diff: consolidate diff algorithm option parsing

[git.git] / midx.c

#include "cache.h"
#include "config.h"
#include "csum-file.h"
#include "dir.h"
#include "lockfile.h"
#include "packfile.h"
#include "object-store.h"
#include "hash-lookup.h"
#include "midx.h"
#include "progress.h"
#include "trace2.h"
#include "run-command.h"
#include "repository.h"
#include "chunk-format.h"
#include "pack.h"
#include "pack-bitmap.h"
#include "refs.h"
#include "revision.h"
#include "list-objects.h"

#define MIDX_SIGNATURE 0x4d494458 /* "MIDX" */
#define MIDX_VERSION 1
#define MIDX_BYTE_FILE_VERSION 4
#define MIDX_BYTE_HASH_VERSION 5
#define MIDX_BYTE_NUM_CHUNKS 6
#define MIDX_BYTE_NUM_PACKS 8
#define MIDX_HEADER_SIZE 12
#define MIDX_MIN_SIZE (MIDX_HEADER_SIZE + the_hash_algo->rawsz)

#define MIDX_CHUNK_ALIGNMENT 4
#define MIDX_CHUNKID_PACKNAMES 0x504e414d /* "PNAM" */
#define MIDX_CHUNKID_OIDFANOUT 0x4f494446 /* "OIDF" */
#define MIDX_CHUNKID_OIDLOOKUP 0x4f49444c /* "OIDL" */
#define MIDX_CHUNKID_OBJECTOFFSETS 0x4f4f4646 /* "OOFF" */
#define MIDX_CHUNKID_LARGEOFFSETS 0x4c4f4646 /* "LOFF" */
#define MIDX_CHUNKID_REVINDEX 0x52494458 /* "RIDX" */
#define MIDX_CHUNK_FANOUT_SIZE (sizeof(uint32_t) * 256)
#define MIDX_CHUNK_OFFSET_WIDTH (2 * sizeof(uint32_t))
#define MIDX_CHUNK_LARGE_OFFSET_WIDTH (sizeof(uint64_t))
#define MIDX_LARGE_OFFSET_NEEDED 0x80000000

#define PACK_EXPIRED UINT_MAX

const unsigned char *get_midx_checksum(struct multi_pack_index *m)
{
        return m->data + m->data_len - the_hash_algo->rawsz;
}

void get_midx_filename(struct strbuf *out, const char *object_dir)
{
        strbuf_addf(out, "%s/pack/multi-pack-index", object_dir);
}

void get_midx_rev_filename(struct strbuf *out, struct multi_pack_index *m)
{
        get_midx_filename(out, m->object_dir);
        strbuf_addf(out, "-%s.rev", hash_to_hex(get_midx_checksum(m)));
}

static int midx_read_oid_fanout(const unsigned char *chunk_start,
                                size_t chunk_size, void *data)
{
        struct multi_pack_index *m = data;
        m->chunk_oid_fanout = (uint32_t *)chunk_start;

        if (chunk_size != 4 * 256) {
                error(_("multi-pack-index OID fanout is of the wrong size"));
                return 1;
        }
        return 0;
}

struct multi_pack_index *load_multi_pack_index(const char *object_dir, int local)
{
        struct multi_pack_index *m = NULL;
        int fd;
        struct stat st;
        size_t midx_size;
        void *midx_map = NULL;
        uint32_t hash_version;
        struct strbuf midx_name = STRBUF_INIT;
        uint32_t i;
        const char *cur_pack_name;
        struct chunkfile *cf = NULL;

        get_midx_filename(&midx_name, object_dir);

        fd = git_open(midx_name.buf);

        if (fd < 0)
                goto cleanup_fail;
        if (fstat(fd, &st)) {
                error_errno(_("failed to read %s"), midx_name.buf);
                goto cleanup_fail;
        }

        midx_size = xsize_t(st.st_size);

        if (midx_size < MIDX_MIN_SIZE) {
                error(_("multi-pack-index file %s is too small"), midx_name.buf);
                goto cleanup_fail;
        }

        strbuf_release(&midx_name);

        midx_map = xmmap(NULL, midx_size, PROT_READ, MAP_PRIVATE, fd, 0);
        close(fd);

        FLEX_ALLOC_STR(m, object_dir, object_dir);
        m->data = midx_map;
        m->data_len = midx_size;
        m->local = local;

        m->signature = get_be32(m->data);
        if (m->signature != MIDX_SIGNATURE)
                die(_("multi-pack-index signature 0x%08x does not match signature 0x%08x"),
                      m->signature, MIDX_SIGNATURE);

        m->version = m->data[MIDX_BYTE_FILE_VERSION];
        if (m->version != MIDX_VERSION)
                die(_("multi-pack-index version %d not recognized"),
                      m->version);

        hash_version = m->data[MIDX_BYTE_HASH_VERSION];
        if (hash_version != oid_version(the_hash_algo)) {
                error(_("multi-pack-index hash version %u does not match version %u"),
                      hash_version, oid_version(the_hash_algo));
                goto cleanup_fail;
        }
        m->hash_len = the_hash_algo->rawsz;

        m->num_chunks = m->data[MIDX_BYTE_NUM_CHUNKS];

        m->num_packs = get_be32(m->data + MIDX_BYTE_NUM_PACKS);

        cf = init_chunkfile(NULL);

        if (read_table_of_contents(cf, m->data, midx_size,
                                   MIDX_HEADER_SIZE, m->num_chunks))
                goto cleanup_fail;

        if (pair_chunk(cf, MIDX_CHUNKID_PACKNAMES, &m->chunk_pack_names) == CHUNK_NOT_FOUND)
                die(_("multi-pack-index missing required pack-name chunk"));
        if (read_chunk(cf, MIDX_CHUNKID_OIDFANOUT, midx_read_oid_fanout, m) == CHUNK_NOT_FOUND)
                die(_("multi-pack-index missing required OID fanout chunk"));
        if (pair_chunk(cf, MIDX_CHUNKID_OIDLOOKUP, &m->chunk_oid_lookup) == CHUNK_NOT_FOUND)
                die(_("multi-pack-index missing required OID lookup chunk"));
        if (pair_chunk(cf, MIDX_CHUNKID_OBJECTOFFSETS, &m->chunk_object_offsets) == CHUNK_NOT_FOUND)
                die(_("multi-pack-index missing required object offsets chunk"));

        pair_chunk(cf, MIDX_CHUNKID_LARGEOFFSETS, &m->chunk_large_offsets);

        if (git_env_bool("GIT_TEST_MIDX_READ_RIDX", 1))
                pair_chunk(cf, MIDX_CHUNKID_REVINDEX, &m->chunk_revindex);

        m->num_objects = ntohl(m->chunk_oid_fanout[255]);

        CALLOC_ARRAY(m->pack_names, m->num_packs);
        CALLOC_ARRAY(m->packs, m->num_packs);

        cur_pack_name = (const char *)m->chunk_pack_names;
        for (i = 0; i < m->num_packs; i++) {
                m->pack_names[i] = cur_pack_name;

                cur_pack_name += strlen(cur_pack_name) + 1;

                if (i && strcmp(m->pack_names[i], m->pack_names[i - 1]) <= 0)
                        die(_("multi-pack-index pack names out of order: '%s' before '%s'"),
                              m->pack_names[i - 1],
                              m->pack_names[i]);
        }

        trace2_data_intmax("midx", the_repository, "load/num_packs", m->num_packs);
        trace2_data_intmax("midx", the_repository, "load/num_objects", m->num_objects);

        free_chunkfile(cf);
        return m;

cleanup_fail:
        free(m);
        strbuf_release(&midx_name);
        free_chunkfile(cf);
        if (midx_map)
                munmap(midx_map, midx_size);
        if (0 <= fd)
                close(fd);
        return NULL;
}

void close_midx(struct multi_pack_index *m)
{
        uint32_t i;

        if (!m)
                return;

        close_midx(m->next);

        munmap((unsigned char *)m->data, m->data_len);

        for (i = 0; i < m->num_packs; i++) {
                if (m->packs[i])
                        m->packs[i]->multi_pack_index = 0;
        }
        FREE_AND_NULL(m->packs);
        FREE_AND_NULL(m->pack_names);
        free(m);
}

int prepare_midx_pack(struct repository *r, struct multi_pack_index *m, uint32_t pack_int_id)
{
        struct strbuf pack_name = STRBUF_INIT;
        struct packed_git *p;

        if (pack_int_id >= m->num_packs)
                die(_("bad pack-int-id: %u (%u total packs)"),
                    pack_int_id, m->num_packs);

        if (m->packs[pack_int_id])
                return 0;

        strbuf_addf(&pack_name, "%s/pack/%s", m->object_dir,
                    m->pack_names[pack_int_id]);

        p = add_packed_git(pack_name.buf, pack_name.len, m->local);
        strbuf_release(&pack_name);

        if (!p)
                return 1;

        p->multi_pack_index = 1;
        m->packs[pack_int_id] = p;
        install_packed_git(r, p);
        list_add_tail(&p->mru, &r->objects->packed_git_mru);

        return 0;
}

int bsearch_midx(const struct object_id *oid, struct multi_pack_index *m, uint32_t *result)
{
        return bsearch_hash(oid->hash, m->chunk_oid_fanout, m->chunk_oid_lookup,
                            the_hash_algo->rawsz, result);
}

struct object_id *nth_midxed_object_oid(struct object_id *oid,
                                        struct multi_pack_index *m,
                                        uint32_t n)
{
        if (n >= m->num_objects)
                return NULL;

        oidread(oid, m->chunk_oid_lookup + m->hash_len * n);
        return oid;
}

off_t nth_midxed_offset(struct multi_pack_index *m, uint32_t pos)
{
        const unsigned char *offset_data;
        uint32_t offset32;

        offset_data = m->chunk_object_offsets + (off_t)pos * MIDX_CHUNK_OFFSET_WIDTH;
        offset32 = get_be32(offset_data + sizeof(uint32_t));

        if (m->chunk_large_offsets && offset32 & MIDX_LARGE_OFFSET_NEEDED) {
                if (sizeof(off_t) < sizeof(uint64_t))
                        die(_("multi-pack-index stores a 64-bit offset, but off_t is too small"));

                offset32 ^= MIDX_LARGE_OFFSET_NEEDED;
                return get_be64(m->chunk_large_offsets + sizeof(uint64_t) * offset32);
        }

        return offset32;
}

uint32_t nth_midxed_pack_int_id(struct multi_pack_index *m, uint32_t pos)
{
        return get_be32(m->chunk_object_offsets +
                        (off_t)pos * MIDX_CHUNK_OFFSET_WIDTH);
}

int fill_midx_entry(struct repository *r,
                    const struct object_id *oid,
                    struct pack_entry *e,
                    struct multi_pack_index *m)
{
        uint32_t pos;
        uint32_t pack_int_id;
        struct packed_git *p;

        if (!bsearch_midx(oid, m, &pos))
                return 0;

        if (pos >= m->num_objects)
                return 0;

        pack_int_id = nth_midxed_pack_int_id(m, pos);

        if (prepare_midx_pack(r, m, pack_int_id))
                return 0;
        p = m->packs[pack_int_id];

        /*
        * We are about to tell the caller where they can locate the
        * requested object.  We better make sure the packfile is
        * still here and can be accessed before supplying that
        * answer, as it may have been deleted since the MIDX was
        * loaded!
        */
        if (!is_pack_valid(p))
                return 0;

        if (oidset_size(&p->bad_objects) &&
            oidset_contains(&p->bad_objects, oid))
                return 0;

        e->offset = nth_midxed_offset(m, pos);
        e->p = p;

        return 1;
}

/* Match "foo.idx" against either "foo.pack" _or_ "foo.idx". */
static int cmp_idx_or_pack_name(const char *idx_or_pack_name,
                                const char *idx_name)
{
        /* Skip past any initial matching prefix. */
        while (*idx_name && *idx_name == *idx_or_pack_name) {
                idx_name++;
                idx_or_pack_name++;
        }

        /*
         * If we didn't match completely, we may have matched "pack-1234." and
         * be left with "idx" and "pack" respectively, which is also OK. We do
         * not have to check for "idx" and "idx", because that would have been
         * a complete match (and in that case these strcmps will be false, but
         * we'll correctly return 0 from the final strcmp() below.
         *
         * Technically this matches "fooidx" and "foopack", but we'd never have
         * such names in the first place.
         */
        if (!strcmp(idx_name, "idx") && !strcmp(idx_or_pack_name, "pack"))
                return 0;

        /*
         * This not only checks for a complete match, but also orders based on
         * the first non-identical character, which means our ordering will
         * match a raw strcmp(). That makes it OK to use this to binary search
         * a naively-sorted list.
         */
        return strcmp(idx_or_pack_name, idx_name);
}

int midx_contains_pack(struct multi_pack_index *m, const char *idx_or_pack_name)
{
        uint32_t first = 0, last = m->num_packs;

        while (first < last) {
                uint32_t mid = first + (last - first) / 2;
                const char *current;
                int cmp;

                current = m->pack_names[mid];
                cmp = cmp_idx_or_pack_name(idx_or_pack_name, current);
                if (!cmp)
                        return 1;
                if (cmp > 0) {
                        first = mid + 1;
                        continue;
                }
                last = mid;
        }

        return 0;
}

int prepare_multi_pack_index_one(struct repository *r, const char *object_dir, int local)
{
        struct multi_pack_index *m;
        struct multi_pack_index *m_search;

        prepare_repo_settings(r);
        if (!r->settings.core_multi_pack_index)
                return 0;

        for (m_search = r->objects->multi_pack_index; m_search; m_search = m_search->next)
                if (!strcmp(object_dir, m_search->object_dir))
                        return 1;

        m = load_multi_pack_index(object_dir, local);

        if (m) {
                struct multi_pack_index *mp = r->objects->multi_pack_index;
                if (mp) {
                        m->next = mp->next;
                        mp->next = m;
                } else
                        r->objects->multi_pack_index = m;
                return 1;
        }

        return 0;
}

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;
        unsigned expired : 1;
};

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;
        uint32_t nr;
        uint32_t alloc;
        struct multi_pack_index *m;
        struct progress *progress;
        unsigned pack_paths_checked;

        struct pack_midx_entry *entries;
        uint32_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;

        struct string_list *to_include;
};

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;

        if (ends_with(file_name, ".idx")) {
                display_progress(ctx->progress, ++ctx->pack_paths_checked);
                /*
                 * 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;
                else if (ctx->to_include &&
                         !string_list_has_string(ctx->to_include, file_name))
                        return;

                ALLOC_GROW(ctx->info, ctx->nr + 1, ctx->alloc);

                ctx->info[ctx->nr].p = add_packed_git(full_path,
                                                      full_path_len,
                                                      0);

                if (!ctx->info[ctx->nr].p) {
                        warning(_("failed to add packfile '%s'"),
                                full_path);
                        return;
                }

                if (open_pack_index(ctx->info[ctx->nr].p)) {
                        warning(_("failed to open pack-index '%s'"),
                                full_path);
                        close_pack(ctx->info[ctx->nr].p);
                        FREE_AND_NULL(ctx->info[ctx->nr].p);
                        return;
                }

                ctx->info[ctx->nr].pack_name = xstrdup(file_name);
                ctx->info[ctx->nr].orig_pack_int_id = ctx->nr;
                ctx->info[ctx->nr].expired = 0;
                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)
                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;
        uint32_t nr;
        uint32_t alloc;
};

static void midx_fanout_grow(struct midx_fanout *fanout, uint32_t 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 = 0, end;
        uint32_t cur_object;

        if (cur_fanout)
                start = ntohl(m->chunk_oid_fanout[cur_fanout - 1]);
        end = 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 struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
                                                  struct pack_info *info,
                                                  uint32_t nr_packs,
                                                  uint32_t *nr_objects,
                                                  int preferred_pack)
{
        uint32_t cur_fanout, cur_pack, cur_object;
        uint32_t alloc_objects, total_objects = 0;
        struct midx_fanout fanout = { 0 };
        struct pack_midx_entry *deduplicated_entries = NULL;
        uint32_t start_pack = m ? m->num_packs : 0;

        for (cur_pack = start_pack; cur_pack < nr_packs; cur_pack++)
                total_objects += 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(deduplicated_entries, alloc_objects);
        *nr_objects = 0;

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

                if (m)
                        midx_fanout_add_midx_fanout(&fanout, m, cur_fanout,
                                                    preferred_pack);

                for (cur_pack = start_pack; cur_pack < nr_packs; cur_pack++) {
                        int preferred = cur_pack == preferred_pack;
                        midx_fanout_add_pack_fanout(&fanout,
                                                    info, cur_pack,
                                                    preferred, cur_fanout);
                }

                if (-1 < preferred_pack && preferred_pack < start_pack)
                        midx_fanout_add_pack_fanout(&fanout, info,
                                                    preferred_pack, 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;

                        ALLOC_GROW(deduplicated_entries, *nr_objects + 1, alloc_objects);
                        memcpy(&deduplicated_entries[*nr_objects],
                               &fanout.entries[cur_object],
                               sizeof(struct pack_midx_entry));
                        (*nr_objects)++;
                }
        }

        free(fanout.entries);
        return deduplicated_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_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;

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

        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;
        uint32_t i;

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

        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);

        ALLOC_ARRAY(pack_order, ctx->entries_nr);
        for (i = 0; i < ctx->entries_nr; i++)
                pack_order[i] = data[i].nr;
        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 clear_midx_files_ext(const char *object_dir, const char *ext,
                                 unsigned char *keep_hash);

static int midx_checksum_valid(struct multi_pack_index *m)
{
        return hashfile_checksum_valid(m->data, m->data_len);
}

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++) {
                struct pack_midx_entry *from = &ctx->entries[ctx->pack_order[i]];
                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 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(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);
                for_each_ref(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 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_show_progress(flags & MIDX_PROGRESS);
        bitmap_writer_build_type_index(pdata, index, pdata->nr_objects);

        /*
         * 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(commits, commits_nr, -1);
        ret = bitmap_writer_build(pdata);
        if (ret < 0)
                goto cleanup;

        bitmap_writer_set_checksum(midx_hash);
        bitmap_writer_finish(index, pdata->nr_objects, bitmap_name, options);

cleanup:
        free(index);
        free(bitmap_name);

        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 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;
        struct hashfile *f = NULL;
        struct lock_file lk;
        struct write_midx_context ctx = { 0 };
        int pack_name_concat_len = 0;
        int dropped_packs = 0;
        int result = 0;
        struct chunkfile *cf;

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

        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) {
                /*
                 * 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.
                 */
                ctx.m = lookup_multi_pack_index(the_repository, object_dir);
        }

        if (ctx.m && !midx_checksum_valid(ctx.m)) {
                warning(_("ignoring existing multi-pack-index; checksum mismatch"));
                ctx.m = NULL;
        }

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

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

                        ctx.info[ctx.nr].orig_pack_int_id = i;
                        ctx.info[ctx.nr].pack_name = xstrdup(ctx.m->pack_names[i]);
                        ctx.info[ctx.nr].p = ctx.m->packs[i];
                        ctx.info[ctx.nr].expired = 0;

                        if (flags & MIDX_WRITE_REV_INDEX) {
                                /*
                                 * If generating a reverse index, need to have
                                 * packed_git's loaded to compare their
                                 * mtimes and object count.
                                 */
                                if (prepare_midx_pack(the_repository, ctx.m, i)) {
                                        error(_("could not load pack"));
                                        result = 1;
                                        goto cleanup;
                                }

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

                        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) &&
            !(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 (preferred_pack_name) {
                int found = 0;
                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;
                                found = 1;
                                break;
                        }
                }

                if (!found)
                        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;
                }
        }

        ctx.entries = get_sorted_entries(ctx.m, ctx.info, ctx.nr, &ctx.entries_nr,
                                         ctx.preferred_pack_idx);

        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)
                        pack_name_concat_len += strlen(ctx.info[i].pack_name) + 1;
        }

        /* 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);

        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));

        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);
        }

        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,
                  (size_t)ctx.entries_nr * the_hash_algo->rawsz,
                  write_midx_oid_lookup);
        add_chunk(cf, MIDX_CHUNKID_OBJECTOFFSETS,
                  (size_t)ctx.entries_nr * MIDX_CHUNK_OFFSET_WIDTH,
                  write_midx_object_offsets);

        if (ctx.large_offsets_needed)
                add_chunk(cf, MIDX_CHUNKID_LARGEOFFSETS,
                        (size_t)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,
                          ctx.entries_nr * sizeof(uint32_t),
                          write_midx_revindex);
        }

        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;
                        goto cleanup;
                }
        }
        /*
         * NOTE: Do not use ctx.entries beyond this point, since it might
         * have been freed in the previous if block.
         */

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

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

        clear_midx_files_ext(object_dir, ".bitmap", midx_hash);
        clear_midx_files_ext(object_dir, ".rev", midx_hash);

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);
        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);
}

struct clear_midx_data {
        char *keep;
        const char *ext;
};

static void clear_midx_file_ext(const char *full_path, size_t full_path_len,
                                const char *file_name, void *_data)
{
        struct clear_midx_data *data = _data;

        if (!(starts_with(file_name, "multi-pack-index-") &&
              ends_with(file_name, data->ext)))
                return;
        if (data->keep && !strcmp(data->keep, file_name))
                return;

        if (unlink(full_path))
                die_errno(_("failed to remove %s"), full_path);
}

static void clear_midx_files_ext(const char *object_dir, const char *ext,
                                 unsigned char *keep_hash)
{
        struct clear_midx_data data;
        memset(&data, 0, sizeof(struct clear_midx_data));

        if (keep_hash)
                data.keep = xstrfmt("multi-pack-index-%s%s",
                                    hash_to_hex(keep_hash), ext);
        data.ext = ext;

        for_each_file_in_pack_dir(object_dir,
                                  clear_midx_file_ext,
                                  &data);

        free(data.keep);
}

void clear_midx_file(struct repository *r)
{
        struct strbuf midx = STRBUF_INIT;

        get_midx_filename(&midx, r->objects->odb->path);

        if (r->objects && r->objects->multi_pack_index) {
                close_midx(r->objects->multi_pack_index);
                r->objects->multi_pack_index = NULL;
        }

        if (remove_path(midx.buf))
                die(_("failed to clear multi-pack-index at %s"), midx.buf);

        clear_midx_files_ext(r->objects->odb->path, ".bitmap", NULL);
        clear_midx_files_ext(r->objects->odb->path, ".rev", NULL);

        strbuf_release(&midx);
}

static int verify_midx_error;

__attribute__((format (printf, 1, 2)))
static void midx_report(const char *fmt, ...)
{
        va_list ap;
        verify_midx_error = 1;
        va_start(ap, fmt);
        vfprintf(stderr, fmt, ap);
        fprintf(stderr, "\n");
        va_end(ap);
}

struct pair_pos_vs_id
{
        uint32_t pos;
        uint32_t pack_int_id;
};

static int compare_pair_pos_vs_id(const void *_a, const void *_b)
{
        struct pair_pos_vs_id *a = (struct pair_pos_vs_id *)_a;
        struct pair_pos_vs_id *b = (struct pair_pos_vs_id *)_b;

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

/*
 * Limit calls to display_progress() for performance reasons.
 * The interval here was arbitrarily chosen.
 */
#define SPARSE_PROGRESS_INTERVAL (1 << 12)
#define midx_display_sparse_progress(progress, n) \
        do { \
                uint64_t _n = (n); \
                if ((_n & (SPARSE_PROGRESS_INTERVAL - 1)) == 0) \
                        display_progress(progress, _n); \
        } while (0)

int verify_midx_file(struct repository *r, const char *object_dir, unsigned flags)
{
        struct pair_pos_vs_id *pairs = NULL;
        uint32_t i;
        struct progress *progress = NULL;
        struct multi_pack_index *m = load_multi_pack_index(object_dir, 1);
        verify_midx_error = 0;

        if (!m) {
                int result = 0;
                struct stat sb;
                struct strbuf filename = STRBUF_INIT;

                get_midx_filename(&filename, object_dir);

                if (!stat(filename.buf, &sb)) {
                        error(_("multi-pack-index file exists, but failed to parse"));
                        result = 1;
                }
                strbuf_release(&filename);
                return result;
        }

        if (!midx_checksum_valid(m))
                midx_report(_("incorrect checksum"));

        if (flags & MIDX_PROGRESS)
                progress = start_delayed_progress(_("Looking for referenced packfiles"),
                                          m->num_packs);
        for (i = 0; i < m->num_packs; i++) {
                if (prepare_midx_pack(r, m, i))
                        midx_report("failed to load pack in position %d", i);

                display_progress(progress, i + 1);
        }
        stop_progress(&progress);

        for (i = 0; i < 255; i++) {
                uint32_t oid_fanout1 = ntohl(m->chunk_oid_fanout[i]);
                uint32_t oid_fanout2 = ntohl(m->chunk_oid_fanout[i + 1]);

                if (oid_fanout1 > oid_fanout2)
                        midx_report(_("oid fanout out of order: fanout[%d] = %"PRIx32" > %"PRIx32" = fanout[%d]"),
                                    i, oid_fanout1, oid_fanout2, i + 1);
        }

        if (m->num_objects == 0) {
                midx_report(_("the midx contains no oid"));
                /*
                 * Remaining tests assume that we have objects, so we can
                 * return here.
                 */
                goto cleanup;
        }

        if (flags & MIDX_PROGRESS)
                progress = start_sparse_progress(_("Verifying OID order in multi-pack-index"),
                                                 m->num_objects - 1);
        for (i = 0; i < m->num_objects - 1; i++) {
                struct object_id oid1, oid2;

                nth_midxed_object_oid(&oid1, m, i);
                nth_midxed_object_oid(&oid2, m, i + 1);

                if (oidcmp(&oid1, &oid2) >= 0)
                        midx_report(_("oid lookup out of order: oid[%d] = %s >= %s = oid[%d]"),
                                    i, oid_to_hex(&oid1), oid_to_hex(&oid2), i + 1);

                midx_display_sparse_progress(progress, i + 1);
        }
        stop_progress(&progress);

        /*
         * Create an array mapping each object to its packfile id.  Sort it
         * to group the objects by packfile.  Use this permutation to visit
         * each of the objects and only require 1 packfile to be open at a
         * time.
         */
        ALLOC_ARRAY(pairs, m->num_objects);
        for (i = 0; i < m->num_objects; i++) {
                pairs[i].pos = i;
                pairs[i].pack_int_id = nth_midxed_pack_int_id(m, i);
        }

        if (flags & MIDX_PROGRESS)
                progress = start_sparse_progress(_("Sorting objects by packfile"),
                                                 m->num_objects);
        display_progress(progress, 0); /* TODO: Measure QSORT() progress */
        QSORT(pairs, m->num_objects, compare_pair_pos_vs_id);
        stop_progress(&progress);

        if (flags & MIDX_PROGRESS)
                progress = start_sparse_progress(_("Verifying object offsets"), m->num_objects);
        for (i = 0; i < m->num_objects; i++) {
                struct object_id oid;
                struct pack_entry e;
                off_t m_offset, p_offset;

                if (i > 0 && pairs[i-1].pack_int_id != pairs[i].pack_int_id &&
                    m->packs[pairs[i-1].pack_int_id])
                {
                        close_pack_fd(m->packs[pairs[i-1].pack_int_id]);
                        close_pack_index(m->packs[pairs[i-1].pack_int_id]);
                }

                nth_midxed_object_oid(&oid, m, pairs[i].pos);

                if (!fill_midx_entry(r, &oid, &e, m)) {
                        midx_report(_("failed to load pack entry for oid[%d] = %s"),
                                    pairs[i].pos, oid_to_hex(&oid));
                        continue;
                }

                if (open_pack_index(e.p)) {
                        midx_report(_("failed to load pack-index for packfile %s"),
                                    e.p->pack_name);
                        break;
                }

                m_offset = e.offset;
                p_offset = find_pack_entry_one(oid.hash, e.p);

                if (m_offset != p_offset)
                        midx_report(_("incorrect object offset for oid[%d] = %s: %"PRIx64" != %"PRIx64),
                                    pairs[i].pos, oid_to_hex(&oid), m_offset, p_offset);

                midx_display_sparse_progress(progress, i + 1);
        }
        stop_progress(&progress);

cleanup:
        free(pairs);
        close_midx(m);

        return verify_midx_error;
}

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;

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

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

        for (i = 0; i < m->num_packs; i++) {
                if (prepare_midx_pack(r, m, i))
                        continue;
                if (!pack_kept_objects && m->packs[i]->pack_keep)
                        continue;
                if (m->packs[i]->is_cruft)
                        continue;

                include_pack[i] = 1;
                count++;
        }

        return count < 2;
}

static int fill_included_packs_batch(struct repository *r,
                                     struct multi_pack_index *m,
                                     unsigned char *include_pack,
                                     size_t batch_size)
{
        uint32_t i, packs_to_repack;
        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;
        packs_to_repack = 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 (!p)
                        continue;
                if (!pack_kept_objects && p->pack_keep)
                        continue;
                if (p->is_cruft)
                        continue;
                if (open_pack_index(p) || !p->num_objects)
                        continue;

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

                if (expected_size >= batch_size)
                        continue;

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

        free(pack_info);

        if (packs_to_repack < 2)
                return 1;

        return 0;
}

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

        CALLOC_ARRAY(include_pack, m->num_packs);

        if (batch_size) {
                if (fill_included_packs_batch(r, m, include_pack, batch_size))
                        goto cleanup;
        } else if (fill_included_packs_all(r, m, include_pack))
                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");

        strbuf_addstr(&base_name, object_dir);
        strbuf_addstr(&base_name, "/pack/pack");
        strvec_push(&cmd.args, base_name.buf);

        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");

        strbuf_release(&base_name);

        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;
}