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

enum mmap_strategy {
        /*
         * Don't use mmap() at all for reading `packed-refs`.
         */
        MMAP_NONE,

        /*
         * Can use mmap() for reading `packed-refs`, but the file must
         * not remain mmapped. This is the usual option on Windows,
         * where you cannot rename a new version of a file onto a file
         * that is currently mmapped.
         */
        MMAP_TEMPORARY,

        /*
         * It is OK to leave the `packed-refs` file mmapped while
         * arbitrary other code is running.
         */
        MMAP_OK
};

#if defined(NO_MMAP)
static enum mmap_strategy mmap_strategy = MMAP_NONE;
#elif defined(MMAP_PREVENTS_DELETE)
static enum mmap_strategy mmap_strategy = MMAP_TEMPORARY;
#else
static enum mmap_strategy mmap_strategy = MMAP_OK;
#endif

struct packed_ref_store;

struct packed_ref_cache {
        /*
         * A back-pointer to the packed_ref_store with which this
         * cache is associated:
         */
        struct packed_ref_store *refs;

        /* Is the `packed-refs` file currently mmapped? */
        int mmapped;

        /*
         * The contents of the `packed-refs` file. If the file was
         * already sorted, this points at the mmapped contents of the
         * file. If not, this points at heap-allocated memory
         * containing the contents, sorted. If there were no contents
         * (e.g., because the file didn't exist), `buf` and `eof` are
         * both NULL.
         */
        char *buf, *eof;

        /* The size of the header line, if any; otherwise, 0: */
        size_t header_len;

        /*
         * What is the peeled state of this cache? (This is usually
         * determined from the header of the "packed-refs" file.)
         */
        enum { PEELED_NONE, PEELED_TAGS, PEELED_FULLY } peeled;

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

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

/*
 * A container for `packed-refs`-related data. It is not (yet) a
 * `ref_store`.
 */
struct packed_ref_store {
        struct ref_store base;

        unsigned int store_flags;

        /* The path of the "packed-refs" file: */
        char *path;

        /*
         * A cache of the values read from the `packed-refs` file, if
         * it might still be current; otherwise, NULL.
         */
        struct packed_ref_cache *cache;

        /*
         * Lock used for the "packed-refs" file. Note that this (and
         * thus the enclosing `packed_ref_store`) must not be freed.
         */
        struct lock_file lock;

        /*
         * Temporary file used when rewriting new contents to the
         * "packed-refs" file. Note that this (and thus the enclosing
         * `packed_ref_store`) must not be freed.
         */
        struct tempfile tempfile;
};

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

/*
 * If the buffer in `packed_refs` is active, then either munmap the
 * memory and close the file, or free the memory. Then set the buffer
 * pointers to NULL.
 */
static void release_packed_ref_buffer(struct packed_ref_cache *packed_refs)
{
        if (packed_refs->mmapped) {
                if (munmap(packed_refs->buf,
                           packed_refs->eof - packed_refs->buf))
                        die_errno("error ummapping packed-refs file %s",
                                  packed_refs->refs->path);
                packed_refs->mmapped = 0;
        } else {
                free(packed_refs->buf);
        }
        packed_refs->buf = packed_refs->eof = NULL;
        packed_refs->header_len = 0;
}

/*
 * 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) {
                stat_validity_clear(&packed_refs->validity);
                release_packed_ref_buffer(packed_refs);
                free(packed_refs);
                return 1;
        } else {
                return 0;
        }
}

struct ref_store *packed_ref_store_create(const char *path,
                                          unsigned int store_flags)
{
        struct packed_ref_store *refs = xcalloc(1, sizeof(*refs));
        struct ref_store *ref_store = (struct ref_store *)refs;

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

        refs->path = xstrdup(path);
        return ref_store;
}

/*
 * Downcast `ref_store` to `packed_ref_store`. Die if `ref_store` is
 * not a `packed_ref_store`. Also die if `packed_ref_store` doesn't
 * support at least the flags specified in `required_flags`. `caller`
 * is used in any necessary error messages.
 */
static struct packed_ref_store *packed_downcast(struct ref_store *ref_store,
                                                unsigned int required_flags,
                                                const char *caller)
{
        struct packed_ref_store *refs;

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

        refs = (struct packed_ref_store *)ref_store;

        if ((refs->store_flags & required_flags) != required_flags)
                die("BUG: unallowed operation (%s), requires %x, has %x\n",
                    caller, required_flags, refs->store_flags);

        return refs;
}

static void clear_packed_ref_cache(struct packed_ref_store *refs)
{
        if (refs->cache) {
                struct packed_ref_cache *cache = refs->cache;

                refs->cache = NULL;
                release_packed_ref_cache(cache);
        }
}

static NORETURN void die_unterminated_line(const char *path,
                                           const char *p, size_t len)
{
        if (len < 80)
                die("unterminated line in %s: %.*s", path, (int)len, p);
        else
                die("unterminated line in %s: %.75s...", path, p);
}

static NORETURN void die_invalid_line(const char *path,
                                      const char *p, size_t len)
{
        const char *eol = memchr(p, '\n', len);

        if (!eol)
                die_unterminated_line(path, p, len);
        else if (eol - p < 80)
                die("unexpected line in %s: %.*s", path, (int)(eol - p), p);
        else
                die("unexpected line in %s: %.75s...", path, p);

}

/*
 * This value is set in `base.flags` if the peeled value of the
 * current reference is known. In that case, `peeled` 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 0x40

/*
 * An iterator over a packed-refs file that is currently mmapped.
 */
struct mmapped_ref_iterator {
        struct ref_iterator base;

        struct packed_ref_cache *packed_refs;

        /* The current position in the mmapped file: */
        const char *pos;

        /* The end of the mmapped file: */
        const char *eof;

        struct object_id oid, peeled;

        struct strbuf refname_buf;
};

static int mmapped_ref_iterator_advance(struct ref_iterator *ref_iterator)
{
        struct mmapped_ref_iterator *iter =
                (struct mmapped_ref_iterator *)ref_iterator;
        const char *p = iter->pos, *eol;

        strbuf_reset(&iter->refname_buf);

        if (iter->pos == iter->eof)
                return ref_iterator_abort(ref_iterator);

        iter->base.flags = REF_ISPACKED;

        if (iter->eof - p < GIT_SHA1_HEXSZ + 2 ||
            parse_oid_hex(p, &iter->oid, &p) ||
            !isspace(*p++))
                die_invalid_line(iter->packed_refs->refs->path,
                                 iter->pos, iter->eof - iter->pos);

        eol = memchr(p, '\n', iter->eof - p);
        if (!eol)
                die_unterminated_line(iter->packed_refs->refs->path,
                                      iter->pos, iter->eof - iter->pos);

        strbuf_add(&iter->refname_buf, p, eol - p);
        iter->base.refname = iter->refname_buf.buf;

        if (check_refname_format(iter->base.refname, REFNAME_ALLOW_ONELEVEL)) {
                if (!refname_is_safe(iter->base.refname))
                        die("packed refname is dangerous: %s",
                            iter->base.refname);
                oidclr(&iter->oid);
                iter->base.flags |= REF_BAD_NAME | REF_ISBROKEN;
        }
        if (iter->packed_refs->peeled == PEELED_FULLY ||
            (iter->packed_refs->peeled == PEELED_TAGS &&
             starts_with(iter->base.refname, "refs/tags/")))
                iter->base.flags |= REF_KNOWS_PEELED;

        iter->pos = eol + 1;

        if (iter->pos < iter->eof && *iter->pos == '^') {
                p = iter->pos + 1;
                if (iter->eof - p < GIT_SHA1_HEXSZ + 1 ||
                    parse_oid_hex(p, &iter->peeled, &p) ||
                    *p++ != '\n')
                        die_invalid_line(iter->packed_refs->refs->path,
                                         iter->pos, iter->eof - iter->pos);
                iter->pos = p;

                /*
                 * Regardless of what the file header said, we
                 * definitely know the value of *this* reference. But
                 * we suppress it if the reference is broken:
                 */
                if ((iter->base.flags & REF_ISBROKEN)) {
                        oidclr(&iter->peeled);
                        iter->base.flags &= ~REF_KNOWS_PEELED;
                } else {
                        iter->base.flags |= REF_KNOWS_PEELED;
                }
        } else {
                oidclr(&iter->peeled);
        }

        return ITER_OK;
}

static int mmapped_ref_iterator_peel(struct ref_iterator *ref_iterator,
                                    struct object_id *peeled)
{
        struct mmapped_ref_iterator *iter =
                (struct mmapped_ref_iterator *)ref_iterator;

        if ((iter->base.flags & REF_KNOWS_PEELED)) {
                oidcpy(peeled, &iter->peeled);
                return is_null_oid(&iter->peeled) ? -1 : 0;
        } else if ((iter->base.flags & (REF_ISBROKEN | REF_ISSYMREF))) {
                return -1;
        } else {
                return !!peel_object(iter->oid.hash, peeled->hash);
        }
}

static int mmapped_ref_iterator_abort(struct ref_iterator *ref_iterator)
{
        struct mmapped_ref_iterator *iter =
                (struct mmapped_ref_iterator *)ref_iterator;

        release_packed_ref_cache(iter->packed_refs);
        strbuf_release(&iter->refname_buf);
        base_ref_iterator_free(ref_iterator);
        return ITER_DONE;
}

static struct ref_iterator_vtable mmapped_ref_iterator_vtable = {
        mmapped_ref_iterator_advance,
        mmapped_ref_iterator_peel,
        mmapped_ref_iterator_abort
};

struct ref_iterator *mmapped_ref_iterator_begin(
                struct packed_ref_cache *packed_refs,
                const char *pos, const char *eof)
{
        struct mmapped_ref_iterator *iter = xcalloc(1, sizeof(*iter));
        struct ref_iterator *ref_iterator = &iter->base;

        if (!packed_refs->buf)
                return empty_ref_iterator_begin();

        base_ref_iterator_init(ref_iterator, &mmapped_ref_iterator_vtable, 1);

        iter->packed_refs = packed_refs;
        acquire_packed_ref_cache(iter->packed_refs);
        iter->pos = pos;
        iter->eof = eof;
        strbuf_init(&iter->refname_buf, 0);

        iter->base.oid = &iter->oid;

        return ref_iterator;
}

struct packed_ref_entry {
        const char *start;
        size_t len;
};

static int cmp_packed_ref_entries(const void *v1, const void *v2)
{
        const struct packed_ref_entry *e1 = v1, *e2 = v2;
        const char *r1 = e1->start + GIT_SHA1_HEXSZ + 1;
        const char *r2 = e2->start + GIT_SHA1_HEXSZ + 1;

        while (1) {
                if (*r1 == '\n')
                        return *r2 == '\n' ? 0 : -1;
                if (*r1 != *r2) {
                        if (*r2 == '\n')
                                return 1;
                        else
                                return (unsigned char)*r1 < (unsigned char)*r2 ? -1 : +1;
                }
                r1++;
                r2++;
        }
}

/*
 * Compare a packed-refs record pointed to by `rec` to the specified
 * NUL-terminated refname.
 */
static int cmp_entry_to_refname(const char *rec, const char *refname)
{
        const char *r1 = rec + GIT_SHA1_HEXSZ + 1;
        const char *r2 = refname;

        while (1) {
                if (*r1 == '\n')
                        return *r2 ? -1 : 0;
                if (!*r2)
                        return 1;
                if (*r1 != *r2)
                        return (unsigned char)*r1 < (unsigned char)*r2 ? -1 : +1;
                r1++;
                r2++;
        }
}

/*
 * `packed_refs->buf` is not known to be sorted. Check whether it is,
 * and if not, sort it into new memory and munmap/free the old
 * storage.
 */
static void sort_packed_refs(struct packed_ref_cache *packed_refs)
{
        struct packed_ref_entry *entries = NULL;
        size_t alloc = 0, nr = 0;
        int sorted = 1;
        const char *pos, *eof, *eol;
        size_t len, i;
        char *new_buffer, *dst;

        pos = packed_refs->buf + packed_refs->header_len;
        eof = packed_refs->eof;
        len = eof - pos;

        if (!len)
                return;

        /*
         * Initialize entries based on a crude estimate of the number
         * of references in the file (we'll grow it below if needed):
         */
        ALLOC_GROW(entries, len / 80 + 20, alloc);

        while (pos < eof) {
                eol = memchr(pos, '\n', eof - pos);
                if (!eol)
                        /* The safety check should prevent this. */
                        BUG("unterminated line found in packed-refs");
                if (eol - pos < GIT_SHA1_HEXSZ + 2)
                        die_invalid_line(packed_refs->refs->path,
                                         pos, eof - pos);
                eol++;
                if (eol < eof && *eol == '^') {
                        /*
                         * Keep any peeled line together with its
                         * reference:
                         */
                        const char *peeled_start = eol;

                        eol = memchr(peeled_start, '\n', eof - peeled_start);
                        if (!eol)
                                /* The safety check should prevent this. */
                                BUG("unterminated peeled line found in packed-refs");
                        eol++;
                }

                ALLOC_GROW(entries, nr + 1, alloc);
                entries[nr].start = pos;
                entries[nr].len = eol - pos;
                nr++;

                if (sorted &&
                    nr > 1 &&
                    cmp_packed_ref_entries(&entries[nr - 2],
                                           &entries[nr - 1]) >= 0)
                        sorted = 0;

                pos = eol;
        }

        if (sorted)
                goto cleanup;

        /* We need to sort the memory. First we sort the entries array: */
        QSORT(entries, nr, cmp_packed_ref_entries);

        /*
         * Allocate a new chunk of memory, and copy the old memory to
         * the new in the order indicated by `entries` (not bothering
         * with the header line):
         */
        new_buffer = xmalloc(len);
        for (dst = new_buffer, i = 0; i < nr; i++) {
                memcpy(dst, entries[i].start, entries[i].len);
                dst += entries[i].len;
        }

        /*
         * Now munmap the old buffer and use the sorted buffer in its
         * place:
         */
        release_packed_ref_buffer(packed_refs);
        packed_refs->buf = new_buffer;
        packed_refs->eof = new_buffer + len;
        packed_refs->header_len = 0;

cleanup:
        free(entries);
}

/*
 * Return a pointer to the start of the record that contains the
 * character `*p` (which must be within the buffer). If no other
 * record start is found, return `buf`.
 */
static const char *find_start_of_record(const char *buf, const char *p)
{
        while (p > buf && (p[-1] != '\n' || p[0] == '^'))
                p--;
        return p;
}

/*
 * Return a pointer to the start of the record following the record
 * that contains `*p`. If none is found before `end`, return `end`.
 */
static const char *find_end_of_record(const char *p, const char *end)
{
        while (++p < end && (p[-1] != '\n' || p[0] == '^'))
                ;
        return p;
}

/*
 * We want to be able to compare mmapped reference records quickly,
 * without totally parsing them. We can do so because the records are
 * LF-terminated, and the refname should start exactly (GIT_SHA1_HEXSZ
 * + 1) bytes past the beginning of the record.
 *
 * But what if the `packed-refs` file contains garbage? We're willing
 * to tolerate not detecting the problem, as long as we don't produce
 * totally garbled output (we can't afford to check the integrity of
 * the whole file during every Git invocation). But we do want to be
 * sure that we never read past the end of the buffer in memory and
 * perform an illegal memory access.
 *
 * Guarantee that minimum level of safety by verifying that the last
 * record in the file is LF-terminated, and that it has at least
 * (GIT_SHA1_HEXSZ + 1) characters before the LF. Die if either of
 * these checks fails.
 */
static void verify_buffer_safe(struct packed_ref_cache *packed_refs)
{
        const char *buf = packed_refs->buf + packed_refs->header_len;
        const char *eof = packed_refs->eof;
        const char *last_line;

        if (buf == eof)
                return;

        last_line = find_start_of_record(buf, eof - 1);
        if (*(eof - 1) != '\n' || eof - last_line < GIT_SHA1_HEXSZ + 2)
                die_invalid_line(packed_refs->refs->path,
                                 last_line, eof - last_line);
}

/*
 * Depending on `mmap_strategy`, either mmap or read the contents of
 * the `packed-refs` file into the `packed_refs` instance. Return 1 if
 * the file existed and was read, or 0 if the file was absent. Die on
 * errors.
 */
static int load_contents(struct packed_ref_cache *packed_refs)
{
        int fd;
        struct stat st;
        size_t size;
        ssize_t bytes_read;

        fd = open(packed_refs->refs->path, O_RDONLY);
        if (fd < 0) {
                if (errno == ENOENT) {
                        /*
                         * This is OK; it just means that no
                         * "packed-refs" file has been written yet,
                         * which is equivalent to it being empty,
                         * which is its state when initialized with
                         * zeros.
                         */
                        return 0;
                } else {
                        die_errno("couldn't read %s", packed_refs->refs->path);
                }
        }

        stat_validity_update(&packed_refs->validity, fd);

        if (fstat(fd, &st) < 0)
                die_errno("couldn't stat %s", packed_refs->refs->path);
        size = xsize_t(st.st_size);

        switch (mmap_strategy) {
        case MMAP_NONE:
                packed_refs->buf = xmalloc(size);
                bytes_read = read_in_full(fd, packed_refs->buf, size);
                if (bytes_read < 0 || bytes_read != size)
                        die_errno("couldn't read %s", packed_refs->refs->path);
                packed_refs->eof = packed_refs->buf + size;
                packed_refs->mmapped = 0;
                break;
        case MMAP_TEMPORARY:
        case MMAP_OK:
                packed_refs->buf = xmmap(NULL, size, PROT_READ, MAP_PRIVATE, fd, 0);
                packed_refs->eof = packed_refs->buf + size;
                packed_refs->mmapped = 1;
                break;
        }
        close(fd);

        return 1;
}

/*
 * Find the place in `cache->buf` where the start of the record for
 * `refname` starts. If `mustexist` is true and the reference doesn't
 * exist, then return NULL. If `mustexist` is false and the reference
 * doesn't exist, then return the point where that reference would be
 * inserted. In the latter mode, `refname` doesn't have to be a proper
 * reference name; for example, one could search for "refs/replace/"
 * to find the start of any replace references.
 *
 * The record is sought using a binary search, so `cache->buf` must be
 * sorted.
 */
static const char *find_reference_location(struct packed_ref_cache *cache,
                                           const char *refname, int mustexist)
{
        /*
         * This is not *quite* a garden-variety binary search, because
         * the data we're searching is made up of records, and we
         * always need to find the beginning of a record to do a
         * comparison. A "record" here is one line for the reference
         * itself and zero or one peel lines that start with '^'. Our
         * loop invariant is described in the next two comments.
         */

        /*
         * A pointer to the character at the start of a record whose
         * preceding records all have reference names that come
         * *before* `refname`.
         */
        const char *lo = cache->buf + cache->header_len;

        /*
         * A pointer to a the first character of a record whose
         * reference name comes *after* `refname`.
         */
        const char *hi = cache->eof;

        while (lo < hi) {
                const char *mid, *rec;
                int cmp;

                mid = lo + (hi - lo) / 2;
                rec = find_start_of_record(lo, mid);
                cmp = cmp_entry_to_refname(rec, refname);
                if (cmp < 0) {
                        lo = find_end_of_record(mid, hi);
                } else if (cmp > 0) {
                        hi = rec;
                } else {
                        return rec;
                }
        }

        if (mustexist)
                return NULL;
        else
                return lo;
}

/*
 * Read from the `packed-refs` file into a newly-allocated
 * `packed_ref_cache` and return it. The return value will already
 * have its reference count incremented.
 *
 * A comment line of the form "# pack-refs with: " may contain zero or
 * more traits. We interpret the traits as follows:
 *
 *   Neither `peeled` nor `fully-peeled`:
 *
 *      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).
 *
 *   `sorted`:
 *
 *      The references in this file are known to be sorted by refname.
 */
static struct packed_ref_cache *read_packed_refs(struct packed_ref_store *refs)
{
        struct packed_ref_cache *packed_refs = xcalloc(1, sizeof(*packed_refs));
        int sorted = 0;

        packed_refs->refs = refs;
        acquire_packed_ref_cache(packed_refs);
        packed_refs->peeled = PEELED_NONE;

        if (!load_contents(packed_refs))
                return packed_refs;

        /* If the file has a header line, process it: */
        if (packed_refs->buf < packed_refs->eof && *packed_refs->buf == '#') {
                struct strbuf tmp = STRBUF_INIT;
                char *p;
                const char *eol;
                struct string_list traits = STRING_LIST_INIT_NODUP;

                eol = memchr(packed_refs->buf, '\n',
                             packed_refs->eof - packed_refs->buf);
                if (!eol)
                        die_unterminated_line(refs->path,
                                              packed_refs->buf,
                                              packed_refs->eof - packed_refs->buf);

                strbuf_add(&tmp, packed_refs->buf, eol - packed_refs->buf);

                if (!skip_prefix(tmp.buf, "# pack-refs with:", (const char **)&p))
                        die_invalid_line(refs->path,
                                         packed_refs->buf,
                                         packed_refs->eof - packed_refs->buf);

                string_list_split_in_place(&traits, p, ' ', -1);

                if (unsorted_string_list_has_string(&traits, "fully-peeled"))
                        packed_refs->peeled = PEELED_FULLY;
                else if (unsorted_string_list_has_string(&traits, "peeled"))
                        packed_refs->peeled = PEELED_TAGS;

                sorted = unsorted_string_list_has_string(&traits, "sorted");

                /* perhaps other traits later as well */

                /* The "+ 1" is for the LF character. */
                packed_refs->header_len = eol + 1 - packed_refs->buf;

                string_list_clear(&traits, 0);
                strbuf_release(&tmp);
        }

        verify_buffer_safe(packed_refs);

        if (!sorted) {
                sort_packed_refs(packed_refs);

                /*
                 * Reordering the records might have moved a short one
                 * to the end of the buffer, so verify the buffer's
                 * safety again:
                 */
                verify_buffer_safe(packed_refs);
        }

        if (mmap_strategy != MMAP_OK && packed_refs->mmapped) {
                /*
                 * We don't want to leave the file mmapped, so we are
                 * forced to make a copy now:
                 */
                size_t size = packed_refs->eof -
                        (packed_refs->buf + packed_refs->header_len);
                char *buf_copy = xmalloc(size);

                memcpy(buf_copy, packed_refs->buf + packed_refs->header_len, size);
                release_packed_ref_buffer(packed_refs);
                packed_refs->buf = buf_copy;
                packed_refs->eof = buf_copy + size;
        }

        return packed_refs;
}

/*
 * Check that the packed refs cache (if any) still reflects the
 * contents of the file. If not, clear the cache.
 */
static void validate_packed_ref_cache(struct packed_ref_store *refs)
{
        if (refs->cache &&
            !stat_validity_check(&refs->cache->validity, refs->path))
                clear_packed_ref_cache(refs);
}

/*
 * Get the packed_ref_cache for the specified packed_ref_store,
 * creating and populating it if it hasn't been read before or if the
 * file has been changed (according to its `validity` field) since it
 * was last read. On the other hand, if we hold the lock, then assume
 * that the file hasn't been changed out from under us, so skip the
 * extra `stat()` call in `stat_validity_check()`.
 */
static struct packed_ref_cache *get_packed_ref_cache(struct packed_ref_store *refs)
{
        if (!is_lock_file_locked(&refs->lock))
                validate_packed_ref_cache(refs);

        if (!refs->cache)
                refs->cache = read_packed_refs(refs);

        return refs->cache;
}

static int packed_read_raw_ref(struct ref_store *ref_store,
                               const char *refname, unsigned char *sha1,
                               struct strbuf *referent, unsigned int *type)
{
        struct packed_ref_store *refs =
                packed_downcast(ref_store, REF_STORE_READ, "read_raw_ref");
        struct packed_ref_cache *packed_refs = get_packed_ref_cache(refs);
        const char *rec;

        *type = 0;

        rec = find_reference_location(packed_refs, refname, 1);

        if (!rec) {
                /* refname is not a packed reference. */
                errno = ENOENT;
                return -1;
        }

        if (get_sha1_hex(rec, sha1))
                die_invalid_line(refs->path, rec, packed_refs->eof - rec);

        *type = REF_ISPACKED;
        return 0;
}

struct packed_ref_iterator {
        struct ref_iterator base;

        struct packed_ref_cache *cache;
        struct ref_iterator *iter0;
        unsigned int flags;
};

static int packed_ref_iterator_advance(struct ref_iterator *ref_iterator)
{
        struct packed_ref_iterator *iter =
                (struct packed_ref_iterator *)ref_iterator;
        int ok;

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

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

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

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

        return ok;
}

static int packed_ref_iterator_peel(struct ref_iterator *ref_iterator,
                                   struct object_id *peeled)
{
        struct packed_ref_iterator *iter =
                (struct packed_ref_iterator *)ref_iterator;

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

static int packed_ref_iterator_abort(struct ref_iterator *ref_iterator)
{
        struct packed_ref_iterator *iter =
                (struct packed_ref_iterator *)ref_iterator;
        int ok = ITER_DONE;

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

        release_packed_ref_cache(iter->cache);
        base_ref_iterator_free(ref_iterator);
        return ok;
}

static struct ref_iterator_vtable packed_ref_iterator_vtable = {
        packed_ref_iterator_advance,
        packed_ref_iterator_peel,
        packed_ref_iterator_abort
};

static struct ref_iterator *packed_ref_iterator_begin(
                struct ref_store *ref_store,
                const char *prefix, unsigned int flags)
{
        struct packed_ref_store *refs;
        struct packed_ref_cache *packed_refs;
        const char *start;
        struct packed_ref_iterator *iter;
        struct ref_iterator *ref_iterator;
        unsigned int required_flags = REF_STORE_READ;

        if (!(flags & DO_FOR_EACH_INCLUDE_BROKEN))
                required_flags |= REF_STORE_ODB;
        refs = packed_downcast(ref_store, required_flags, "ref_iterator_begin");

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

        /*
         * Note that get_packed_ref_cache() internally checks whether
         * the packed-ref cache is up to date with what is on disk,
         * and re-reads it if not.
         */
        iter->cache = packed_refs = get_packed_ref_cache(refs);
        acquire_packed_ref_cache(packed_refs);

        if (prefix && *prefix)
                start = find_reference_location(packed_refs, prefix, 0);
        else
                start = packed_refs->buf + packed_refs->header_len;

        iter->iter0 = mmapped_ref_iterator_begin(packed_refs,
                                                 start, packed_refs->eof);

        iter->flags = flags;

        if (prefix && *prefix)
                /* Stop iteration after we've gone *past* prefix: */
                ref_iterator = prefix_ref_iterator_begin(ref_iterator, prefix, 0);

        return ref_iterator;
}

/*
 * 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. On
 * error, return a nonzero value and leave errno set at the value left
 * by the failing call to `fprintf()`.
 */
static int write_packed_entry(FILE *fh, const char *refname,
                              const unsigned char *sha1,
                              const unsigned char *peeled)
{
        if (fprintf(fh, "%s %s\n", sha1_to_hex(sha1), refname) < 0 ||
            (peeled && fprintf(fh, "^%s\n", sha1_to_hex(peeled)) < 0))
                return -1;

        return 0;
}

int packed_refs_lock(struct ref_store *ref_store, int flags, struct strbuf *err)
{
        struct packed_ref_store *refs =
                packed_downcast(ref_store, REF_STORE_WRITE | REF_STORE_MAIN,
                                "packed_refs_lock");
        static int timeout_configured = 0;
        static int timeout_value = 1000;

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

        /*
         * Note that we close the lockfile immediately because we
         * don't write new content to it, but rather to a separate
         * tempfile.
         */
        if (hold_lock_file_for_update_timeout(
                            &refs->lock,
                            refs->path,
                            flags, timeout_value) < 0) {
                unable_to_lock_message(refs->path, errno, err);
                return -1;
        }

        if (close_lock_file(&refs->lock)) {
                strbuf_addf(err, "unable to close %s: %s", refs->path, strerror(errno));
                return -1;
        }

        /*
         * Now that we hold the `packed-refs` lock, make sure that our
         * cache matches the current version of the file. Normally
         * `get_packed_ref_cache()` does that for us, but that
         * function assumes that when the file is locked, any existing
         * cache is still valid. We've just locked the file, but it
         * might have changed the moment *before* we locked it.
         */
        validate_packed_ref_cache(refs);

        /*
         * Now make sure that the packed-refs file as it exists in the
         * locked state is loaded into the cache:
         */
        get_packed_ref_cache(refs);
        return 0;
}

void packed_refs_unlock(struct ref_store *ref_store)
{
        struct packed_ref_store *refs = packed_downcast(
                        ref_store,
                        REF_STORE_READ | REF_STORE_WRITE,
                        "packed_refs_unlock");

        if (!is_lock_file_locked(&refs->lock))
                die("BUG: packed_refs_unlock() called when not locked");
        rollback_lock_file(&refs->lock);
}

int packed_refs_is_locked(struct ref_store *ref_store)
{
        struct packed_ref_store *refs = packed_downcast(
                        ref_store,
                        REF_STORE_READ | REF_STORE_WRITE,
                        "packed_refs_is_locked");

        return is_lock_file_locked(&refs->lock);
}

/*
 * The packed-refs header line that we write out.  Perhaps other
 * traits will be added later.
 *
 * Note that earlier versions of Git used to parse these traits by
 * looking for " trait " in the line. For this reason, the space after
 * the colon and the trailing space are required.
 */
static const char PACKED_REFS_HEADER[] =
        "# pack-refs with: peeled fully-peeled sorted \n";

static int packed_init_db(struct ref_store *ref_store, struct strbuf *err)
{
        /* Nothing to do. */
        return 0;
}

/*
 * Write the packed-refs from the cache to the packed-refs tempfile,
 * incorporating any changes from `updates`. `updates` must be a
 * sorted string list whose keys are the refnames and whose util
 * values are `struct ref_update *`. On error, rollback the tempfile,
 * write an error message to `err`, and return a nonzero value.
 *
 * The packfile must be locked before calling this function and will
 * remain locked when it is done.
 */
static int write_with_updates(struct packed_ref_store *refs,
                              struct string_list *updates,
                              struct strbuf *err)
{
        struct ref_iterator *iter = NULL;
        size_t i;
        int ok;
        FILE *out;
        struct strbuf sb = STRBUF_INIT;
        char *packed_refs_path;

        if (!is_lock_file_locked(&refs->lock))
                die("BUG: write_with_updates() called while unlocked");

        /*
         * If packed-refs is a symlink, we want to overwrite the
         * symlinked-to file, not the symlink itself. Also, put the
         * staging file next to it:
         */
        packed_refs_path = get_locked_file_path(&refs->lock);
        strbuf_addf(&sb, "%s.new", packed_refs_path);
        free(packed_refs_path);
        if (create_tempfile(&refs->tempfile, sb.buf) < 0) {
                strbuf_addf(err, "unable to create file %s: %s",
                            sb.buf, strerror(errno));
                strbuf_release(&sb);
                return -1;
        }
        strbuf_release(&sb);

        out = fdopen_tempfile(&refs->tempfile, "w");
        if (!out) {
                strbuf_addf(err, "unable to fdopen packed-refs tempfile: %s",
                            strerror(errno));
                goto error;
        }

        if (fprintf(out, "%s", PACKED_REFS_HEADER) < 0)
                goto write_error;

        /*
         * We iterate in parallel through the current list of refs and
         * the list of updates, processing an entry from at least one
         * of the lists each time through the loop. When the current
         * list of refs is exhausted, set iter to NULL. When the list
         * of updates is exhausted, leave i set to updates->nr.
         */
        iter = packed_ref_iterator_begin(&refs->base, "",
                                         DO_FOR_EACH_INCLUDE_BROKEN);
        if ((ok = ref_iterator_advance(iter)) != ITER_OK)
                iter = NULL;

        i = 0;

        while (iter || i < updates->nr) {
                struct ref_update *update = NULL;
                int cmp;

                if (i >= updates->nr) {
                        cmp = -1;
                } else {
                        update = updates->items[i].util;

                        if (!iter)
                                cmp = +1;
                        else
                                cmp = strcmp(iter->refname, update->refname);
                }

                if (!cmp) {
                        /*
                         * There is both an old value and an update
                         * for this reference. Check the old value if
                         * necessary:
                         */
                        if ((update->flags & REF_HAVE_OLD)) {
                                if (is_null_oid(&update->old_oid)) {
                                        strbuf_addf(err, "cannot update ref '%s': "
                                                    "reference already exists",
                                                    update->refname);
                                        goto error;
                                } else if (oidcmp(&update->old_oid, iter->oid)) {
                                        strbuf_addf(err, "cannot update ref '%s': "
                                                    "is at %s but expected %s",
                                                    update->refname,
                                                    oid_to_hex(iter->oid),
                                                    oid_to_hex(&update->old_oid));
                                        goto error;
                                }
                        }

                        /* Now figure out what to use for the new value: */
                        if ((update->flags & REF_HAVE_NEW)) {
                                /*
                                 * The update takes precedence. Skip
                                 * the iterator over the unneeded
                                 * value.
                                 */
                                if ((ok = ref_iterator_advance(iter)) != ITER_OK)
                                        iter = NULL;
                                cmp = +1;
                        } else {
                                /*
                                 * The update doesn't actually want to
                                 * change anything. We're done with it.
                                 */
                                i++;
                                cmp = -1;
                        }
                } else if (cmp > 0) {
                        /*
                         * There is no old value but there is an
                         * update for this reference. Make sure that
                         * the update didn't expect an existing value:
                         */
                        if ((update->flags & REF_HAVE_OLD) &&
                            !is_null_oid(&update->old_oid)) {
                                strbuf_addf(err, "cannot update ref '%s': "
                                            "reference is missing but expected %s",
                                            update->refname,
                                            oid_to_hex(&update->old_oid));
                                goto error;
                        }
                }

                if (cmp < 0) {
                        /* Pass the old reference through. */

                        struct object_id peeled;
                        int peel_error = ref_iterator_peel(iter, &peeled);

                        if (write_packed_entry(out, iter->refname,
                                               iter->oid->hash,
                                               peel_error ? NULL : peeled.hash))
                                goto write_error;

                        if ((ok = ref_iterator_advance(iter)) != ITER_OK)
                                iter = NULL;
                } else if (is_null_oid(&update->new_oid)) {
                        /*
                         * The update wants to delete the reference,
                         * and the reference either didn't exist or we
                         * have already skipped it. So we're done with
                         * the update (and don't have to write
                         * anything).
                         */
                        i++;
                } else {
                        struct object_id peeled;
                        int peel_error = peel_object(update->new_oid.hash,
                                                     peeled.hash);

                        if (write_packed_entry(out, update->refname,
                                               update->new_oid.hash,
                                               peel_error ? NULL : peeled.hash))
                                goto write_error;

                        i++;
                }
        }

        if (ok != ITER_DONE) {
                strbuf_addf(err, "unable to write packed-refs file: "
                            "error iterating over old contents");
                goto error;
        }

        if (close_tempfile(&refs->tempfile)) {
                strbuf_addf(err, "error closing file %s: %s",
                            get_tempfile_path(&refs->tempfile),
                            strerror(errno));
                strbuf_release(&sb);
                return -1;
        }

        return 0;

write_error:
        strbuf_addf(err, "error writing to %s: %s",
                    get_tempfile_path(&refs->tempfile), strerror(errno));

error:
        if (iter)
                ref_iterator_abort(iter);

        delete_tempfile(&refs->tempfile);
        return -1;
}

struct packed_transaction_backend_data {
        /* True iff the transaction owns the packed-refs lock. */
        int own_lock;

        struct string_list updates;
};

static void packed_transaction_cleanup(struct packed_ref_store *refs,
                                       struct ref_transaction *transaction)
{
        struct packed_transaction_backend_data *data = transaction->backend_data;

        if (data) {
                string_list_clear(&data->updates, 0);

                if (is_tempfile_active(&refs->tempfile))
                        delete_tempfile(&refs->tempfile);

                if (data->own_lock && is_lock_file_locked(&refs->lock)) {
                        packed_refs_unlock(&refs->base);
                        data->own_lock = 0;
                }

                free(data);
                transaction->backend_data = NULL;
        }

        transaction->state = REF_TRANSACTION_CLOSED;
}

static int packed_transaction_prepare(struct ref_store *ref_store,
                                      struct ref_transaction *transaction,
                                      struct strbuf *err)
{
        struct packed_ref_store *refs = packed_downcast(
                        ref_store,
                        REF_STORE_READ | REF_STORE_WRITE | REF_STORE_ODB,
                        "ref_transaction_prepare");
        struct packed_transaction_backend_data *data;
        size_t i;
        int ret = TRANSACTION_GENERIC_ERROR;

        /*
         * Note that we *don't* skip transactions with zero updates,
         * because such a transaction might be executed for the side
         * effect of ensuring that all of the references are peeled.
         * If the caller wants to optimize away empty transactions, it
         * should do so itself.
         */

        data = xcalloc(1, sizeof(*data));
        string_list_init(&data->updates, 0);

        transaction->backend_data = data;

        /*
         * Stick the updates in a string list by refname so that we
         * can sort them:
         */
        for (i = 0; i < transaction->nr; i++) {
                struct ref_update *update = transaction->updates[i];
                struct string_list_item *item =
                        string_list_append(&data->updates, update->refname);

                /* Store a pointer to update in item->util: */
                item->util = update;
        }
        string_list_sort(&data->updates);

        if (ref_update_reject_duplicates(&data->updates, err))
                goto failure;

        if (!is_lock_file_locked(&refs->lock)) {
                if (packed_refs_lock(ref_store, 0, err))
                        goto failure;
                data->own_lock = 1;
        }

        if (write_with_updates(refs, &data->updates, err))
                goto failure;

        transaction->state = REF_TRANSACTION_PREPARED;
        return 0;

failure:
        packed_transaction_cleanup(refs, transaction);
        return ret;
}

static int packed_transaction_abort(struct ref_store *ref_store,
                                    struct ref_transaction *transaction,
                                    struct strbuf *err)
{
        struct packed_ref_store *refs = packed_downcast(
                        ref_store,
                        REF_STORE_READ | REF_STORE_WRITE | REF_STORE_ODB,
                        "ref_transaction_abort");

        packed_transaction_cleanup(refs, transaction);
        return 0;
}

static int packed_transaction_finish(struct ref_store *ref_store,
                                     struct ref_transaction *transaction,
                                     struct strbuf *err)
{
        struct packed_ref_store *refs = packed_downcast(
                        ref_store,
                        REF_STORE_READ | REF_STORE_WRITE | REF_STORE_ODB,
                        "ref_transaction_finish");
        int ret = TRANSACTION_GENERIC_ERROR;
        char *packed_refs_path;

        clear_packed_ref_cache(refs);

        packed_refs_path = get_locked_file_path(&refs->lock);
        if (rename_tempfile(&refs->tempfile, packed_refs_path)) {
                strbuf_addf(err, "error replacing %s: %s",
                            refs->path, strerror(errno));
                goto cleanup;
        }

        ret = 0;

cleanup:
        free(packed_refs_path);
        packed_transaction_cleanup(refs, transaction);
        return ret;
}

static int packed_initial_transaction_commit(struct ref_store *ref_store,
                                            struct ref_transaction *transaction,
                                            struct strbuf *err)
{
        return ref_transaction_commit(transaction, err);
}

static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
                             struct string_list *refnames, unsigned int flags)
{
        struct packed_ref_store *refs =
                packed_downcast(ref_store, REF_STORE_WRITE, "delete_refs");
        struct strbuf err = STRBUF_INIT;
        struct ref_transaction *transaction;
        struct string_list_item *item;
        int ret;

        (void)refs; /* We need the check above, but don't use the variable */

        if (!refnames->nr)
                return 0;

        /*
         * Since we don't check the references' old_oids, the
         * individual updates can't fail, so we can pack all of the
         * updates into a single transaction.
         */

        transaction = ref_store_transaction_begin(ref_store, &err);
        if (!transaction)
                return -1;

        for_each_string_list_item(item, refnames) {
                if (ref_transaction_delete(transaction, item->string, NULL,
                                           flags, msg, &err)) {
                        warning(_("could not delete reference %s: %s"),
                                item->string, err.buf);
                        strbuf_reset(&err);
                }
        }

        ret = ref_transaction_commit(transaction, &err);

        if (ret) {
                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);
        }

        ref_transaction_free(transaction);
        strbuf_release(&err);
        return ret;
}

static int packed_pack_refs(struct ref_store *ref_store, unsigned int flags)
{
        /*
         * Packed refs are already packed. It might be that loose refs
         * are packed *into* a packed refs store, but that is done by
         * updating the packed references via a transaction.
         */
        return 0;
}

static int packed_create_symref(struct ref_store *ref_store,
                               const char *refname, const char *target,
                               const char *logmsg)
{
        die("BUG: packed reference store does not support symrefs");
}

static int packed_rename_ref(struct ref_store *ref_store,
                            const char *oldrefname, const char *newrefname,
                            const char *logmsg)
{
        die("BUG: packed reference store does not support renaming references");
}

static struct ref_iterator *packed_reflog_iterator_begin(struct ref_store *ref_store)
{
        return empty_ref_iterator_begin();
}

static int packed_for_each_reflog_ent(struct ref_store *ref_store,
                                      const char *refname,
                                      each_reflog_ent_fn fn, void *cb_data)
{
        return 0;
}

static int packed_for_each_reflog_ent_reverse(struct ref_store *ref_store,
                                              const char *refname,
                                              each_reflog_ent_fn fn,
                                              void *cb_data)
{
        return 0;
}

static int packed_reflog_exists(struct ref_store *ref_store,
                               const char *refname)
{
        return 0;
}

static int packed_create_reflog(struct ref_store *ref_store,
                               const char *refname, int force_create,
                               struct strbuf *err)
{
        die("BUG: packed reference store does not support reflogs");
}

static int packed_delete_reflog(struct ref_store *ref_store,
                               const char *refname)
{
        return 0;
}

static int packed_reflog_expire(struct ref_store *ref_store,
                                const char *refname, const unsigned char *sha1,
                                unsigned int flags,
                                reflog_expiry_prepare_fn prepare_fn,
                                reflog_expiry_should_prune_fn should_prune_fn,
                                reflog_expiry_cleanup_fn cleanup_fn,
                                void *policy_cb_data)
{
        return 0;
}

struct ref_storage_be refs_be_packed = {
        NULL,
        "packed",
        packed_ref_store_create,
        packed_init_db,
        packed_transaction_prepare,
        packed_transaction_finish,
        packed_transaction_abort,
        packed_initial_transaction_commit,

        packed_pack_refs,
        packed_create_symref,
        packed_delete_refs,
        packed_rename_ref,

        packed_ref_iterator_begin,
        packed_read_raw_ref,

        packed_reflog_iterator_begin,
        packed_for_each_reflog_ent,
        packed_for_each_reflog_ent_reverse,
        packed_reflog_exists,
        packed_create_reflog,
        packed_delete_reflog,
        packed_reflog_expire
};