midx: use midx in approximate_object_count

[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"

#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_HASH_VERSION 1
#define MIDX_HEADER_SIZE 12
#define MIDX_HASH_LEN 20
#define MIDX_MIN_SIZE (MIDX_HEADER_SIZE + MIDX_HASH_LEN)

#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

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)
{
        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);

        FLEX_ALLOC_MEM(m, object_dir, object_dir, strlen(object_dir));
        m->fd = fd;
        m->data = midx_map;
        m->data_len = midx_size;

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

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

        hash_version = m->data[MIDX_BYTE_HASH_VERSION];
        if (hash_version != MIDX_HASH_VERSION) {
                error(_("hash version %u does not match"), hash_version);
                goto cleanup_fail;
        }
        m->hash_len = MIDX_HASH_LEN;

        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);

                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) {
                        error(_("multi-pack-index pack names out of order: '%s' before '%s'"),
                              m->pack_names[i - 1],
                              m->pack_names[i]);
                        goto cleanup_fail;
                }
        }

        return m;

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

static void close_midx(struct multi_pack_index *m)
{
        uint32_t i;
        munmap((unsigned char *)m->data, m->data_len);
        close(m->fd);
        m->fd = -1;

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

static int prepare_midx_pack(struct multi_pack_index *m, uint32_t pack_int_id)
{
        struct strbuf pack_name = STRBUF_INIT;

        if (pack_int_id >= m->num_packs)
                BUG("bad pack-int-id");

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

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

        m->packs[pack_int_id] = add_packed_git(pack_name.buf, pack_name.len, 1);
        strbuf_release(&pack_name);
        return !m->packs[pack_int_id];
}

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,
                            MIDX_HASH_LEN, 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(offset32) < 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 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(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;

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

        return 1;
}

int fill_midx_entry(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(m, e, pos);
}

int midx_contains_pack(struct multi_pack_index *m, const char *idx_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 = strcmp(idx_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)
{
        struct multi_pack_index *m = r->objects->multi_pack_index;
        struct multi_pack_index *m_search;
        int config_value;

        if (repo_config_get_bool(r, "core.multipackindex", &config_value) ||
            !config_value)
                return 0;

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

        r->objects->multi_pack_index = load_multi_pack_index(object_dir);

        if (r->objects->multi_pack_index) {
                r->objects->multi_pack_index->next = 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] = MIDX_HASH_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_list {
        struct packed_git **list;
        char **names;
        uint32_t nr;
        uint32_t alloc_list;
        uint32_t alloc_names;
        size_t pack_name_concat_len;
        struct multi_pack_index *m;
};

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")) {
                if (packs->m && midx_contains_pack(packs->m, file_name))
                        return;

                ALLOC_GROW(packs->list, packs->nr + 1, packs->alloc_list);
                ALLOC_GROW(packs->names, packs->nr + 1, packs->alloc_names);

                packs->list[packs->nr] = add_packed_git(full_path,
                                                        full_path_len,
                                                        0);

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

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

                packs->names[packs->nr] = xstrdup(file_name);
                packs->pack_name_concat_len += strlen(file_name) + 1;
                packs->nr++;
        }
}

struct pack_pair {
        uint32_t pack_int_id;
        char *pack_name;
};

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

static void sort_packs_by_name(char **pack_names, uint32_t nr_packs, uint32_t *perm)
{
        uint32_t i;
        struct pack_pair *pairs;

        ALLOC_ARRAY(pairs, nr_packs);

        for (i = 0; i < nr_packs; i++) {
                pairs[i].pack_int_id = i;
                pairs[i].pack_name = pack_names[i];
        }

        QSORT(pairs, nr_packs, pack_pair_compare);

        for (i = 0; i < nr_packs; i++) {
                pack_names[i] = pairs[i].pack_name;
                perm[pairs[i].pack_int_id] = i;
        }

        free(pairs);
}

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,
                                      uint32_t *pack_perm,
                                      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 = pack_perm[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_oid(&entry->oid, p, cur_object))
                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 packed_git **p,
                                                  uint32_t *perm,
                                                  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 += p[cur_pack]->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, perm,
                                                           &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(p[cur_pack], cur_fanout - 1);
                        end = get_pack_fanout(p[cur_pack], cur_fanout);

                        for (cur_object = start; cur_object < end; cur_object++) {
                                ALLOC_GROW(entries_by_fanout, nr_fanout + 1, alloc_fanout);
                                fill_pack_entry(perm[cur_pack], p[cur_pack], 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 && !oidcmp(&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,
                                    char **pack_names,
                                    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 = strlen(pack_names[i]) + 1;

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

                hashwrite(f, pack_names[i], 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,
                                        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++;

                hashwrite_be32(f, 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;
        size_t written = 0;

        while (nr_large_offset) {
                struct pack_midx_entry *obj = list++;
                uint64_t 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;
}

int write_midx_file(const char *object_dir)
{
        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;
        int large_offsets_needed = 0;

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

        packs.m = load_multi_pack_index(object_dir);

        packs.nr = 0;
        packs.alloc_list = packs.m ? packs.m->num_packs : 16;
        packs.alloc_names = packs.alloc_list;
        packs.list = NULL;
        packs.names = NULL;
        packs.pack_name_concat_len = 0;
        ALLOC_ARRAY(packs.list, packs.alloc_list);
        ALLOC_ARRAY(packs.names, packs.alloc_names);

        if (packs.m) {
                for (i = 0; i < packs.m->num_packs; i++) {
                        ALLOC_GROW(packs.list, packs.nr + 1, packs.alloc_list);
                        ALLOC_GROW(packs.names, packs.nr + 1, packs.alloc_names);

                        packs.list[packs.nr] = NULL;
                        packs.names[packs.nr] = xstrdup(packs.m->pack_names[i]);
                        packs.pack_name_concat_len += strlen(packs.names[packs.nr]) + 1;
                        packs.nr++;
                }
        }

        for_each_file_in_pack_dir(object_dir, add_pack_to_midx, &packs);

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

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

        ALLOC_ARRAY(pack_perm, packs.nr);
        sort_packs_by_name(packs.names, packs.nr, pack_perm);

        entries = get_sorted_entries(packs.m, packs.list, pack_perm, 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;
        }

        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;

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

        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] + packs.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 * MIDX_HASH_LEN;

        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;
        }

        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.names, 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, MIDX_HASH_LEN, entries, nr_entries);
                                break;

                        case MIDX_CHUNKID_OBJECTOFFSETS:
                                written += write_midx_object_offsets(f, large_offsets_needed, 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]);
                }
        }

        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.list[i]) {
                        close_pack(packs.list[i]);
                        free(packs.list[i]);
                }
                free(packs.names[i]);
        }

        free(packs.list);
        free(packs.names);
        free(entries);
        free(pack_perm);
        free(midx_name);
        return 0;
}