(path, &st) < 0) {
                if (errno != ENOENT)
                        goto out;
                if (resolve_missing_loose_ref(refname, sha1, 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;
                }
        }

        /* 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 (resolve_missing_loose_ref(refname, sha1, 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)
                        /* 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 verify_refname_available_dir() 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 verify_refname_available_dir() 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(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);
        *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);
        strbuf_git_path(&ref_file, "%s", 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 (verify_refname_available(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
                                 * verify_refname_available_dir() 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(lock->lk, ref_file.buf, LOCK_NO_DEREF) < 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 (read_raw_ref(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 (verify_refname_available_dir(
                                                    refname, extras, skip,
                                                    get_loose_refs(&ref_cache),
                                                    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 whose
                 * name begins with our refname, nor a packed ref
                 * whose name is a proper prefix of our refname.
                 */
                if (verify_refname_available_dir(
                                    refname, extras, skip,
                                    get_packed_refs(&ref_cache),
                                    err)) {
                        goto error_return;
                }
        }

        ret = 0;
        goto out;

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

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

/*
 * Peel the entry (if possible) and return its new peel_status.  If
 * repeel is true, re-peel the entry even if there is an old peeled
 * value that is already stored in it.
 *
 * It is OK to call this function with a packed reference entry that
 * might be stale and might even refer to an object that has since
 * been garbage-collected.  In such a case, if the entry has
 * REF_KNOWS_PEELED then leave the status unchanged and return
 * PEEL_PEELED or PEEL_NON_TAG; otherwise, return PEEL_INVALID.
 */
static enum peel_status peel_entry(struct ref_entry *entry, int repeel)
{
        enum peel_status status;

        if (entry->flag & REF_KNOWS_PEELED) {
                if (repeel) {
                        entry->flag &= ~REF_KNOWS_PEELED;
                        oidclr(&entry->u.value.peeled);
                } else {
                        return is_null_oid(&entry->u.value.peeled) ?
                                PEEL_NON_TAG : PEEL_PEELED;
                }
        }
        if (entry->flag & REF_ISBROKEN)
                return PEEL_BROKEN;
        if (entry->flag & REF_ISSYMREF)
                return PEEL_IS_SYMREF;

        status = peel_object(entry->u.value.oid.hash, entry->u.value.peeled.hash);
        if (status == PEEL_PEELED || status == PEEL_NON_TAG)
                entry->flag |= REF_KNOWS_PEELED;
        return status;
}

int peel_ref(const char *refname, unsigned char *sha1)
{
        int flag;
        unsigned char base[20];

        if (current_ref && (current_ref->name == refname
                            || !strcmp(current_ref->name, refname))) {
                if (peel_entry(current_ref, 0))
                        return -1;
                hashcpy(sha1, current_ref->u.value.peeled.hash);
                return 0;
        }

        if (read_ref_full(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) {
                struct ref_entry *r = get_packed_ref(refname);
                if (r) {
                        if (peel_entry(r, 0))
                                return -1;
                        hashcpy(sha1, r->u.value.peeled.hash);
                        return 0;
                }
        }

        return peel_object(base, sha1);
}

struct files_ref_iterator {
        struct ref_iterator base;

        struct packed_ref_cache *packed_ref_cache;
        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_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);

        release_packed_ref_cache(iter->packed_ref_cache);
        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
};

struct ref_iterator *files_ref_iterator_begin(
                const char *submodule,
                const char *prefix, unsigned int flags)
{
        struct ref_cache *refs = get_ref_cache(submodule);
        struct ref_dir *loose_dir, *packed_dir;
        struct ref_iterator *loose_iter, *packed_iter;
        struct files_ref_iterator *iter;
        struct ref_iterator *ref_iterator;

        if (!refs)
                return empty_ref_iterator_begin();

        if (ref_paranoia < 0)
                ref_paranoia = git_env_bool("GIT_REF_PARANOIA", 0);
        if (ref_paranoia)
                flags |= DO_FOR_EACH_INCLUDE_BROKEN;

        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 prime_ref_dir(), which pre-reads the loose
         * references for the subtree 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 get_packed_ref_cache(), which
         * internally checks whether the packed-ref cache is up to
         * date with what is on disk, and re-reads it if not.
         */

        loose_dir = get_loose_refs(refs);

        if (prefix && *prefix)
                loose_dir = find_containing_dir(loose_dir, prefix, 0);

        if (loose_dir) {
                prime_ref_dir(loose_dir);
                loose_iter = cache_ref_iterator_begin(loose_dir);
        } else {
                /* There's nothing to iterate over. */
                loose_iter = empty_ref_iterator_begin();
        }

        iter->packed_ref_cache = get_packed_ref_cache(refs);
        acquire_packed_ref_cache(iter->packed_ref_cache);
        packed_dir = get_packed_ref_dir(iter->packed_ref_cache);

        if (prefix && *prefix)
                packed_dir = find_containing_dir(packed_dir, prefix, 0);

        if (packed_dir) {
                packed_iter = cache_ref_iterator_begin(packed_dir);
        } else {
                /* There's nothing to iterate over. */
                packed_iter = empty_ref_iterator_begin();
        }

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

        return ref_iterator;
}

/*
 * Call fn for each reference in the specified ref_cache, omitting
 * references not in the containing_dir of prefix. Call fn for all
 * references, including broken ones. If fn ever returns a non-zero
 * value, stop the iteration and return that value; otherwise, return
 * 0.
 */
static int do_for_each_entry(struct ref_cache *refs, const char *prefix,
                             each_ref_entry_fn fn, void *cb_data)
{
        struct packed_ref_cache *packed_ref_cache;
        struct ref_dir *loose_dir;
        struct ref_dir *packed_dir;
        int retval = 0;

        /*
         * We must make sure that all loose refs are read before accessing the
         * packed-refs file; this avoids a race condition in which loose refs
         * are migrated to the packed-refs file by a simultaneous process, but
         * our in-memory view is from before the migration. get_packed_ref_cache()
         * takes care of making sure our view is up to date with what is on
         * disk.
         */
        loose_dir = get_loose_refs(refs);
        if (prefix && *prefix) {
                loose_dir = find_containing_dir(loose_dir, prefix, 0);
        }
        if (loose_dir)
                prime_ref_dir(loose_dir);

        packed_ref_cache = get_packed_ref_cache(refs);
        acquire_packed_ref_cache(packed_ref_cache);
        packed_dir = get_packed_ref_dir(packed_ref_cache);
        if (prefix && *prefix) {
                packed_dir = find_containing_dir(packed_dir, prefix, 0);
        }

        if (packed_dir && loose_dir) {
                sort_ref_dir(packed_dir);
                sort_ref_dir(loose_dir);
                retval = do_for_each_entry_in_dirs(
                                packed_dir, loose_dir, fn, cb_data);
        } else if (packed_dir) {
                sort_ref_dir(packed_dir);
                retval = do_for_each_entry_in_dir(
                                packed_dir, 0, fn, cb_data);
        } else if (loose_dir) {
                sort_ref_dir(loose_dir);
                retval = do_for_each_entry_in_dir(
                                loose_dir, 0, fn, cb_data);
        }

        release_packed_ref_cache(packed_ref_cache);
        return retval;
}

int do_for_each_ref(const char *submodule, const char *prefix,
                    each_ref_fn fn, int trim, int flags, void *cb_data)
{
        struct ref_entry_cb data;
        struct ref_cache *refs;

        refs = get_ref_cache(submodule);
        if (!refs)
                return 0;

        data.prefix = prefix;
        data.trim = trim;
        data.flags = flags;
        data.fn = fn;
        data.cb_data = cb_data;

        if (ref_paranoia < 0)
                ref_paranoia = git_env_bool("GIT_REF_PARANOIA", 0);
        if (ref_paranoia)
                data.flags |= DO_FOR_EACH_INCLUDE_BROKEN;

        return do_for_each_entry(refs, prefix, do_one_ref, &data);
}

/*
 * 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_lock *lock,
                       const unsigned char *old_sha1, int mustexist,
                       struct strbuf *err)
{
        assert(err);

        if (read_ref_full(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 {
                        hashclr(lock->old_oid.hash);
                        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,
                            sha1_to_hex(lock->old_oid.hash),
                            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);
}

/*
 * 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(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 lflags = LOCK_NO_DEREF;
        int mustexist = (old_sha1 && !is_null_sha1(old_sha1));
        int resolve_flags = RESOLVE_REF_NO_RECURSE;
        int attempts_remaining = 3;
        int resolved;

        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;

        strbuf_git_path(&ref_file, "%s", refname);
        resolved = !!resolve_ref_unsafe(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 (!verify_refname_available_dir(refname, extras, skip,
                                                          get_loose_refs(&ref_cache), err))
                                strbuf_addf(err, "there are still refs under '%s'",
                                            refname);
                        goto error_return;
                }
                resolved = !!resolve_ref_unsafe(refname, resolve_flags,
                                                lock->old_oid.hash, type);
        }
        if (!resolved) {
                last_errno = errno;
                if (last_errno != ENOTDIR ||
                    !verify_refname_available_dir(refname, extras, skip,
                                                  get_loose_refs(&ref_cache), 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) &&
            verify_refname_available_dir(refname, extras, skip,
                                         get_packed_refs(&ref_cache), err)) {
                last_errno = ENOTDIR;
                goto error_return;
        }

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

        lock->ref_name = xstrdup(refname);

 retry:
        switch (safe_create_leading_directories_const(ref_file.buf)) {
        case SCLD_OK:
                break; /* success */
        case SCLD_VANISHED:
                if (--attempts_remaining > 0)
                        goto retry;
                /* fall through */
        default:
                last_errno = errno;
                strbuf_addf(err, "unable to create directory for '%s'",
                            ref_file.buf);
                goto error_return;
        }

        if (hold_lock_file_for_update(lock->lk, ref_file.buf, lflags) < 0) {
                last_errno = errno;
                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;
                }
        }
        if (verify_lock(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;
}

/*
 * Write an entry to the packed-refs file for the specified refname.
 * If peeled is non-NULL, write it as the entry's peeled value.
 */
static void write_packed_entry(FILE *fh, char *refname, unsigned char *sha1,
                               unsigned char *peeled)
{
        fprintf_or_die(fh, "%s %s\n", sha1_to_hex(sha1), refname);
        if (peeled)
                fprintf_or_die(fh, "^%s\n", sha1_to_hex(peeled));
}

/*
 * An each_ref_entry_fn that writes the entry to a packed-refs file.
 */
static int write_packed_entry_fn(struct ref_entry *entry, void *cb_data)
{
        enum peel_status peel_status = peel_entry(entry, 0);

        if (peel_status != PEEL_PEELED && peel_status != PEEL_NON_TAG)
                error("internal error: %s is not a valid packed reference!",
                      entry->name);
        write_packed_entry(cb_data, entry->name, entry->u.value.oid.hash,
                           peel_status == PEEL_PEELED ?
                           entry->u.value.peeled.hash : NULL);
        return 0;
}

/*
 * Lock the packed-refs file for writing. Flags is passed to
 * hold_lock_file_for_update(). Return 0 on success. On errors, set
 * errno appropriately and return a nonzero value.
 */
static int lock_packed_refs(int flags)
{
        static int timeout_configured = 0;
        static int timeout_value = 1000;

        struct packed_ref_cache *packed_ref_cache;

        if (!timeout_configured) {
                git_config_get_int("core.packedrefstimeout", &timeout_value);
                timeout_configured = 1;
        }

        if (hold_lock_file_for_update_timeout(
                            &packlock, git_path("packed-refs"),
                            flags, timeout_value) < 0)
                return -1;
        /*
         * Get the current packed-refs while holding the lock.  If the
         * packed-refs file has been modified since we last read it,
         * this will automatically invalidate the cache and re-read
         * the packed-refs file.
         */
        packed_ref_cache = get_packed_ref_cache(&ref_cache);
        packed_ref_cache->lock = &packlock;
        /* Increment the reference count to prevent it from being freed: */
        acquire_packed_ref_cache(packed_ref_cache);
        return 0;
}

/*
 * Write the current version of the packed refs cache from memory to
 * disk. The packed-refs file must already be locked for writing (see
 * lock_packed_refs()). Return zero on success. On errors, set errno
 * and return a nonzero value
 */
static int commit_packed_refs(void)
{
        struct packed_ref_cache *packed_ref_cache =
                get_packed_ref_cache(&ref_cache);
        int error = 0;
        int save_errno = 0;
        FILE *out;

        if (!packed_ref_cache->lock)
                die("internal error: packed-refs not locked");

        out = fdopen_lock_file(packed_ref_cache->lock, "w");
        if (!out)
                die_errno("unable to fdopen packed-refs descriptor");

        fprintf_or_die(out, "%s", PACKED_REFS_HEADER);
        do_for_each_entry_in_dir(get_packed_ref_dir(packed_ref_cache),
                                 0, write_packed_entry_fn, out);

        if (commit_lock_file(packed_ref_cache->lock)) {
                save_errno = errno;
                error = -1;
        }
        packed_ref_cache->lock = NULL;
        release_packed_ref_cache(packed_ref_cache);
        errno = save_errno;
        return error;
}

/*
 * Rollback the lockfile for the packed-refs file, and discard the
 * in-memory packed reference cache.  (The packed-refs file will be
 * read anew if it is needed again after this function is called.)
 */
static void rollback_packed_refs(void)
{
        struct packed_ref_cache *packed_ref_cache =
                get_packed_ref_cache(&ref_cache);

        if (!packed_ref_cache->lock)
                die("internal error: packed-refs not locked");
        rollback_lock_file(packed_ref_cache->lock);
        packed_ref_cache->lock = NULL;
        release_packed_ref_cache(packed_ref_cache);
        clear_packed_ref_cache(&ref_cache);
}

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

struct pack_refs_cb_data {
        unsigned int flags;
        struct ref_dir *packed_refs;
        struct ref_to_prune *ref_to_prune;
};

/*
 * An each_ref_entry_fn that is run over loose references only.  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
 * ref_to_prune in the pack_refs_cb_data.
 */
static int pack_if_possible_fn(struct ref_entry *entry, void *cb_data)
{
        struct pack_refs_cb_data *cb = cb_data;
        enum peel_status peel_status;
        struct ref_entry *packed_entry;
        int is_tag_ref = starts_with(entry->name, "refs/tags/");

        /* Do not pack per-worktree refs: */
        if (ref_type(entry->name) != REF_TYPE_NORMAL)
                return 0;

        /* ALWAYS pack tags */
        if (!(cb->flags & PACK_REFS_ALL) && !is_tag_ref)
                return 0;

        /* Do not pack symbolic or broken refs: */
        if ((entry->flag & REF_ISSYMREF) || !entry_resolves_to_object(entry))
                return 0;

        /* Add a packed ref cache entry equivalent to the loose entry. */
        peel_status = peel_entry(entry, 1);
        if (peel_status != PEEL_PEELED && peel_status != PEEL_NON_TAG)
                die("internal error peeling reference %s (%s)",
                    entry->name, oid_to_hex(&entry->u.value.oid));
        packed_entry = find_ref(cb->packed_refs, entry->name);
        if (packed_entry) {
                /* Overwrite existing packed entry with info from loose entry */
                packed_entry->flag = REF_ISPACKED | REF_KNOWS_PEELED;
                oidcpy(&packed_entry->u.value.oid, &entry->u.value.oid);
        } else {
                packed_entry = create_ref_entry(entry->name, entry->u.value.oid.hash,
                                                REF_ISPACKED | REF_KNOWS_PEELED, 0);
                add_ref(cb->packed_refs, packed_entry);
        }
        oidcpy(&packed_entry->u.value.peeled, &entry->u.value.peeled);

        /* Schedule the loose reference for pruning if requested. */
        if ((cb->flags & PACK_REFS_PRUNE)) {
                struct ref_to_prune *n;
                FLEX_ALLOC_STR(n, name, entry->name);
                hashcpy(n->sha1, entry->u.value.oid.hash);
                n->next = cb->ref_to_prune;
                cb->ref_to_prune = n;
        }
        return 0;
}

/*
 * Remove empty parents, but spare refs/ and immediate subdirs.
 * Note: munges *name.
 */
static void try_remove_empty_parents(char *name)
{
        char *p, *q;
        int i;
        p = name;
        for (i = 0; i < 2; i++) { /* refs/{heads,tags,...}/ */
                while (*p && *p != '/')
                        p++;
                /* tolerate duplicate slashes; see check_refname_format() */
                while (*p == '/')
                        p++;
        }
        for (q = p; *q; q++)
                ;
        while (1) {
                while (q > p && *q != '/')
                        q--;
                while (q > p && *(q-1) == '/')
                        q--;
                if (q == p)
                        break;
                *q = '\0';
                if (rmdir(git_path("%s", name)))
                        break;
        }
}

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

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

        transaction = ref_transaction_begin(&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);
        try_remove_empty_parents(r->name);
}

static void prune_refs(struct ref_to_prune *r)
{
        while (r) {
                prune_ref(r);
                r = r->next;
        }
}

int pack_refs(unsigned int flags)
{
        struct pack_refs_cb_data cbdata;

        memset(&cbdata, 0, sizeof(cbdata));
        cbdata.flags = flags;

        lock_packed_refs(LOCK_DIE_ON_ERROR);
        cbdata.packed_refs = get_packed_refs(&ref_cache);

        do_for_each_entry_in_dir(get_loose_refs(&ref_cache), 0,
                                 pack_if_possible_fn, &cbdata);

        if (commit_packed_refs())
                die_errno("unable to overwrite old ref-pack file");

        prune_refs(cbdata.ref_to_prune);
        return 0;
}

/*
 * Rewrite the packed-refs file, omitting any refs listed in
 * 'refnames'. On error, leave packed-refs unchanged, write an error
 * message to 'err', and return a nonzero value.
 *
 * The refs in 'refnames' needn't be sorted. `err` must not be NULL.
 */
static int repack_without_refs(struct string_list *refnames, struct strbuf *err)
{
        struct ref_dir *packed;
        struct string_list_item *refname;
        int ret, needs_repacking = 0, removed = 0;

        assert(err);

        /* Look for a packed ref */
        for_each_string_list_item(refname, refnames) {
                if (get_packed_ref(refname->string)) {
                        needs_repacking = 1;
                        break;
                }
        }

        /* Avoid locking if we have nothing to do */
        if (!needs_repacking)
                return 0; /* no refname exists in packed refs */

        if (lock_packed_refs(0)) {
                unable_to_lock_message(git_path("packed-refs"), errno, err);
                return -1;
        }
        packed = get_packed_refs(&ref_cache);

        /* Remove refnames from the cache */
        for_each_string_list_item(refname, refnames)
                if (remove_entry(packed, refname->string) != -1)
                        removed = 1;
        if (!removed) {
                /*
                 * All packed entries disappeared while we were
                 * acquiring the lock.
                 */
                rollback_packed_refs();
                return 0;
        }

        /* Write what remains */
        ret = commit_packed_refs();
        if (ret)
                strbuf_addf(err, "unable to overwrite old ref-pack file: %s",
                            strerror(errno));
        return ret;
}

static int delete_ref_loose(struct ref_lock *lock, int flag, struct strbuf *err)
{
        assert(err);

        if (!(flag & REF_ISPACKED) || flag & REF_ISSYMREF) {
                /*
                 * loose.  The loose file name is the same as the
                 * lockfile name, minus ".lock":
                 */
                char *loose_filename = get_locked_file_path(lock->lk);
                int res = unlink_or_msg(loose_filename, err);
                free(loose_filename);
                if (res)
                        return 1;
        }
        return 0;
}

int delete_refs(struct string_list *refnames, unsigned int flags)
{
        struct strbuf err = STRBUF_INIT;
        int i, result = 0;

        if (!refnames->nr)
                return 0;

        result = repack_without_refs(refnames, &err);
        if (result) {
                /*
                 * 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);

                goto out;
        }

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

                if (delete_ref(refname, NULL, flags))
                        result |= error(_("could not remove reference %s"), refname);
        }

out:
        strbuf_release(&err);
        return result;
}

/*
 * 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  "logs/refs/.tmp-renamed-log"

static int rename_tmp_log(const char *newrefname)
{
        int attempts_remaining = 4;
        struct strbuf path = STRBUF_INIT;
        int ret = -1;

 retry:
        strbuf_reset(&path);
        strbuf_git_path(&path, "logs/%s", newrefname);
        switch (safe_create_leading_directories_const(path.buf)) {
        case SCLD_OK:
                break; /* success */
        case SCLD_VANISHED:
                if (--attempts_remaining > 0)
                        goto retry;
                /* fall through */
        default:
                error("unable to create directory for %s", newrefname);
                goto out;
        }

        if (rename(git_path(TMP_RENAMED_LOG), path.buf)) {
                if ((errno==EISDIR || errno==ENOTDIR) && --attempts_remaining > 0) {
                        /*
                         * rename(a, b) when b is an existing
                         * directory ought to result in ISDIR, but
                         * Solaris 5.8 gives ENOTDIR.  Sheesh.
                         */
                        if (remove_empty_directories(&path)) {
                                error("Directory not empty: logs/%s", newrefname);
                                goto out;
                        }
                        goto retry;
                } else if (errno == ENOENT && --attempts_remaining > 0) {
                        /*
                         * Maybe another process just deleted one of
                         * the directories in the path to newrefname.
                         * Try again from the beginning.
                         */
                        goto retry;
                } else {
                        error("unable to move logfile "TMP_RENAMED_LOG" to logs/%s: %s",
                                newrefname, strerror(errno));
                        goto out;
                }
        }
        ret = 0;
out:
        strbuf_release(&path);
        return ret;
}

int verify_refname_available(const char *newname,
                             const struct string_list *extras,
                             const struct string_list *skip,
                             struct strbuf *err)
{
        struct ref_dir *packed_refs = get_packed_refs(&ref_cache);
        struct ref_dir *loose_refs = get_loose_refs(&ref_cache);

        if (verify_refname_available_dir(newname, extras, skip,
                                         packed_refs, err) ||
            verify_refname_available_dir(newname, extras, skip,
                                         loose_refs, err))
                return -1;

        return 0;
}

static int write_ref_to_lockfile(struct ref_lock *lock,
                                 const unsigned char *sha1, struct strbuf *err);
static int commit_ref_update(struct ref_lock *lock,
                             const unsigned char *sha1, const char *logmsg,
                             struct strbuf *err);

int rename_ref(const char *oldrefname, const char *newrefname, const char *logmsg)
{
        unsigned char sha1[20], orig_sha1[20];
        int flag = 0, logmoved = 0;
        struct ref_lock *lock;
        struct stat loginfo;
        int log = !lstat(git_path("logs/%s", oldrefname), &loginfo);
        struct strbuf err = STRBUF_INIT;

        if (log && S_ISLNK(loginfo.st_mode))
                return error("reflog for %s is a symlink", oldrefname);

        if (!resolve_ref_unsafe(oldrefname, RESOLVE_REF_READING | RESOLVE_REF_NO_RECURSE,
                                orig_sha1, &flag))
                return error("refname %s not found", oldrefname);

        if (flag & REF_ISSYMREF)
                return error("refname %s is a symbolic ref, renaming it is not supported",
                        oldrefname);
        if (!rename_ref_available(oldrefname, newrefname))
                return 1;

        if (log && rename(git_path("logs/%s", oldrefname), git_path(TMP_RENAMED_LOG)))
                return error("unable to move logfile logs/%s to "TMP_RENAMED_LOG": %s",
                        oldrefname, strerror(errno));

        if (delete_ref(oldrefname, orig_sha1, REF_NODEREF)) {
                error("unable to delete old %s", oldrefname);
                goto rollback;
        }

        /*
         * Since we are doing a shallow lookup, sha1 is not the
         * correct value to pass to delete_ref as old_sha1. But that
         * doesn't matter, because an old_sha1 check wouldn't add to
         * the safety anyway; we want to delete the reference whatever
         * its current value.
         */
        if (!read_ref_full(newrefname, RESOLVE_REF_READING | RESOLVE_REF_NO_RECURSE,
                           sha1, NULL) &&
            delete_ref(newrefname, NULL, REF_NODEREF)) {
                if (errno==EISDIR) {
                        struct strbuf path = STRBUF_INIT;
                        int result;

                        strbuf_git_path(&path, "%s", 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(newrefname))
                goto rollback;

        logmoved = log;

        lock = lock_ref_sha1_basic(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;
        }
        hashcpy(lock->old_oid.hash, orig_sha1);

        if (write_ref_to_lockfile(lock, orig_sha1, &err) ||
            commit_ref_update(lock, orig_sha1, logmsg, &err)) {
                error("unable to write current sha1 into %s: %s", newrefname, err.buf);
                strbuf_release(&err);
                goto rollback;
        }

        return 0;

 rollback:
        lock = lock_ref_sha1_basic(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 = 0;
        if (write_ref_to_lockfile(lock, orig_sha1, &err) ||
            commit_ref_update(lock, orig_sha1, 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(git_path("logs/%s", newrefname), git_path("logs/%s", oldrefname)))
                error("unable to restore logfile %s from %s: %s",
                        oldrefname, newrefname, strerror(errno));
        if (!logmoved && log &&
            rename(git_path(TMP_RENAMED_LOG), git_path("logs/%s", oldrefname)))
                error("unable to restore logfile %s from "TMP_RENAMED_LOG": %s",
                        oldrefname, strerror(errno));

        return 1;
}

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;
}

/*
 * Create a reflog for a ref.  If force_create = 0, the reflog will
 * only be created for certain refs (those for which
 * should_autocreate_reflog returns non-zero.  Otherwise, create it
 * regardless of the ref name.  Fill in *err and return -1 on failure.
 */
static int log_ref_setup(const char *refname, struct strbuf *logfile, struct strbuf *err, int force_create)
{
        int logfd, oflags = O_APPEND | O_WRONLY;

        strbuf_git_path(logfile, "logs/%s", refname);
        if (force_create || should_autocreate_reflog(refname)) {
                if (safe_create_leading_directories(logfile->buf) < 0) {
                        strbuf_addf(err, "unable to create directory for '%s': "
                                    "%s", logfile->buf, strerror(errno));
                        return -1;
                }
                oflags |= O_CREAT;
        }

        logfd = open(logfile->buf, oflags, 0666);
        if (logfd < 0) {
                if (!(oflags & O_CREAT) && (errno == ENOENT || errno == EISDIR))
                        return 0;

                if (errno == EISDIR) {
                        if (remove_empty_directories(logfile)) {
                                strbuf_addf(err, "there are still logs under "
                                            "'%s'", logfile->buf);
                                return -1;
                        }
                        logfd = open(logfile->buf, oflags, 0666);
                }

                if (logfd < 0) {
                        strbuf_addf(err, "unable to append to '%s': %s",
                                    logfile->buf, strerror(errno));
                        return -1;
                }
        }

        adjust_shared_perm(logfile->buf);
        close(logfd);
        return 0;
}


int safe_create_reflog(const char *refname, int force_create, struct strbuf *err)
{
        int ret;
        struct strbuf sb = STRBUF_INIT;

        ret = log_ref_setup(refname, &sb, err, force_create);
        strbuf_release(&sb);
        return ret;
}

static int log_ref_write_fd(int fd, const unsigned char *old_sha1,
                            const unsigned char *new_sha1,
                            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",
                        sha1_to_hex(old_sha1),
                        sha1_to_hex(new_sha1),
                        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 log_ref_write_1(const char *refname, const unsigned char *old_sha1,
                           const unsigned char *new_sha1, const char *msg,
                           struct strbuf *logfile, int flags,
                           struct strbuf *err)
{
        int logfd, result, oflags = O_APPEND | O_WRONLY;

        if (log_all_ref_updates < 0)
                log_all_ref_updates = !is_bare_repository();

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

        if (result)
                return result;

        logfd = open(logfile->buf, oflags);
        if (logfd < 0)
                return 0;
        result = log_ref_write_fd(logfd, old_sha1, new_sha1,
                                  git_committer_info(0), msg);
        if (result) {
                strbuf_addf(err, "unable to append to '%s': %s", logfile->buf,
                            strerror(errno));
                close(logfd);
                return -1;
        }
        if (close(logfd)) {
                strbuf_addf(err, "unable to append to '%s': %s", logfile->buf,
                            strerror(errno));
                return -1;
        }
        return 0;
}

static int log_ref_write(const char *refname, const unsigned char *old_sha1,
                         const unsigned char *new_sha1, const char *msg,
                         int flags, struct strbuf *err)
{
        return files_log_ref_write(refname, old_sha1, new_sha1, msg, flags,
                                   err);
}

int files_log_ref_write(const char *refname, const unsigned char *old_sha1,
                        const unsigned char *new_sha1, const char *msg,
                        int flags, struct strbuf *err)
{
        struct strbuf sb = STRBUF_INIT;
        int ret = log_ref_write_1(refname, old_sha1, new_sha1, msg, &sb, flags,
                                  err);
        strbuf_release(&sb);
        return ret;
}

/*
 * 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 unsigned char *sha1, struct strbuf *err)
{
        static char term = '\n';
        struct object *o;
        int fd;

        o = parse_object(sha1);
        if (!o) {
                strbuf_addf(err,
                            "trying to write ref '%s' with nonexistent object %s",
                            lock->ref_name, sha1_to_hex(sha1));
                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'",
                            sha1_to_hex(sha1), lock->ref_name);
                unlock_ref(lock);
                return -1;
        }
        fd = get_lock_file_fd(lock->lk);
        if (write_in_full(fd, sha1_to_hex(sha1), 40) != 40 ||
            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 ref_lock *lock,
                             const unsigned char *sha1, const char *logmsg,
                             struct strbuf *err)
{
        clear_loose_ref_cache(&ref_cache);
        if (log_ref_write(lock->ref_name, lock->old_oid.hash, sha1, 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).
                 */
                unsigned char head_sha1[20];
                int head_flag;
                const char *head_ref;

                head_ref = resolve_ref_unsafe("HEAD", RESOLVE_REF_READING,
                                              head_sha1, &head_flag);
                if (head_ref && (head_flag & REF_ISSYMREF) &&
                    !strcmp(head_ref, lock->ref_name)) {
                        struct strbuf log_err = STRBUF_INIT;
                        if (log_ref_write("HEAD", lock->old_oid.hash, sha1,
                                          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 ref_lock *lock, const char *refname,
                                 const char *target, const char *logmsg)
{
        struct strbuf err = STRBUF_INIT;
        unsigned char new_sha1[20];
        if (logmsg && !read_ref(target, new_sha1) &&
            log_ref_write(refname, lock->old_oid.hash, new_sha1, logmsg, 0, &err)) {
                error("%s", err.buf);
                strbuf_release(&err);
        }
}

static int create_symref_locked(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(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(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;
}

int create_symref(const char *refname, const char *target, const char *logmsg)
{
        struct strbuf err = STRBUF_INIT;
        struct ref_lock *lock;
        int ret;

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

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

int set_worktree_head_symref(const char *gitdir, const char *target)
{
        static struct lock_file head_lock;
        struct ref_lock *lock;
        struct strbuf head_path = STRBUF_INIT;
        const char *head_rel;
        int ret;

        strbuf_addf(&head_path, "%s/HEAD", absolute_path(gitdir));
        if (hold_lock_file_for_update(&head_lock, head_path.buf,
                                      LOCK_NO_DEREF) < 0) {
                struct strbuf err = STRBUF_INIT;
                unable_to_lock_message(head_path.buf, errno, &err);
                error("%s", err.buf);
                strbuf_release(&err);
                strbuf_release(&head_path);
                return -1;
        }

        /* head_rel will be "HEAD" for the main tree, "worktrees/wt/HEAD" for
           linked trees */
        head_rel = remove_leading_path(head_path.buf,
                                       absolute_path(get_git_common_dir()));
        /* to make use of create_symref_locked(), initialize ref_lock */
        lock = xcalloc(1, sizeof(struct ref_lock));
        lock->lk = &head_lock;
        lock->ref_name = xstrdup(head_rel);

        ret = create_symref_locked(lock, head_rel, target, NULL);

        unlock_ref(lock); /* will free lock */
        strbuf_release(&head_path);
        return ret;
}

int reflog_exists(const char *refname)
{
        struct stat st;

        return !lstat(git_path("logs/%s", refname), &st) &&
                S_ISREG(st.st_mode);
}

int delete_reflog(const char *refname)
{
        return remove_path(git_path("logs/%s", refname));
}

static int show_one_reflog_ent(struct strbuf *sb, each_reflog_ent_fn fn, void *cb_data)
{
        unsigned char osha1[20], nsha1[20];
        char *email_end, *message;
        unsigned long timestamp;
        int tz;

        /* old SP new SP name <email> SP time TAB msg LF */
        if (sb->len < 83 || sb->buf[sb->len - 1] != '\n' ||
            get_sha1_hex(sb->buf, osha1) || sb->buf[40] != ' ' ||
            get_sha1_hex(sb->buf + 41, nsha1) || sb->buf[81] != ' ' ||
            !(email_end = strchr(sb->buf + 82, '>')) ||
            email_end[1] != ' ' ||
            !(timestamp = strtoul(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(osha1, nsha1, sb->buf + 82, 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;
}

int for_each_reflog_ent_reverse(const char *refname, each_reflog_ent_fn fn, void *cb_data)
{
        struct strbuf sb = STRBUF_INIT;
        FILE *logfp;
        long pos;
        int ret = 0, at_tail = 1;

        logfp = fopen(git_path("logs/%s", refname), "r");
        if (!logfp)
                return -1;

        /* Jump to the end */
        if (fseek(logfp, 0, SEEK_END) < 0)
                return 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))
                        return error("cannot seek back reflog for %s: %s",
                                     refname, strerror(errno));
                nread = fread(buf, cnt, 1, logfp);
                if (nread != 1)
                        return error("cannot read %d bytes from reflog for %s: %s",
                                     cnt, refname, strerror(errno));
                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;
}

int for_each_reflog_ent(const char *refname, each_reflog_ent_fn fn, void *cb_data)
{
        FILE *logfp;
        struct strbuf sb = STRBUF_INIT;
        int ret = 0;

        logfp = fopen(git_path("logs/%s", refname), "r");
        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;
}
/*
 * Call fn for each reflog in the namespace indicated by name.  name
 * must be empty or end with '/'.  Name will be used as a scratch
 * space, but its contents will be restored before return.
 */
static int do_for_each_reflog(struct strbuf *name, each_ref_fn fn, void *cb_data)
{
        DIR *d = opendir(git_path("logs/%s", name->buf));
        int retval = 0;
        struct dirent *de;
        int oldlen = name->len;

        if (!d)
                return name->len ? errno : 0;

        while ((de = readdir(d)) != NULL) {
                struct stat st;

                if (de->d_name[0] == '.')
                        continue;
                if (ends_with(de->d_name, ".lock"))
                        continue;
                strbuf_addstr(name, de->d_name);
                if (stat(git_path("logs/%s", name->buf), &st) < 0) {
                        ; /* silently ignore */
                } else {
                        if (S_ISDIR(st.st_mode)) {
                                strbuf_addch(name, '/');
                                retval = do_for_each_reflog(name, fn, cb_data);
                        } else {
                                struct object_id oid;

                                if (read_ref_full(name->buf, 0, oid.hash, NULL))
                                        retval = error("bad ref for %s", name->buf);
                                else
                                        retval = fn(name->buf, &oid, 0, cb_data);
                        }
                        if (retval)
                                break;
                }
                strbuf_setlen(name, oldlen);
        }
        closedir(d);
        return retval;
}

int for_each_reflog(each_ref_fn fn, void *cb_data)
{
        int retval;
        struct strbuf name;
        strbuf_init(&name, PATH_MAX);
        retval = do_for_each_reflog(&name, fn, cb_data);
        strbuf_release(&name);
        return retval;
}

static int ref_update_reject_duplicates(struct string_list *refnames,
                                        struct strbuf *err)
{
        int i, n = refnames->nr;

        assert(err);

        for (i = 1; i < n; i++)
                if (!strcmp(refnames->items[i - 1].string, refnames->items[i].string)) {
                        strbuf_addf(err,
                                    "multiple updates for ref '%s' not allowed.",
                                    refnames->items[i].string);
                        return 1;
                }
        return 0;
}

/*
 * 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_sha1, update->old_sha1,
                        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 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_sha1, update->old_sha1,
                        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;
}

/*
 * 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 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_sha1(update->old_sha1);
        int ret;
        struct ref_lock *lock;

        if ((update->flags & REF_HAVE_NEW) && is_null_sha1(update->new_sha1))
                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(update->refname, mustexist,
                           affected_refnames, NULL,
                           &update->lock, &referent,
                           &update->type, err);

        if (ret) {
                char *reason;

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

        lock = update->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 (read_ref_full(update->refname,
                                          mustexist ? RESOLVE_REF_READING : 0,
                                          lock->old_oid.hash, NULL)) {
                                if (update->flags & REF_HAVE_OLD) {
                                        strbuf_addf(err, "cannot lock ref '%s': "
                                                    "can't resolve old value",
                                                    update->refname);
                                        return TRANSACTION_GENERIC_ERROR;
                                } else {
                                        hashclr(lock->old_oid.hash);
                                }
                        }
                        if ((update->flags & REF_HAVE_OLD) &&
                            hashcmp(lock->old_oid.hash, update->old_sha1)) {
                                strbuf_addf(err, "cannot lock ref '%s': "
                                            "is at %s but expected %s",
                                            update->refname,
                                            sha1_to_hex(lock->old_oid.hash),
                                            sha1_to_hex(update->old_sha1));
                                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(update, referent.buf, transaction,
                                                  affected_refnames, err);
                        if (ret)
                                return ret;
                }
        } else {
                struct ref_update *parent_update;

                /*
                 * 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) {
                        oidcpy(&parent_update->lock->old_oid, &lock->old_oid);
                }

                if ((update->flags & REF_HAVE_OLD) &&
                    hashcmp(lock->old_oid.hash, update->old_sha1)) {
                        if (is_null_sha1(update->old_sha1))
                                strbuf_addf(err, "cannot lock ref '%s': reference already exists",
                                            original_update_refname(update));
                        else
                                strbuf_addf(err, "cannot lock ref '%s': is at %s but expected %s",
                                            original_update_refname(update),
                                            sha1_to_hex(lock->old_oid.hash),
                                            sha1_to_hex(update->old_sha1));

                        return TRANSACTION_GENERIC_ERROR;
                }
        }

        if ((update->flags & REF_HAVE_NEW) &&
            !(update->flags & REF_DELETING) &&