pack-revindex: read multi-pack reverse indexes

[git/debian.git] / pack-revindex.c

#include "cache.h"
#include "pack-revindex.h"
#include "object-store.h"
#include "packfile.h"
#include "config.h"
#include "midx.h"

struct revindex_entry {
        off_t offset;
        unsigned int nr;
};

/*
 * Pack index for existing packs give us easy access to the offsets into
 * corresponding pack file where each object's data starts, but the entries
 * do not store the size of the compressed representation (uncompressed
 * size is easily available by examining the pack entry header).  It is
 * also rather expensive to find the sha1 for an object given its offset.
 *
 * The pack index file is sorted by object name mapping to offset;
 * this revindex array is a list of offset/index_nr pairs
 * ordered by offset, so if you know the offset of an object, next offset
 * is where its packed representation ends and the index_nr can be used to
 * get the object sha1 from the main index.
 */

/*
 * This is a least-significant-digit radix sort.
 *
 * It sorts each of the "n" items in "entries" by its offset field. The "max"
 * parameter must be at least as large as the largest offset in the array,
 * and lets us quit the sort early.
 */
static void sort_revindex(struct revindex_entry *entries, unsigned n, off_t max)
{
        /*
         * We use a "digit" size of 16 bits. That keeps our memory
         * usage reasonable, and we can generally (for a 4G or smaller
         * packfile) quit after two rounds of radix-sorting.
         */
#define DIGIT_SIZE (16)
#define BUCKETS (1 << DIGIT_SIZE)
        /*
         * We want to know the bucket that a[i] will go into when we are using
         * the digit that is N bits from the (least significant) end.
         */
#define BUCKET_FOR(a, i, bits) (((a)[(i)].offset >> (bits)) & (BUCKETS-1))

        /*
         * We need O(n) temporary storage. Rather than do an extra copy of the
         * partial results into "entries", we sort back and forth between the
         * real array and temporary storage. In each iteration of the loop, we
         * keep track of them with alias pointers, always sorting from "from"
         * to "to".
         */
        struct revindex_entry *tmp, *from, *to;
        int bits;
        unsigned *pos;

        ALLOC_ARRAY(pos, BUCKETS);
        ALLOC_ARRAY(tmp, n);
        from = entries;
        to = tmp;

        /*
         * If (max >> bits) is zero, then we know that the radix digit we are
         * on (and any higher) will be zero for all entries, and our loop will
         * be a no-op, as everybody lands in the same zero-th bucket.
         */
        for (bits = 0; max >> bits; bits += DIGIT_SIZE) {
                unsigned i;

                memset(pos, 0, BUCKETS * sizeof(*pos));

                /*
                 * We want pos[i] to store the index of the last element that
                 * will go in bucket "i" (actually one past the last element).
                 * To do this, we first count the items that will go in each
                 * bucket, which gives us a relative offset from the last
                 * bucket. We can then cumulatively add the index from the
                 * previous bucket to get the true index.
                 */
                for (i = 0; i < n; i++)
                        pos[BUCKET_FOR(from, i, bits)]++;
                for (i = 1; i < BUCKETS; i++)
                        pos[i] += pos[i-1];

                /*
                 * Now we can drop the elements into their correct buckets (in
                 * our temporary array).  We iterate the pos counter backwards
                 * to avoid using an extra index to count up. And since we are
                 * going backwards there, we must also go backwards through the
                 * array itself, to keep the sort stable.
                 *
                 * Note that we use an unsigned iterator to make sure we can
                 * handle 2^32-1 objects, even on a 32-bit system. But this
                 * means we cannot use the more obvious "i >= 0" loop condition
                 * for counting backwards, and must instead check for
                 * wrap-around with UINT_MAX.
                 */
                for (i = n - 1; i != UINT_MAX; i--)
                        to[--pos[BUCKET_FOR(from, i, bits)]] = from[i];

                /*
                 * Now "to" contains the most sorted list, so we swap "from" and
                 * "to" for the next iteration.
                 */
                SWAP(from, to);
        }

        /*
         * If we ended with our data in the original array, great. If not,
         * we have to move it back from the temporary storage.
         */
        if (from != entries)
                COPY_ARRAY(entries, tmp, n);
        free(tmp);
        free(pos);

#undef BUCKET_FOR
#undef BUCKETS
#undef DIGIT_SIZE
}

/*
 * Ordered list of offsets of objects in the pack.
 */
static void create_pack_revindex(struct packed_git *p)
{
        const unsigned num_ent = p->num_objects;
        unsigned i;
        const char *index = p->index_data;
        const unsigned hashsz = the_hash_algo->rawsz;

        ALLOC_ARRAY(p->revindex, num_ent + 1);
        index += 4 * 256;

        if (p->index_version > 1) {
                const uint32_t *off_32 =
                        (uint32_t *)(index + 8 + (size_t)p->num_objects * (hashsz + 4));
                const uint32_t *off_64 = off_32 + p->num_objects;
                for (i = 0; i < num_ent; i++) {
                        const uint32_t off = ntohl(*off_32++);
                        if (!(off & 0x80000000)) {
                                p->revindex[i].offset = off;
                        } else {
                                p->revindex[i].offset = get_be64(off_64);
                                off_64 += 2;
                        }
                        p->revindex[i].nr = i;
                }
        } else {
                for (i = 0; i < num_ent; i++) {
                        const uint32_t hl = *((uint32_t *)(index + (hashsz + 4) * i));
                        p->revindex[i].offset = ntohl(hl);
                        p->revindex[i].nr = i;
                }
        }

        /*
         * This knows the pack format -- the hash trailer
         * follows immediately after the last object data.
         */
        p->revindex[num_ent].offset = p->pack_size - hashsz;
        p->revindex[num_ent].nr = -1;
        sort_revindex(p->revindex, num_ent, p->pack_size);
}

static int create_pack_revindex_in_memory(struct packed_git *p)
{
        if (git_env_bool(GIT_TEST_REV_INDEX_DIE_IN_MEMORY, 0))
                die("dying as requested by '%s'",
                    GIT_TEST_REV_INDEX_DIE_IN_MEMORY);
        if (open_pack_index(p))
                return -1;
        create_pack_revindex(p);
        return 0;
}

static char *pack_revindex_filename(struct packed_git *p)
{
        size_t len;
        if (!strip_suffix(p->pack_name, ".pack", &len))
                BUG("pack_name does not end in .pack");
        return xstrfmt("%.*s.rev", (int)len, p->pack_name);
}

#define RIDX_HEADER_SIZE (12)
#define RIDX_MIN_SIZE (RIDX_HEADER_SIZE + (2 * the_hash_algo->rawsz))

struct revindex_header {
        uint32_t signature;
        uint32_t version;
        uint32_t hash_id;
};

static int load_revindex_from_disk(char *revindex_name,
                                   uint32_t num_objects,
                                   const uint32_t **data_p, size_t *len_p)
{
        int fd, ret = 0;
        struct stat st;
        void *data = NULL;
        size_t revindex_size;
        struct revindex_header *hdr;

        fd = git_open(revindex_name);

        if (fd < 0) {
                ret = -1;
                goto cleanup;
        }
        if (fstat(fd, &st)) {
                ret = error_errno(_("failed to read %s"), revindex_name);
                goto cleanup;
        }

        revindex_size = xsize_t(st.st_size);

        if (revindex_size < RIDX_MIN_SIZE) {
                ret = error(_("reverse-index file %s is too small"), revindex_name);
                goto cleanup;
        }

        if (revindex_size - RIDX_MIN_SIZE != st_mult(sizeof(uint32_t), num_objects)) {
                ret = error(_("reverse-index file %s is corrupt"), revindex_name);
                goto cleanup;
        }

        data = xmmap(NULL, revindex_size, PROT_READ, MAP_PRIVATE, fd, 0);
        hdr = data;

        if (ntohl(hdr->signature) != RIDX_SIGNATURE) {
                ret = error(_("reverse-index file %s has unknown signature"), revindex_name);
                goto cleanup;
        }
        if (ntohl(hdr->version) != 1) {
                ret = error(_("reverse-index file %s has unsupported version %"PRIu32),
                            revindex_name, ntohl(hdr->version));
                goto cleanup;
        }
        if (!(ntohl(hdr->hash_id) == 1 || ntohl(hdr->hash_id) == 2)) {
                ret = error(_("reverse-index file %s has unsupported hash id %"PRIu32),
                            revindex_name, ntohl(hdr->hash_id));
                goto cleanup;
        }

cleanup:
        if (ret) {
                if (data)
                        munmap(data, revindex_size);
        } else {
                *len_p = revindex_size;
                *data_p = (const uint32_t *)data;
        }

        close(fd);
        return ret;
}

static int load_pack_revindex_from_disk(struct packed_git *p)
{
        char *revindex_name;
        int ret;
        if (open_pack_index(p))
                return -1;

        revindex_name = pack_revindex_filename(p);

        ret = load_revindex_from_disk(revindex_name,
                                      p->num_objects,
                                      &p->revindex_map,
                                      &p->revindex_size);
        if (ret)
                goto cleanup;

        p->revindex_data = (const uint32_t *)((const char *)p->revindex_map + RIDX_HEADER_SIZE);

cleanup:
        free(revindex_name);
        return ret;
}

int load_pack_revindex(struct packed_git *p)
{
        if (p->revindex || p->revindex_data)
                return 0;

        if (!load_pack_revindex_from_disk(p))
                return 0;
        else if (!create_pack_revindex_in_memory(p))
                return 0;
        return -1;
}

int load_midx_revindex(struct multi_pack_index *m)
{
        char *revindex_name;
        int ret;
        if (m->revindex_data)
                return 0;

        revindex_name = get_midx_rev_filename(m);

        ret = load_revindex_from_disk(revindex_name,
                                      m->num_objects,
                                      &m->revindex_map,
                                      &m->revindex_len);
        if (ret)
                goto cleanup;

        m->revindex_data = (const uint32_t *)((const char *)m->revindex_map + RIDX_HEADER_SIZE);

cleanup:
        free(revindex_name);
        return ret;
}

int close_midx_revindex(struct multi_pack_index *m)
{
        if (!m || !m->revindex_map)
                return 0;

        munmap((void*)m->revindex_map, m->revindex_len);

        m->revindex_map = NULL;
        m->revindex_data = NULL;
        m->revindex_len = 0;

        return 0;
}

int offset_to_pack_pos(struct packed_git *p, off_t ofs, uint32_t *pos)
{
        unsigned lo, hi;

        if (load_pack_revindex(p) < 0)
                return -1;

        lo = 0;
        hi = p->num_objects + 1;

        do {
                const unsigned mi = lo + (hi - lo) / 2;
                off_t got = pack_pos_to_offset(p, mi);

                if (got == ofs) {
                        *pos = mi;
                        return 0;
                } else if (ofs < got)
                        hi = mi;
                else
                        lo = mi + 1;
        } while (lo < hi);

        error("bad offset for revindex");
        return -1;
}

uint32_t pack_pos_to_index(struct packed_git *p, uint32_t pos)
{
        if (!(p->revindex || p->revindex_data))
                BUG("pack_pos_to_index: reverse index not yet loaded");
        if (p->num_objects <= pos)
                BUG("pack_pos_to_index: out-of-bounds object at %"PRIu32, pos);

        if (p->revindex)
                return p->revindex[pos].nr;
        else
                return get_be32(p->revindex_data + pos);
}

off_t pack_pos_to_offset(struct packed_git *p, uint32_t pos)
{
        if (!(p->revindex || p->revindex_data))
                BUG("pack_pos_to_index: reverse index not yet loaded");
        if (p->num_objects < pos)
                BUG("pack_pos_to_offset: out-of-bounds object at %"PRIu32, pos);

        if (p->revindex)
                return p->revindex[pos].offset;
        else if (pos == p->num_objects)
                return p->pack_size - the_hash_algo->rawsz;
        else
                return nth_packed_object_offset(p, pack_pos_to_index(p, pos));
}

uint32_t pack_pos_to_midx(struct multi_pack_index *m, uint32_t pos)
{
        if (!m->revindex_data)
                BUG("pack_pos_to_midx: reverse index not yet loaded");
        if (m->num_objects <= pos)
                BUG("pack_pos_to_midx: out-of-bounds object at %"PRIu32, pos);
        return get_be32(m->revindex_data + pos);
}

struct midx_pack_key {
        uint32_t pack;
        off_t offset;

        uint32_t preferred_pack;
        struct multi_pack_index *midx;
};

static int midx_pack_order_cmp(const void *va, const void *vb)
{
        const struct midx_pack_key *key = va;
        struct multi_pack_index *midx = key->midx;

        uint32_t versus = pack_pos_to_midx(midx, (uint32_t*)vb - (const uint32_t *)midx->revindex_data);
        uint32_t versus_pack = nth_midxed_pack_int_id(midx, versus);
        off_t versus_offset;

        uint32_t key_preferred = key->pack == key->preferred_pack;
        uint32_t versus_preferred = versus_pack == key->preferred_pack;

        /*
         * First, compare the preferred-ness, noting that the preferred pack
         * comes first.
         */
        if (key_preferred && !versus_preferred)
                return -1;
        else if (!key_preferred && versus_preferred)
                return 1;

        /* Then, break ties first by comparing the pack IDs. */
        if (key->pack < versus_pack)
                return -1;
        else if (key->pack > versus_pack)
                return 1;

        /* Finally, break ties by comparing offsets within a pack. */
        versus_offset = nth_midxed_offset(midx, versus);
        if (key->offset < versus_offset)
                return -1;
        else if (key->offset > versus_offset)
                return 1;

        return 0;
}

int midx_to_pack_pos(struct multi_pack_index *m, uint32_t at, uint32_t *pos)
{
        struct midx_pack_key key;
        uint32_t *found;

        if (!m->revindex_data)
                BUG("midx_to_pack_pos: reverse index not yet loaded");
        if (m->num_objects <= at)
                BUG("midx_to_pack_pos: out-of-bounds object at %"PRIu32, at);

        key.pack = nth_midxed_pack_int_id(m, at);
        key.offset = nth_midxed_offset(m, at);
        key.midx = m;
        /*
         * The preferred pack sorts first, so determine its identifier by
         * looking at the first object in pseudo-pack order.
         *
         * Note that if no --preferred-pack is explicitly given when writing a
         * multi-pack index, then whichever pack has the lowest identifier
         * implicitly is preferred (and includes all its objects, since ties are
         * broken first by pack identifier).
         */
        key.preferred_pack = nth_midxed_pack_int_id(m, pack_pos_to_midx(m, 0));

        found = bsearch(&key, m->revindex_data, m->num_objects,
                        sizeof(*m->revindex_data), midx_pack_order_cmp);

        if (!found)
                return error("bad offset for revindex");

        *pos = found - m->revindex_data;
        return 0;
}