pack-revindex.c

   1 #include "cache.h"
   2 #include "pack-revindex.h"
   3 #include "object-store.h"
   4 #include "packfile.h"
   5 #include "config.h"
   6
   7 struct revindex_entry {
   8         off_t offset;
   9         unsigned int nr;
  10 };
  11
  12 /*
  13  * Pack index for existing packs give us easy access to the offsets into
  14  * corresponding pack file where each object's data starts, but the entries
  15  * do not store the size of the compressed representation (uncompressed
  16  * size is easily available by examining the pack entry header).  It is
  17  * also rather expensive to find the sha1 for an object given its offset.
  18  *
  19  * The pack index file is sorted by object name mapping to offset;
  20  * this revindex array is a list of offset/index_nr pairs
  21  * ordered by offset, so if you know the offset of an object, next offset
  22  * is where its packed representation ends and the index_nr can be used to
  23  * get the object sha1 from the main index.
  24  */
  25
  26 /*
  27  * This is a least-significant-digit radix sort.
  28  *
  29  * It sorts each of the "n" items in "entries" by its offset field. The "max"
  30  * parameter must be at least as large as the largest offset in the array,
  31  * and lets us quit the sort early.
  32  */
  33 static void sort_revindex(struct revindex_entry *entries, unsigned n, off_t max)
  34 {
  35         /*
  36          * We use a "digit" size of 16 bits. That keeps our memory
  37          * usage reasonable, and we can generally (for a 4G or smaller
  38          * packfile) quit after two rounds of radix-sorting.
  39          */
  40 #define DIGIT_SIZE (16)
  41 #define BUCKETS (1 << DIGIT_SIZE)
  42         /*
  43          * We want to know the bucket that a[i] will go into when we are using
  44          * the digit that is N bits from the (least significant) end.
  45          */
  46 #define BUCKET_FOR(a, i, bits) (((a)[(i)].offset >> (bits)) & (BUCKETS-1))
  47
  48         /*
  49          * We need O(n) temporary storage. Rather than do an extra copy of the
  50          * partial results into "entries", we sort back and forth between the
  51          * real array and temporary storage. In each iteration of the loop, we
  52          * keep track of them with alias pointers, always sorting from "from"
  53          * to "to".
  54          */
  55         struct revindex_entry *tmp, *from, *to;
  56         int bits;
  57         unsigned *pos;
  58
  59         ALLOC_ARRAY(pos, BUCKETS);
  60         ALLOC_ARRAY(tmp, n);
  61         from = entries;
  62         to = tmp;
  63
  64         /*
  65          * If (max >> bits) is zero, then we know that the radix digit we are
  66          * on (and any higher) will be zero for all entries, and our loop will
  67          * be a no-op, as everybody lands in the same zero-th bucket.
  68          */
  69         for (bits = 0; max >> bits; bits += DIGIT_SIZE) {
  70                 unsigned i;
  71
  72                 memset(pos, 0, BUCKETS * sizeof(*pos));
  73
  74                 /*
  75                  * We want pos[i] to store the index of the last element that
  76                  * will go in bucket "i" (actually one past the last element).
  77                  * To do this, we first count the items that will go in each
  78                  * bucket, which gives us a relative offset from the last
  79                  * bucket. We can then cumulatively add the index from the
  80                  * previous bucket to get the true index.
  81                  */
  82                 for (i = 0; i < n; i++)
  83                         pos[BUCKET_FOR(from, i, bits)]++;
  84                 for (i = 1; i < BUCKETS; i++)
  85                         pos[i] += pos[i-1];
  86
  87                 /*
  88                  * Now we can drop the elements into their correct buckets (in
  89                  * our temporary array).  We iterate the pos counter backwards
  90                  * to avoid using an extra index to count up. And since we are
  91                  * going backwards there, we must also go backwards through the
  92                  * array itself, to keep the sort stable.
  93                  *
  94                  * Note that we use an unsigned iterator to make sure we can
  95                  * handle 2^32-1 objects, even on a 32-bit system. But this
  96                  * means we cannot use the more obvious "i >= 0" loop condition
  97                  * for counting backwards, and must instead check for
  98                  * wrap-around with UINT_MAX.
  99                  */
 100                 for (i = n - 1; i != UINT_MAX; i--)
 101                         to[--pos[BUCKET_FOR(from, i, bits)]] = from[i];
 102
 103                 /*
 104                  * Now "to" contains the most sorted list, so we swap "from" and
 105                  * "to" for the next iteration.
 106                  */
 107                 SWAP(from, to);
 108         }
 109
 110         /*
 111          * If we ended with our data in the original array, great. If not,
 112          * we have to move it back from the temporary storage.
 113          */
 114         if (from != entries)
 115                 COPY_ARRAY(entries, tmp, n);
 116         free(tmp);
 117         free(pos);
 118
 119 #undef BUCKET_FOR
 120 #undef BUCKETS
 121 #undef DIGIT_SIZE
 122 }
 123
 124 /*
 125  * Ordered list of offsets of objects in the pack.
 126  */
 127 static void create_pack_revindex(struct packed_git *p)
 128 {
 129         const unsigned num_ent = p->num_objects;
 130         unsigned i;
 131         const char *index = p->index_data;
 132         const unsigned hashsz = the_hash_algo->rawsz;
 133
 134         ALLOC_ARRAY(p->revindex, num_ent + 1);
 135         index += 4 * 256;
 136
 137         if (p->index_version > 1) {
 138                 const uint32_t *off_32 =
 139                         (uint32_t *)(index + 8 + (size_t)p->num_objects * (hashsz + 4));
 140                 const uint32_t *off_64 = off_32 + p->num_objects;
 141                 for (i = 0; i < num_ent; i++) {
 142                         const uint32_t off = ntohl(*off_32++);
 143                         if (!(off & 0x80000000)) {
 144                                 p->revindex[i].offset = off;
 145                         } else {
 146                                 p->revindex[i].offset = get_be64(off_64);
 147                                 off_64 += 2;
 148                         }
 149                         p->revindex[i].nr = i;
 150                 }
 151         } else {
 152                 for (i = 0; i < num_ent; i++) {
 153                         const uint32_t hl = *((uint32_t *)(index + (hashsz + 4) * i));
 154                         p->revindex[i].offset = ntohl(hl);
 155                         p->revindex[i].nr = i;
 156                 }
 157         }
 158
 159         /*
 160          * This knows the pack format -- the hash trailer
 161          * follows immediately after the last object data.
 162          */
 163         p->revindex[num_ent].offset = p->pack_size - hashsz;
 164         p->revindex[num_ent].nr = -1;
 165         sort_revindex(p->revindex, num_ent, p->pack_size);
 166 }
 167
 168 static int create_pack_revindex_in_memory(struct packed_git *p)
 169 {
 170         if (git_env_bool(GIT_TEST_REV_INDEX_DIE_IN_MEMORY, 0))
 171                 die("dying as requested by '%s'",
 172                     GIT_TEST_REV_INDEX_DIE_IN_MEMORY);
 173         if (open_pack_index(p))
 174                 return -1;
 175         create_pack_revindex(p);
 176         return 0;
 177 }
 178
 179 static char *pack_revindex_filename(struct packed_git *p)
 180 {
 181         size_t len;
 182         if (!strip_suffix(p->pack_name, ".pack", &len))
 183                 BUG("pack_name does not end in .pack");
 184         return xstrfmt("%.*s.rev", (int)len, p->pack_name);
 185 }
 186
 187 #define RIDX_HEADER_SIZE (12)
 188 #define RIDX_MIN_SIZE (RIDX_HEADER_SIZE + (2 * the_hash_algo->rawsz))
 189
 190 struct revindex_header {
 191         uint32_t signature;
 192         uint32_t version;
 193         uint32_t hash_id;
 194 };
 195
 196 static int load_revindex_from_disk(char *revindex_name,
 197                                    uint32_t num_objects,
 198                                    const uint32_t **data_p, size_t *len_p)
 199 {
 200         int fd, ret = 0;
 201         struct stat st;
 202         void *data = NULL;
 203         size_t revindex_size;
 204         struct revindex_header *hdr;
 205
 206         fd = git_open(revindex_name);
 207
 208         if (fd < 0) {
 209                 ret = -1;
 210                 goto cleanup;
 211         }
 212         if (fstat(fd, &st)) {
 213                 ret = error_errno(_("failed to read %s"), revindex_name);
 214                 goto cleanup;
 215         }
 216
 217         revindex_size = xsize_t(st.st_size);
 218
 219         if (revindex_size < RIDX_MIN_SIZE) {
 220                 ret = error(_("reverse-index file %s is too small"), revindex_name);
 221                 goto cleanup;
 222         }
 223
 224         if (revindex_size - RIDX_MIN_SIZE != st_mult(sizeof(uint32_t), num_objects)) {
 225                 ret = error(_("reverse-index file %s is corrupt"), revindex_name);
 226                 goto cleanup;
 227         }
 228
 229         data = xmmap(NULL, revindex_size, PROT_READ, MAP_PRIVATE, fd, 0);
 230         hdr = data;
 231
 232         if (ntohl(hdr->signature) != RIDX_SIGNATURE) {
 233                 ret = error(_("reverse-index file %s has unknown signature"), revindex_name);
 234                 goto cleanup;
 235         }
 236         if (ntohl(hdr->version) != 1) {
 237                 ret = error(_("reverse-index file %s has unsupported version %"PRIu32),
 238                             revindex_name, ntohl(hdr->version));
 239                 goto cleanup;
 240         }
 241         if (!(ntohl(hdr->hash_id) == 1 || ntohl(hdr->hash_id) == 2)) {
 242                 ret = error(_("reverse-index file %s has unsupported hash id %"PRIu32),
 243                             revindex_name, ntohl(hdr->hash_id));
 244                 goto cleanup;
 245         }
 246
 247 cleanup:
 248         if (ret) {
 249                 if (data)
 250                         munmap(data, revindex_size);
 251         } else {
 252                 *len_p = revindex_size;
 253                 *data_p = (const uint32_t *)data;
 254         }
 255
 256         close(fd);
 257         return ret;
 258 }
 259
 260 static int load_pack_revindex_from_disk(struct packed_git *p)
 261 {
 262         char *revindex_name;
 263         int ret;
 264         if (open_pack_index(p))
 265                 return -1;
 266
 267         revindex_name = pack_revindex_filename(p);
 268
 269         ret = load_revindex_from_disk(revindex_name,
 270                                       p->num_objects,
 271                                       &p->revindex_map,
 272                                       &p->revindex_size);
 273         if (ret)
 274                 goto cleanup;
 275
 276         p->revindex_data = (const uint32_t *)((const char *)p->revindex_map + RIDX_HEADER_SIZE);
 277
 278 cleanup:
 279         free(revindex_name);
 280         return ret;
 281 }
 282
 283 int load_pack_revindex(struct packed_git *p)
 284 {
 285         if (p->revindex || p->revindex_data)
 286                 return 0;
 287
 288         if (!load_pack_revindex_from_disk(p))
 289                 return 0;
 290         else if (!create_pack_revindex_in_memory(p))
 291                 return 0;
 292         return -1;
 293 }
 294
 295 int offset_to_pack_pos(struct packed_git *p, off_t ofs, uint32_t *pos)
 296 {
 297         unsigned lo, hi;
 298
 299         if (load_pack_revindex(p) < 0)
 300                 return -1;
 301
 302         lo = 0;
 303         hi = p->num_objects + 1;
 304
 305         do {
 306                 const unsigned mi = lo + (hi - lo) / 2;
 307                 off_t got = pack_pos_to_offset(p, mi);
 308
 309                 if (got == ofs) {
 310                         *pos = mi;
 311                         return 0;
 312                 } else if (ofs < got)
 313                         hi = mi;
 314                 else
 315                         lo = mi + 1;
 316         } while (lo < hi);
 317
 318         error("bad offset for revindex");
 319         return -1;
 320 }
 321
 322 uint32_t pack_pos_to_index(struct packed_git *p, uint32_t pos)
 323 {
 324         if (!(p->revindex || p->revindex_data))
 325                 BUG("pack_pos_to_index: reverse index not yet loaded");
 326         if (p->num_objects <= pos)
 327                 BUG("pack_pos_to_index: out-of-bounds object at %"PRIu32, pos);
 328
 329         if (p->revindex)
 330                 return p->revindex[pos].nr;
 331         else
 332                 return get_be32(p->revindex_data + pos);
 333 }
 334
 335 off_t pack_pos_to_offset(struct packed_git *p, uint32_t pos)
 336 {
 337         if (!(p->revindex || p->revindex_data))
 338                 BUG("pack_pos_to_index: reverse index not yet loaded");
 339         if (p->num_objects < pos)
 340                 BUG("pack_pos_to_offset: out-of-bounds object at %"PRIu32, pos);
 341
 342         if (p->revindex)
 343                 return p->revindex[pos].offset;
 344         else if (pos == p->num_objects)
 345                 return p->pack_size - the_hash_algo->rawsz;
 346         else
 347                 return nth_packed_object_offset(p, pack_pos_to_index(p, pos));
 348 }