remote-mediawiki doc: correct link to GitHub project

[thirdparty/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 "sha1-lookup.h"
#include "midx.h"
#include "progress.h"
#include "trace2.h"
#include "run-command.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_MAX_CHUNKS 5
#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_CHUNKLOOKUP_WIDTH (sizeof(uint32_t) + sizeof(uint64_t))
#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

static uint8_t oid_version(void)
{
        switch (hash_algo_by_ptr(the_hash_algo)) {
        case GIT_HASH_SHA1:
                return 1;
        case GIT_HASH_SHA256:
                return 2;
        default:
                die(_("invalid hash version"));
        }
}

static char *get_midx_filename(const char *object_dir)
{
        return xstrfmt("%s/pack/multi-pack-index", object_dir);
}

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;
        char *midx_name = get_midx_filename(object_dir);
        uint32_t i;
        const char *cur_pack_name;

        fd = git_open(midx_name);

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

        FREE_AND_NULL(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()) {
                error(_("multi-pack-index hash version %u does not match version %u"),
                      hash_version, oid_version());
                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);

        for (i = 0; i < m->num_chunks; i++) {
                uint32_t chunk_id = get_be32(m->data + MIDX_HEADER_SIZE +
                                             MIDX_CHUNKLOOKUP_WIDTH * i);
                uint64_t chunk_offset = get_be64(m->data + MIDX_HEADER_SIZE + 4 +
                                                 MIDX_CHUNKLOOKUP_WIDTH * i);

                if (chunk_offset >= m->data_len)
                        die(_("invalid chunk offset (too large)"));

                switch (chunk_id) {
                        case MIDX_CHUNKID_PACKNAMES:
                                m->chunk_pack_names = m->data + chunk_offset;
                                break;

                        case MIDX_CHUNKID_OIDFANOUT:
                                m->chunk_oid_fanout = (uint32_t *)(m->data + chunk_offset);
                                break;

                        case MIDX_CHUNKID_OIDLOOKUP:
                                m->chunk_oid_lookup = m->data + chunk_offset;
                                break;

                        case MIDX_CHUNKID_OBJECTOFFSETS:
                                m->chunk_object_offsets = m->data + chunk_offset;
                                break;

                        case MIDX_CHUNKID_LARGEOFFSETS:
                                m->chunk_large_offsets = m->data + chunk_offset;
                                break;

                        case 0:
                                die(_("terminating multi-pack-index chunk id appears earlier than expected"));
                                break;

                        default:
                                /*
                                 * Do nothing on unrecognized chunks, allowing future
                                 * extensions to add optional chunks.
                                 */
                                break;
                }
        }

        if (!m->chunk_pack_names)
                die(_("multi-pack-index missing required pack-name chunk"));
        if (!m->chunk_oid_fanout)
                die(_("multi-pack-index missing required OID fanout chunk"));
        if (!m->chunk_oid_lookup)
                die(_("multi-pack-index missing required OID lookup chunk"));
        if (!m->chunk_object_offsets)
                die(_("multi-pack-index missing required object offsets chunk"));

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

        m->pack_names = xcalloc(m->num_packs, sizeof(*m->pack_names));
        m->packs = xcalloc(m->num_packs, sizeof(*m->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);

        return m;

cleanup_fail:
        free(m);
        free(midx_name);
        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;

        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);
}

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;

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

static 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 + 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;
}

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

static int nth_midxed_pack_entry(struct repository *r,
                                 struct multi_pack_index *m,
                                 struct pack_entry *e,
                                 uint32_t pos)
{
        uint32_t pack_int_id;
        struct packed_git *p;

        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))
                die(_("error preparing packfile from multi-pack-index"));
        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 (p->num_bad_objects) {
                uint32_t i;
                struct object_id oid;
                nth_midxed_object_oid(&oid, m, pos);
                for (i = 0; i < p->num_bad_objects; i++)
                        if (hasheq(oid.hash,
                                   p->bad_object_sha1 + the_hash_algo->rawsz * i))
                                return 0;
        }

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

        return 1;
}

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

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

        return nth_midxed_pack_entry(r, m, e, pos);
}

/* 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;
        int config_value;
        static int env_value = -1;

        if (env_value < 0)
                env_value = git_env_bool(GIT_TEST_MULTI_PACK_INDEX, 0);

        if (!env_value &&
            (repo_config_get_bool(r, "core.multipackindex", &config_value) ||
            !config_value))
                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)
{
        unsigned char byte_values[4];

        hashwrite_be32(f, MIDX_SIGNATURE);
        byte_values[0] = MIDX_VERSION;
        byte_values[1] = oid_version();
        byte_values[2] = num_chunks;
        byte_values[3] = 0; /* unused */
        hashwrite(f, byte_values, sizeof(byte_values));
        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);
}

struct pack_list {
        struct pack_info *info;
        uint32_t nr;
        uint32_t alloc;
        struct multi_pack_index *m;
        struct progress *progress;
        unsigned pack_paths_checked;
};

static void add_pack_to_midx(const char *full_path, size_t full_path_len,
                             const char *file_name, void *data)
{
        struct pack_list *packs = (struct pack_list *)data;

        if (ends_with(file_name, ".idx")) {
                display_progress(packs->progress, ++packs->pack_paths_checked);
                if (packs->m && midx_contains_pack(packs->m, file_name))
                        return;

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

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

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

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

                packs->info[packs->nr].pack_name = xstrdup(file_name);
                packs->info[packs->nr].orig_pack_int_id = packs->nr;
                packs->info[packs->nr].expired = 0;
                packs->nr++;
        }
}

struct pack_midx_entry {
        struct object_id oid;
        uint32_t pack_int_id;
        time_t pack_mtime;
        uint64_t offset;
};

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;

        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)
{
        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);
}

/*
 * 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)
{
        uint32_t cur_fanout, cur_pack, cur_object;
        uint32_t alloc_fanout, alloc_objects, total_objects = 0;
        struct pack_midx_entry *entries_by_fanout = NULL;
        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 = alloc_fanout = total_objects > 3200 ? total_objects / 200 : 16;

        ALLOC_ARRAY(entries_by_fanout, alloc_fanout);
        ALLOC_ARRAY(deduplicated_entries, alloc_objects);
        *nr_objects = 0;

        for (cur_fanout = 0; cur_fanout < 256; cur_fanout++) {
                uint32_t nr_fanout = 0;

                if (m) {
                        uint32_t start = 0, end;

                        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++) {
                                ALLOC_GROW(entries_by_fanout, nr_fanout + 1, alloc_fanout);
                                nth_midxed_pack_midx_entry(m,
                                                           &entries_by_fanout[nr_fanout],
                                                           cur_object);
                                nr_fanout++;
                        }
                }

                for (cur_pack = start_pack; cur_pack < nr_packs; cur_pack++) {
                        uint32_t start = 0, end;

                        if (cur_fanout)
                                start = get_pack_fanout(info[cur_pack].p, cur_fanout - 1);
                        end = get_pack_fanout(info[cur_pack].p, cur_fanout);

                        for (cur_object = start; cur_object < end; cur_object++) {
                                ALLOC_GROW(entries_by_fanout, nr_fanout + 1, alloc_fanout);
                                fill_pack_entry(cur_pack, info[cur_pack].p, cur_object, &entries_by_fanout[nr_fanout]);
                                nr_fanout++;
                        }
                }

                QSORT(entries_by_fanout, nr_fanout, midx_oid_compare);

                /*
                 * The batch is now sorted by OID and then mtime (descending).
                 * Take only the first duplicate.
                 */
                for (cur_object = 0; cur_object < nr_fanout; cur_object++) {
                        if (cur_object && oideq(&entries_by_fanout[cur_object - 1].oid,
                                                &entries_by_fanout[cur_object].oid))
                                continue;

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

        free(entries_by_fanout);
        return deduplicated_entries;
}

static size_t write_midx_pack_names(struct hashfile *f,
                                    struct pack_info *info,
                                    uint32_t num_packs)
{
        uint32_t i;
        unsigned char padding[MIDX_CHUNK_ALIGNMENT];
        size_t written = 0;

        for (i = 0; i < num_packs; i++) {
                size_t writelen;

                if (info[i].expired)
                        continue;

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

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

        return written;
}

static size_t write_midx_oid_fanout(struct hashfile *f,
                                    struct pack_midx_entry *objects,
                                    uint32_t nr_objects)
{
        struct pack_midx_entry *list = objects;
        struct pack_midx_entry *last = objects + nr_objects;
        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 MIDX_CHUNK_FANOUT_SIZE;
}

static size_t write_midx_oid_lookup(struct hashfile *f, unsigned char hash_len,
                                    struct pack_midx_entry *objects,
                                    uint32_t nr_objects)
{
        struct pack_midx_entry *list = objects;
        uint32_t i;
        size_t written = 0;

        for (i = 0; i < nr_objects; i++) {
                struct pack_midx_entry *obj = list++;

                if (i < nr_objects - 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);
                written += hash_len;
        }

        return written;
}

static size_t write_midx_object_offsets(struct hashfile *f, int large_offset_needed,
                                        uint32_t *perm,
                                        struct pack_midx_entry *objects, uint32_t nr_objects)
{
        struct pack_midx_entry *list = objects;
        uint32_t i, nr_large_offset = 0;
        size_t written = 0;

        for (i = 0; i < nr_objects; i++) {
                struct pack_midx_entry *obj = list++;

                if (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, perm[obj->pack_int_id]);

                if (large_offset_needed && obj->offset >> 31)
                        hashwrite_be32(f, MIDX_LARGE_OFFSET_NEEDED | nr_large_offset++);
                else if (!large_offset_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);

                written += MIDX_CHUNK_OFFSET_WIDTH;
        }

        return written;
}

static size_t write_midx_large_offsets(struct hashfile *f, uint32_t nr_large_offset,
                                       struct pack_midx_entry *objects, uint32_t nr_objects)
{
        struct pack_midx_entry *list = objects, *end = objects + nr_objects;
        size_t written = 0;

        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_be32(f, offset >> 32);
                hashwrite_be32(f, offset & 0xffffffffUL);
                written += 2 * sizeof(uint32_t);

                nr_large_offset--;
        }

        return written;
}

static int write_midx_internal(const char *object_dir, struct multi_pack_index *m,
                               struct string_list *packs_to_drop, unsigned flags)
{
        unsigned char cur_chunk, num_chunks = 0;
        char *midx_name;
        uint32_t i;
        struct hashfile *f = NULL;
        struct lock_file lk;
        struct pack_list packs;
        uint32_t *pack_perm = NULL;
        uint64_t written = 0;
        uint32_t chunk_ids[MIDX_MAX_CHUNKS + 1];
        uint64_t chunk_offsets[MIDX_MAX_CHUNKS + 1];
        uint32_t nr_entries, num_large_offsets = 0;
        struct pack_midx_entry *entries = NULL;
        struct progress *progress = NULL;
        int large_offsets_needed = 0;
        int pack_name_concat_len = 0;
        int dropped_packs = 0;
        int result = 0;

        midx_name = get_midx_filename(object_dir);
        if (safe_create_leading_directories(midx_name))
                die_errno(_("unable to create leading directories of %s"),
                          midx_name);

        if (m)
                packs.m = m;
        else
                packs.m = load_multi_pack_index(object_dir, 1);

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

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

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

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

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

        if (packs.m && packs.nr == packs.m->num_packs && !packs_to_drop)
                goto cleanup;

        entries = get_sorted_entries(packs.m, packs.info, packs.nr, &nr_entries);

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

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

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

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

                        if (!cmp) {
                                drop_index++;
                                packs.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 {
                                packs.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(pack_perm, packs.nr);
        for (i = 0; i < packs.nr; i++) {
                if (packs.info[i].expired) {
                        dropped_packs++;
                        pack_perm[packs.info[i].orig_pack_int_id] = PACK_EXPIRED;
                } else {
                        pack_perm[packs.info[i].orig_pack_int_id] = i - dropped_packs;
                }
        }

        for (i = 0; i < packs.nr; i++) {
                if (!packs.info[i].expired)
                        pack_name_concat_len += strlen(packs.info[i].pack_name) + 1;
        }

        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, LOCK_DIE_ON_ERROR);
        f = hashfd(lk.tempfile->fd, lk.tempfile->filename.buf);
        FREE_AND_NULL(midx_name);

        if (packs.m)
                close_midx(packs.m);

        cur_chunk = 0;
        num_chunks = large_offsets_needed ? 5 : 4;

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

        written = write_midx_header(f, num_chunks, packs.nr - dropped_packs);

        chunk_ids[cur_chunk] = MIDX_CHUNKID_PACKNAMES;
        chunk_offsets[cur_chunk] = written + (num_chunks + 1) * MIDX_CHUNKLOOKUP_WIDTH;

        cur_chunk++;
        chunk_ids[cur_chunk] = MIDX_CHUNKID_OIDFANOUT;
        chunk_offsets[cur_chunk] = chunk_offsets[cur_chunk - 1] + pack_name_concat_len;

        cur_chunk++;
        chunk_ids[cur_chunk] = MIDX_CHUNKID_OIDLOOKUP;
        chunk_offsets[cur_chunk] = chunk_offsets[cur_chunk - 1] + MIDX_CHUNK_FANOUT_SIZE;

        cur_chunk++;
        chunk_ids[cur_chunk] = MIDX_CHUNKID_OBJECTOFFSETS;
        chunk_offsets[cur_chunk] = chunk_offsets[cur_chunk - 1] + nr_entries * the_hash_algo->rawsz;

        cur_chunk++;
        chunk_offsets[cur_chunk] = chunk_offsets[cur_chunk - 1] + nr_entries * MIDX_CHUNK_OFFSET_WIDTH;
        if (large_offsets_needed) {
                chunk_ids[cur_chunk] = MIDX_CHUNKID_LARGEOFFSETS;

                cur_chunk++;
                chunk_offsets[cur_chunk] = chunk_offsets[cur_chunk - 1] +
                                           num_large_offsets * MIDX_CHUNK_LARGE_OFFSET_WIDTH;
        }

        chunk_ids[cur_chunk] = 0;

        for (i = 0; i <= num_chunks; i++) {
                if (i && chunk_offsets[i] < chunk_offsets[i - 1])
                        BUG("incorrect chunk offsets: %"PRIu64" before %"PRIu64,
                            chunk_offsets[i - 1],
                            chunk_offsets[i]);

                if (chunk_offsets[i] % MIDX_CHUNK_ALIGNMENT)
                        BUG("chunk offset %"PRIu64" is not properly aligned",
                            chunk_offsets[i]);

                hashwrite_be32(f, chunk_ids[i]);
                hashwrite_be32(f, chunk_offsets[i] >> 32);
                hashwrite_be32(f, chunk_offsets[i]);

                written += MIDX_CHUNKLOOKUP_WIDTH;
        }

        if (flags & MIDX_PROGRESS)
                progress = start_progress(_("Writing chunks to multi-pack-index"),
                                          num_chunks);
        for (i = 0; i < num_chunks; i++) {
                if (written != chunk_offsets[i])
                        BUG("incorrect chunk offset (%"PRIu64" != %"PRIu64") for chunk id %"PRIx32,
                            chunk_offsets[i],
                            written,
                            chunk_ids[i]);

                switch (chunk_ids[i]) {
                        case MIDX_CHUNKID_PACKNAMES:
                                written += write_midx_pack_names(f, packs.info, packs.nr);
                                break;

                        case MIDX_CHUNKID_OIDFANOUT:
                                written += write_midx_oid_fanout(f, entries, nr_entries);
                                break;

                        case MIDX_CHUNKID_OIDLOOKUP:
                                written += write_midx_oid_lookup(f, the_hash_algo->rawsz, entries, nr_entries);
                                break;

                        case MIDX_CHUNKID_OBJECTOFFSETS:
                                written += write_midx_object_offsets(f, large_offsets_needed, pack_perm, entries, nr_entries);
                                break;

                        case MIDX_CHUNKID_LARGEOFFSETS:
                                written += write_midx_large_offsets(f, num_large_offsets, entries, nr_entries);
                                break;

                        default:
                                BUG("trying to write unknown chunk id %"PRIx32,
                                    chunk_ids[i]);
                }

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

        if (written != chunk_offsets[num_chunks])
                BUG("incorrect final offset %"PRIu64" != %"PRIu64,
                    written,
                    chunk_offsets[num_chunks]);

        finalize_hashfile(f, NULL, CSUM_FSYNC | CSUM_HASH_IN_STREAM);
        commit_lock_file(&lk);

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

        free(packs.info);
        free(entries);
        free(pack_perm);
        free(midx_name);
        return result;
}

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

void clear_midx_file(struct repository *r)
{
        char *midx = get_midx_filename(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))
                die(_("failed to clear multi-pack-index at %s"), midx);

        free(midx);
}

static int verify_midx_error;

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;
                char *filename = get_midx_filename(object_dir);
                if (!stat(filename, &sb)) {
                        error(_("multi-pack-index file exists, but failed to parse"));
                        result = 1;
                }
                free(filename);
                return result;
        }

        if (flags & MIDX_PROGRESS)
                progress = start_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.
                 */
                return verify_midx_error;
        }

        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);

        free(pairs);

        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 = load_multi_pack_index(object_dir, 1);
        struct progress *progress = NULL;

        if (!m)
                return 0;

        count = xcalloc(m->num_packs, sizeof(uint32_t));

        if (flags & MIDX_PROGRESS)
                progress = start_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_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)
                        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, m, &packs_to_drop, 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;

                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 = xcalloc(m->num_packs, sizeof(struct repack_info));
        int pack_kept_objects = 0;

        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; batch_size && 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 (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 = load_multi_pack_index(object_dir, 1);

        /*
         * 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;

        include_pack = xcalloc(m->num_packs, sizeof(unsigned char));

        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, m, NULL, flags);
        m = NULL;

cleanup:
        if (m)
                close_midx(m);
        free(include_pack);
        return result;
}