Sync with 'maint'

[git/gitster.git] / midx-write.c

#define USE_THE_REPOSITORY_VARIABLE

#include "git-compat-util.h"
#include "abspath.h"
#include "config.h"
#include "hex.h"
#include "lockfile.h"
#include "packfile.h"
#include "object-file.h"
#include "hash-lookup.h"
#include "midx.h"
#include "progress.h"
#include "trace2.h"
#include "run-command.h"
#include "chunk-format.h"
#include "pack-bitmap.h"
#include "refs.h"
#include "revision.h"
#include "list-objects.h"
#include "path.h"
#include "pack-revindex.h"

#define PACK_EXPIRED UINT_MAX
#define BITMAP_POS_UNKNOWN (~((uint32_t)0))
#define MIDX_CHUNK_FANOUT_SIZE (sizeof(uint32_t) * 256)
#define MIDX_CHUNK_LARGE_OFFSET_WIDTH (sizeof(uint64_t))

extern int midx_checksum_valid(struct multi_pack_index *m);
extern void clear_midx_files_ext(const char *object_dir, const char *ext,
                                 const char *keep_hash);
extern void clear_incremental_midx_files_ext(const char *object_dir,
                                             const char *ext,
                                             const char **keep_hashes,
                                             uint32_t hashes_nr);
extern int cmp_idx_or_pack_name(const char *idx_or_pack_name,
                                const char *idx_name);

static size_t write_midx_header(struct hashfile *f,
                                unsigned char num_chunks,
                                uint32_t num_packs)
{
        hashwrite_be32(f, MIDX_SIGNATURE);
        hashwrite_u8(f, MIDX_VERSION);
        hashwrite_u8(f, oid_version(the_hash_algo));
        hashwrite_u8(f, num_chunks);
        hashwrite_u8(f, 0); /* unused */
        hashwrite_be32(f, num_packs);

        return MIDX_HEADER_SIZE;
}

struct pack_info {
        uint32_t orig_pack_int_id;
        char *pack_name;
        struct packed_git *p;

        uint32_t bitmap_pos;
        uint32_t bitmap_nr;

        unsigned expired : 1;
};

static void fill_pack_info(struct pack_info *info,
                           struct packed_git *p, const char *pack_name,
                           uint32_t orig_pack_int_id)
{
        memset(info, 0, sizeof(struct pack_info));

        info->orig_pack_int_id = orig_pack_int_id;
        info->pack_name = xstrdup(pack_name);
        info->p = p;
        info->bitmap_pos = BITMAP_POS_UNKNOWN;
}

static int pack_info_compare(const void *_a, const void *_b)
{
        struct pack_info *a = (struct pack_info *)_a;
        struct pack_info *b = (struct pack_info *)_b;
        return strcmp(a->pack_name, b->pack_name);
}

static int idx_or_pack_name_cmp(const void *_va, const void *_vb)
{
        const char *pack_name = _va;
        const struct pack_info *compar = _vb;

        return cmp_idx_or_pack_name(pack_name, compar->pack_name);
}

struct write_midx_context {
        struct pack_info *info;
        size_t nr;
        size_t alloc;
        struct multi_pack_index *m;
        struct multi_pack_index *base_midx;
        struct progress *progress;
        unsigned pack_paths_checked;

        struct pack_midx_entry *entries;
        size_t entries_nr;

        uint32_t *pack_perm;
        uint32_t *pack_order;
        unsigned large_offsets_needed:1;
        uint32_t num_large_offsets;

        int preferred_pack_idx;

        int incremental;
        uint32_t num_multi_pack_indexes_before;

        struct string_list *to_include;
};

static int should_include_pack(const struct write_midx_context *ctx,
                               const char *file_name)
{
        /*
         * Note that at most one of ctx->m and ctx->to_include are set,
         * so we are testing midx_contains_pack() and
         * string_list_has_string() independently (guarded by the
         * appropriate NULL checks).
         *
         * We could support passing to_include while reusing an existing
         * MIDX, but don't currently since the reuse process drags
         * forward all packs from an existing MIDX (without checking
         * whether or not they appear in the to_include list).
         *
         * If we added support for that, these next two conditional
         * should be performed independently (likely checking
         * to_include before the existing MIDX).
         */
        if (ctx->m && midx_contains_pack(ctx->m, file_name))
                return 0;
        else if (ctx->base_midx && midx_contains_pack(ctx->base_midx,
                                                      file_name))
                return 0;
        else if (ctx->to_include &&
                 !string_list_has_string(ctx->to_include, file_name))
                return 0;
        return 1;
}

static void add_pack_to_midx(const char *full_path, size_t full_path_len,
                             const char *file_name, void *data)
{
        struct write_midx_context *ctx = data;
        struct packed_git *p;

        if (ends_with(file_name, ".idx")) {
                display_progress(ctx->progress, ++ctx->pack_paths_checked);

                if (!should_include_pack(ctx, file_name))
                        return;

                ALLOC_GROW(ctx->info, ctx->nr + 1, ctx->alloc);
                p = add_packed_git(full_path, full_path_len, 0);
                if (!p) {
                        warning(_("failed to add packfile '%s'"),
                                full_path);
                        return;
                }

                if (open_pack_index(p)) {
                        warning(_("failed to open pack-index '%s'"),
                                full_path);
                        close_pack(p);
                        free(p);
                        return;
                }

                fill_pack_info(&ctx->info[ctx->nr], p, file_name, ctx->nr);
                ctx->nr++;
        }
}

struct pack_midx_entry {
        struct object_id oid;
        uint32_t pack_int_id;
        time_t pack_mtime;
        uint64_t offset;
        unsigned preferred : 1;
};

static int midx_oid_compare(const void *_a, const void *_b)
{
        const struct pack_midx_entry *a = (const struct pack_midx_entry *)_a;
        const struct pack_midx_entry *b = (const struct pack_midx_entry *)_b;
        int cmp = oidcmp(&a->oid, &b->oid);

        if (cmp)
                return cmp;

        /* Sort objects in a preferred pack first when multiple copies exist. */
        if (a->preferred > b->preferred)
                return -1;
        if (a->preferred < b->preferred)
                return 1;

        if (a->pack_mtime > b->pack_mtime)
                return -1;
        else if (a->pack_mtime < b->pack_mtime)
                return 1;

        return a->pack_int_id - b->pack_int_id;
}

static int nth_midxed_pack_midx_entry(struct multi_pack_index *m,
                                      struct pack_midx_entry *e,
                                      uint32_t pos)
{
        if (pos >= m->num_objects + m->num_objects_in_base)
                return 1;

        nth_midxed_object_oid(&e->oid, m, pos);
        e->pack_int_id = nth_midxed_pack_int_id(m, pos);
        e->offset = nth_midxed_offset(m, pos);

        /* consider objects in midx to be from "old" packs */
        e->pack_mtime = 0;
        return 0;
}

static void fill_pack_entry(uint32_t pack_int_id,
                            struct packed_git *p,
                            uint32_t cur_object,
                            struct pack_midx_entry *entry,
                            int preferred)
{
        if (nth_packed_object_id(&entry->oid, p, cur_object) < 0)
                die(_("failed to locate object %d in packfile"), cur_object);

        entry->pack_int_id = pack_int_id;
        entry->pack_mtime = p->mtime;

        entry->offset = nth_packed_object_offset(p, cur_object);
        entry->preferred = !!preferred;
}

struct midx_fanout {
        struct pack_midx_entry *entries;
        size_t nr, alloc;
};

static void midx_fanout_grow(struct midx_fanout *fanout, size_t nr)
{
        if (nr < fanout->nr)
                BUG("negative growth in midx_fanout_grow() (%"PRIuMAX" < %"PRIuMAX")",
                    (uintmax_t)nr, (uintmax_t)fanout->nr);
        ALLOC_GROW(fanout->entries, nr, fanout->alloc);
}

static void midx_fanout_sort(struct midx_fanout *fanout)
{
        QSORT(fanout->entries, fanout->nr, midx_oid_compare);
}

static void midx_fanout_add_midx_fanout(struct midx_fanout *fanout,
                                        struct multi_pack_index *m,
                                        uint32_t cur_fanout,
                                        int preferred_pack)
{
        uint32_t start = m->num_objects_in_base, end;
        uint32_t cur_object;

        if (m->base_midx)
                midx_fanout_add_midx_fanout(fanout, m->base_midx, cur_fanout,
                                            preferred_pack);

        if (cur_fanout)
                start += ntohl(m->chunk_oid_fanout[cur_fanout - 1]);
        end = m->num_objects_in_base + ntohl(m->chunk_oid_fanout[cur_fanout]);

        for (cur_object = start; cur_object < end; cur_object++) {
                if ((preferred_pack > -1) &&
                    (preferred_pack == nth_midxed_pack_int_id(m, cur_object))) {
                        /*
                         * Objects from preferred packs are added
                         * separately.
                         */
                        continue;
                }

                midx_fanout_grow(fanout, fanout->nr + 1);
                nth_midxed_pack_midx_entry(m,
                                           &fanout->entries[fanout->nr],
                                           cur_object);
                fanout->entries[fanout->nr].preferred = 0;
                fanout->nr++;
        }
}

static void midx_fanout_add_pack_fanout(struct midx_fanout *fanout,
                                        struct pack_info *info,
                                        uint32_t cur_pack,
                                        int preferred,
                                        uint32_t cur_fanout)
{
        struct packed_git *pack = info[cur_pack].p;
        uint32_t start = 0, end;
        uint32_t cur_object;

        if (cur_fanout)
                start = get_pack_fanout(pack, cur_fanout - 1);
        end = get_pack_fanout(pack, cur_fanout);

        for (cur_object = start; cur_object < end; cur_object++) {
                midx_fanout_grow(fanout, fanout->nr + 1);
                fill_pack_entry(cur_pack,
                                info[cur_pack].p,
                                cur_object,
                                &fanout->entries[fanout->nr],
                                preferred);
                fanout->nr++;
        }
}

/*
 * It is possible to artificially get into a state where there are many
 * duplicate copies of objects. That can create high memory pressure if
 * we are to create a list of all objects before de-duplication. To reduce
 * this memory pressure without a significant performance drop, automatically
 * group objects by the first byte of their object id. Use the IDX fanout
 * tables to group the data, copy to a local array, then sort.
 *
 * Copy only the de-duplicated entries (selected by most-recent modified time
 * of a packfile containing the object).
 */
static void compute_sorted_entries(struct write_midx_context *ctx,
                                   uint32_t start_pack)
{
        uint32_t cur_fanout, cur_pack, cur_object;
        size_t alloc_objects, total_objects = 0;
        struct midx_fanout fanout = { 0 };

        for (cur_pack = start_pack; cur_pack < ctx->nr; cur_pack++)
                total_objects = st_add(total_objects,
                                       ctx->info[cur_pack].p->num_objects);

        /*
         * As we de-duplicate by fanout value, we expect the fanout
         * slices to be evenly distributed, with some noise. Hence,
         * allocate slightly more than one 256th.
         */
        alloc_objects = fanout.alloc = total_objects > 3200 ? total_objects / 200 : 16;

        ALLOC_ARRAY(fanout.entries, fanout.alloc);
        ALLOC_ARRAY(ctx->entries, alloc_objects);
        ctx->entries_nr = 0;

        for (cur_fanout = 0; cur_fanout < 256; cur_fanout++) {
                fanout.nr = 0;

                if (ctx->m && !ctx->incremental)
                        midx_fanout_add_midx_fanout(&fanout, ctx->m, cur_fanout,
                                                    ctx->preferred_pack_idx);

                for (cur_pack = start_pack; cur_pack < ctx->nr; cur_pack++) {
                        int preferred = cur_pack == ctx->preferred_pack_idx;
                        midx_fanout_add_pack_fanout(&fanout,
                                                    ctx->info, cur_pack,
                                                    preferred, cur_fanout);
                }

                if (-1 < ctx->preferred_pack_idx && ctx->preferred_pack_idx < start_pack)
                        midx_fanout_add_pack_fanout(&fanout, ctx->info,
                                                    ctx->preferred_pack_idx, 1,
                                                    cur_fanout);

                midx_fanout_sort(&fanout);

                /*
                 * The batch is now sorted by OID and then mtime (descending).
                 * Take only the first duplicate.
                 */
                for (cur_object = 0; cur_object < fanout.nr; cur_object++) {
                        if (cur_object && oideq(&fanout.entries[cur_object - 1].oid,
                                                &fanout.entries[cur_object].oid))
                                continue;
                        if (ctx->incremental && ctx->base_midx &&
                            midx_has_oid(ctx->base_midx,
                                         &fanout.entries[cur_object].oid))
                                continue;

                        ALLOC_GROW(ctx->entries, st_add(ctx->entries_nr, 1),
                                   alloc_objects);
                        memcpy(&ctx->entries[ctx->entries_nr],
                               &fanout.entries[cur_object],
                               sizeof(struct pack_midx_entry));
                        ctx->entries_nr++;
                }
        }

        free(fanout.entries);
}

static int write_midx_pack_names(struct hashfile *f, void *data)
{
        struct write_midx_context *ctx = data;
        uint32_t i;
        unsigned char padding[MIDX_CHUNK_ALIGNMENT];
        size_t written = 0;

        for (i = 0; i < ctx->nr; i++) {
                size_t writelen;

                if (ctx->info[i].expired)
                        continue;

                if (i && strcmp(ctx->info[i].pack_name, ctx->info[i - 1].pack_name) <= 0)
                        BUG("incorrect pack-file order: %s before %s",
                            ctx->info[i - 1].pack_name,
                            ctx->info[i].pack_name);

                writelen = strlen(ctx->info[i].pack_name) + 1;
                hashwrite(f, ctx->info[i].pack_name, writelen);
                written += writelen;
        }

        /* add padding to be aligned */
        i = MIDX_CHUNK_ALIGNMENT - (written % MIDX_CHUNK_ALIGNMENT);
        if (i < MIDX_CHUNK_ALIGNMENT) {
                memset(padding, 0, sizeof(padding));
                hashwrite(f, padding, i);
        }

        return 0;
}

static int write_midx_bitmapped_packs(struct hashfile *f, void *data)
{
        struct write_midx_context *ctx = data;
        size_t i;

        for (i = 0; i < ctx->nr; i++) {
                struct pack_info *pack = &ctx->info[i];
                if (pack->expired)
                        continue;

                if (pack->bitmap_pos == BITMAP_POS_UNKNOWN && pack->bitmap_nr)
                        BUG("pack '%s' has no bitmap position, but has %d bitmapped object(s)",
                            pack->pack_name, pack->bitmap_nr);

                hashwrite_be32(f, pack->bitmap_pos);
                hashwrite_be32(f, pack->bitmap_nr);
        }
        return 0;
}

static int write_midx_oid_fanout(struct hashfile *f,
                                 void *data)
{
        struct write_midx_context *ctx = data;
        struct pack_midx_entry *list = ctx->entries;
        struct pack_midx_entry *last = ctx->entries + ctx->entries_nr;
        uint32_t count = 0;
        uint32_t i;

        /*
        * Write the first-level table (the list is sorted,
        * but we use a 256-entry lookup to be able to avoid
        * having to do eight extra binary search iterations).
        */
        for (i = 0; i < 256; i++) {
                struct pack_midx_entry *next = list;

                while (next < last && next->oid.hash[0] == i) {
                        count++;
                        next++;
                }

                hashwrite_be32(f, count);
                list = next;
        }

        return 0;
}

static int write_midx_oid_lookup(struct hashfile *f,
                                 void *data)
{
        struct write_midx_context *ctx = data;
        unsigned char hash_len = the_hash_algo->rawsz;
        struct pack_midx_entry *list = ctx->entries;
        uint32_t i;

        for (i = 0; i < ctx->entries_nr; i++) {
                struct pack_midx_entry *obj = list++;

                if (i < ctx->entries_nr - 1) {
                        struct pack_midx_entry *next = list;
                        if (oidcmp(&obj->oid, &next->oid) >= 0)
                                BUG("OIDs not in order: %s >= %s",
                                    oid_to_hex(&obj->oid),
                                    oid_to_hex(&next->oid));
                }

                hashwrite(f, obj->oid.hash, (int)hash_len);
        }

        return 0;
}

static int write_midx_object_offsets(struct hashfile *f,
                                     void *data)
{
        struct write_midx_context *ctx = data;
        struct pack_midx_entry *list = ctx->entries;
        uint32_t i, nr_large_offset = 0;

        for (i = 0; i < ctx->entries_nr; i++) {
                struct pack_midx_entry *obj = list++;

                if (ctx->pack_perm[obj->pack_int_id] == PACK_EXPIRED)
                        BUG("object %s is in an expired pack with int-id %d",
                            oid_to_hex(&obj->oid),
                            obj->pack_int_id);

                hashwrite_be32(f, ctx->pack_perm[obj->pack_int_id]);

                if (ctx->large_offsets_needed && obj->offset >> 31)
                        hashwrite_be32(f, MIDX_LARGE_OFFSET_NEEDED | nr_large_offset++);
                else if (!ctx->large_offsets_needed && obj->offset >> 32)
                        BUG("object %s requires a large offset (%"PRIx64") but the MIDX is not writing large offsets!",
                            oid_to_hex(&obj->oid),
                            obj->offset);
                else
                        hashwrite_be32(f, (uint32_t)obj->offset);
        }

        return 0;
}

static int write_midx_large_offsets(struct hashfile *f,
                                    void *data)
{
        struct write_midx_context *ctx = data;
        struct pack_midx_entry *list = ctx->entries;
        struct pack_midx_entry *end = ctx->entries + ctx->entries_nr;
        uint32_t nr_large_offset = ctx->num_large_offsets;

        while (nr_large_offset) {
                struct pack_midx_entry *obj;
                uint64_t offset;

                if (list >= end)
                        BUG("too many large-offset objects");

                obj = list++;
                offset = obj->offset;

                if (!(offset >> 31))
                        continue;

                hashwrite_be64(f, offset);

                nr_large_offset--;
        }

        return 0;
}

static int write_midx_revindex(struct hashfile *f,
                               void *data)
{
        struct write_midx_context *ctx = data;
        uint32_t i, nr_base;

        if (ctx->incremental && ctx->base_midx)
                nr_base = ctx->base_midx->num_objects +
                        ctx->base_midx->num_objects_in_base;
        else
                nr_base = 0;

        for (i = 0; i < ctx->entries_nr; i++)
                hashwrite_be32(f, ctx->pack_order[i] + nr_base);

        return 0;
}

struct midx_pack_order_data {
        uint32_t nr;
        uint32_t pack;
        off_t offset;
};

static int midx_pack_order_cmp(const void *va, const void *vb)
{
        const struct midx_pack_order_data *a = va, *b = vb;
        if (a->pack < b->pack)
                return -1;
        else if (a->pack > b->pack)
                return 1;
        else if (a->offset < b->offset)
                return -1;
        else if (a->offset > b->offset)
                return 1;
        else
                return 0;
}

static uint32_t *midx_pack_order(struct write_midx_context *ctx)
{
        struct midx_pack_order_data *data;
        uint32_t *pack_order, base_objects = 0;
        uint32_t i;

        trace2_region_enter("midx", "midx_pack_order", the_repository);

        if (ctx->incremental && ctx->base_midx)
                base_objects = ctx->base_midx->num_objects +
                        ctx->base_midx->num_objects_in_base;

        ALLOC_ARRAY(pack_order, ctx->entries_nr);
        ALLOC_ARRAY(data, ctx->entries_nr);

        for (i = 0; i < ctx->entries_nr; i++) {
                struct pack_midx_entry *e = &ctx->entries[i];
                data[i].nr = i;
                data[i].pack = ctx->pack_perm[e->pack_int_id];
                if (!e->preferred)
                        data[i].pack |= (1U << 31);
                data[i].offset = e->offset;
        }

        QSORT(data, ctx->entries_nr, midx_pack_order_cmp);

        for (i = 0; i < ctx->entries_nr; i++) {
                struct pack_midx_entry *e = &ctx->entries[data[i].nr];
                struct pack_info *pack = &ctx->info[ctx->pack_perm[e->pack_int_id]];
                if (pack->bitmap_pos == BITMAP_POS_UNKNOWN)
                        pack->bitmap_pos = i + base_objects;
                pack->bitmap_nr++;
                pack_order[i] = data[i].nr;
        }
        for (i = 0; i < ctx->nr; i++) {
                struct pack_info *pack = &ctx->info[ctx->pack_perm[i]];
                if (pack->bitmap_pos == BITMAP_POS_UNKNOWN)
                        pack->bitmap_pos = 0;
        }
        free(data);

        trace2_region_leave("midx", "midx_pack_order", the_repository);

        return pack_order;
}

static void write_midx_reverse_index(char *midx_name, unsigned char *midx_hash,
                                     struct write_midx_context *ctx)
{
        struct strbuf buf = STRBUF_INIT;
        const char *tmp_file;

        trace2_region_enter("midx", "write_midx_reverse_index", the_repository);

        strbuf_addf(&buf, "%s-%s.rev", midx_name, hash_to_hex(midx_hash));

        tmp_file = write_rev_file_order(NULL, ctx->pack_order, ctx->entries_nr,
                                        midx_hash, WRITE_REV);

        if (finalize_object_file(tmp_file, buf.buf))
                die(_("cannot store reverse index file"));

        strbuf_release(&buf);

        trace2_region_leave("midx", "write_midx_reverse_index", the_repository);
}

static void prepare_midx_packing_data(struct packing_data *pdata,
                                      struct write_midx_context *ctx)
{
        uint32_t i;

        trace2_region_enter("midx", "prepare_midx_packing_data", the_repository);

        memset(pdata, 0, sizeof(struct packing_data));
        prepare_packing_data(the_repository, pdata);

        for (i = 0; i < ctx->entries_nr; i++) {
                uint32_t pos = ctx->pack_order[i];
                struct pack_midx_entry *from = &ctx->entries[pos];
                struct object_entry *to = packlist_alloc(pdata, &from->oid);

                oe_set_in_pack(pdata, to,
                               ctx->info[ctx->pack_perm[from->pack_int_id]].p);
        }

        trace2_region_leave("midx", "prepare_midx_packing_data", the_repository);
}

static int add_ref_to_pending(const char *refname, const char *referent UNUSED,
                              const struct object_id *oid,
                              int flag, void *cb_data)
{
        struct rev_info *revs = (struct rev_info*)cb_data;
        struct object_id peeled;
        struct object *object;

        if ((flag & REF_ISSYMREF) && (flag & REF_ISBROKEN)) {
                warning("symbolic ref is dangling: %s", refname);
                return 0;
        }

        if (!peel_iterated_oid(the_repository, oid, &peeled))
                oid = &peeled;

        object = parse_object_or_die(oid, refname);
        if (object->type != OBJ_COMMIT)
                return 0;

        add_pending_object(revs, object, "");
        if (bitmap_is_preferred_refname(revs->repo, refname))
                object->flags |= NEEDS_BITMAP;
        return 0;
}

struct bitmap_commit_cb {
        struct commit **commits;
        size_t commits_nr, commits_alloc;

        struct write_midx_context *ctx;
};

static const struct object_id *bitmap_oid_access(size_t index,
                                                 const void *_entries)
{
        const struct pack_midx_entry *entries = _entries;
        return &entries[index].oid;
}

static void bitmap_show_commit(struct commit *commit, void *_data)
{
        struct bitmap_commit_cb *data = _data;
        int pos = oid_pos(&commit->object.oid, data->ctx->entries,
                          data->ctx->entries_nr,
                          bitmap_oid_access);
        if (pos < 0)
                return;

        ALLOC_GROW(data->commits, data->commits_nr + 1, data->commits_alloc);
        data->commits[data->commits_nr++] = commit;
}

static int read_refs_snapshot(const char *refs_snapshot,
                              struct rev_info *revs)
{
        struct strbuf buf = STRBUF_INIT;
        struct object_id oid;
        FILE *f = xfopen(refs_snapshot, "r");

        while (strbuf_getline(&buf, f) != EOF) {
                struct object *object;
                int preferred = 0;
                char *hex = buf.buf;
                const char *end = NULL;

                if (buf.len && *buf.buf == '+') {
                        preferred = 1;
                        hex = &buf.buf[1];
                }

                if (parse_oid_hex(hex, &oid, &end) < 0)
                        die(_("could not parse line: %s"), buf.buf);
                if (*end)
                        die(_("malformed line: %s"), buf.buf);

                object = parse_object_or_die(&oid, NULL);
                if (preferred)
                        object->flags |= NEEDS_BITMAP;

                add_pending_object(revs, object, "");
        }

        fclose(f);
        strbuf_release(&buf);
        return 0;
}
static struct commit **find_commits_for_midx_bitmap(uint32_t *indexed_commits_nr_p,
                                                    const char *refs_snapshot,
                                                    struct write_midx_context *ctx)
{
        struct rev_info revs;
        struct bitmap_commit_cb cb = {0};

        trace2_region_enter("midx", "find_commits_for_midx_bitmap",
                            the_repository);

        cb.ctx = ctx;

        repo_init_revisions(the_repository, &revs, NULL);
        if (refs_snapshot) {
                read_refs_snapshot(refs_snapshot, &revs);
        } else {
                setup_revisions(0, NULL, &revs, NULL);
                refs_for_each_ref(get_main_ref_store(the_repository),
                                  add_ref_to_pending, &revs);
        }

        /*
         * Skipping promisor objects here is intentional, since it only excludes
         * them from the list of reachable commits that we want to select from
         * when computing the selection of MIDX'd commits to receive bitmaps.
         *
         * Reachability bitmaps do require that their objects be closed under
         * reachability, but fetching any objects missing from promisors at this
         * point is too late. But, if one of those objects can be reached from
         * an another object that is included in the bitmap, then we will
         * complain later that we don't have reachability closure (and fail
         * appropriately).
         */
        fetch_if_missing = 0;
        revs.exclude_promisor_objects = 1;

        if (prepare_revision_walk(&revs))
                die(_("revision walk setup failed"));

        traverse_commit_list(&revs, bitmap_show_commit, NULL, &cb);
        if (indexed_commits_nr_p)
                *indexed_commits_nr_p = cb.commits_nr;

        release_revisions(&revs);

        trace2_region_leave("midx", "find_commits_for_midx_bitmap",
                            the_repository);

        return cb.commits;
}

static int write_midx_bitmap(const char *midx_name,
                             const unsigned char *midx_hash,
                             struct packing_data *pdata,
                             struct commit **commits,
                             uint32_t commits_nr,
                             uint32_t *pack_order,
                             unsigned flags)
{
        int ret, i;
        uint16_t options = 0;
        struct bitmap_writer writer;
        struct pack_idx_entry **index;
        char *bitmap_name = xstrfmt("%s-%s.bitmap", midx_name,
                                        hash_to_hex(midx_hash));

        trace2_region_enter("midx", "write_midx_bitmap", the_repository);

        if (flags & MIDX_WRITE_BITMAP_HASH_CACHE)
                options |= BITMAP_OPT_HASH_CACHE;

        if (flags & MIDX_WRITE_BITMAP_LOOKUP_TABLE)
                options |= BITMAP_OPT_LOOKUP_TABLE;

        /*
         * Build the MIDX-order index based on pdata.objects (which is already
         * in MIDX order; c.f., 'midx_pack_order_cmp()' for the definition of
         * this order).
         */
        ALLOC_ARRAY(index, pdata->nr_objects);
        for (i = 0; i < pdata->nr_objects; i++)
                index[i] = &pdata->objects[i].idx;

        bitmap_writer_init(&writer, the_repository, pdata);
        bitmap_writer_show_progress(&writer, flags & MIDX_PROGRESS);
        bitmap_writer_build_type_index(&writer, index);

        /*
         * bitmap_writer_finish expects objects in lex order, but pack_order
         * gives us exactly that. use it directly instead of re-sorting the
         * array.
         *
         * This changes the order of objects in 'index' between
         * bitmap_writer_build_type_index and bitmap_writer_finish.
         *
         * The same re-ordering takes place in the single-pack bitmap code via
         * write_idx_file(), which is called by finish_tmp_packfile(), which
         * happens between bitmap_writer_build_type_index() and
         * bitmap_writer_finish().
         */
        for (i = 0; i < pdata->nr_objects; i++)
                index[pack_order[i]] = &pdata->objects[i].idx;

        bitmap_writer_select_commits(&writer, commits, commits_nr);
        ret = bitmap_writer_build(&writer);
        if (ret < 0)
                goto cleanup;

        bitmap_writer_set_checksum(&writer, midx_hash);
        bitmap_writer_finish(&writer, index, bitmap_name, options);

cleanup:
        free(index);
        free(bitmap_name);
        bitmap_writer_free(&writer);

        trace2_region_leave("midx", "write_midx_bitmap", the_repository);

        return ret;
}

static struct multi_pack_index *lookup_multi_pack_index(struct repository *r,
                                                        const char *object_dir)
{
        struct multi_pack_index *result = NULL;
        struct multi_pack_index *cur;
        char *obj_dir_real = real_pathdup(object_dir, 1);
        struct strbuf cur_path_real = STRBUF_INIT;

        /* Ensure the given object_dir is local, or a known alternate. */
        find_odb(r, obj_dir_real);

        for (cur = get_multi_pack_index(r); cur; cur = cur->next) {
                strbuf_realpath(&cur_path_real, cur->object_dir, 1);
                if (!strcmp(obj_dir_real, cur_path_real.buf)) {
                        result = cur;
                        goto cleanup;
                }
        }

cleanup:
        free(obj_dir_real);
        strbuf_release(&cur_path_real);
        return result;
}

static int fill_packs_from_midx(struct write_midx_context *ctx,
                                const char *preferred_pack_name, uint32_t flags)
{
        struct multi_pack_index *m;

        for (m = ctx->m; m; m = m->base_midx) {
                uint32_t i;

                for (i = 0; i < m->num_packs; i++) {
                        ALLOC_GROW(ctx->info, ctx->nr + 1, ctx->alloc);

                        /*
                         * If generating a reverse index, need to have
                         * packed_git's loaded to compare their
                         * mtimes and object count.
                         *
                         * If a preferred pack is specified, need to
                         * have packed_git's loaded to ensure the chosen
                         * preferred pack has a non-zero object count.
                         */
                        if (flags & MIDX_WRITE_REV_INDEX ||
                            preferred_pack_name) {
                                if (prepare_midx_pack(the_repository, m,
                                                      m->num_packs_in_base + i)) {
                                        error(_("could not load pack"));
                                        return 1;
                                }

                                if (open_pack_index(m->packs[i]))
                                        die(_("could not open index for %s"),
                                            m->packs[i]->pack_name);
                        }

                        fill_pack_info(&ctx->info[ctx->nr++], m->packs[i],
                                       m->pack_names[i],
                                       m->num_packs_in_base + i);
                }
        }
        return 0;
}

static struct {
        const char *non_split;
        const char *split;
} midx_exts[] = {
        {NULL, MIDX_EXT_MIDX},
        {MIDX_EXT_BITMAP, MIDX_EXT_BITMAP},
        {MIDX_EXT_REV, MIDX_EXT_REV},
};

static int link_midx_to_chain(struct multi_pack_index *m)
{
        struct strbuf from = STRBUF_INIT;
        struct strbuf to = STRBUF_INIT;
        int ret = 0;
        size_t i;

        if (!m || m->has_chain) {
                /*
                 * Either no MIDX previously existed, or it was already
                 * part of a MIDX chain. In both cases, we have nothing
                 * to link, so return early.
                 */
                goto done;
        }

        for (i = 0; i < ARRAY_SIZE(midx_exts); i++) {
                const unsigned char *hash = get_midx_checksum(m);

                get_midx_filename_ext(&from, m->object_dir, hash,
                                      midx_exts[i].non_split);
                get_split_midx_filename_ext(&to, m->object_dir, hash,
                                            midx_exts[i].split);

                if (link(from.buf, to.buf) < 0 && errno != ENOENT) {
                        ret = error_errno(_("unable to link '%s' to '%s'"),
                                          from.buf, to.buf);
                        goto done;
                }

                strbuf_reset(&from);
                strbuf_reset(&to);
        }

done:
        strbuf_release(&from);
        strbuf_release(&to);
        return ret;
}

static void clear_midx_files(const char *object_dir,
                             const char **hashes,
                             uint32_t hashes_nr,
                             unsigned incremental)
{
        /*
         * if incremental:
         *   - remove all non-incremental MIDX files
         *   - remove any incremental MIDX files not in the current one
         *
         * if non-incremental:
         *   - remove all incremental MIDX files
         *   - remove any non-incremental MIDX files not matching the current
         *     hash
         */
        struct strbuf buf = STRBUF_INIT;
        const char *exts[] = { MIDX_EXT_BITMAP, MIDX_EXT_REV, MIDX_EXT_MIDX };
        uint32_t i, j;

        for (i = 0; i < ARRAY_SIZE(exts); i++) {
                clear_incremental_midx_files_ext(object_dir, exts[i],
                                                 hashes, hashes_nr);
                for (j = 0; j < hashes_nr; j++)
                        clear_midx_files_ext(object_dir, exts[i], hashes[j]);
        }

        if (incremental)
                get_midx_filename(&buf, object_dir);
        else
                get_midx_chain_filename(&buf, object_dir);

        if (unlink(buf.buf) && errno != ENOENT)
                die_errno(_("failed to clear multi-pack-index at %s"), buf.buf);

        strbuf_release(&buf);
}

static int write_midx_internal(const char *object_dir,
                               struct string_list *packs_to_include,
                               struct string_list *packs_to_drop,
                               const char *preferred_pack_name,
                               const char *refs_snapshot,
                               unsigned flags)
{
        struct strbuf midx_name = STRBUF_INIT;
        unsigned char midx_hash[GIT_MAX_RAWSZ];
        uint32_t i, start_pack;
        struct hashfile *f = NULL;
        struct lock_file lk;
        struct tempfile *incr;
        struct write_midx_context ctx = { 0 };
        int bitmapped_packs_concat_len = 0;
        int pack_name_concat_len = 0;
        int dropped_packs = 0;
        int result = 0;
        const char **keep_hashes = NULL;
        struct chunkfile *cf;

        trace2_region_enter("midx", "write_midx_internal", the_repository);

        ctx.incremental = !!(flags & MIDX_WRITE_INCREMENTAL);
        if (ctx.incremental && (flags & MIDX_WRITE_BITMAP))
                die(_("cannot write incremental MIDX with bitmap"));

        if (ctx.incremental)
                strbuf_addf(&midx_name,
                            "%s/pack/multi-pack-index.d/tmp_midx_XXXXXX",
                            object_dir);
        else
                get_midx_filename(&midx_name, object_dir);
        if (safe_create_leading_directories(midx_name.buf))
                die_errno(_("unable to create leading directories of %s"),
                          midx_name.buf);

        if (!packs_to_include || ctx.incremental) {
                struct multi_pack_index *m = lookup_multi_pack_index(the_repository,
                                                                     object_dir);
                if (m && !midx_checksum_valid(m)) {
                        warning(_("ignoring existing multi-pack-index; checksum mismatch"));
                        m = NULL;
                }

                if (m) {
                        /*
                         * Only reference an existing MIDX when not filtering
                         * which packs to include, since all packs and objects
                         * are copied blindly from an existing MIDX if one is
                         * present.
                         */
                        if (ctx.incremental)
                                ctx.base_midx = m;
                        else if (!packs_to_include)
                                ctx.m = m;
                }
        }

        ctx.nr = 0;
        ctx.alloc = ctx.m ? ctx.m->num_packs + ctx.m->num_packs_in_base : 16;
        ctx.info = NULL;
        ALLOC_ARRAY(ctx.info, ctx.alloc);

        if (ctx.incremental) {
                struct multi_pack_index *m = ctx.base_midx;
                while (m) {
                        ctx.num_multi_pack_indexes_before++;
                        m = m->base_midx;
                }
        } else if (ctx.m && fill_packs_from_midx(&ctx, preferred_pack_name,
                                                 flags) < 0) {
                goto cleanup;
        }

        start_pack = ctx.nr;

        ctx.pack_paths_checked = 0;
        if (flags & MIDX_PROGRESS)
                ctx.progress = start_delayed_progress(_("Adding packfiles to multi-pack-index"), 0);
        else
                ctx.progress = NULL;

        ctx.to_include = packs_to_include;

        for_each_file_in_pack_dir(object_dir, add_pack_to_midx, &ctx);
        stop_progress(&ctx.progress);

        if ((ctx.m && ctx.nr == ctx.m->num_packs + ctx.m->num_packs_in_base) &&
            !ctx.incremental &&
            !(packs_to_include || packs_to_drop)) {
                struct bitmap_index *bitmap_git;
                int bitmap_exists;
                int want_bitmap = flags & MIDX_WRITE_BITMAP;

                bitmap_git = prepare_midx_bitmap_git(ctx.m);
                bitmap_exists = bitmap_git && bitmap_is_midx(bitmap_git);
                free_bitmap_index(bitmap_git);

                if (bitmap_exists || !want_bitmap) {
                        /*
                         * The correct MIDX already exists, and so does a
                         * corresponding bitmap (or one wasn't requested).
                         */
                        if (!want_bitmap)
                                clear_midx_files_ext(object_dir, "bitmap", NULL);
                        goto cleanup;
                }
        }

        if (ctx.incremental && !ctx.nr)
                goto cleanup; /* nothing to do */

        if (preferred_pack_name) {
                ctx.preferred_pack_idx = -1;

                for (i = 0; i < ctx.nr; i++) {
                        if (!cmp_idx_or_pack_name(preferred_pack_name,
                                                  ctx.info[i].pack_name)) {
                                ctx.preferred_pack_idx = i;
                                break;
                        }
                }

                if (ctx.preferred_pack_idx == -1)
                        warning(_("unknown preferred pack: '%s'"),
                                preferred_pack_name);
        } else if (ctx.nr &&
                   (flags & (MIDX_WRITE_REV_INDEX | MIDX_WRITE_BITMAP))) {
                struct packed_git *oldest = ctx.info[ctx.preferred_pack_idx].p;
                ctx.preferred_pack_idx = 0;

                if (packs_to_drop && packs_to_drop->nr)
                        BUG("cannot write a MIDX bitmap during expiration");

                /*
                 * set a preferred pack when writing a bitmap to ensure that
                 * the pack from which the first object is selected in pseudo
                 * pack-order has all of its objects selected from that pack
                 * (and not another pack containing a duplicate)
                 */
                for (i = 1; i < ctx.nr; i++) {
                        struct packed_git *p = ctx.info[i].p;

                        if (!oldest->num_objects || p->mtime < oldest->mtime) {
                                oldest = p;
                                ctx.preferred_pack_idx = i;
                        }
                }

                if (!oldest->num_objects) {
                        /*
                         * If all packs are empty; unset the preferred index.
                         * This is acceptable since there will be no duplicate
                         * objects to resolve, so the preferred value doesn't
                         * matter.
                         */
                        ctx.preferred_pack_idx = -1;
                }
        } else {
                /*
                 * otherwise don't mark any pack as preferred to avoid
                 * interfering with expiration logic below
                 */
                ctx.preferred_pack_idx = -1;
        }

        if (ctx.preferred_pack_idx > -1) {
                struct packed_git *preferred = ctx.info[ctx.preferred_pack_idx].p;
                if (!preferred->num_objects) {
                        error(_("cannot select preferred pack %s with no objects"),
                              preferred->pack_name);
                        result = 1;
                        goto cleanup;
                }
        }

        compute_sorted_entries(&ctx, start_pack);

        ctx.large_offsets_needed = 0;
        for (i = 0; i < ctx.entries_nr; i++) {
                if (ctx.entries[i].offset > 0x7fffffff)
                        ctx.num_large_offsets++;
                if (ctx.entries[i].offset > 0xffffffff)
                        ctx.large_offsets_needed = 1;
        }

        QSORT(ctx.info, ctx.nr, pack_info_compare);

        if (packs_to_drop && packs_to_drop->nr) {
                int drop_index = 0;
                int missing_drops = 0;

                for (i = 0; i < ctx.nr && drop_index < packs_to_drop->nr; i++) {
                        int cmp = strcmp(ctx.info[i].pack_name,
                                         packs_to_drop->items[drop_index].string);

                        if (!cmp) {
                                drop_index++;
                                ctx.info[i].expired = 1;
                        } else if (cmp > 0) {
                                error(_("did not see pack-file %s to drop"),
                                      packs_to_drop->items[drop_index].string);
                                drop_index++;
                                missing_drops++;
                                i--;
                        } else {
                                ctx.info[i].expired = 0;
                        }
                }

                if (missing_drops) {
                        result = 1;
                        goto cleanup;
                }
        }

        /*
         * pack_perm stores a permutation between pack-int-ids from the
         * previous multi-pack-index to the new one we are writing:
         *
         * pack_perm[old_id] = new_id
         */
        ALLOC_ARRAY(ctx.pack_perm, ctx.nr);
        for (i = 0; i < ctx.nr; i++) {
                if (ctx.info[i].expired) {
                        dropped_packs++;
                        ctx.pack_perm[ctx.info[i].orig_pack_int_id] = PACK_EXPIRED;
                } else {
                        ctx.pack_perm[ctx.info[i].orig_pack_int_id] = i - dropped_packs;
                }
        }

        for (i = 0; i < ctx.nr; i++) {
                if (ctx.info[i].expired)
                        continue;
                pack_name_concat_len += strlen(ctx.info[i].pack_name) + 1;
                bitmapped_packs_concat_len += 2 * sizeof(uint32_t);
        }

        /* Check that the preferred pack wasn't expired (if given). */
        if (preferred_pack_name) {
                struct pack_info *preferred = bsearch(preferred_pack_name,
                                                      ctx.info, ctx.nr,
                                                      sizeof(*ctx.info),
                                                      idx_or_pack_name_cmp);
                if (preferred) {
                        uint32_t perm = ctx.pack_perm[preferred->orig_pack_int_id];
                        if (perm == PACK_EXPIRED)
                                warning(_("preferred pack '%s' is expired"),
                                        preferred_pack_name);
                }
        }

        if (pack_name_concat_len % MIDX_CHUNK_ALIGNMENT)
                pack_name_concat_len += MIDX_CHUNK_ALIGNMENT -
                                        (pack_name_concat_len % MIDX_CHUNK_ALIGNMENT);

        if (ctx.nr - dropped_packs == 0) {
                error(_("no pack files to index."));
                result = 1;
                goto cleanup;
        }

        if (!ctx.entries_nr) {
                if (flags & MIDX_WRITE_BITMAP)
                        warning(_("refusing to write multi-pack .bitmap without any objects"));
                flags &= ~(MIDX_WRITE_REV_INDEX | MIDX_WRITE_BITMAP);
        }

        if (ctx.incremental) {
                struct strbuf lock_name = STRBUF_INIT;

                get_midx_chain_filename(&lock_name, object_dir);
                hold_lock_file_for_update(&lk, lock_name.buf, LOCK_DIE_ON_ERROR);
                strbuf_release(&lock_name);

                incr = mks_tempfile_m(midx_name.buf, 0444);
                if (!incr) {
                        error(_("unable to create temporary MIDX layer"));
                        return -1;
                }

                if (adjust_shared_perm(get_tempfile_path(incr))) {
                        error(_("unable to adjust shared permissions for '%s'"),
                              get_tempfile_path(incr));
                        return -1;
                }

                f = hashfd(get_tempfile_fd(incr), get_tempfile_path(incr));
        } else {
                hold_lock_file_for_update(&lk, midx_name.buf, LOCK_DIE_ON_ERROR);
                f = hashfd(get_lock_file_fd(&lk), get_lock_file_path(&lk));
        }

        cf = init_chunkfile(f);

        add_chunk(cf, MIDX_CHUNKID_PACKNAMES, pack_name_concat_len,
                  write_midx_pack_names);
        add_chunk(cf, MIDX_CHUNKID_OIDFANOUT, MIDX_CHUNK_FANOUT_SIZE,
                  write_midx_oid_fanout);
        add_chunk(cf, MIDX_CHUNKID_OIDLOOKUP,
                  st_mult(ctx.entries_nr, the_hash_algo->rawsz),
                  write_midx_oid_lookup);
        add_chunk(cf, MIDX_CHUNKID_OBJECTOFFSETS,
                  st_mult(ctx.entries_nr, MIDX_CHUNK_OFFSET_WIDTH),
                  write_midx_object_offsets);

        if (ctx.large_offsets_needed)
                add_chunk(cf, MIDX_CHUNKID_LARGEOFFSETS,
                        st_mult(ctx.num_large_offsets,
                                MIDX_CHUNK_LARGE_OFFSET_WIDTH),
                        write_midx_large_offsets);

        if (flags & (MIDX_WRITE_REV_INDEX | MIDX_WRITE_BITMAP)) {
                ctx.pack_order = midx_pack_order(&ctx);
                add_chunk(cf, MIDX_CHUNKID_REVINDEX,
                          st_mult(ctx.entries_nr, sizeof(uint32_t)),
                          write_midx_revindex);
                add_chunk(cf, MIDX_CHUNKID_BITMAPPEDPACKS,
                          bitmapped_packs_concat_len,
                          write_midx_bitmapped_packs);
        }

        write_midx_header(f, get_num_chunks(cf), ctx.nr - dropped_packs);
        write_chunkfile(cf, &ctx);

        finalize_hashfile(f, midx_hash, FSYNC_COMPONENT_PACK_METADATA,
                          CSUM_FSYNC | CSUM_HASH_IN_STREAM);
        free_chunkfile(cf);

        if (flags & MIDX_WRITE_REV_INDEX &&
            git_env_bool("GIT_TEST_MIDX_WRITE_REV", 0))
                write_midx_reverse_index(midx_name.buf, midx_hash, &ctx);

        if (flags & MIDX_WRITE_BITMAP) {
                struct packing_data pdata;
                struct commit **commits;
                uint32_t commits_nr;

                if (!ctx.entries_nr)
                        BUG("cannot write a bitmap without any objects");

                prepare_midx_packing_data(&pdata, &ctx);

                commits = find_commits_for_midx_bitmap(&commits_nr, refs_snapshot, &ctx);

                /*
                 * The previous steps translated the information from
                 * 'entries' into information suitable for constructing
                 * bitmaps. We no longer need that array, so clear it to
                 * reduce memory pressure.
                 */
                FREE_AND_NULL(ctx.entries);
                ctx.entries_nr = 0;

                if (write_midx_bitmap(midx_name.buf, midx_hash, &pdata,
                                      commits, commits_nr, ctx.pack_order,
                                      flags) < 0) {
                        error(_("could not write multi-pack bitmap"));
                        result = 1;
                        clear_packing_data(&pdata);
                        free(commits);
                        goto cleanup;
                }

                clear_packing_data(&pdata);
                free(commits);
        }
        /*
         * NOTE: Do not use ctx.entries beyond this point, since it might
         * have been freed in the previous if block.
         */

        CALLOC_ARRAY(keep_hashes, ctx.num_multi_pack_indexes_before + 1);

        if (ctx.incremental) {
                FILE *chainf = fdopen_lock_file(&lk, "w");
                struct strbuf final_midx_name = STRBUF_INIT;
                struct multi_pack_index *m = ctx.base_midx;

                if (!chainf) {
                        error_errno(_("unable to open multi-pack-index chain file"));
                        return -1;
                }

                if (link_midx_to_chain(ctx.base_midx) < 0)
                        return -1;

                get_split_midx_filename_ext(&final_midx_name, object_dir,
                                            midx_hash, MIDX_EXT_MIDX);

                if (rename_tempfile(&incr, final_midx_name.buf) < 0) {
                        error_errno(_("unable to rename new multi-pack-index layer"));
                        return -1;
                }

                keep_hashes[ctx.num_multi_pack_indexes_before] =
                        xstrdup(hash_to_hex(midx_hash));

                for (i = 0; i < ctx.num_multi_pack_indexes_before; i++) {
                        uint32_t j = ctx.num_multi_pack_indexes_before - i - 1;

                        keep_hashes[j] = xstrdup(hash_to_hex(get_midx_checksum(m)));
                        m = m->base_midx;
                }

                for (i = 0; i < ctx.num_multi_pack_indexes_before + 1; i++)
                        fprintf(get_lock_file_fp(&lk), "%s\n", keep_hashes[i]);
        } else {
                keep_hashes[ctx.num_multi_pack_indexes_before] =
                        xstrdup(hash_to_hex(midx_hash));
        }

        if (ctx.m || ctx.base_midx)
                close_object_store(the_repository->objects);

        if (commit_lock_file(&lk) < 0)
                die_errno(_("could not write multi-pack-index"));

        clear_midx_files(object_dir, keep_hashes,
                         ctx.num_multi_pack_indexes_before + 1,
                         ctx.incremental);

cleanup:
        for (i = 0; i < ctx.nr; i++) {
                if (ctx.info[i].p) {
                        close_pack(ctx.info[i].p);
                        free(ctx.info[i].p);
                }
                free(ctx.info[i].pack_name);
        }

        free(ctx.info);
        free(ctx.entries);
        free(ctx.pack_perm);
        free(ctx.pack_order);
        if (keep_hashes) {
                for (i = 0; i < ctx.num_multi_pack_indexes_before + 1; i++)
                        free((char *)keep_hashes[i]);
                free(keep_hashes);
        }
        strbuf_release(&midx_name);

        trace2_region_leave("midx", "write_midx_internal", the_repository);

        return result;
}

int write_midx_file(const char *object_dir,
                    const char *preferred_pack_name,
                    const char *refs_snapshot,
                    unsigned flags)
{
        return write_midx_internal(object_dir, NULL, NULL, preferred_pack_name,
                                   refs_snapshot, flags);
}

int write_midx_file_only(const char *object_dir,
                         struct string_list *packs_to_include,
                         const char *preferred_pack_name,
                         const char *refs_snapshot,
                         unsigned flags)
{
        return write_midx_internal(object_dir, packs_to_include, NULL,
                                   preferred_pack_name, refs_snapshot, flags);
}

int expire_midx_packs(struct repository *r, const char *object_dir, unsigned flags)
{
        uint32_t i, *count, result = 0;
        struct string_list packs_to_drop = STRING_LIST_INIT_DUP;
        struct multi_pack_index *m = lookup_multi_pack_index(r, object_dir);
        struct progress *progress = NULL;

        if (!m)
                return 0;

        if (m->base_midx)
                die(_("cannot expire packs from an incremental multi-pack-index"));

        CALLOC_ARRAY(count, m->num_packs);

        if (flags & MIDX_PROGRESS)
                progress = start_delayed_progress(_("Counting referenced objects"),
                                          m->num_objects);
        for (i = 0; i < m->num_objects; i++) {
                int pack_int_id = nth_midxed_pack_int_id(m, i);
                count[pack_int_id]++;
                display_progress(progress, i + 1);
        }
        stop_progress(&progress);

        if (flags & MIDX_PROGRESS)
                progress = start_delayed_progress(_("Finding and deleting unreferenced packfiles"),
                                          m->num_packs);
        for (i = 0; i < m->num_packs; i++) {
                char *pack_name;
                display_progress(progress, i + 1);

                if (count[i])
                        continue;

                if (prepare_midx_pack(r, m, i))
                        continue;

                if (m->packs[i]->pack_keep || m->packs[i]->is_cruft)
                        continue;

                pack_name = xstrdup(m->packs[i]->pack_name);
                close_pack(m->packs[i]);

                string_list_insert(&packs_to_drop, m->pack_names[i]);
                unlink_pack_path(pack_name, 0);
                free(pack_name);
        }
        stop_progress(&progress);

        free(count);

        if (packs_to_drop.nr)
                result = write_midx_internal(object_dir, NULL, &packs_to_drop, NULL, NULL, flags);

        string_list_clear(&packs_to_drop, 0);

        return result;
}

struct repack_info {
        timestamp_t mtime;
        uint32_t referenced_objects;
        uint32_t pack_int_id;
};

static int compare_by_mtime(const void *a_, const void *b_)
{
        const struct repack_info *a, *b;

        a = (const struct repack_info *)a_;
        b = (const struct repack_info *)b_;

        if (a->mtime < b->mtime)
                return -1;
        if (a->mtime > b->mtime)
                return 1;
        return 0;
}

static int want_included_pack(struct repository *r,
                              struct multi_pack_index *m,
                              int pack_kept_objects,
                              uint32_t pack_int_id)
{
        struct packed_git *p;
        if (prepare_midx_pack(r, m, pack_int_id))
                return 0;
        p = m->packs[pack_int_id];
        if (!pack_kept_objects && p->pack_keep)
                return 0;
        if (p->is_cruft)
                return 0;
        if (open_pack_index(p) || !p->num_objects)
                return 0;
        return 1;
}

static void fill_included_packs_all(struct repository *r,
                                    struct multi_pack_index *m,
                                    unsigned char *include_pack)
{
        uint32_t i;
        int pack_kept_objects = 0;

        repo_config_get_bool(r, "repack.packkeptobjects", &pack_kept_objects);

        for (i = 0; i < m->num_packs; i++) {
                if (!want_included_pack(r, m, pack_kept_objects, i))
                        continue;

                include_pack[i] = 1;
        }
}

static void fill_included_packs_batch(struct repository *r,
                                      struct multi_pack_index *m,
                                      unsigned char *include_pack,
                                      size_t batch_size)
{
        uint32_t i;
        size_t total_size;
        struct repack_info *pack_info;
        int pack_kept_objects = 0;

        CALLOC_ARRAY(pack_info, m->num_packs);

        repo_config_get_bool(r, "repack.packkeptobjects", &pack_kept_objects);

        for (i = 0; i < m->num_packs; i++) {
                pack_info[i].pack_int_id = i;

                if (prepare_midx_pack(r, m, i))
                        continue;

                pack_info[i].mtime = m->packs[i]->mtime;
        }

        for (i = 0; i < m->num_objects; i++) {
                uint32_t pack_int_id = nth_midxed_pack_int_id(m, i);
                pack_info[pack_int_id].referenced_objects++;
        }

        QSORT(pack_info, m->num_packs, compare_by_mtime);

        total_size = 0;
        for (i = 0; total_size < batch_size && i < m->num_packs; i++) {
                int pack_int_id = pack_info[i].pack_int_id;
                struct packed_git *p = m->packs[pack_int_id];
                size_t expected_size;

                if (!want_included_pack(r, m, pack_kept_objects, pack_int_id))
                        continue;

                expected_size = st_mult(p->pack_size,
                                        pack_info[i].referenced_objects);
                expected_size /= p->num_objects;

                if (expected_size >= batch_size)
                        continue;

                total_size += expected_size;
                include_pack[pack_int_id] = 1;
        }

        free(pack_info);
}

int midx_repack(struct repository *r, const char *object_dir, size_t batch_size, unsigned flags)
{
        int result = 0;
        uint32_t i, packs_to_repack = 0;
        unsigned char *include_pack;
        struct child_process cmd = CHILD_PROCESS_INIT;
        FILE *cmd_in;
        struct multi_pack_index *m = lookup_multi_pack_index(r, object_dir);

        /*
         * When updating the default for these configuration
         * variables in builtin/repack.c, these must be adjusted
         * to match.
         */
        int delta_base_offset = 1;
        int use_delta_islands = 0;

        if (!m)
                return 0;
        if (m->base_midx)
                die(_("cannot repack an incremental multi-pack-index"));

        CALLOC_ARRAY(include_pack, m->num_packs);

        if (batch_size)
                fill_included_packs_batch(r, m, include_pack, batch_size);
        else
                fill_included_packs_all(r, m, include_pack);

        for (i = 0; i < m->num_packs; i++) {
                if (include_pack[i])
                        packs_to_repack++;
        }
        if (packs_to_repack <= 1)
                goto cleanup;

        repo_config_get_bool(r, "repack.usedeltabaseoffset", &delta_base_offset);
        repo_config_get_bool(r, "repack.usedeltaislands", &use_delta_islands);

        strvec_push(&cmd.args, "pack-objects");

        strvec_pushf(&cmd.args, "%s/pack/pack", object_dir);

        if (delta_base_offset)
                strvec_push(&cmd.args, "--delta-base-offset");
        if (use_delta_islands)
                strvec_push(&cmd.args, "--delta-islands");

        if (flags & MIDX_PROGRESS)
                strvec_push(&cmd.args, "--progress");
        else
                strvec_push(&cmd.args, "-q");

        cmd.git_cmd = 1;
        cmd.in = cmd.out = -1;

        if (start_command(&cmd)) {
                error(_("could not start pack-objects"));
                result = 1;
                goto cleanup;
        }

        cmd_in = xfdopen(cmd.in, "w");

        for (i = 0; i < m->num_objects; i++) {
                struct object_id oid;
                uint32_t pack_int_id = nth_midxed_pack_int_id(m, i);

                if (!include_pack[pack_int_id])
                        continue;

                nth_midxed_object_oid(&oid, m, i);
                fprintf(cmd_in, "%s\n", oid_to_hex(&oid));
        }
        fclose(cmd_in);

        if (finish_command(&cmd)) {
                error(_("could not finish pack-objects"));
                result = 1;
                goto cleanup;
        }

        result = write_midx_internal(object_dir, NULL, NULL, NULL, NULL, flags);

cleanup:
        free(include_pack);
        return result;
}