http*: copy string returned by sha1_to_hex

[git/jrn.git] / refs.c

#include "cache.h"
#include "refs.h"
#include "object.h"
#include "tag.h"
#include "dir.h"

/* ISSYMREF=01 and ISPACKED=02 are public interfaces */
#define REF_KNOWS_PEELED 04

struct ref_list {
        struct ref_list *next;
        unsigned char flag; /* ISSYMREF? ISPACKED? */
        unsigned char sha1[20];
        unsigned char peeled[20];
        char name[FLEX_ARRAY];
};

static const char *parse_ref_line(char *line, unsigned char *sha1)
{
        /*
         * 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)
         */
        int len = strlen(line) - 42;

        if (len <= 0)
                return NULL;
        if (get_sha1_hex(line, sha1) < 0)
                return NULL;
        if (!isspace(line[40]))
                return NULL;
        line += 41;
        if (isspace(*line))
                return NULL;
        if (line[len] != '\n')
                return NULL;
        line[len] = 0;

        return line;
}

static struct ref_list *add_ref(const char *name, const unsigned char *sha1,
                                int flag, struct ref_list *list,
                                struct ref_list **new_entry)
{
        int len;
        struct ref_list *entry;

        /* Allocate it and add it in.. */
        len = strlen(name) + 1;
        entry = xmalloc(sizeof(struct ref_list) + len);
        hashcpy(entry->sha1, sha1);
        hashclr(entry->peeled);
        memcpy(entry->name, name, len);
        entry->flag = flag;
        entry->next = list;
        if (new_entry)
                *new_entry = entry;
        return entry;
}

/* merge sort the ref list */
static struct ref_list *sort_ref_list(struct ref_list *list)
{
        int psize, qsize, last_merge_count, cmp;
        struct ref_list *p, *q, *l, *e;
        struct ref_list *new_list = list;
        int k = 1;
        int merge_count = 0;

        if (!list)
                return list;

        do {
                last_merge_count = merge_count;
                merge_count = 0;

                psize = 0;

                p = new_list;
                q = new_list;
                new_list = NULL;
                l = NULL;

                while (p) {
                        merge_count++;

                        while (psize < k && q->next) {
                                q = q->next;
                                psize++;
                        }
                        qsize = k;

                        while ((psize > 0) || (qsize > 0 && q)) {
                                if (qsize == 0 || !q) {
                                        e = p;
                                        p = p->next;
                                        psize--;
                                } else if (psize == 0) {
                                        e = q;
                                        q = q->next;
                                        qsize--;
                                } else {
                                        cmp = strcmp(q->name, p->name);
                                        if (cmp < 0) {
                                                e = q;
                                                q = q->next;
                                                qsize--;
                                        } else if (cmp > 0) {
                                                e = p;
                                                p = p->next;
                                                psize--;
                                        } else {
                                                if (hashcmp(q->sha1, p->sha1))
                                                        die("Duplicated ref, and SHA1s don't match: %s",
                                                            q->name);
                                                warning("Duplicated ref: %s", q->name);
                                                e = q;
                                                q = q->next;
                                                qsize--;
                                                free(e);
                                                e = p;
                                                p = p->next;
                                                psize--;
                                        }
                                }

                                e->next = NULL;

                                if (l)
                                        l->next = e;
                                if (!new_list)
                                        new_list = e;
                                l = e;
                        }

                        p = q;
                };

                k = k * 2;
        } while ((last_merge_count != merge_count) || (last_merge_count != 1));

        return new_list;
}

/*
 * Future: need to be in "struct repository"
 * when doing a full libification.
 */
static struct cached_refs {
        char did_loose;
        char did_packed;
        struct ref_list *loose;
        struct ref_list *packed;
} cached_refs;
static struct ref_list *current_ref;

static struct ref_list *extra_refs;

static void free_ref_list(struct ref_list *list)
{
        struct ref_list *next;
        for ( ; list; list = next) {
                next = list->next;
                free(list);
        }
}

static void invalidate_cached_refs(void)
{
        struct cached_refs *ca = &cached_refs;

        if (ca->did_loose && ca->loose)
                free_ref_list(ca->loose);
        if (ca->did_packed && ca->packed)
                free_ref_list(ca->packed);
        ca->loose = ca->packed = NULL;
        ca->did_loose = ca->did_packed = 0;
}

static void read_packed_refs(FILE *f, struct cached_refs *cached_refs)
{
        struct ref_list *list = NULL;
        struct ref_list *last = NULL;
        char refline[PATH_MAX];
        int flag = REF_ISPACKED;

        while (fgets(refline, sizeof(refline), f)) {
                unsigned char sha1[20];
                const char *name;
                static const char header[] = "# pack-refs with:";

                if (!strncmp(refline, header, sizeof(header)-1)) {
                        const char *traits = refline + sizeof(header) - 1;
                        if (strstr(traits, " peeled "))
                                flag |= REF_KNOWS_PEELED;
                        /* perhaps other traits later as well */
                        continue;
                }

                name = parse_ref_line(refline, sha1);
                if (name) {
                        list = add_ref(name, sha1, flag, list, &last);
                        continue;
                }
                if (last &&
                    refline[0] == '^' &&
                    strlen(refline) == 42 &&
                    refline[41] == '\n' &&
                    !get_sha1_hex(refline + 1, sha1))
                        hashcpy(last->peeled, sha1);
        }
        cached_refs->packed = sort_ref_list(list);
}

void add_extra_ref(const char *name, const unsigned char *sha1, int flag)
{
        extra_refs = add_ref(name, sha1, flag, extra_refs, NULL);
}

void clear_extra_refs(void)
{
        free_ref_list(extra_refs);
        extra_refs = NULL;
}

static struct ref_list *get_packed_refs(void)
{
        if (!cached_refs.did_packed) {
                FILE *f = fopen(git_path("packed-refs"), "r");
                cached_refs.packed = NULL;
                if (f) {
                        read_packed_refs(f, &cached_refs);
                        fclose(f);
                }
                cached_refs.did_packed = 1;
        }
        return cached_refs.packed;
}

static struct ref_list *get_ref_dir(const char *base, struct ref_list *list)
{
        DIR *dir = opendir(git_path("%s", base));

        if (dir) {
                struct dirent *de;
                int baselen = strlen(base);
                char *ref = xmalloc(baselen + 257);

                memcpy(ref, base, baselen);
                if (baselen && base[baselen-1] != '/')
                        ref[baselen++] = '/';

                while ((de = readdir(dir)) != NULL) {
                        unsigned char sha1[20];
                        struct stat st;
                        int flag;
                        int namelen;

                        if (de->d_name[0] == '.')
                                continue;
                        namelen = strlen(de->d_name);
                        if (namelen > 255)
                                continue;
                        if (has_extension(de->d_name, ".lock"))
                                continue;
                        memcpy(ref + baselen, de->d_name, namelen+1);
                        if (stat(git_path("%s", ref), &st) < 0)
                                continue;
                        if (S_ISDIR(st.st_mode)) {
                                list = get_ref_dir(ref, list);
                                continue;
                        }
                        if (!resolve_ref(ref, sha1, 1, &flag))
                                hashclr(sha1);
                        list = add_ref(ref, sha1, flag, list, NULL);
                }
                free(ref);
                closedir(dir);
        }
        return sort_ref_list(list);
}

struct warn_if_dangling_data {
        const char *refname;
        const char *msg_fmt;
};

static int warn_if_dangling_symref(const char *refname, const unsigned char *sha1,
                                   int flags, void *cb_data)
{
        struct warn_if_dangling_data *d = cb_data;
        const char *resolves_to;
        unsigned char junk[20];

        if (!(flags & REF_ISSYMREF))
                return 0;

        resolves_to = resolve_ref(refname, junk, 0, NULL);
        if (!resolves_to || strcmp(resolves_to, d->refname))
                return 0;

        printf(d->msg_fmt, refname);
        return 0;
}

void warn_dangling_symref(const char *msg_fmt, const char *refname)
{
        struct warn_if_dangling_data data = { refname, msg_fmt };
        for_each_rawref(warn_if_dangling_symref, &data);
}

static struct ref_list *get_loose_refs(void)
{
        if (!cached_refs.did_loose) {
                cached_refs.loose = get_ref_dir("refs", NULL);
                cached_refs.did_loose = 1;
        }
        return cached_refs.loose;
}

/* We allow "recursive" symbolic refs. Only within reason, though */
#define MAXDEPTH 5
#define MAXREFLEN (1024)

static int resolve_gitlink_packed_ref(char *name, int pathlen, const char *refname, unsigned char *result)
{
        FILE *f;
        struct cached_refs refs;
        struct ref_list *ref;
        int retval;

        strcpy(name + pathlen, "packed-refs");
        f = fopen(name, "r");
        if (!f)
                return -1;
        read_packed_refs(f, &refs);
        fclose(f);
        ref = refs.packed;
        retval = -1;
        while (ref) {
                if (!strcmp(ref->name, refname)) {
                        retval = 0;
                        memcpy(result, ref->sha1, 20);
                        break;
                }
                ref = ref->next;
        }
        free_ref_list(refs.packed);
        return retval;
}

static int resolve_gitlink_ref_recursive(char *name, int pathlen, const char *refname, unsigned char *result, int recursion)
{
        int fd, len = strlen(refname);
        char buffer[128], *p;

        if (recursion > MAXDEPTH || len > MAXREFLEN)
                return -1;
        memcpy(name + pathlen, refname, len+1);
        fd = open(name, O_RDONLY);
        if (fd < 0)
                return resolve_gitlink_packed_ref(name, pathlen, refname, result);

        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, result))
                return 0;

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

        return resolve_gitlink_ref_recursive(name, pathlen, p, result, recursion+1);
}

int resolve_gitlink_ref(const char *path, const char *refname, unsigned char *result)
{
        int len = strlen(path), retval;
        char *gitdir;
        const char *tmp;

        while (len && path[len-1] == '/')
                len--;
        if (!len)
                return -1;
        gitdir = xmalloc(len + MAXREFLEN + 8);
        memcpy(gitdir, path, len);
        memcpy(gitdir + len, "/.git", 6);
        len += 5;

        tmp = read_gitfile_gently(gitdir);
        if (tmp) {
                free(gitdir);
                len = strlen(tmp);
                gitdir = xmalloc(len + MAXREFLEN + 3);
                memcpy(gitdir, tmp, len);
        }
        gitdir[len] = '/';
        gitdir[++len] = '\0';
        retval = resolve_gitlink_ref_recursive(gitdir, len, refname, result, 0);
        free(gitdir);
        return retval;
}

/*
 * If the "reading" argument is set, this function finds out what _object_
 * the ref points at by "reading" the ref.  The ref, if it is not symbolic,
 * has to exist, and if it is symbolic, it has to point at an existing ref,
 * because the "read" goes through the symref to the ref it points at.
 *
 * The access that is not "reading" may often be "writing", but does not
 * have to; it can be merely checking _where it leads to_. If it is a
 * prelude to "writing" to the ref, a write to a symref that points at
 * yet-to-be-born ref will create the real ref pointed by the symref.
 * reading=0 allows the caller to check where such a symref leads to.
 */
const char *resolve_ref(const char *ref, unsigned char *sha1, int reading, int *flag)
{
        int depth = MAXDEPTH;
        ssize_t len;
        char buffer[256];
        static char ref_buffer[256];

        if (flag)
                *flag = 0;

        for (;;) {
                char path[PATH_MAX];
                struct stat st;
                char *buf;
                int fd;

                if (--depth < 0)
                        return NULL;

                git_snpath(path, sizeof(path), "%s", ref);
                /* Special case: non-existing file. */
                if (lstat(path, &st) < 0) {
                        struct ref_list *list = get_packed_refs();
                        while (list) {
                                if (!strcmp(ref, list->name)) {
                                        hashcpy(sha1, list->sha1);
                                        if (flag)
                                                *flag |= REF_ISPACKED;
                                        return ref;
                                }
                                list = list->next;
                        }
                        if (reading || errno != ENOENT)
                                return NULL;
                        hashclr(sha1);
                        return ref;
                }

                /* Follow "normalized" - ie "refs/.." symlinks by hand */
                if (S_ISLNK(st.st_mode)) {
                        len = readlink(path, buffer, sizeof(buffer)-1);
                        if (len >= 5 && !memcmp("refs/", buffer, 5)) {
                                buffer[len] = 0;
                                strcpy(ref_buffer, buffer);
                                ref = ref_buffer;
                                if (flag)
                                        *flag |= REF_ISSYMREF;
                                continue;
                        }
                }

                /* Is it a directory? */
                if (S_ISDIR(st.st_mode)) {
                        errno = EISDIR;
                        return NULL;
                }

                /*
                 * Anything else, just open it and try to use it as
                 * a ref
                 */
                fd = open(path, O_RDONLY);
                if (fd < 0)
                        return NULL;
                len = read_in_full(fd, buffer, sizeof(buffer)-1);
                close(fd);

                /*
                 * Is it a symbolic ref?
                 */
                if (len < 4 || memcmp("ref:", buffer, 4))
                        break;
                buf = buffer + 4;
                len -= 4;
                while (len && isspace(*buf))
                        buf++, len--;
                while (len && isspace(buf[len-1]))
                        len--;
                buf[len] = 0;
                memcpy(ref_buffer, buf, len + 1);
                ref = ref_buffer;
                if (flag)
                        *flag |= REF_ISSYMREF;
        }
        if (len < 40 || get_sha1_hex(buffer, sha1))
                return NULL;
        return ref;
}

int read_ref(const char *ref, unsigned char *sha1)
{
        if (resolve_ref(ref, sha1, 1, NULL))
                return 0;
        return -1;
}

#define DO_FOR_EACH_INCLUDE_BROKEN 01
static int do_one_ref(const char *base, each_ref_fn fn, int trim,
                      int flags, void *cb_data, struct ref_list *entry)
{
        if (strncmp(base, entry->name, trim))
                return 0;
        if (!(flags & DO_FOR_EACH_INCLUDE_BROKEN)) {
                if (is_null_sha1(entry->sha1))
                        return 0;
                if (!has_sha1_file(entry->sha1)) {
                        error("%s does not point to a valid object!", entry->name);
                        return 0;
                }
        }
        current_ref = entry;
        return fn(entry->name + trim, entry->sha1, entry->flag, cb_data);
}

int peel_ref(const char *ref, unsigned char *sha1)
{
        int flag;
        unsigned char base[20];
        struct object *o;

        if (current_ref && (current_ref->name == ref
                || !strcmp(current_ref->name, ref))) {
                if (current_ref->flag & REF_KNOWS_PEELED) {
                        hashcpy(sha1, current_ref->peeled);
                        return 0;
                }
                hashcpy(base, current_ref->sha1);
                goto fallback;
        }

        if (!resolve_ref(ref, base, 1, &flag))
                return -1;

        if ((flag & REF_ISPACKED)) {
                struct ref_list *list = get_packed_refs();

                while (list) {
                        if (!strcmp(list->name, ref)) {
                                if (list->flag & REF_KNOWS_PEELED) {
                                        hashcpy(sha1, list->peeled);
                                        return 0;
                                }
                                /* older pack-refs did not leave peeled ones */
                                break;
                        }
                        list = list->next;
                }
        }

fallback:
        o = parse_object(base);
        if (o && o->type == OBJ_TAG) {
                o = deref_tag(o, ref, 0);
                if (o) {
                        hashcpy(sha1, o->sha1);
                        return 0;
                }
        }
        return -1;
}

static int do_for_each_ref(const char *base, each_ref_fn fn, int trim,
                           int flags, void *cb_data)
{
        int retval = 0;
        struct ref_list *packed = get_packed_refs();
        struct ref_list *loose = get_loose_refs();

        struct ref_list *extra;

        for (extra = extra_refs; extra; extra = extra->next)
                retval = do_one_ref(base, fn, trim, flags, cb_data, extra);

        while (packed && loose) {
                struct ref_list *entry;
                int cmp = strcmp(packed->name, loose->name);
                if (!cmp) {
                        packed = packed->next;
                        continue;
                }
                if (cmp > 0) {
                        entry = loose;
                        loose = loose->next;
                } else {
                        entry = packed;
                        packed = packed->next;
                }
                retval = do_one_ref(base, fn, trim, flags, cb_data, entry);
                if (retval)
                        goto end_each;
        }

        for (packed = packed ? packed : loose; packed; packed = packed->next) {
                retval = do_one_ref(base, fn, trim, flags, cb_data, packed);
                if (retval)
                        goto end_each;
        }

end_each:
        current_ref = NULL;
        return retval;
}

int head_ref(each_ref_fn fn, void *cb_data)
{
        unsigned char sha1[20];
        int flag;

        if (resolve_ref("HEAD", sha1, 1, &flag))
                return fn("HEAD", sha1, flag, cb_data);
        return 0;
}

int for_each_ref(each_ref_fn fn, void *cb_data)
{
        return do_for_each_ref("refs/", fn, 0, 0, cb_data);
}

int for_each_ref_in(const char *prefix, each_ref_fn fn, void *cb_data)
{
        return do_for_each_ref(prefix, fn, strlen(prefix), 0, cb_data);
}

int for_each_tag_ref(each_ref_fn fn, void *cb_data)
{
        return for_each_ref_in("refs/tags/", fn, cb_data);
}

int for_each_branch_ref(each_ref_fn fn, void *cb_data)
{
        return for_each_ref_in("refs/heads/", fn, cb_data);
}

int for_each_remote_ref(each_ref_fn fn, void *cb_data)
{
        return for_each_ref_in("refs/remotes/", fn, cb_data);
}

int for_each_rawref(each_ref_fn fn, void *cb_data)
{
        return do_for_each_ref("refs/", fn, 0,
                               DO_FOR_EACH_INCLUDE_BROKEN, cb_data);
}

/*
 * Make sure "ref" is something reasonable to have under ".git/refs/";
 * We do not like it if:
 *
 * - any path component of it begins with ".", or
 * - it has double dots "..", or
 * - it has ASCII control character, "~", "^", ":" or SP, anywhere, or
 * - it ends with a "/".
 * - it ends with ".lock"
 */

static inline int bad_ref_char(int ch)
{
        if (((unsigned) ch) <= ' ' ||
            ch == '~' || ch == '^' || ch == ':')
                return 1;
        /* 2.13 Pattern Matching Notation */
        if (ch == '?' || ch == '[') /* Unsupported */
                return 1;
        if (ch == '*') /* Supported at the end */
                return 2;
        return 0;
}

int check_ref_format(const char *ref)
{
        int ch, level, bad_type, last;
        int ret = CHECK_REF_FORMAT_OK;
        const char *cp = ref;

        level = 0;
        while (1) {
                while ((ch = *cp++) == '/')
                        ; /* tolerate duplicated slashes */
                if (!ch)
                        /* should not end with slashes */
                        return CHECK_REF_FORMAT_ERROR;

                /* we are at the beginning of the path component */
                if (ch == '.')
                        return CHECK_REF_FORMAT_ERROR;
                bad_type = bad_ref_char(ch);
                if (bad_type) {
                        if (bad_type == 2 && (!*cp || *cp == '/') &&
                            ret == CHECK_REF_FORMAT_OK)
                                ret = CHECK_REF_FORMAT_WILDCARD;
                        else
                                return CHECK_REF_FORMAT_ERROR;
                }

                last = ch;
                /* scan the rest of the path component */
                while ((ch = *cp++) != 0) {
                        bad_type = bad_ref_char(ch);
                        if (bad_type)
                                return CHECK_REF_FORMAT_ERROR;
                        if (ch == '/')
                                break;
                        if (last == '.' && ch == '.')
                                return CHECK_REF_FORMAT_ERROR;
                        if (last == '@' && ch == '{')
                                return CHECK_REF_FORMAT_ERROR;
                        last = ch;
                }
                level++;
                if (!ch) {
                        if (ref <= cp - 2 && cp[-2] == '.')
                                return CHECK_REF_FORMAT_ERROR;
                        if (level < 2)
                                return CHECK_REF_FORMAT_ONELEVEL;
                        if (has_extension(ref, ".lock"))
                                return CHECK_REF_FORMAT_ERROR;
                        return ret;
                }
        }
}

const char *prettify_ref(const struct ref *ref)
{
        const char *name = ref->name;
        return name + (
                !prefixcmp(name, "refs/heads/") ? 11 :
                !prefixcmp(name, "refs/tags/") ? 10 :
                !prefixcmp(name, "refs/remotes/") ? 13 :
                0);
}

const char *ref_rev_parse_rules[] = {
        "%.*s",
        "refs/%.*s",
        "refs/tags/%.*s",
        "refs/heads/%.*s",
        "refs/remotes/%.*s",
        "refs/remotes/%.*s/HEAD",
        NULL
};

const char *ref_fetch_rules[] = {
        "%.*s",
        "refs/%.*s",
        "refs/heads/%.*s",
        NULL
};

int refname_match(const char *abbrev_name, const char *full_name, const char **rules)
{
        const char **p;
        const int abbrev_name_len = strlen(abbrev_name);

        for (p = rules; *p; p++) {
                if (!strcmp(full_name, mkpath(*p, abbrev_name_len, abbrev_name))) {
                        return 1;
                }
        }

        return 0;
}

static struct ref_lock *verify_lock(struct ref_lock *lock,
        const unsigned char *old_sha1, int mustexist)
{
        if (!resolve_ref(lock->ref_name, lock->old_sha1, mustexist, NULL)) {
                error("Can't verify ref %s", lock->ref_name);
                unlock_ref(lock);
                return NULL;
        }
        if (hashcmp(lock->old_sha1, old_sha1)) {
                error("Ref %s is at %s but expected %s", lock->ref_name,
                        sha1_to_hex(lock->old_sha1), sha1_to_hex(old_sha1));
                unlock_ref(lock);
                return NULL;
        }
        return lock;
}

static int remove_empty_directories(const char *file)
{
        /* 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.
         */
        struct strbuf path;
        int result;

        strbuf_init(&path, 20);
        strbuf_addstr(&path, file);

        result = remove_dir_recursively(&path, 1);

        strbuf_release(&path);

        return result;
}

static int is_refname_available(const char *ref, const char *oldref,
                                struct ref_list *list, int quiet)
{
        int namlen = strlen(ref); /* e.g. 'foo/bar' */
        while (list) {
                /* list->name could be 'foo' or 'foo/bar/baz' */
                if (!oldref || strcmp(oldref, list->name)) {
                        int len = strlen(list->name);
                        int cmplen = (namlen < len) ? namlen : len;
                        const char *lead = (namlen < len) ? list->name : ref;
                        if (!strncmp(ref, list->name, cmplen) &&
                            lead[cmplen] == '/') {
                                if (!quiet)
                                        error("'%s' exists; cannot create '%s'",
                                              list->name, ref);
                                return 0;
                        }
                }
                list = list->next;
        }
        return 1;
}

static struct ref_lock *lock_ref_sha1_basic(const char *ref, const unsigned char *old_sha1, int flags, int *type_p)
{
        char *ref_file;
        const char *orig_ref = ref;
        struct ref_lock *lock;
        int last_errno = 0;
        int type, lflags;
        int mustexist = (old_sha1 && !is_null_sha1(old_sha1));
        int missing = 0;

        lock = xcalloc(1, sizeof(struct ref_lock));
        lock->lock_fd = -1;

        ref = resolve_ref(ref, lock->old_sha1, mustexist, &type);
        if (!ref && 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.
                 */
                ref_file = git_path("%s", orig_ref);
                if (remove_empty_directories(ref_file)) {
                        last_errno = errno;
                        error("there are still refs under '%s'", orig_ref);
                        goto error_return;
                }
                ref = resolve_ref(orig_ref, lock->old_sha1, mustexist, &type);
        }
        if (type_p)
            *type_p = type;
        if (!ref) {
                last_errno = errno;
                error("unable to resolve reference %s: %s",
                        orig_ref, strerror(errno));
                goto error_return;
        }
        missing = is_null_sha1(lock->old_sha1);
        /* When the ref did not exist and we are creating it,
         * make sure there is no existing ref that is packed
         * whose name begins with our refname, nor a ref whose
         * name is a proper prefix of our refname.
         */
        if (missing &&
            !is_refname_available(ref, NULL, get_packed_refs(), 0))
                goto error_return;

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

        lflags = LOCK_DIE_ON_ERROR;
        if (flags & REF_NODEREF) {
                ref = orig_ref;
                lflags |= LOCK_NODEREF;
        }
        lock->ref_name = xstrdup(ref);
        lock->orig_ref_name = xstrdup(orig_ref);
        ref_file = git_path("%s", ref);
        if (missing)
                lock->force_write = 1;
        if ((flags & REF_NODEREF) && (type & REF_ISSYMREF))
                lock->force_write = 1;

        if (safe_create_leading_directories(ref_file)) {
                last_errno = errno;
                error("unable to create directory for %s", ref_file);
                goto error_return;
        }

        lock->lock_fd = hold_lock_file_for_update(lock->lk, ref_file, lflags);
        return old_sha1 ? verify_lock(lock, old_sha1, mustexist) : lock;

 error_return:
        unlock_ref(lock);
        errno = last_errno;
        return NULL;
}

struct ref_lock *lock_ref_sha1(const char *ref, const unsigned char *old_sha1)
{
        char refpath[PATH_MAX];
        if (check_ref_format(ref))
                return NULL;
        strcpy(refpath, mkpath("refs/%s", ref));
        return lock_ref_sha1_basic(refpath, old_sha1, 0, NULL);
}

struct ref_lock *lock_any_ref_for_update(const char *ref, const unsigned char *old_sha1, int flags)
{
        switch (check_ref_format(ref)) {
        default:
                return NULL;
        case 0:
        case CHECK_REF_FORMAT_ONELEVEL:
                return lock_ref_sha1_basic(ref, old_sha1, flags, NULL);
        }
}

static struct lock_file packlock;

static int repack_without_ref(const char *refname)
{
        struct ref_list *list, *packed_ref_list;
        int fd;
        int found = 0;

        packed_ref_list = get_packed_refs();
        for (list = packed_ref_list; list; list = list->next) {
                if (!strcmp(refname, list->name)) {
                        found = 1;
                        break;
                }
        }
        if (!found)
                return 0;
        fd = hold_lock_file_for_update(&packlock, git_path("packed-refs"), 0);
        if (fd < 0)
                return error("cannot delete '%s' from packed refs", refname);

        for (list = packed_ref_list; list; list = list->next) {
                char line[PATH_MAX + 100];
                int len;

                if (!strcmp(refname, list->name))
                        continue;
                len = snprintf(line, sizeof(line), "%s %s\n",
                               sha1_to_hex(list->sha1), list->name);
                /* this should not happen but just being defensive */
                if (len > sizeof(line))
                        die("too long a refname '%s'", list->name);
                write_or_die(fd, line, len);
        }
        return commit_lock_file(&packlock);
}

int delete_ref(const char *refname, const unsigned char *sha1, int delopt)
{
        struct ref_lock *lock;
        int err, i = 0, ret = 0, flag = 0;

        lock = lock_ref_sha1_basic(refname, sha1, 0, &flag);
        if (!lock)
                return 1;
        if (!(flag & REF_ISPACKED) || flag & REF_ISSYMREF) {
                /* loose */
                const char *path;

                if (!(delopt & REF_NODEREF)) {
                        i = strlen(lock->lk->filename) - 5; /* .lock */
                        lock->lk->filename[i] = 0;
                        path = lock->lk->filename;
                } else {
                        path = git_path("%s", refname);
                }
                err = unlink_or_warn(path);
                if (err && errno != ENOENT)
                        ret = 1;

                if (!(delopt & REF_NODEREF))
                        lock->lk->filename[i] = '.';
        }
        /* removing the loose one could have resurrected an earlier
         * packed one.  Also, if it was not loose we need to repack
         * without it.
         */
        ret |= repack_without_ref(refname);

        unlink_or_warn(git_path("logs/%s", lock->ref_name));
        invalidate_cached_refs();
        unlock_ref(lock);
        return ret;
}

int rename_ref(const char *oldref, const char *newref, const char *logmsg)
{
        static const char renamed_ref[] = "RENAMED-REF";
        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", oldref), &loginfo);
        const char *symref = NULL;

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

        symref = resolve_ref(oldref, orig_sha1, 1, &flag);
        if (flag & REF_ISSYMREF)
                return error("refname %s is a symbolic ref, renaming it is not supported",
                        oldref);
        if (!symref)
                return error("refname %s not found", oldref);

        if (!is_refname_available(newref, oldref, get_packed_refs(), 0))
                return 1;

        if (!is_refname_available(newref, oldref, get_loose_refs(), 0))
                return 1;

        lock = lock_ref_sha1_basic(renamed_ref, NULL, 0, NULL);
        if (!lock)
                return error("unable to lock %s", renamed_ref);
        lock->force_write = 1;
        if (write_ref_sha1(lock, orig_sha1, logmsg))
                return error("unable to save current sha1 in %s", renamed_ref);

        if (log && rename(git_path("logs/%s", oldref), git_path("tmp-renamed-log")))
                return error("unable to move logfile logs/%s to tmp-renamed-log: %s",
                        oldref, strerror(errno));

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

        if (resolve_ref(newref, sha1, 1, &flag) && delete_ref(newref, sha1, REF_NODEREF)) {
                if (errno==EISDIR) {
                        if (remove_empty_directories(git_path("%s", newref))) {
                                error("Directory not empty: %s", newref);
                                goto rollback;
                        }
                } else {
                        error("unable to delete existing %s", newref);
                        goto rollback;
                }
        }

        if (log && safe_create_leading_directories(git_path("logs/%s", newref))) {
                error("unable to create directory for %s", newref);
                goto rollback;
        }

 retry:
        if (log && rename(git_path("tmp-renamed-log"), git_path("logs/%s", newref))) {
                if (errno==EISDIR || errno==ENOTDIR) {
                        /*
                         * 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(git_path("logs/%s", newref))) {
                                error("Directory not empty: logs/%s", newref);
                                goto rollback;
                        }
                        goto retry;
                } else {
                        error("unable to move logfile tmp-renamed-log to logs/%s: %s",
                                newref, strerror(errno));
                        goto rollback;
                }
        }
        logmoved = log;

        lock = lock_ref_sha1_basic(newref, NULL, 0, NULL);
        if (!lock) {
                error("unable to lock %s for update", newref);
                goto rollback;
        }
        lock->force_write = 1;
        hashcpy(lock->old_sha1, orig_sha1);
        if (write_ref_sha1(lock, orig_sha1, logmsg)) {
                error("unable to write current sha1 into %s", newref);
                goto rollback;
        }

        return 0;

 rollback:
        lock = lock_ref_sha1_basic(oldref, NULL, 0, NULL);
        if (!lock) {
                error("unable to lock %s for rollback", oldref);
                goto rollbacklog;
        }

        lock->force_write = 1;
        flag = log_all_ref_updates;
        log_all_ref_updates = 0;
        if (write_ref_sha1(lock, orig_sha1, NULL))
                error("unable to write current sha1 into %s", oldref);
        log_all_ref_updates = flag;

 rollbacklog:
        if (logmoved && rename(git_path("logs/%s", newref), git_path("logs/%s", oldref)))
                error("unable to restore logfile %s from %s: %s",
                        oldref, newref, strerror(errno));
        if (!logmoved && log &&
            rename(git_path("tmp-renamed-log"), git_path("logs/%s", oldref)))
                error("unable to restore logfile %s from tmp-renamed-log: %s",
                        oldref, strerror(errno));

        return 1;
}

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

int commit_ref(struct ref_lock *lock)
{
        if (commit_lock_file(lock->lk))
                return -1;
        lock->lock_fd = -1;
        return 0;
}

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->orig_ref_name);
        free(lock);
}

/*
 * copy the reflog message msg to buf, which has been allocated sufficiently
 * large, while cleaning up the whitespaces.  Especially, convert LF to space,
 * because reflog file is one line per entry.
 */
static int copy_msg(char *buf, const char *msg)
{
        char *cp = buf;
        char c;
        int wasspace = 1;

        *cp++ = '\t';
        while ((c = *msg++)) {
                if (wasspace && isspace(c))
                        continue;
                wasspace = isspace(c);
                if (wasspace)
                        c = ' ';
                *cp++ = c;
        }
        while (buf < cp && isspace(cp[-1]))
                cp--;
        *cp++ = '\n';
        return cp - buf;
}

static int log_ref_write(const char *ref_name, const unsigned char *old_sha1,
                         const unsigned char *new_sha1, const char *msg)
{
        int logfd, written, oflags = O_APPEND | O_WRONLY;
        unsigned maxlen, len;
        int msglen;
        char log_file[PATH_MAX];
        char *logrec;
        const char *committer;

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

        git_snpath(log_file, sizeof(log_file), "logs/%s", ref_name);

        if (log_all_ref_updates &&
            (!prefixcmp(ref_name, "refs/heads/") ||
             !prefixcmp(ref_name, "refs/remotes/") ||
             !strcmp(ref_name, "HEAD"))) {
                if (safe_create_leading_directories(log_file) < 0)
                        return error("unable to create directory for %s",
                                     log_file);
                oflags |= O_CREAT;
        }

        logfd = open(log_file, oflags, 0666);
        if (logfd < 0) {
                if (!(oflags & O_CREAT) && errno == ENOENT)
                        return 0;

                if ((oflags & O_CREAT) && errno == EISDIR) {
                        if (remove_empty_directories(log_file)) {
                                return error("There are still logs under '%s'",
                                             log_file);
                        }
                        logfd = open(log_file, oflags, 0666);
                }

                if (logfd < 0)
                        return error("Unable to append to %s: %s",
                                     log_file, strerror(errno));
        }

        adjust_shared_perm(log_file);

        msglen = msg ? strlen(msg) : 0;
        committer = git_committer_info(0);
        maxlen = strlen(committer) + msglen + 100;
        logrec = xmalloc(maxlen);
        len = sprintf(logrec, "%s %s %s\n",
                      sha1_to_hex(old_sha1),
                      sha1_to_hex(new_sha1),
                      committer);
        if (msglen)
                len += copy_msg(logrec + len - 1, msg) - 1;
        written = len <= maxlen ? write_in_full(logfd, logrec, len) : -1;
        free(logrec);
        if (close(logfd) != 0 || written != len)
                return error("Unable to append to %s", log_file);
        return 0;
}

static int is_branch(const char *refname)
{
        return !strcmp(refname, "HEAD") || !prefixcmp(refname, "refs/heads/");
}

int write_ref_sha1(struct ref_lock *lock,
        const unsigned char *sha1, const char *logmsg)
{
        static char term = '\n';
        struct object *o;

        if (!lock)
                return -1;
        if (!lock->force_write && !hashcmp(lock->old_sha1, sha1)) {
                unlock_ref(lock);
                return 0;
        }
        o = parse_object(sha1);
        if (!o) {
                error("Trying to write ref %s with nonexistant object %s",
                        lock->ref_name, sha1_to_hex(sha1));
                unlock_ref(lock);
                return -1;
        }
        if (o->type != OBJ_COMMIT && is_branch(lock->ref_name)) {
                error("Trying to write non-commit object %s to branch %s",
                        sha1_to_hex(sha1), lock->ref_name);
                unlock_ref(lock);
                return -1;
        }
        if (write_in_full(lock->lock_fd, sha1_to_hex(sha1), 40) != 40 ||
            write_in_full(lock->lock_fd, &term, 1) != 1
                || close_ref(lock) < 0) {
                error("Couldn't write %s", lock->lk->filename);
                unlock_ref(lock);
                return -1;
        }
        invalidate_cached_refs();
        if (log_ref_write(lock->ref_name, lock->old_sha1, sha1, logmsg) < 0 ||
            (strcmp(lock->ref_name, lock->orig_ref_name) &&
             log_ref_write(lock->orig_ref_name, lock->old_sha1, sha1, logmsg) < 0)) {
                unlock_ref(lock);
                return -1;
        }
        if (strcmp(lock->orig_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("HEAD", head_sha1, 1, &head_flag);
                if (head_ref && (head_flag & REF_ISSYMREF) &&
                    !strcmp(head_ref, lock->ref_name))
                        log_ref_write("HEAD", lock->old_sha1, sha1, logmsg);
        }
        if (commit_ref(lock)) {
                error("Couldn't set %s", lock->ref_name);
                unlock_ref(lock);
                return -1;
        }
        unlock_ref(lock);
        return 0;
}

int create_symref(const char *ref_target, const char *refs_heads_master,
                  const char *logmsg)
{
        const char *lockpath;
        char ref[1000];
        int fd, len, written;
        char *git_HEAD = git_pathdup("%s", ref_target);
        unsigned char old_sha1[20], new_sha1[20];

        if (logmsg && read_ref(ref_target, old_sha1))
                hashclr(old_sha1);

        if (safe_create_leading_directories(git_HEAD) < 0)
                return error("unable to create directory for %s", git_HEAD);

#ifndef NO_SYMLINK_HEAD
        if (prefer_symlink_refs) {
                unlink(git_HEAD);
                if (!symlink(refs_heads_master, git_HEAD))
                        goto done;
                fprintf(stderr, "no symlink - falling back to symbolic ref\n");
        }
#endif

        len = snprintf(ref, sizeof(ref), "ref: %s\n", refs_heads_master);
        if (sizeof(ref) <= len) {
                error("refname too long: %s", refs_heads_master);
                goto error_free_return;
        }
        lockpath = mkpath("%s.lock", git_HEAD);
        fd = open(lockpath, O_CREAT | O_EXCL | O_WRONLY, 0666);
        if (fd < 0) {
                error("Unable to open %s for writing", lockpath);
                goto error_free_return;
        }
        written = write_in_full(fd, ref, len);
        if (close(fd) != 0 || written != len) {
                error("Unable to write to %s", lockpath);
                goto error_unlink_return;
        }
        if (rename(lockpath, git_HEAD) < 0) {
                error("Unable to create %s", git_HEAD);
                goto error_unlink_return;
        }
        if (adjust_shared_perm(git_HEAD)) {
                error("Unable to fix permissions on %s", lockpath);
        error_unlink_return:
                unlink_or_warn(lockpath);
        error_free_return:
                free(git_HEAD);
                return -1;
        }

#ifndef NO_SYMLINK_HEAD
        done:
#endif
        if (logmsg && !read_ref(refs_heads_master, new_sha1))
                log_ref_write(ref_target, old_sha1, new_sha1, logmsg);

        free(git_HEAD);
        return 0;
}

static char *ref_msg(const char *line, const char *endp)
{
        const char *ep;
        line += 82;
        ep = memchr(line, '\n', endp - line);
        if (!ep)
                ep = endp;
        return xmemdupz(line, ep - line);
}

int read_ref_at(const char *ref, unsigned long at_time, int cnt, unsigned char *sha1, char **msg, unsigned long *cutoff_time, int *cutoff_tz, int *cutoff_cnt)
{
        const char *logfile, *logdata, *logend, *rec, *lastgt, *lastrec;
        char *tz_c;
        int logfd, tz, reccnt = 0;
        struct stat st;
        unsigned long date;
        unsigned char logged_sha1[20];
        void *log_mapped;
        size_t mapsz;

        logfile = git_path("logs/%s", ref);
        logfd = open(logfile, O_RDONLY, 0);
        if (logfd < 0)
                die("Unable to read log %s: %s", logfile, strerror(errno));
        fstat(logfd, &st);
        if (!st.st_size)
                die("Log %s is empty.", logfile);
        mapsz = xsize_t(st.st_size);
        log_mapped = xmmap(NULL, mapsz, PROT_READ, MAP_PRIVATE, logfd, 0);
        logdata = log_mapped;
        close(logfd);

        lastrec = NULL;
        rec = logend = logdata + st.st_size;
        while (logdata < rec) {
                reccnt++;
                if (logdata < rec && *(rec-1) == '\n')
                        rec--;
                lastgt = NULL;
                while (logdata < rec && *(rec-1) != '\n') {
                        rec--;
                        if (*rec == '>')
                                lastgt = rec;
                }
                if (!lastgt)
                        die("Log %s is corrupt.", logfile);
                date = strtoul(lastgt + 1, &tz_c, 10);
                if (date <= at_time || cnt == 0) {
                        tz = strtoul(tz_c, NULL, 10);
                        if (msg)
                                *msg = ref_msg(rec, logend);
                        if (cutoff_time)
                                *cutoff_time = date;
                        if (cutoff_tz)
                                *cutoff_tz = tz;
                        if (cutoff_cnt)
                                *cutoff_cnt = reccnt - 1;
                        if (lastrec) {
                                if (get_sha1_hex(lastrec, logged_sha1))
                                        die("Log %s is corrupt.", logfile);
                                if (get_sha1_hex(rec + 41, sha1))
                                        die("Log %s is corrupt.", logfile);
                                if (hashcmp(logged_sha1, sha1)) {
                                        warning("Log %s has gap after %s.",
                                                logfile, show_date(date, tz, DATE_RFC2822));
                                }
                        }
                        else if (date == at_time) {
                                if (get_sha1_hex(rec + 41, sha1))
                                        die("Log %s is corrupt.", logfile);
                        }
                        else {
                                if (get_sha1_hex(rec + 41, logged_sha1))
                                        die("Log %s is corrupt.", logfile);
                                if (hashcmp(logged_sha1, sha1)) {
                                        warning("Log %s unexpectedly ended on %s.",
                                                logfile, show_date(date, tz, DATE_RFC2822));
                                }
                        }
                        munmap(log_mapped, mapsz);
                        return 0;
                }
                lastrec = rec;
                if (cnt > 0)
                        cnt--;
        }

        rec = logdata;
        while (rec < logend && *rec != '>' && *rec != '\n')
                rec++;
        if (rec == logend || *rec == '\n')
                die("Log %s is corrupt.", logfile);
        date = strtoul(rec + 1, &tz_c, 10);
        tz = strtoul(tz_c, NULL, 10);
        if (get_sha1_hex(logdata, sha1))
                die("Log %s is corrupt.", logfile);
        if (is_null_sha1(sha1)) {
                if (get_sha1_hex(logdata + 41, sha1))
                        die("Log %s is corrupt.", logfile);
        }
        if (msg)
                *msg = ref_msg(logdata, logend);
        munmap(log_mapped, mapsz);

        if (cutoff_time)
                *cutoff_time = date;
        if (cutoff_tz)
                *cutoff_tz = tz;
        if (cutoff_cnt)
                *cutoff_cnt = reccnt;
        return 1;
}

int for_each_recent_reflog_ent(const char *ref, each_reflog_ent_fn fn, long ofs, void *cb_data)
{
        const char *logfile;
        FILE *logfp;
        char buf[1024];
        int ret = 0;

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

        if (ofs) {
                struct stat statbuf;
                if (fstat(fileno(logfp), &statbuf) ||
                    statbuf.st_size < ofs ||
                    fseek(logfp, -ofs, SEEK_END) ||
                    fgets(buf, sizeof(buf), logfp))
                        return -1;
        }

        while (fgets(buf, sizeof(buf), logfp)) {
                unsigned char osha1[20], nsha1[20];
                char *email_end, *message;
                unsigned long timestamp;
                int len, tz;

                /* old SP new SP name <email> SP time TAB msg LF */
                len = strlen(buf);
                if (len < 83 || buf[len-1] != '\n' ||
                    get_sha1_hex(buf, osha1) || buf[40] != ' ' ||
                    get_sha1_hex(buf + 41, nsha1) || buf[81] != ' ' ||
                    !(email_end = strchr(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]))
                        continue; /* corrupt? */
                email_end[1] = '\0';
                tz = strtol(message + 1, NULL, 10);
                if (message[6] != '\t')
                        message += 6;
                else
                        message += 7;
                ret = fn(osha1, nsha1, buf+82, timestamp, tz, message, cb_data);
                if (ret)
                        break;
        }
        fclose(logfp);
        return ret;
}

int for_each_reflog_ent(const char *ref, each_reflog_ent_fn fn, void *cb_data)
{
        return for_each_recent_reflog_ent(ref, fn, 0, cb_data);
}

static int do_for_each_reflog(const char *base, each_ref_fn fn, void *cb_data)
{
        DIR *dir = opendir(git_path("logs/%s", base));
        int retval = 0;

        if (dir) {
                struct dirent *de;
                int baselen = strlen(base);
                char *log = xmalloc(baselen + 257);

                memcpy(log, base, baselen);
                if (baselen && base[baselen-1] != '/')
                        log[baselen++] = '/';

                while ((de = readdir(dir)) != NULL) {
                        struct stat st;
                        int namelen;

                        if (de->d_name[0] == '.')
                                continue;
                        namelen = strlen(de->d_name);
                        if (namelen > 255)
                                continue;
                        if (has_extension(de->d_name, ".lock"))
                                continue;
                        memcpy(log + baselen, de->d_name, namelen+1);
                        if (stat(git_path("logs/%s", log), &st) < 0)
                                continue;
                        if (S_ISDIR(st.st_mode)) {
                                retval = do_for_each_reflog(log, fn, cb_data);
                        } else {
                                unsigned char sha1[20];
                                if (!resolve_ref(log, sha1, 0, NULL))
                                        retval = error("bad ref for %s", log);
                                else
                                        retval = fn(log, sha1, 0, cb_data);
                        }
                        if (retval)
                                break;
                }
                free(log);
                closedir(dir);
        }
        else if (*base)
                return errno;
        return retval;
}

int for_each_reflog(each_ref_fn fn, void *cb_data)
{
        return do_for_each_reflog("", fn, cb_data);
}

int update_ref(const char *action, const char *refname,
                const unsigned char *sha1, const unsigned char *oldval,
                int flags, enum action_on_err onerr)
{
        static struct ref_lock *lock;
        lock = lock_any_ref_for_update(refname, oldval, flags);
        if (!lock) {
                const char *str = "Cannot lock the ref '%s'.";
                switch (onerr) {
                case MSG_ON_ERR: error(str, refname); break;
                case DIE_ON_ERR: die(str, refname); break;
                case QUIET_ON_ERR: break;
                }
                return 1;
        }
        if (write_ref_sha1(lock, sha1, action) < 0) {
                const char *str = "Cannot update the ref '%s'.";
                switch (onerr) {
                case MSG_ON_ERR: error(str, refname); break;
                case DIE_ON_ERR: die(str, refname); break;
                case QUIET_ON_ERR: break;
                }
                return 1;
        }
        return 0;
}

struct ref *find_ref_by_name(const struct ref *list, const char *name)
{
        for ( ; list; list = list->next)
                if (!strcmp(list->name, name))
                        return (struct ref *)list;
        return NULL;
}

/*
 * generate a format suitable for scanf from a ref_rev_parse_rules
 * rule, that is replace the "%.*s" spec with a "%s" spec
 */
static void gen_scanf_fmt(char *scanf_fmt, const char *rule)
{
        char *spec;

        spec = strstr(rule, "%.*s");
        if (!spec || strstr(spec + 4, "%.*s"))
                die("invalid rule in ref_rev_parse_rules: %s", rule);

        /* copy all until spec */
        strncpy(scanf_fmt, rule, spec - rule);
        scanf_fmt[spec - rule] = '\0';
        /* copy new spec */
        strcat(scanf_fmt, "%s");
        /* copy remaining rule */
        strcat(scanf_fmt, spec + 4);

        return;
}

char *shorten_unambiguous_ref(const char *ref, int strict)
{
        int i;
        static char **scanf_fmts;
        static int nr_rules;
        char *short_name;

        /* pre generate scanf formats from ref_rev_parse_rules[] */
        if (!nr_rules) {
                size_t total_len = 0;

                /* the rule list is NULL terminated, count them first */
                for (; ref_rev_parse_rules[nr_rules]; nr_rules++)
                        /* no +1 because strlen("%s") < strlen("%.*s") */
                        total_len += strlen(ref_rev_parse_rules[nr_rules]);

                scanf_fmts = xmalloc(nr_rules * sizeof(char *) + total_len);

                total_len = 0;
                for (i = 0; i < nr_rules; i++) {
                        scanf_fmts[i] = (char *)&scanf_fmts[nr_rules]
                                        + total_len;
                        gen_scanf_fmt(scanf_fmts[i], ref_rev_parse_rules[i]);
                        total_len += strlen(ref_rev_parse_rules[i]);
                }
        }

        /* bail out if there are no rules */
        if (!nr_rules)
                return xstrdup(ref);

        /* buffer for scanf result, at most ref must fit */
        short_name = xstrdup(ref);

        /* skip first rule, it will always match */
        for (i = nr_rules - 1; i > 0 ; --i) {
                int j;
                int rules_to_fail = i;
                int short_name_len;

                if (1 != sscanf(ref, scanf_fmts[i], short_name))
                        continue;

                short_name_len = strlen(short_name);

                /*
                 * in strict mode, all (except the matched one) rules
                 * must fail to resolve to a valid non-ambiguous ref
                 */
                if (strict)
                        rules_to_fail = nr_rules;

                /*
                 * check if the short name resolves to a valid ref,
                 * but use only rules prior to the matched one
                 */
                for (j = 0; j < rules_to_fail; j++) {
                        const char *rule = ref_rev_parse_rules[j];
                        unsigned char short_objectname[20];
                        char refname[PATH_MAX];

                        /* skip matched rule */
                        if (i == j)
                                continue;

                        /*
                         * the short name is ambiguous, if it resolves
                         * (with this previous rule) to a valid ref
                         * read_ref() returns 0 on success
                         */
                        mksnpath(refname, sizeof(refname),
                                 rule, short_name_len, short_name);
                        if (!read_ref(refname, short_objectname))
                                break;
                }

                /*
                 * short name is non-ambiguous if all previous rules
                 * haven't resolved to a valid ref
                 */
                if (j == rules_to_fail)
                        return short_name;
        }

        free(short_name);
        return xstrdup(ref);
}