reftable/writer.c

   1 /*
   2 Copyright 2020 Google LLC
   3
   4 Use of this source code is governed by a BSD-style
   5 license that can be found in the LICENSE file or at
   6 https://developers.google.com/open-source/licenses/bsd
   7 */
   8
   9 #include "writer.h"
  10
  11 #include "system.h"
  12
  13 #include "block.h"
  14 #include "constants.h"
  15 #include "record.h"
  16 #include "tree.h"
  17 #include "reftable-error.h"
  18
  19 /* finishes a block, and writes it to storage */
  20 static int writer_flush_block(struct reftable_writer *w);
  21
  22 /* deallocates memory related to the index */
  23 static void writer_clear_index(struct reftable_writer *w);
  24
  25 /* finishes writing a 'r' (refs) or 'g' (reflogs) section */
  26 static int writer_finish_public_section(struct reftable_writer *w);
  27
  28 static struct reftable_block_stats *
  29 writer_reftable_block_stats(struct reftable_writer *w, uint8_t typ)
  30 {
  31         switch (typ) {
  32         case 'r':
  33                 return &w->stats.ref_stats;
  34         case 'o':
  35                 return &w->stats.obj_stats;
  36         case 'i':
  37                 return &w->stats.idx_stats;
  38         case 'g':
  39                 return &w->stats.log_stats;
  40         }
  41         abort();
  42         return NULL;
  43 }
  44
  45 /* write data, queuing the padding for the next write. Returns negative for
  46  * error. */
  47 static int padded_write(struct reftable_writer *w, uint8_t *data, size_t len,
  48                         int padding)
  49 {
  50         int n = 0;
  51         if (w->pending_padding > 0) {
  52                 uint8_t *zeroed = reftable_calloc(w->pending_padding, sizeof(*zeroed));
  53                 int n = w->write(w->write_arg, zeroed, w->pending_padding);
  54                 if (n < 0)
  55                         return n;
  56
  57                 w->pending_padding = 0;
  58                 reftable_free(zeroed);
  59         }
  60
  61         w->pending_padding = padding;
  62         n = w->write(w->write_arg, data, len);
  63         if (n < 0)
  64                 return n;
  65         n += padding;
  66         return 0;
  67 }
  68
  69 static void options_set_defaults(struct reftable_write_options *opts)
  70 {
  71         if (opts->restart_interval == 0) {
  72                 opts->restart_interval = 16;
  73         }
  74
  75         if (opts->hash_id == 0) {
  76                 opts->hash_id = GIT_SHA1_FORMAT_ID;
  77         }
  78         if (opts->block_size == 0) {
  79                 opts->block_size = DEFAULT_BLOCK_SIZE;
  80         }
  81 }
  82
  83 static int writer_version(struct reftable_writer *w)
  84 {
  85         return (w->opts.hash_id == 0 || w->opts.hash_id == GIT_SHA1_FORMAT_ID) ?
  86                              1 :
  87                              2;
  88 }
  89
  90 static int writer_write_header(struct reftable_writer *w, uint8_t *dest)
  91 {
  92         memcpy(dest, "REFT", 4);
  93
  94         dest[4] = writer_version(w);
  95
  96         put_be24(dest + 5, w->opts.block_size);
  97         put_be64(dest + 8, w->min_update_index);
  98         put_be64(dest + 16, w->max_update_index);
  99         if (writer_version(w) == 2) {
 100                 put_be32(dest + 24, w->opts.hash_id);
 101         }
 102         return header_size(writer_version(w));
 103 }
 104
 105 static void writer_reinit_block_writer(struct reftable_writer *w, uint8_t typ)
 106 {
 107         int block_start = 0;
 108         if (w->next == 0) {
 109                 block_start = header_size(writer_version(w));
 110         }
 111
 112         strbuf_reset(&w->last_key);
 113         block_writer_init(&w->block_writer_data, typ, w->block,
 114                           w->opts.block_size, block_start,
 115                           hash_size(w->opts.hash_id));
 116         w->block_writer = &w->block_writer_data;
 117         w->block_writer->restart_interval = w->opts.restart_interval;
 118 }
 119
 120 static struct strbuf reftable_empty_strbuf = STRBUF_INIT;
 121
 122 struct reftable_writer *
 123 reftable_new_writer(ssize_t (*writer_func)(void *, const void *, size_t),
 124                     int (*flush_func)(void *),
 125                     void *writer_arg, struct reftable_write_options *opts)
 126 {
 127         struct reftable_writer *wp = reftable_calloc(1, sizeof(*wp));
 128         strbuf_init(&wp->block_writer_data.last_key, 0);
 129         options_set_defaults(opts);
 130         if (opts->block_size >= (1 << 24)) {
 131                 /* TODO - error return? */
 132                 abort();
 133         }
 134         wp->last_key = reftable_empty_strbuf;
 135         REFTABLE_CALLOC_ARRAY(wp->block, opts->block_size);
 136         wp->write = writer_func;
 137         wp->write_arg = writer_arg;
 138         wp->opts = *opts;
 139         wp->flush = flush_func;
 140         writer_reinit_block_writer(wp, BLOCK_TYPE_REF);
 141
 142         return wp;
 143 }
 144
 145 void reftable_writer_set_limits(struct reftable_writer *w, uint64_t min,
 146                                 uint64_t max)
 147 {
 148         w->min_update_index = min;
 149         w->max_update_index = max;
 150 }
 151
 152 static void writer_release(struct reftable_writer *w)
 153 {
 154         if (w) {
 155                 reftable_free(w->block);
 156                 w->block = NULL;
 157                 block_writer_release(&w->block_writer_data);
 158                 w->block_writer = NULL;
 159                 writer_clear_index(w);
 160                 strbuf_release(&w->last_key);
 161         }
 162 }
 163
 164 void reftable_writer_free(struct reftable_writer *w)
 165 {
 166         writer_release(w);
 167         reftable_free(w);
 168 }
 169
 170 struct obj_index_tree_node {
 171         struct strbuf hash;
 172         uint64_t *offsets;
 173         size_t offset_len;
 174         size_t offset_cap;
 175 };
 176
 177 #define OBJ_INDEX_TREE_NODE_INIT    \
 178         {                           \
 179                 .hash = STRBUF_INIT \
 180         }
 181
 182 static int obj_index_tree_node_compare(const void *a, const void *b)
 183 {
 184         return strbuf_cmp(&((const struct obj_index_tree_node *)a)->hash,
 185                           &((const struct obj_index_tree_node *)b)->hash);
 186 }
 187
 188 static void writer_index_hash(struct reftable_writer *w, struct strbuf *hash)
 189 {
 190         uint64_t off = w->next;
 191
 192         struct obj_index_tree_node want = { .hash = *hash };
 193
 194         struct tree_node *node = tree_search(&want, &w->obj_index_tree,
 195                                              &obj_index_tree_node_compare, 0);
 196         struct obj_index_tree_node *key = NULL;
 197         if (!node) {
 198                 struct obj_index_tree_node empty = OBJ_INDEX_TREE_NODE_INIT;
 199                 key = reftable_malloc(sizeof(struct obj_index_tree_node));
 200                 *key = empty;
 201
 202                 strbuf_reset(&key->hash);
 203                 strbuf_addbuf(&key->hash, hash);
 204                 tree_search((void *)key, &w->obj_index_tree,
 205                             &obj_index_tree_node_compare, 1);
 206         } else {
 207                 key = node->key;
 208         }
 209
 210         if (key->offset_len > 0 && key->offsets[key->offset_len - 1] == off) {
 211                 return;
 212         }
 213
 214         REFTABLE_ALLOC_GROW(key->offsets, key->offset_len + 1, key->offset_cap);
 215         key->offsets[key->offset_len++] = off;
 216 }
 217
 218 static int writer_add_record(struct reftable_writer *w,
 219                              struct reftable_record *rec)
 220 {
 221         struct strbuf key = STRBUF_INIT;
 222         int err;
 223
 224         reftable_record_key(rec, &key);
 225         if (strbuf_cmp(&w->last_key, &key) >= 0) {
 226                 err = REFTABLE_API_ERROR;
 227                 goto done;
 228         }
 229
 230         strbuf_reset(&w->last_key);
 231         strbuf_addbuf(&w->last_key, &key);
 232         if (!w->block_writer)
 233                 writer_reinit_block_writer(w, reftable_record_type(rec));
 234
 235         if (block_writer_type(w->block_writer) != reftable_record_type(rec))
 236                 BUG("record of type %d added to writer of type %d",
 237                     reftable_record_type(rec), block_writer_type(w->block_writer));
 238
 239         /*
 240          * Try to add the record to the writer. If this succeeds then we're
 241          * done. Otherwise the block writer may have hit the block size limit
 242          * and needs to be flushed.
 243          */
 244         if (!block_writer_add(w->block_writer, rec)) {
 245                 err = 0;
 246                 goto done;
 247         }
 248
 249         /*
 250          * The current block is full, so we need to flush and reinitialize the
 251          * writer to start writing the next block.
 252          */
 253         err = writer_flush_block(w);
 254         if (err < 0)
 255                 goto done;
 256         writer_reinit_block_writer(w, reftable_record_type(rec));
 257
 258         /*
 259          * Try to add the record to the writer again. If this still fails then
 260          * the record does not fit into the block size.
 261          *
 262          * TODO: it would be great to have `block_writer_add()` return proper
 263          *       error codes so that we don't have to second-guess the failure
 264          *       mode here.
 265          */
 266         err = block_writer_add(w->block_writer, rec);
 267         if (err) {
 268                 err = REFTABLE_ENTRY_TOO_BIG_ERROR;
 269                 goto done;
 270         }
 271
 272 done:
 273         strbuf_release(&key);
 274         return err;
 275 }
 276
 277 int reftable_writer_add_ref(struct reftable_writer *w,
 278                             struct reftable_ref_record *ref)
 279 {
 280         struct reftable_record rec = {
 281                 .type = BLOCK_TYPE_REF,
 282                 .u = {
 283                         .ref = *ref
 284                 },
 285         };
 286         int err = 0;
 287
 288         if (!ref->refname)
 289                 return REFTABLE_API_ERROR;
 290         if (ref->update_index < w->min_update_index ||
 291             ref->update_index > w->max_update_index)
 292                 return REFTABLE_API_ERROR;
 293
 294         rec.u.ref.update_index -= w->min_update_index;
 295
 296         err = writer_add_record(w, &rec);
 297         if (err < 0)
 298                 return err;
 299
 300         if (!w->opts.skip_index_objects && reftable_ref_record_val1(ref)) {
 301                 struct strbuf h = STRBUF_INIT;
 302                 strbuf_add(&h, (char *)reftable_ref_record_val1(ref),
 303                            hash_size(w->opts.hash_id));
 304                 writer_index_hash(w, &h);
 305                 strbuf_release(&h);
 306         }
 307
 308         if (!w->opts.skip_index_objects && reftable_ref_record_val2(ref)) {
 309                 struct strbuf h = STRBUF_INIT;
 310                 strbuf_add(&h, reftable_ref_record_val2(ref),
 311                            hash_size(w->opts.hash_id));
 312                 writer_index_hash(w, &h);
 313                 strbuf_release(&h);
 314         }
 315         return 0;
 316 }
 317
 318 int reftable_writer_add_refs(struct reftable_writer *w,
 319                              struct reftable_ref_record *refs, int n)
 320 {
 321         int err = 0;
 322         int i = 0;
 323         QSORT(refs, n, reftable_ref_record_compare_name);
 324         for (i = 0; err == 0 && i < n; i++) {
 325                 err = reftable_writer_add_ref(w, &refs[i]);
 326         }
 327         return err;
 328 }
 329
 330 static int reftable_writer_add_log_verbatim(struct reftable_writer *w,
 331                                             struct reftable_log_record *log)
 332 {
 333         struct reftable_record rec = {
 334                 .type = BLOCK_TYPE_LOG,
 335                 .u = {
 336                         .log = *log,
 337                 },
 338         };
 339         if (w->block_writer &&
 340             block_writer_type(w->block_writer) == BLOCK_TYPE_REF) {
 341                 int err = writer_finish_public_section(w);
 342                 if (err < 0)
 343                         return err;
 344         }
 345
 346         w->next -= w->pending_padding;
 347         w->pending_padding = 0;
 348         return writer_add_record(w, &rec);
 349 }
 350
 351 int reftable_writer_add_log(struct reftable_writer *w,
 352                             struct reftable_log_record *log)
 353 {
 354         char *input_log_message = NULL;
 355         struct strbuf cleaned_message = STRBUF_INIT;
 356         int err = 0;
 357
 358         if (log->value_type == REFTABLE_LOG_DELETION)
 359                 return reftable_writer_add_log_verbatim(w, log);
 360
 361         if (!log->refname)
 362                 return REFTABLE_API_ERROR;
 363
 364         input_log_message = log->value.update.message;
 365         if (!w->opts.exact_log_message && log->value.update.message) {
 366                 strbuf_addstr(&cleaned_message, log->value.update.message);
 367                 while (cleaned_message.len &&
 368                        cleaned_message.buf[cleaned_message.len - 1] == '\n')
 369                         strbuf_setlen(&cleaned_message,
 370                                       cleaned_message.len - 1);
 371                 if (strchr(cleaned_message.buf, '\n')) {
 372                         /* multiple lines not allowed. */
 373                         err = REFTABLE_API_ERROR;
 374                         goto done;
 375                 }
 376                 strbuf_addstr(&cleaned_message, "\n");
 377                 log->value.update.message = cleaned_message.buf;
 378         }
 379
 380         err = reftable_writer_add_log_verbatim(w, log);
 381         log->value.update.message = input_log_message;
 382 done:
 383         strbuf_release(&cleaned_message);
 384         return err;
 385 }
 386
 387 int reftable_writer_add_logs(struct reftable_writer *w,
 388                              struct reftable_log_record *logs, int n)
 389 {
 390         int err = 0;
 391         int i = 0;
 392         QSORT(logs, n, reftable_log_record_compare_key);
 393
 394         for (i = 0; err == 0 && i < n; i++) {
 395                 err = reftable_writer_add_log(w, &logs[i]);
 396         }
 397         return err;
 398 }
 399
 400 static int writer_finish_section(struct reftable_writer *w)
 401 {
 402         struct reftable_block_stats *bstats = NULL;
 403         uint8_t typ = block_writer_type(w->block_writer);
 404         uint64_t index_start = 0;
 405         int max_level = 0;
 406         size_t threshold = w->opts.unpadded ? 1 : 3;
 407         int before_blocks = w->stats.idx_stats.blocks;
 408         int err;
 409
 410         err = writer_flush_block(w);
 411         if (err < 0)
 412                 return err;
 413
 414         /*
 415          * When the section we are about to index has a lot of blocks then the
 416          * index itself may span across multiple blocks, as well. This would
 417          * require a linear scan over index blocks only to find the desired
 418          * indexed block, which is inefficient. Instead, we write a multi-level
 419          * index where index records of level N+1 will refer to index blocks of
 420          * level N. This isn't constant time, either, but at least logarithmic.
 421          *
 422          * This loop handles writing this multi-level index. Note that we write
 423          * the lowest-level index pointing to the indexed blocks first. We then
 424          * continue writing additional index levels until the current level has
 425          * less blocks than the threshold so that the highest level will be at
 426          * the end of the index section.
 427          *
 428          * Readers are thus required to start reading the index section from
 429          * its end, which is why we set `index_start` to the beginning of the
 430          * last index section.
 431          */
 432         while (w->index_len > threshold) {
 433                 struct reftable_index_record *idx = NULL;
 434                 size_t i, idx_len;
 435
 436                 max_level++;
 437                 index_start = w->next;
 438                 writer_reinit_block_writer(w, BLOCK_TYPE_INDEX);
 439
 440                 idx = w->index;
 441                 idx_len = w->index_len;
 442
 443                 w->index = NULL;
 444                 w->index_len = 0;
 445                 w->index_cap = 0;
 446                 for (i = 0; i < idx_len; i++) {
 447                         struct reftable_record rec = {
 448                                 .type = BLOCK_TYPE_INDEX,
 449                                 .u = {
 450                                         .idx = idx[i],
 451                                 },
 452                         };
 453
 454                         err = writer_add_record(w, &rec);
 455                         if (err < 0)
 456                                 return err;
 457                 }
 458
 459                 err = writer_flush_block(w);
 460                 if (err < 0)
 461                         return err;
 462
 463                 for (i = 0; i < idx_len; i++)
 464                         strbuf_release(&idx[i].last_key);
 465                 reftable_free(idx);
 466         }
 467
 468         /*
 469          * The index may still contain a number of index blocks lower than the
 470          * threshold. Clear it so that these entries don't leak into the next
 471          * index section.
 472          */
 473         writer_clear_index(w);
 474
 475         bstats = writer_reftable_block_stats(w, typ);
 476         bstats->index_blocks = w->stats.idx_stats.blocks - before_blocks;
 477         bstats->index_offset = index_start;
 478         bstats->max_index_level = max_level;
 479
 480         /* Reinit lastKey, as the next section can start with any key. */
 481         strbuf_reset(&w->last_key);
 482
 483         return 0;
 484 }
 485
 486 struct common_prefix_arg {
 487         struct strbuf *last;
 488         int max;
 489 };
 490
 491 static void update_common(void *void_arg, void *key)
 492 {
 493         struct common_prefix_arg *arg = void_arg;
 494         struct obj_index_tree_node *entry = key;
 495         if (arg->last) {
 496                 int n = common_prefix_size(&entry->hash, arg->last);
 497                 if (n > arg->max) {
 498                         arg->max = n;
 499                 }
 500         }
 501         arg->last = &entry->hash;
 502 }
 503
 504 struct write_record_arg {
 505         struct reftable_writer *w;
 506         int err;
 507 };
 508
 509 static void write_object_record(void *void_arg, void *key)
 510 {
 511         struct write_record_arg *arg = void_arg;
 512         struct obj_index_tree_node *entry = key;
 513         struct reftable_record
 514                 rec = { .type = BLOCK_TYPE_OBJ,
 515                         .u.obj = {
 516                                 .hash_prefix = (uint8_t *)entry->hash.buf,
 517                                 .hash_prefix_len = arg->w->stats.object_id_len,
 518                                 .offsets = entry->offsets,
 519                                 .offset_len = entry->offset_len,
 520                         } };
 521         if (arg->err < 0)
 522                 goto done;
 523
 524         arg->err = block_writer_add(arg->w->block_writer, &rec);
 525         if (arg->err == 0)
 526                 goto done;
 527
 528         arg->err = writer_flush_block(arg->w);
 529         if (arg->err < 0)
 530                 goto done;
 531
 532         writer_reinit_block_writer(arg->w, BLOCK_TYPE_OBJ);
 533         arg->err = block_writer_add(arg->w->block_writer, &rec);
 534         if (arg->err == 0)
 535                 goto done;
 536
 537         rec.u.obj.offset_len = 0;
 538         arg->err = block_writer_add(arg->w->block_writer, &rec);
 539
 540         /* Should be able to write into a fresh block. */
 541         assert(arg->err == 0);
 542
 543 done:;
 544 }
 545
 546 static void object_record_free(void *void_arg, void *key)
 547 {
 548         struct obj_index_tree_node *entry = key;
 549
 550         FREE_AND_NULL(entry->offsets);
 551         strbuf_release(&entry->hash);
 552         reftable_free(entry);
 553 }
 554
 555 static int writer_dump_object_index(struct reftable_writer *w)
 556 {
 557         struct write_record_arg closure = { .w = w };
 558         struct common_prefix_arg common = {
 559                 .max = 1,               /* obj_id_len should be >= 2. */
 560         };
 561         if (w->obj_index_tree) {
 562                 infix_walk(w->obj_index_tree, &update_common, &common);
 563         }
 564         w->stats.object_id_len = common.max + 1;
 565
 566         writer_reinit_block_writer(w, BLOCK_TYPE_OBJ);
 567
 568         if (w->obj_index_tree) {
 569                 infix_walk(w->obj_index_tree, &write_object_record, &closure);
 570         }
 571
 572         if (closure.err < 0)
 573                 return closure.err;
 574         return writer_finish_section(w);
 575 }
 576
 577 static int writer_finish_public_section(struct reftable_writer *w)
 578 {
 579         uint8_t typ = 0;
 580         int err = 0;
 581
 582         if (!w->block_writer)
 583                 return 0;
 584
 585         typ = block_writer_type(w->block_writer);
 586         err = writer_finish_section(w);
 587         if (err < 0)
 588                 return err;
 589         if (typ == BLOCK_TYPE_REF && !w->opts.skip_index_objects &&
 590             w->stats.ref_stats.index_blocks > 0) {
 591                 err = writer_dump_object_index(w);
 592                 if (err < 0)
 593                         return err;
 594         }
 595
 596         if (w->obj_index_tree) {
 597                 infix_walk(w->obj_index_tree, &object_record_free, NULL);
 598                 tree_free(w->obj_index_tree);
 599                 w->obj_index_tree = NULL;
 600         }
 601
 602         w->block_writer = NULL;
 603         return 0;
 604 }
 605
 606 int reftable_writer_close(struct reftable_writer *w)
 607 {
 608         uint8_t footer[72];
 609         uint8_t *p = footer;
 610         int err = writer_finish_public_section(w);
 611         int empty_table = w->next == 0;
 612         if (err != 0)
 613                 goto done;
 614         w->pending_padding = 0;
 615         if (empty_table) {
 616                 /* Empty tables need a header anyway. */
 617                 uint8_t header[28];
 618                 int n = writer_write_header(w, header);
 619                 err = padded_write(w, header, n, 0);
 620                 if (err < 0)
 621                         goto done;
 622         }
 623
 624         p += writer_write_header(w, footer);
 625         put_be64(p, w->stats.ref_stats.index_offset);
 626         p += 8;
 627         put_be64(p, (w->stats.obj_stats.offset) << 5 | w->stats.object_id_len);
 628         p += 8;
 629         put_be64(p, w->stats.obj_stats.index_offset);
 630         p += 8;
 631
 632         put_be64(p, w->stats.log_stats.offset);
 633         p += 8;
 634         put_be64(p, w->stats.log_stats.index_offset);
 635         p += 8;
 636
 637         put_be32(p, crc32(0, footer, p - footer));
 638         p += 4;
 639
 640         err = w->flush(w->write_arg);
 641         if (err < 0) {
 642                 err = REFTABLE_IO_ERROR;
 643                 goto done;
 644         }
 645
 646         err = padded_write(w, footer, footer_size(writer_version(w)), 0);
 647         if (err < 0)
 648                 goto done;
 649
 650         if (empty_table) {
 651                 err = REFTABLE_EMPTY_TABLE_ERROR;
 652                 goto done;
 653         }
 654
 655 done:
 656         writer_release(w);
 657         return err;
 658 }
 659
 660 static void writer_clear_index(struct reftable_writer *w)
 661 {
 662         for (size_t i = 0; w->index && i < w->index_len; i++)
 663                 strbuf_release(&w->index[i].last_key);
 664         FREE_AND_NULL(w->index);
 665         w->index_len = 0;
 666         w->index_cap = 0;
 667 }
 668
 669 static int writer_flush_nonempty_block(struct reftable_writer *w)
 670 {
 671         struct reftable_index_record index_record = {
 672                 .last_key = STRBUF_INIT,
 673         };
 674         uint8_t typ = block_writer_type(w->block_writer);
 675         struct reftable_block_stats *bstats;
 676         int raw_bytes, padding = 0, err;
 677         uint64_t block_typ_off;
 678
 679         /*
 680          * Finish the current block. This will cause the block writer to emit
 681          * restart points and potentially compress records in case we are
 682          * writing a log block.
 683          *
 684          * Note that this is still happening in memory.
 685          */
 686         raw_bytes = block_writer_finish(w->block_writer);
 687         if (raw_bytes < 0)
 688                 return raw_bytes;
 689
 690         /*
 691          * By default, all records except for log records are padded to the
 692          * block size.
 693          */
 694         if (!w->opts.unpadded && typ != BLOCK_TYPE_LOG)
 695                 padding = w->opts.block_size - raw_bytes;
 696
 697         bstats = writer_reftable_block_stats(w, typ);
 698         block_typ_off = (bstats->blocks == 0) ? w->next : 0;
 699         if (block_typ_off > 0)
 700                 bstats->offset = block_typ_off;
 701         bstats->entries += w->block_writer->entries;
 702         bstats->restarts += w->block_writer->restart_len;
 703         bstats->blocks++;
 704         w->stats.blocks++;
 705
 706         /*
 707          * If this is the first block we're writing to the table then we need
 708          * to also write the reftable header.
 709          */
 710         if (!w->next)
 711                 writer_write_header(w, w->block);
 712
 713         err = padded_write(w, w->block, raw_bytes, padding);
 714         if (err < 0)
 715                 return err;
 716
 717         /*
 718          * Add an index record for every block that we're writing. If we end up
 719          * having more than a threshold of index records we will end up writing
 720          * an index section in `writer_finish_section()`. Each index record
 721          * contains the last record key of the block it is indexing as well as
 722          * the offset of that block.
 723          *
 724          * Note that this also applies when flushing index blocks, in which
 725          * case we will end up with a multi-level index.
 726          */
 727         REFTABLE_ALLOC_GROW(w->index, w->index_len + 1, w->index_cap);
 728         index_record.offset = w->next;
 729         strbuf_reset(&index_record.last_key);
 730         strbuf_addbuf(&index_record.last_key, &w->block_writer->last_key);
 731         w->index[w->index_len] = index_record;
 732         w->index_len++;
 733
 734         w->next += padding + raw_bytes;
 735         w->block_writer = NULL;
 736
 737         return 0;
 738 }
 739
 740 static int writer_flush_block(struct reftable_writer *w)
 741 {
 742         if (!w->block_writer)
 743                 return 0;
 744         if (w->block_writer->entries == 0)
 745                 return 0;
 746         return writer_flush_nonempty_block(w);
 747 }
 748
 749 const struct reftable_stats *reftable_writer_stats(struct reftable_writer *w)
 750 {
 751         return &w->stats;
 752 }