git.ozlabs.org Git - ccan/blob - ccan/tdb2/hash.c

   1  /*
   2    Trivial Database 2: hash handling
   3    Copyright (C) Rusty Russell 2010
   4
   5    This library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 3 of the License, or (at your option) any later version.
   9
  10    This library is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with this library; if not, see <http://www.gnu.org/licenses/>.
  17 */
  18 #include "private.h"
  19 #include <assert.h>
  20 #include <ccan/hash/hash.h>
  21
  22 static uint64_t jenkins_hash(const void *key, size_t length, uint64_t seed,
  23                              void *arg)
  24 {
  25         uint64_t ret;
  26         /* hash64_stable assumes lower bits are more important; they are a
  27          * slightly better hash.  We use the upper bits first, so swap them. */
  28         ret = hash64_stable((const unsigned char *)key, length, seed);
  29         return (ret >> 32) | (ret << 32);
  30 }
  31
  32 void tdb_hash_init(struct tdb_context *tdb)
  33 {
  34         tdb->khash = jenkins_hash;
  35         tdb->hash_priv = NULL;
  36 }
  37
  38 uint64_t tdb_hash(struct tdb_context *tdb, const void *ptr, size_t len)
  39 {
  40         return tdb->khash(ptr, len, tdb->hash_seed, tdb->hash_priv);
  41 }
  42
  43 uint64_t hash_record(struct tdb_context *tdb, tdb_off_t off)
  44 {
  45         struct tdb_used_record pad, *r;
  46         const void *key;
  47         uint64_t klen, hash;
  48
  49         r = tdb_get(tdb, off, &pad, sizeof(pad));
  50         if (!r)
  51                 /* FIXME */
  52                 return 0;
  53
  54         klen = rec_key_length(r);
  55         key = tdb_access_read(tdb, off + sizeof(pad), klen, false);
  56         if (!key)
  57                 return 0;
  58
  59         hash = tdb_hash(tdb, key, klen);
  60         tdb_access_release(tdb, key);
  61         return hash;
  62 }
  63
  64 /* Get bits from a value. */
  65 static uint32_t bits(uint64_t val, unsigned start, unsigned num)
  66 {
  67         assert(num <= 32);
  68         return (val >> start) & ((1U << num) - 1);
  69 }
  70
  71 /* We take bits from the top: that way we can lock whole sections of the hash
  72  * by using lock ranges. */
  73 static uint32_t use_bits(struct hash_info *h, unsigned num)
  74 {
  75         h->hash_used += num;
  76         return bits(h->h, 64 - h->hash_used, num);
  77 }
  78
  79 /* Does entry match? */
  80 static bool match(struct tdb_context *tdb,
  81                   struct hash_info *h,
  82                   const struct tdb_data *key,
  83                   tdb_off_t val,
  84                   struct tdb_used_record *rec)
  85 {
  86         bool ret;
  87         const unsigned char *rkey;
  88         tdb_off_t off;
  89
  90         /* FIXME: Handle hash value truncated. */
  91         if (bits(val, TDB_OFF_HASH_TRUNCATED_BIT, 1))
  92                 abort();
  93
  94         /* Desired bucket must match. */
  95         if (h->home_bucket != (val & TDB_OFF_HASH_GROUP_MASK))
  96                 return false;
  97
  98         /* Top bits of offset == next bits of hash. */
  99         if (bits(val, TDB_OFF_HASH_EXTRA_BIT, TDB_OFF_UPPER_STEAL_EXTRA)
 100             != bits(h->h, 64 - h->hash_used - TDB_OFF_UPPER_STEAL_EXTRA,
 101                     TDB_OFF_UPPER_STEAL_EXTRA))
 102                 return false;
 103
 104         off = val & TDB_OFF_MASK;
 105         if (tdb_read_convert(tdb, off, rec, sizeof(*rec)) == -1)
 106                 return false;
 107
 108         /* FIXME: check extra bits in header? */
 109         if (rec_key_length(rec) != key->dsize)
 110                 return false;
 111
 112         rkey = tdb_access_read(tdb, off + sizeof(*rec), key->dsize, false);
 113         if (!rkey)
 114                 return false;
 115         ret = (memcmp(rkey, key->dptr, key->dsize) == 0);
 116         tdb_access_release(tdb, rkey);
 117         return ret;
 118 }
 119
 120 static tdb_off_t hbucket_off(tdb_off_t group_start, unsigned bucket)
 121 {
 122         return group_start
 123                 + (bucket % (1 << TDB_HASH_GROUP_BITS)) * sizeof(tdb_off_t);
 124 }
 125
 126 /* Truncated hashes can't be all 1: that's how we spot a sub-hash */
 127 bool is_subhash(tdb_off_t val)
 128 {
 129         return val >> (64-TDB_OFF_UPPER_STEAL) == (1<<TDB_OFF_UPPER_STEAL) - 1;
 130 }
 131
 132 /* This is the core routine which searches the hashtable for an entry.
 133  * On error, no locks are held and TDB_OFF_ERR is returned.
 134  * Otherwise, hinfo is filled in (and the optional tinfo).
 135  * If not found, the return value is 0.
 136  * If found, the return value is the offset, and *rec is the record. */
 137 tdb_off_t find_and_lock(struct tdb_context *tdb,
 138                         struct tdb_data key,
 139                         int ltype,
 140                         struct hash_info *h,
 141                         struct tdb_used_record *rec,
 142                         struct traverse_info *tinfo)
 143 {
 144         uint32_t i, group;
 145         tdb_off_t hashtable;
 146
 147         h->h = tdb_hash(tdb, key.dptr, key.dsize);
 148         h->hash_used = 0;
 149         group = use_bits(h, TDB_TOPLEVEL_HASH_BITS - TDB_HASH_GROUP_BITS);
 150         h->home_bucket = use_bits(h, TDB_HASH_GROUP_BITS);
 151
 152         /* FIXME: Guess the depth, don't over-lock! */
 153         h->hlock_start = (tdb_off_t)group
 154                 << (64 - (TDB_TOPLEVEL_HASH_BITS - TDB_HASH_GROUP_BITS));
 155         h->hlock_range = 1ULL << (64 - (TDB_TOPLEVEL_HASH_BITS
 156                                         - TDB_HASH_GROUP_BITS));
 157         if (tdb_lock_hashes(tdb, h->hlock_start, h->hlock_range, ltype,
 158                             TDB_LOCK_WAIT))
 159                 return TDB_OFF_ERR;
 160
 161         hashtable = offsetof(struct tdb_header, hashtable);
 162         if (tinfo) {
 163                 tinfo->toplevel_group = group;
 164                 tinfo->num_levels = 1;
 165                 tinfo->levels[0].entry = 0;
 166                 tinfo->levels[0].hashtable = hashtable
 167                         + (group << TDB_HASH_GROUP_BITS) * sizeof(tdb_off_t);
 168                 tinfo->levels[0].total_buckets = 1 << TDB_HASH_GROUP_BITS;
 169         }
 170
 171         while (likely(h->hash_used < 64)) {
 172                 /* Read in the hash group. */
 173                 h->group_start = hashtable
 174                         + group * (sizeof(tdb_off_t) << TDB_HASH_GROUP_BITS);
 175
 176                 if (tdb_read_convert(tdb, h->group_start, &h->group,
 177                                      sizeof(h->group)) == -1)
 178                         goto fail;
 179
 180                 /* Pointer to another hash table?  Go down... */
 181                 if (is_subhash(h->group[h->home_bucket])) {
 182                         hashtable = (h->group[h->home_bucket] & TDB_OFF_MASK)
 183                                 + sizeof(struct tdb_used_record);
 184                         if (tinfo) {
 185                                 /* When we come back, use *next* bucket */
 186                                 tinfo->levels[tinfo->num_levels-1].entry
 187                                         += h->home_bucket + 1;
 188                         }
 189                         group = use_bits(h, TDB_SUBLEVEL_HASH_BITS
 190                                          - TDB_HASH_GROUP_BITS);
 191                         h->home_bucket = use_bits(h, TDB_HASH_GROUP_BITS);
 192                         if (tinfo) {
 193                                 tinfo->levels[tinfo->num_levels].hashtable
 194                                         = hashtable;
 195                                 tinfo->levels[tinfo->num_levels].total_buckets
 196                                         = 1 << TDB_SUBLEVEL_HASH_BITS;
 197                                 tinfo->levels[tinfo->num_levels].entry
 198                                         = group << TDB_HASH_GROUP_BITS;
 199                                 tinfo->num_levels++;
 200                         }
 201                         continue;
 202                 }
 203
 204                 /* It's in this group: search (until 0 or all searched) */
 205                 for (i = 0, h->found_bucket = h->home_bucket;
 206                      i < (1 << TDB_HASH_GROUP_BITS);
 207                      i++, h->found_bucket = ((h->found_bucket+1)
 208                                              % (1 << TDB_HASH_GROUP_BITS))) {
 209                         if (is_subhash(h->group[h->found_bucket]))
 210                                 continue;
 211
 212                         if (!h->group[h->found_bucket])
 213                                 break;
 214
 215                         if (match(tdb, h, &key, h->group[h->found_bucket],
 216                                   rec)) {
 217                                 if (tinfo) {
 218                                         tinfo->levels[tinfo->num_levels-1].entry
 219                                                 += h->found_bucket;
 220                                 }
 221                                 return h->group[h->found_bucket] & TDB_OFF_MASK;
 222                         }
 223                 }
 224                 /* Didn't find it: h indicates where it would go. */
 225                 return 0;
 226         }
 227
 228         /* FIXME: We hit the bottom.  Chain! */
 229         abort();
 230
 231 fail:
 232         tdb_unlock_hashes(tdb, h->hlock_start, h->hlock_range, ltype);
 233         return TDB_OFF_ERR;
 234 }
 235
 236 /* I wrote a simple test, expanding a hash to 2GB, for the following
 237  * cases:
 238  * 1) Expanding all the buckets at once,
 239  * 2) Expanding the most-populated bucket,
 240  * 3) Expanding the bucket we wanted to place the new entry ito.
 241  *
 242  * I measured the worst/average/best density during this process.
 243  * 1) 3%/16%/30%
 244  * 2) 4%/20%/38%
 245  * 3) 6%/22%/41%
 246  *
 247  * So we figure out the busiest bucket for the moment.
 248  */
 249 static unsigned fullest_bucket(struct tdb_context *tdb,
 250                                const tdb_off_t *group,
 251                                unsigned new_bucket)
 252 {
 253         unsigned counts[1 << TDB_HASH_GROUP_BITS] = { 0 };
 254         unsigned int i, best_bucket;
 255
 256         /* Count the new entry. */
 257         counts[new_bucket]++;
 258         best_bucket = new_bucket;
 259
 260         for (i = 0; i < (1 << TDB_HASH_GROUP_BITS); i++) {
 261                 unsigned this_bucket;
 262
 263                 if (is_subhash(group[i]))
 264                         continue;
 265                 this_bucket = group[i] & TDB_OFF_HASH_GROUP_MASK;
 266                 if (++counts[this_bucket] > counts[best_bucket])
 267                         best_bucket = this_bucket;
 268         }
 269
 270         return best_bucket;
 271 }
 272
 273 static bool put_into_group(tdb_off_t *group,
 274                            unsigned bucket, tdb_off_t encoded)
 275 {
 276         unsigned int i;
 277
 278         for (i = 0; i < (1 << TDB_HASH_GROUP_BITS); i++) {
 279                 unsigned b = (bucket + i) % (1 << TDB_HASH_GROUP_BITS);
 280
 281                 if (group[b] == 0) {
 282                         group[b] = encoded;
 283                         return true;
 284                 }
 285         }
 286         return false;
 287 }
 288
 289 static void force_into_group(tdb_off_t *group,
 290                              unsigned bucket, tdb_off_t encoded)
 291 {
 292         if (!put_into_group(group, bucket, encoded))
 293                 abort();
 294 }
 295
 296 static tdb_off_t encode_offset(tdb_off_t new_off, struct hash_info *h)
 297 {
 298         return h->home_bucket
 299                 | new_off
 300                 | ((uint64_t)bits(h->h,
 301                                   64 - h->hash_used - TDB_OFF_UPPER_STEAL_EXTRA,
 302                                   TDB_OFF_UPPER_STEAL_EXTRA)
 303                    << TDB_OFF_HASH_EXTRA_BIT);
 304 }
 305
 306 /* Simply overwrite the hash entry we found before. */
 307 int replace_in_hash(struct tdb_context *tdb,
 308                     struct hash_info *h,
 309                     tdb_off_t new_off)
 310 {
 311         return tdb_write_off(tdb, hbucket_off(h->group_start, h->found_bucket),
 312                              encode_offset(new_off, h));
 313 }
 314
 315 /* Add into a newly created subhash. */
 316 static int add_to_subhash(struct tdb_context *tdb, tdb_off_t subhash,
 317                           unsigned hash_used, tdb_off_t val)
 318 {
 319         tdb_off_t off = (val & TDB_OFF_MASK), *group;
 320         struct hash_info h;
 321         unsigned int gnum;
 322
 323         h.hash_used = hash_used;
 324
 325         /* FIXME chain if hash_used == 64 */
 326         if (hash_used + TDB_SUBLEVEL_HASH_BITS > 64)
 327                 abort();
 328
 329         /* FIXME: Do truncated hash bits if we can! */
 330         h.h = hash_record(tdb, off);
 331         gnum = use_bits(&h, TDB_SUBLEVEL_HASH_BITS-TDB_HASH_GROUP_BITS);
 332         h.group_start = subhash + sizeof(struct tdb_used_record)
 333                 + gnum * (sizeof(tdb_off_t) << TDB_HASH_GROUP_BITS);
 334         h.home_bucket = use_bits(&h, TDB_HASH_GROUP_BITS);
 335
 336         group = tdb_access_write(tdb, h.group_start,
 337                                  sizeof(*group) << TDB_HASH_GROUP_BITS, true);
 338         if (!group)
 339                 return -1;
 340         force_into_group(group, h.home_bucket, encode_offset(off, &h));
 341         return tdb_access_commit(tdb, group);
 342 }
 343
 344 static int expand_group(struct tdb_context *tdb, struct hash_info *h)
 345 {
 346         unsigned bucket, num_vals, i;
 347         tdb_off_t subhash;
 348         tdb_off_t vals[1 << TDB_HASH_GROUP_BITS];
 349
 350         /* Attach new empty subhash under fullest bucket. */
 351         bucket = fullest_bucket(tdb, h->group, h->home_bucket);
 352
 353         subhash = alloc(tdb, 0, sizeof(tdb_off_t) << TDB_SUBLEVEL_HASH_BITS,
 354                         0, false);
 355         if (subhash == TDB_OFF_ERR)
 356                 return -1;
 357
 358         if (zero_out(tdb, subhash + sizeof(struct tdb_used_record),
 359                      sizeof(tdb_off_t) << TDB_SUBLEVEL_HASH_BITS) == -1)
 360                 return -1;
 361
 362         /* Remove any which are destined for bucket or are in wrong place. */
 363         num_vals = 0;
 364         for (i = 0; i < (1 << TDB_HASH_GROUP_BITS); i++) {
 365                 unsigned home_bucket = h->group[i] & TDB_OFF_HASH_GROUP_MASK;
 366                 if (!h->group[i] || is_subhash(h->group[i]))
 367                         continue;
 368                 if (home_bucket == bucket || home_bucket != i) {
 369                         vals[num_vals++] = h->group[i];
 370                         h->group[i] = 0;
 371                 }
 372         }
 373         /* FIXME: This assert is valid, but we do this during unit test :( */
 374         /* assert(num_vals); */
 375
 376         /* Overwrite expanded bucket with subhash pointer. */
 377         h->group[bucket] = subhash | ~((1ULL << (64 - TDB_OFF_UPPER_STEAL))-1);
 378
 379         /* Put values back. */
 380         for (i = 0; i < num_vals; i++) {
 381                 unsigned this_bucket = vals[i] & TDB_OFF_HASH_GROUP_MASK;
 382
 383                 if (this_bucket == bucket) {
 384                         if (add_to_subhash(tdb, subhash, h->hash_used, vals[i]))
 385                                 return -1;
 386                 } else {
 387                         /* There should be room to put this back. */
 388                         force_into_group(h->group, this_bucket, vals[i]);
 389                 }
 390         }
 391         return 0;
 392 }
 393
 394 int delete_from_hash(struct tdb_context *tdb, struct hash_info *h)
 395 {
 396         unsigned int i, num_movers = 0;
 397         tdb_off_t movers[1 << TDB_HASH_GROUP_BITS];
 398
 399         h->group[h->found_bucket] = 0;
 400         for (i = 1; i < (1 << TDB_HASH_GROUP_BITS); i++) {
 401                 unsigned this_bucket;
 402
 403                 this_bucket = (h->found_bucket+i) % (1 << TDB_HASH_GROUP_BITS);
 404                 /* Empty bucket?  We're done. */
 405                 if (!h->group[this_bucket])
 406                         break;
 407
 408                 /* Ignore subhashes. */
 409                 if (is_subhash(h->group[this_bucket]))
 410                         continue;
 411
 412                 /* If this one is not happy where it is, we'll move it. */
 413                 if ((h->group[this_bucket] & TDB_OFF_HASH_GROUP_MASK)
 414                     != this_bucket) {
 415                         movers[num_movers++] = h->group[this_bucket];
 416                         h->group[this_bucket] = 0;
 417                 }
 418         }
 419
 420         /* Put back the ones we erased. */
 421         for (i = 0; i < num_movers; i++) {
 422                 force_into_group(h->group, movers[i] & TDB_OFF_HASH_GROUP_MASK,
 423                                  movers[i]);
 424         }
 425
 426         /* Now we write back the hash group */
 427         return tdb_write_convert(tdb, h->group_start,
 428                                  h->group, sizeof(h->group));
 429 }
 430
 431 int add_to_hash(struct tdb_context *tdb, struct hash_info *h, tdb_off_t new_off)
 432 {
 433         /* FIXME: chain! */
 434         if (h->hash_used >= 64)
 435                 abort();
 436
 437         /* We hit an empty bucket during search?  That's where it goes. */
 438         if (!h->group[h->found_bucket]) {
 439                 h->group[h->found_bucket] = encode_offset(new_off, h);
 440                 /* Write back the modified group. */
 441                 return tdb_write_convert(tdb, h->group_start,
 442                                          h->group, sizeof(h->group));
 443         }
 444
 445         /* We're full.  Expand. */
 446         if (expand_group(tdb, h) == -1)
 447                 return -1;
 448
 449         if (is_subhash(h->group[h->home_bucket])) {
 450                 /* We were expanded! */
 451                 tdb_off_t hashtable;
 452                 unsigned int gnum;
 453
 454                 /* Write back the modified group. */
 455                 if (tdb_write_convert(tdb, h->group_start, h->group,
 456                                       sizeof(h->group)))
 457                         return -1;
 458
 459                 /* Move hashinfo down a level. */
 460                 hashtable = (h->group[h->home_bucket] & TDB_OFF_MASK)
 461                         + sizeof(struct tdb_used_record);
 462                 gnum = use_bits(h,TDB_SUBLEVEL_HASH_BITS - TDB_HASH_GROUP_BITS);
 463                 h->home_bucket = use_bits(h, TDB_HASH_GROUP_BITS);
 464                 h->group_start = hashtable
 465                         + gnum * (sizeof(tdb_off_t) << TDB_HASH_GROUP_BITS);
 466                 if (tdb_read_convert(tdb, h->group_start, &h->group,
 467                                      sizeof(h->group)) == -1)
 468                         return -1;
 469         }
 470
 471         /* Expanding the group must have made room if it didn't choose this
 472          * bucket. */
 473         if (put_into_group(h->group, h->home_bucket, encode_offset(new_off, h)))
 474                 return tdb_write_convert(tdb, h->group_start,
 475                                          h->group, sizeof(h->group));
 476
 477         /* This can happen if all hashes in group (and us) dropped into same
 478          * group in subhash. */
 479         return add_to_hash(tdb, h, new_off);
 480 }
 481
 482 /* Traverse support: returns offset of record, or 0 or TDB_OFF_ERR. */
 483 static tdb_off_t iterate_hash(struct tdb_context *tdb,
 484                               struct traverse_info *tinfo)
 485 {
 486         tdb_off_t off, val;
 487         unsigned int i;
 488         struct traverse_level *tlevel;
 489
 490         tlevel = &tinfo->levels[tinfo->num_levels-1];
 491
 492 again:
 493         for (i = tdb_find_nonzero_off(tdb, tlevel->hashtable,
 494                                       tlevel->entry, tlevel->total_buckets);
 495              i != tlevel->total_buckets;
 496              i = tdb_find_nonzero_off(tdb, tlevel->hashtable,
 497                                       i+1, tlevel->total_buckets)) {
 498                 val = tdb_read_off(tdb, tlevel->hashtable+sizeof(tdb_off_t)*i);
 499                 if (unlikely(val == TDB_OFF_ERR))
 500                         return TDB_OFF_ERR;
 501
 502                 off = val & TDB_OFF_MASK;
 503
 504                 /* This makes the delete-all-in-traverse case work
 505                  * (and simplifies our logic a little). */
 506                 if (off == tinfo->prev)
 507                         continue;
 508
 509                 tlevel->entry = i;
 510
 511                 if (!is_subhash(val)) {
 512                         /* Found one. */
 513                         tinfo->prev = off;
 514                         return off;
 515                 }
 516
 517                 /* When we come back, we want the next one */
 518                 tlevel->entry++;
 519                 tinfo->num_levels++;
 520                 tlevel++;
 521                 tlevel->hashtable = off + sizeof(struct tdb_used_record);
 522                 tlevel->entry = 0;
 523                 tlevel->total_buckets = (1 << TDB_SUBLEVEL_HASH_BITS);
 524                 goto again;
 525         }
 526
 527         /* Nothing there? */
 528         if (tinfo->num_levels == 1)
 529                 return 0;
 530
 531         /* Go back up and keep searching. */
 532         tinfo->num_levels--;
 533         tlevel--;
 534         goto again;
 535 }
 536
 537 /* Return 1 if we find something, 0 if not, -1 on error. */
 538 int next_in_hash(struct tdb_context *tdb, int ltype,
 539                  struct traverse_info *tinfo,
 540                  TDB_DATA *kbuf, unsigned int *dlen)
 541 {
 542         const unsigned group_bits = TDB_TOPLEVEL_HASH_BITS-TDB_HASH_GROUP_BITS;
 543         tdb_off_t hlock_start, hlock_range, off;
 544
 545         while (tinfo->toplevel_group < (1 << group_bits)) {
 546                 hlock_start = (tdb_off_t)tinfo->toplevel_group
 547                         << (64 - group_bits);
 548                 hlock_range = 1ULL << group_bits;
 549                 if (tdb_lock_hashes(tdb, hlock_start, hlock_range, ltype,
 550                                     TDB_LOCK_WAIT) != 0)
 551                         return -1;
 552
 553                 off = iterate_hash(tdb, tinfo);
 554                 if (off) {
 555                         struct tdb_used_record rec;
 556
 557                         if (tdb_read_convert(tdb, off, &rec, sizeof(rec))) {
 558                                 tdb_unlock_hashes(tdb,
 559                                                   hlock_start, hlock_range,
 560                                                   ltype);
 561                                 return -1;
 562                         }
 563                         kbuf->dsize = rec_key_length(&rec);
 564
 565                         /* They want data as well? */
 566                         if (dlen) {
 567                                 *dlen = rec_data_length(&rec);
 568                                 kbuf->dptr = tdb_alloc_read(tdb,
 569                                                             off + sizeof(rec),
 570                                                             kbuf->dsize
 571                                                             + *dlen);
 572                         } else {
 573                                 kbuf->dptr = tdb_alloc_read(tdb,
 574                                                             off + sizeof(rec),
 575                                                             kbuf->dsize);
 576                         }
 577                         tdb_unlock_hashes(tdb, hlock_start, hlock_range, ltype);
 578                         return kbuf->dptr ? 1 : -1;
 579                 }
 580
 581                 tdb_unlock_hashes(tdb, hlock_start, hlock_range, ltype);
 582
 583                 tinfo->toplevel_group++;
 584                 tinfo->levels[0].hashtable
 585                         += (sizeof(tdb_off_t) << TDB_HASH_GROUP_BITS);
 586                 tinfo->levels[0].entry = 0;
 587         }
 588         return 0;
 589 }
 590
 591 /* Return 1 if we find something, 0 if not, -1 on error. */
 592 int first_in_hash(struct tdb_context *tdb, int ltype,
 593                   struct traverse_info *tinfo,
 594                   TDB_DATA *kbuf, unsigned int *dlen)
 595 {
 596         tinfo->prev = 0;
 597         tinfo->toplevel_group = 0;
 598         tinfo->num_levels = 1;
 599         tinfo->levels[0].hashtable = offsetof(struct tdb_header, hashtable);
 600         tinfo->levels[0].entry = 0;
 601         tinfo->levels[0].total_buckets = (1 << TDB_HASH_GROUP_BITS);
 602
 603         return next_in_hash(tdb, ltype, tinfo, kbuf, dlen);
 604 }