Training courses

Kernel and Embedded Linux

Bootlin training courses

Embedded Linux, kernel,
Yocto Project, Buildroot, real-time,
graphics, boot time, debugging...

Bootlin logo

Elixir Cross Referencer

   1
   2
   3
   4
   5
   6
   7
   8
   9
  10
  11
  12
  13
  14
  15
  16
  17
  18
  19
  20
  21
  22
  23
  24
  25
  26
  27
  28
  29
  30
  31
  32
  33
  34
  35
  36
  37
  38
  39
  40
  41
  42
  43
  44
  45
  46
  47
  48
  49
  50
  51
  52
  53
  54
  55
  56
  57
  58
  59
  60
  61
  62
  63
  64
  65
  66
  67
  68
  69
  70
  71
  72
  73
  74
  75
  76
  77
  78
  79
  80
  81
  82
  83
  84
  85
  86
  87
  88
  89
  90
  91
  92
  93
  94
  95
  96
  97
  98
  99
 100
 101
 102
 103
 104
 105
 106
 107
 108
 109
 110
 111
 112
 113
 114
 115
 116
 117
 118
 119
 120
 121
 122
 123
 124
 125
 126
 127
 128
 129
 130
 131
 132
 133
 134
 135
 136
 137
 138
 139
 140
 141
 142
 143
 144
 145
 146
 147
 148
 149
 150
 151
 152
 153
 154
 155
 156
 157
 158
 159
 160
 161
 162
 163
 164
 165
 166
 167
 168
 169
 170
 171
 172
 173
 174
 175
 176
 177
 178
 179
 180
 181
 182
 183
 184
 185
 186
 187
 188
 189
 190
 191
 192
 193
 194
 195
 196
 197
 198
 199
 200
 201
 202
 203
 204
 205
 206
 207
 208
 209
 210
 211
 212
 213
 214
 215
 216
 217
 218
 219
 220
 221
 222
 223
 224
 225
 226
 227
 228
 229
 230
 231
 232
 233
 234
 235
 236
 237
 238
 239
 240
 241
 242
 243
 244
 245
 246
 247
 248
 249
 250
 251
 252
 253
 254
 255
 256
 257
 258
 259
 260
 261
 262
 263
 264
 265
 266
 267
 268
 269
 270
 271
 272
 273
 274
 275
 276
 277
 278
 279
 280
 281
 282
 283
 284
 285
 286
 287
 288
 289
 290
 291
 292
 293
 294
 295
 296
 297
 298
 299
 300
 301
 302
 303
 304
 305
 306
 307
 308
 309
 310
 311
 312
 313
 314
 315
 316
 317
 318
 319
 320
 321
 322
 323
 324
 325
 326
 327
 328
 329
 330
 331
 332
 333
 334
 335
 336
 337
 338
 339
 340
 341
 342
 343
 344
 345
 346
 347
 348
 349
 350
 351
 352
 353
 354
 355
 356
 357
 358
 359
 360
 361
 362
 363
 364
 365
 366
 367
 368
 369
 370
 371
 372
 373
 374
 375
 376
 377
 378
 379
 380
 381
 382
 383
 384
 385
 386
 387
 388
 389
 390
 391
 392
 393
 394
 395
 396
 397
 398
 399
 400
 401
 402
 403
 404
 405
 406
 407
 408
 409
 410
 411
 412
 413
 414
 415
 416
 417
 418
 419
 420
 421
 422
 423
 424
 425
 426
 427
 428
 429
 430
 431
 432
 433
 434
 435
 436
 437
 438
 439
 440
 441
 442
 443
 444
 445
 446
 447
 448
 449
 450
 451
 452
 453
 454
 455
 456
 457
 458
 459
 460
 461
 462
 463
 464
 465
 466
 467
 468
 469
 470
 471
 472
 473
 474
 475
 476
 477
 478
 479
 480
 481
 482
 483
 484
 485
 486
 487
 488
 489
 490
 491
 492
 493
 494
 495
 496
 497
 498
 499
 500
 501
 502
 503
 504
 505
 506
 507
 508
 509
 510
 511
 512
 513
 514
 515
 516
 517
 518
 519
 520
 521
 522
 523
 524
 525
 526
 527
 528
 529
 530
 531
 532
 533
 534
 535
 536
 537
 538
 539
 540
 541
 542
 543
 544
 545
 546
 547
 548
 549
 550
 551
 552
 553
 554
 555
 556
 557
 558
 559
 560
 561
 562
 563
 564
 565
 566
 567
 568
 569
 570
 571
 572
 573
 574
 575
 576
 577
 578
 579
 580
 581
 582
 583
 584
 585
 586
 587
 588
 589
 590
 591
 592
 593
 594
 595
 596
 597
 598
 599
 600
 601
 602
 603
 604
 605
 606
 607
 608
 609
 610
 611
 612
 613
 614
 615
 616
 617
 618
 619
 620
 621
 622
 623
 624
 625
 626
 627
 628
 629
 630
 631
 632
 633
 634
 635
 636
 637
 638
 639
 640
 641
 642
 643
 644
 645
 646
 647
 648
 649
 650
 651
 652
 653
 654
 655
 656
 657
 658
 659
 660
 661
 662
 663
 664
 665
 666
 667
 668
 669
 670
 671
 672
 673
 674
 675
 676
 677
 678
 679
 680
 681
 682
 683
 684
 685
 686
 687
 688
 689
 690
 691
 692
 693
 694
 695
 696
 697
 698
 699
 700
 701
 702
 703
 704
 705
 706
 707
 708
 709
 710
 711
 712
 713
 714
 715
 716
 717
 718
 719
 720
 721
 722
 723
 724
 725
 726
 727
 728
 729
 730
 731
 732
 733
 734
 735
 736
 737
 738
 739
 740
 741
 742
 743
 744
 745
 746
 747
 748
 749
 750
 751
 752
 753
 754
 755
 756
 757
 758
 759
 760
 761
 762
 763
 764
 765
 766
 767
 768
 769
 770
 771
 772
 773
 774
 775
 776
 777
 778
 779
 780
 781
 782
 783
 784
 785
 786
 787
 788
 789
 790
 791
 792
 793
 794
 795
 796
 797
 798
 799
 800
 801
 802
 803
 804
 805
 806
 807
 808
 809
 810
 811
 812
 813
 814
 815
 816
 817
 818
 819
 820
 821
 822
 823
 824
 825
 826
 827
 828
 829
 830
 831
 832
 833
 834
 835
 836
 837
 838
 839
 840
 841
 842
 843
 844
 845
 846
 847
 848
 849
 850
 851
 852
 853
 854
 855
 856
 857
 858
 859
 860
 861
 862
 863
 864
 865
 866
 867
 868
 869
 870
 871
 872
 873
 874
 875
 876
 877
 878
 879
 880
 881
 882
 883
 884
 885
 886
 887
 888
 889
 890
 891
 892
 893
 894
 895
 896
 897
 898
 899
 900
 901
 902
 903
 904
 905
 906
 907
 908
 909
 910
 911
 912
 913
 914
 915
 916
 917
 918
 919
 920
 921
 922
 923
 924
 925
 926
 927
 928
 929
 930
 931
 932
 933
 934
 935
 936
 937
 938
 939
 940
 941
 942
 943
 944
 945
 946
 947
 948
 949
 950
 951
 952
 953
 954
 955
 956
 957
 958
 959
 960
 961
 962
 963
 964
 965
 966
 967
 968
 969
 970
 971
 972
 973
 974
 975
 976
 977
 978
 979
 980
 981
 982
 983
 984
 985
 986
 987
 988
 989
 990
 991
 992
 993
 994
 995
 996
 997
 998
 999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
/* reps-strings.c : intepreting representations with respect to strings
 *
 * ====================================================================
 *    Licensed to the Apache Software Foundation (ASF) under one
 *    or more contributor license agreements.  See the NOTICE file
 *    distributed with this work for additional information
 *    regarding copyright ownership.  The ASF licenses this file
 *    to you under the Apache License, Version 2.0 (the
 *    "License"); you may not use this file except in compliance
 *    with the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing,
 *    software distributed under the License is distributed on an
 *    "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 *    KIND, either express or implied.  See the License for the
 *    specific language governing permissions and limitations
 *    under the License.
 * ====================================================================
 */

#include <assert.h>

#include "svn_fs.h"
#include "svn_pools.h"

#include "fs.h"
#include "err.h"
#include "trail.h"
#include "reps-strings.h"

#include "bdb/reps-table.h"
#include "bdb/strings-table.h"

#include "../libsvn_fs/fs-loader.h"
#define SVN_WANT_BDB
#include "svn_private_config.h"


/*** Helper Functions ***/


/* Return non-zero iff REP is mutable under transaction TXN_ID. */
static svn_boolean_t rep_is_mutable(representation_t *rep,
                                    const char *txn_id)
{
  if ((! rep->txn_id) || (strcmp(rep->txn_id, txn_id) != 0))
    return FALSE;
  return TRUE;
}

/* Helper macro that evaluates to an error message indicating that
   the representation referred to by X has an unknown node kind. */
#define UNKNOWN_NODE_KIND(x)                                   \
  svn_error_createf                                            \
    (SVN_ERR_FS_CORRUPT, NULL,                                 \
     _("Unknown node kind for representation '%s'"), x)

/* Return a `fulltext' representation, allocated in POOL, which
 * references the string STR_KEY.
 *
 * If TXN_ID is non-zero and non-NULL, make the representation mutable
 * under that TXN_ID.
 *
 * If STR_KEY is non-null, copy it into an allocation from POOL.
 *
 * If MD5_CHECKSUM is non-null, use it as the MD5 checksum for the new
 * rep; else initialize the rep with an all-zero (i.e., always
 * successful) MD5 checksum.
 *
 * If SHA1_CHECKSUM is non-null, use it as the SHA1 checksum for the new
 * rep; else initialize the rep with an all-zero (i.e., always
 * successful) SHA1 checksum.
 */
static representation_t *
make_fulltext_rep(const char *str_key,
                  const char *txn_id,
                  svn_checksum_t *md5_checksum,
                  svn_checksum_t *sha1_checksum,
                  apr_pool_t *pool)

{
  representation_t *rep = apr_pcalloc(pool, sizeof(*rep));
  if (txn_id && *txn_id)
    rep->txn_id = apr_pstrdup(pool, txn_id);
  rep->kind = rep_kind_fulltext;
  rep->md5_checksum = svn_checksum_dup(md5_checksum, pool);
  rep->sha1_checksum = svn_checksum_dup(sha1_checksum, pool);
  rep->contents.fulltext.string_key
    = str_key ? apr_pstrdup(pool, str_key) : NULL;
  return rep;
}


/* Set *KEYS to an array of string keys gleaned from `delta'
   representation REP.  Allocate *KEYS in POOL. */
static svn_error_t *
delta_string_keys(apr_array_header_t **keys,
                  const representation_t *rep,
                  apr_pool_t *pool)
{
  const char *key;
  int i;
  apr_array_header_t *chunks;

  if (rep->kind != rep_kind_delta)
    return svn_error_create
      (SVN_ERR_FS_GENERAL, NULL,
       _("Representation is not of type 'delta'"));

  /* Set up a convenience variable. */
  chunks = rep->contents.delta.chunks;

  /* Initialize *KEYS to an empty array. */
  *keys = apr_array_make(pool, chunks->nelts, sizeof(key));
  if (! chunks->nelts)
    return SVN_NO_ERROR;

  /* Now, push the string keys for each window into *KEYS */
  for (i = 0; i < chunks->nelts; i++)
    {
      rep_delta_chunk_t *chunk = APR_ARRAY_IDX(chunks, i, rep_delta_chunk_t *);

      key = apr_pstrdup(pool, chunk->string_key);
      APR_ARRAY_PUSH(*keys, const char *) = key;
    }

  return SVN_NO_ERROR;
}


/* Delete the strings associated with array KEYS in FS as part of TRAIL.  */
static svn_error_t *
delete_strings(const apr_array_header_t *keys,
               svn_fs_t *fs,
               trail_t *trail,
               apr_pool_t *pool)
{
  int i;
  const char *str_key;
  apr_pool_t *subpool = svn_pool_create(pool);

  for (i = 0; i < keys->nelts; i++)
    {
      svn_pool_clear(subpool);
      str_key = APR_ARRAY_IDX(keys, i, const char *);
      SVN_ERR(svn_fs_bdb__string_delete(fs, str_key, trail, subpool));
    }
  svn_pool_destroy(subpool);
  return SVN_NO_ERROR;
}



/*** Reading the contents from a representation. ***/

struct compose_handler_baton
{
  /* The combined window, and the pool it's allocated from. */
  svn_txdelta_window_t *window;
  apr_pool_t *window_pool;

  /* If the incoming window was self-compressed, and the combined WINDOW
     exists from previous iterations, SOURCE_BUF will point to the
     expanded self-compressed window. */
  char *source_buf;

  /* The trail for this operation. WINDOW_POOL will be a child of
     TRAIL->pool. No allocations will be made from TRAIL->pool itself. */
  trail_t *trail;

  /* TRUE when no more windows have to be read/combined. */
  svn_boolean_t done;

  /* TRUE if we've just started reading a new window. We need this
     because the svndiff handler will push a NULL window at the end of
     the stream, and we have to ignore that; but we must also know
     when it's appropriate to push a NULL window at the combiner. */
  svn_boolean_t init;
};


/* Handle one window. If BATON is emtpy, copy the WINDOW into it;
   otherwise, combine WINDOW with the one in BATON, unless WINDOW
   is self-compressed (i.e., does not copy from the source view),
   in which case expand. */

static svn_error_t *
compose_handler(svn_txdelta_window_t *window, void *baton)
{
  struct compose_handler_baton *cb = baton;
  SVN_ERR_ASSERT(!cb->done || window == NULL);
  SVN_ERR_ASSERT(cb->trail && cb->trail->pool);

  if (!cb->init && !window)
    return SVN_NO_ERROR;

  /* We should never get here if we've already expanded a
     self-compressed window. */
  SVN_ERR_ASSERT(!cb->source_buf);

  if (cb->window)
    {
      if (window && (window->sview_len == 0 || window->src_ops == 0))
        {
          /* This is a self-compressed window. Don't combine it with
             the others, because the combiner may go quadratic. Instead,
             expand it here and signal that the combination has
             ended. */
          apr_size_t source_len = window->tview_len;
          SVN_ERR_ASSERT(cb->window->sview_len == source_len);
          cb->source_buf = apr_palloc(cb->window_pool, source_len);
          svn_txdelta_apply_instructions(window, NULL,
                                         cb->source_buf, &source_len);
          cb->done = TRUE;
        }
      else
        {
          /* Combine the incoming window with whatever's in the baton. */
          apr_pool_t *composite_pool = svn_pool_create(cb->trail->pool);
          svn_txdelta_window_t *composite;

          composite = svn_txdelta_compose_windows(window, cb->window,
                                                  composite_pool);
          svn_pool_destroy(cb->window_pool);
          cb->window = composite;
          cb->window_pool = composite_pool;
          cb->done = (composite->sview_len == 0 || composite->src_ops == 0);
        }
    }
  else if (window)
    {
      /* Copy the (first) window into the baton. */
      apr_pool_t *window_pool = svn_pool_create(cb->trail->pool);
      SVN_ERR_ASSERT(cb->window_pool == NULL);
      cb->window = svn_txdelta_window_dup(window, window_pool);
      cb->window_pool = window_pool;
      cb->done = (window->sview_len == 0 || window->src_ops == 0);
    }
  else
    cb->done = TRUE;

  cb->init = FALSE;
  return SVN_NO_ERROR;
}



/* Read one delta window from REP[CUR_CHUNK] and push it at the
   composition handler. */

static svn_error_t *
get_one_window(struct compose_handler_baton *cb,
               svn_fs_t *fs,
               representation_t *rep,
               int cur_chunk)
{
  svn_stream_t *wstream;
  char diffdata[4096];   /* hunk of svndiff data */
  svn_filesize_t off;    /* offset into svndiff data */
  apr_size_t amt;        /* how much svndiff data to/was read */
  const char *str_key;

  apr_array_header_t *chunks = rep->contents.delta.chunks;
  rep_delta_chunk_t *this_chunk, *first_chunk;

  cb->init = TRUE;
  if (chunks->nelts <= cur_chunk)
    return compose_handler(NULL, cb);

  /* Set up a window handling stream for the svndiff data. */
  wstream = svn_txdelta_parse_svndiff(compose_handler, cb, TRUE,
                                      cb->trail->pool);

  /* First things first:  send the "SVN"{version} header through the
     stream.  ### For now, we will just use the version specified
     in the first chunk, and then verify that no chunks have a
     different version number than the one used.  In the future,
     we might simply convert chunks that use a different version
     of the diff format -- or, heck, a different format
     altogether -- to the format/version of the first chunk.  */
  first_chunk = APR_ARRAY_IDX(chunks, 0, rep_delta_chunk_t*);
  diffdata[0] = 'S';
  diffdata[1] = 'V';
  diffdata[2] = 'N';
  diffdata[3] = (char) (first_chunk->version);
  amt = 4;
  SVN_ERR(svn_stream_write(wstream, diffdata, &amt));
  /* FIXME: The stream write handler is borked; assert (amt == 4); */

  /* Get this string key which holds this window's data.
     ### todo: make sure this is an `svndiff' DIFF skel here. */
  this_chunk = APR_ARRAY_IDX(chunks, cur_chunk, rep_delta_chunk_t*);
  str_key = this_chunk->string_key;

  /* Run through the svndiff data, at least as far as necessary. */
  off = 0;
  do
    {
      amt = sizeof(diffdata);
      SVN_ERR(svn_fs_bdb__string_read(fs, str_key, diffdata,
                                      off, &amt, cb->trail,
                                      cb->trail->pool));
      off += amt;
      SVN_ERR(svn_stream_write(wstream, diffdata, &amt));
    }
  while (amt != 0);
  SVN_ERR(svn_stream_close(wstream));

  SVN_ERR_ASSERT(!cb->init);
  SVN_ERR_ASSERT(cb->window != NULL);
  SVN_ERR_ASSERT(cb->window_pool != NULL);
  return SVN_NO_ERROR;
}


/* Undeltify a range of data. DELTAS is the set of delta windows to
   combine, FULLTEXT is the source text, CUR_CHUNK is the index of the
   delta chunk we're starting from. OFFSET is the relative offset of
   the requested data within the chunk; BUF and LEN are what we're
   undeltifying to. */

static svn_error_t *
rep_undeltify_range(svn_fs_t *fs,
                    const apr_array_header_t *deltas,
                    representation_t *fulltext,
                    int cur_chunk,
                    char *buf,
                    apr_size_t offset,
                    apr_size_t *len,
                    trail_t *trail,
                    apr_pool_t *pool)
{
  apr_size_t len_read = 0;

  do
    {
      struct compose_handler_baton cb = { 0 };
      char *source_buf, *target_buf;
      apr_size_t target_len;
      int cur_rep;

      cb.trail = trail;
      cb.done = FALSE;
      for (cur_rep = 0; !cb.done && cur_rep < deltas->nelts; ++cur_rep)
        {
          representation_t *const rep =
            APR_ARRAY_IDX(deltas, cur_rep, representation_t*);
          SVN_ERR(get_one_window(&cb, fs, rep, cur_chunk));
        }

      if (!cb.window)
          /* That's it, no more source data is available. */
          break;

      /* The source view length should not be 0 if there are source
         copy ops in the window. */
      SVN_ERR_ASSERT(cb.window->sview_len > 0 || cb.window->src_ops == 0);

      /* cb.window is the combined delta window. Read the source text
         into a buffer. */
      if (cb.source_buf)
        {
          /* The combiner already created the source text from a
             self-compressed window. */
          source_buf = cb.source_buf;
        }
      else if (fulltext && cb.window->sview_len > 0 && cb.window->src_ops > 0)
        {
          apr_size_t source_len = cb.window->sview_len;
          source_buf = apr_palloc(cb.window_pool, source_len);
          SVN_ERR(svn_fs_bdb__string_read
                  (fs, fulltext->contents.fulltext.string_key,
                   source_buf, cb.window->sview_offset, &source_len,
                   trail, pool));
          if (source_len != cb.window->sview_len)
            return svn_error_create
                (SVN_ERR_FS_CORRUPT, NULL,
                 _("Svndiff source length inconsistency"));
        }
      else
        {
          source_buf = NULL;    /* Won't read anything from here. */
        }

      if (offset > 0)
        {
          target_len = *len - len_read + offset;
          target_buf = apr_palloc(cb.window_pool, target_len);
        }
      else
        {
          target_len = *len - len_read;
          target_buf = buf;
        }

      svn_txdelta_apply_instructions(cb.window, source_buf,
                                     target_buf, &target_len);
      if (offset > 0)
        {
          SVN_ERR_ASSERT(target_len > offset);
          target_len -= offset;
          memcpy(buf, target_buf + offset, target_len);
          offset = 0; /* Read from the beginning of the next chunk. */
        }
      /* Don't need this window any more. */
      svn_pool_destroy(cb.window_pool);

      len_read += target_len;
      buf += target_len;
      ++cur_chunk;
    }
  while (len_read < *len);

  *len = len_read;
  return SVN_NO_ERROR;
}



/* Calculate the index of the chunk in REP that contains REP_OFFSET,
   and find the relative CHUNK_OFFSET within the chunk.
   Return -1 if offset is beyond the end of the represented data.
   ### The basic assumption is that all delta windows are the same size
   and aligned at the same offset, so this number is the same in all
   dependent deltas.  Oh, and the chunks in REP must be ordered. */

static int
get_chunk_offset(representation_t *rep,
                 svn_filesize_t rep_offset,
                 apr_size_t *chunk_offset)
{
  const apr_array_header_t *chunks = rep->contents.delta.chunks;
  int cur_chunk;
  assert(chunks->nelts);

  /* ### Yes, this is a linear search.  I'll change this to bisection
     the very second we notice it's slowing us down. */
  for (cur_chunk = 0; cur_chunk < chunks->nelts; ++cur_chunk)
  {
    const rep_delta_chunk_t *const this_chunk
      = APR_ARRAY_IDX(chunks, cur_chunk, rep_delta_chunk_t*);

    if ((this_chunk->offset + this_chunk->size) > rep_offset)
      {
        assert(this_chunk->offset <= rep_offset);
        assert(rep_offset - this_chunk->offset < SVN_MAX_OBJECT_SIZE);
        *chunk_offset = (apr_size_t) (rep_offset - this_chunk->offset);
        return cur_chunk;
      }
  }

  return -1;
}

/* Copy into BUF *LEN bytes starting at OFFSET from the string
   represented via REP_KEY in FS, as part of TRAIL.
   The number of bytes actually copied is stored in *LEN.  */
static svn_error_t *
rep_read_range(svn_fs_t *fs,
               const char *rep_key,
               svn_filesize_t offset,
               char *buf,
               apr_size_t *len,
               trail_t *trail,
               apr_pool_t *pool)
{
  representation_t *rep;
  apr_size_t chunk_offset;

  /* Read in our REP. */
  SVN_ERR(svn_fs_bdb__read_rep(&rep, fs, rep_key, trail, pool));
  if (rep->kind == rep_kind_fulltext)
    {
      SVN_ERR(svn_fs_bdb__string_read(fs, rep->contents.fulltext.string_key,
                                      buf, offset, len, trail, pool));
    }
  else if (rep->kind == rep_kind_delta)
    {
      const int cur_chunk = get_chunk_offset(rep, offset, &chunk_offset);
      if (cur_chunk < 0)
        *len = 0;
      else
        {
          svn_error_t *err;
          /* Preserve for potential use in error message. */
          const char *first_rep_key = rep_key;
          /* Make a list of all the rep's we need to undeltify this range.
             We'll have to read them within this trail anyway, so we might
             as well do it once and up front. */
          apr_array_header_t *reps = apr_array_make(pool, 30, sizeof(rep));
          do
            {
              const rep_delta_chunk_t *const first_chunk
                = APR_ARRAY_IDX(rep->contents.delta.chunks,
                                0, rep_delta_chunk_t*);
              const rep_delta_chunk_t *const chunk
                = APR_ARRAY_IDX(rep->contents.delta.chunks,
                                cur_chunk, rep_delta_chunk_t*);

              /* Verify that this chunk is of the same version as the first. */
              if (first_chunk->version != chunk->version)
                return svn_error_createf
                  (SVN_ERR_FS_CORRUPT, NULL,
                   _("Diff version inconsistencies in representation '%s'"),
                   rep_key);

              rep_key = chunk->rep_key;
              APR_ARRAY_PUSH(reps, representation_t *) = rep;
              SVN_ERR(svn_fs_bdb__read_rep(&rep, fs, rep_key,
                                           trail, pool));
            }
          while (rep->kind == rep_kind_delta
                 && rep->contents.delta.chunks->nelts > cur_chunk);

          /* Right. We've either just read the fulltext rep, or a rep that's
             too short, in which case we'll undeltify without source data.*/
          if (rep->kind != rep_kind_delta && rep->kind != rep_kind_fulltext)
            return UNKNOWN_NODE_KIND(rep_key);

          if (rep->kind == rep_kind_delta)
            rep = NULL;         /* Don't use source data */

          err = rep_undeltify_range(fs, reps, rep, cur_chunk, buf,
                                    chunk_offset, len, trail, pool);
          if (err)
            {
              if (err->apr_err == SVN_ERR_FS_CORRUPT)
                return svn_error_createf
                  (SVN_ERR_FS_CORRUPT, err,
                   _("Corruption detected whilst reading delta chain from "
                     "representation '%s' to '%s'"), first_rep_key, rep_key);
              else
                return svn_error_trace(err);
            }
        }
    }
  else /* unknown kind */
    return UNKNOWN_NODE_KIND(rep_key);

  return SVN_NO_ERROR;
}


svn_error_t *
svn_fs_base__get_mutable_rep(const char **new_rep_key,
                             const char *rep_key,
                             svn_fs_t *fs,
                             const char *txn_id,
                             trail_t *trail,
                             apr_pool_t *pool)
{
  representation_t *rep = NULL;
  const char *new_str = NULL;

  /* We were passed an existing REP_KEY, so examine it.  If it is
     mutable already, then just return REP_KEY as the mutable result
     key.  */
  if (rep_key && (rep_key[0] != '\0'))
    {
      SVN_ERR(svn_fs_bdb__read_rep(&rep, fs, rep_key, trail, pool));
      if (rep_is_mutable(rep, txn_id))
        {
          *new_rep_key = rep_key;
          return SVN_NO_ERROR;
        }
    }

  /* Either we weren't provided a base key to examine, or the base key
     we were provided was not mutable.  So, let's make a new
     representation and return its key to the caller. */
  SVN_ERR(svn_fs_bdb__string_append(fs, &new_str, 0, NULL, trail, pool));
  rep = make_fulltext_rep(new_str, txn_id,
                          svn_checksum_empty_checksum(svn_checksum_md5,
                                                      pool),
                          svn_checksum_empty_checksum(svn_checksum_sha1,
                                                      pool),
                          pool);
  return svn_fs_bdb__write_new_rep(new_rep_key, fs, rep, trail, pool);
}


svn_error_t *
svn_fs_base__delete_rep_if_mutable(svn_fs_t *fs,
                                   const char *rep_key,
                                   const char *txn_id,
                                   trail_t *trail,
                                   apr_pool_t *pool)
{
  representation_t *rep;

  SVN_ERR(svn_fs_bdb__read_rep(&rep, fs, rep_key, trail, pool));
  if (! rep_is_mutable(rep, txn_id))
    return SVN_NO_ERROR;

  if (rep->kind == rep_kind_fulltext)
    {
      SVN_ERR(svn_fs_bdb__string_delete(fs,
                                        rep->contents.fulltext.string_key,
                                        trail, pool));
    }
  else if (rep->kind == rep_kind_delta)
    {
      apr_array_header_t *keys;
      SVN_ERR(delta_string_keys(&keys, rep, pool));
      SVN_ERR(delete_strings(keys, fs, trail, pool));
    }
  else /* unknown kind */
    return UNKNOWN_NODE_KIND(rep_key);

  return svn_fs_bdb__delete_rep(fs, rep_key, trail, pool);
}



/*** Reading and writing data via representations. ***/

/** Reading. **/

struct rep_read_baton
{
  /* The FS from which we're reading. */
  svn_fs_t *fs;

  /* The representation skel whose contents we want to read.  If this
     is NULL, the rep has never had any contents, so all reads fetch 0
     bytes.

     Formerly, we cached the entire rep skel here, not just the key.
     That way we didn't have to fetch the rep from the db every time
     we want to read a little bit more of the file.  Unfortunately,
     this has a problem: if, say, a file's representation changes
     while we're reading (changes from fulltext to delta, for
     example), we'll never know it.  So for correctness, we now
     refetch the representation skel every time we want to read
     another chunk.  */
  const char *rep_key;

  /* How many bytes have been read already. */
  svn_filesize_t offset;

  /* If present, the read will be done as part of this trail, and the
     trail's pool will be used.  Otherwise, see `pool' below.  */
  trail_t *trail;

  /* MD5 checksum context.  Initialized when the baton is created, updated as
     we read data, and finalized when the stream is closed. */
  svn_checksum_ctx_t *md5_checksum_ctx;

  /* Final resting place of the checksum created by md5_checksum_cxt. */
  svn_checksum_t *md5_checksum;

  /* SHA1 checksum context.  Initialized when the baton is created, updated as
     we read data, and finalized when the stream is closed. */
  svn_checksum_ctx_t *sha1_checksum_ctx;

  /* Final resting place of the checksum created by sha1_checksum_cxt. */
  svn_checksum_t *sha1_checksum;

  /* The length of the rep's contents (as fulltext, that is,
     independent of how the rep actually stores the data.)  This is
     retrieved when the baton is created, and used to determine when
     we have read the last byte, at which point we compare checksums.

     Getting this at baton creation time makes interleaved reads and
     writes on the same rep in the same trail impossible.  But we're
     not doing that, and probably no one ever should.  And anyway if
     they do, they should see problems immediately. */
  svn_filesize_t size;

  /* Set to FALSE when the baton is created, TRUE when the checksum_ctx
     is digestified. */
  svn_boolean_t checksum_finalized;

  /* Used for temporary allocations.  This pool is cleared at the
     start of each invocation of the relevant stream read function --
     see rep_read_contents().  */
  apr_pool_t *scratch_pool;

};


static svn_error_t *
rep_read_get_baton(struct rep_read_baton **rb_p,
                   svn_fs_t *fs,
                   const char *rep_key,
                   svn_boolean_t use_trail_for_reads,
                   trail_t *trail,
                   apr_pool_t *pool)
{
  struct rep_read_baton *b;

  b = apr_pcalloc(pool, sizeof(*b));
  b->md5_checksum_ctx = svn_checksum_ctx_create(svn_checksum_md5, pool);
  b->sha1_checksum_ctx = svn_checksum_ctx_create(svn_checksum_sha1, pool);

  if (rep_key)
    SVN_ERR(svn_fs_base__rep_contents_size(&(b->size), fs, rep_key,
                                           trail, pool));
  else
    b->size = 0;

  b->checksum_finalized = FALSE;
  b->fs = fs;
  b->trail = use_trail_for_reads ? trail : NULL;
  b->scratch_pool = svn_pool_create(pool);
  b->rep_key = rep_key;
  b->offset = 0;

  *rb_p = b;

  return SVN_NO_ERROR;
}



/*** Retrieving data. ***/

svn_error_t *
svn_fs_base__rep_contents_size(svn_filesize_t *size_p,
                               svn_fs_t *fs,
                               const char *rep_key,
                               trail_t *trail,
                               apr_pool_t *pool)
{
  representation_t *rep;

  SVN_ERR(svn_fs_bdb__read_rep(&rep, fs, rep_key, trail, pool));

  if (rep->kind == rep_kind_fulltext)
    {
      /* Get the size by asking Berkeley for the string's length. */
      SVN_ERR(svn_fs_bdb__string_size(size_p, fs,
                                      rep->contents.fulltext.string_key,
                                      trail, pool));
    }
  else if (rep->kind == rep_kind_delta)
    {
      /* Get the size by finding the last window pkg in the delta and
         adding its offset to its size.  This way, we won't even be
         messed up by overlapping windows, as long as the window pkgs
         are still ordered. */
      apr_array_header_t *chunks = rep->contents.delta.chunks;
      rep_delta_chunk_t *last_chunk;

      SVN_ERR_ASSERT(chunks->nelts);

      last_chunk = APR_ARRAY_IDX(chunks, chunks->nelts - 1,
                                 rep_delta_chunk_t *);
      *size_p = last_chunk->offset + last_chunk->size;
    }
  else /* unknown kind */
    return UNKNOWN_NODE_KIND(rep_key);

  return SVN_NO_ERROR;
}


svn_error_t *
svn_fs_base__rep_contents_checksums(svn_checksum_t **md5_checksum,
                                    svn_checksum_t **sha1_checksum,
                                    svn_fs_t *fs,
                                    const char *rep_key,
                                    trail_t *trail,
                                    apr_pool_t *pool)
{
  representation_t *rep;

  SVN_ERR(svn_fs_bdb__read_rep(&rep, fs, rep_key, trail, pool));
  if (md5_checksum)
    *md5_checksum = svn_checksum_dup(rep->md5_checksum, pool);
  if (sha1_checksum)
    *sha1_checksum = svn_checksum_dup(rep->sha1_checksum, pool);

  return SVN_NO_ERROR;
}


svn_error_t *
svn_fs_base__rep_contents(svn_string_t *str,
                          svn_fs_t *fs,
                          const char *rep_key,
                          trail_t *trail,
                          apr_pool_t *pool)
{
  svn_filesize_t contents_size;
  apr_size_t len;
  char *data;

  SVN_ERR(svn_fs_base__rep_contents_size(&contents_size, fs, rep_key,
                                         trail, pool));

  /* What if the contents are larger than we can handle? */
  if (contents_size > SVN_MAX_OBJECT_SIZE)
    return svn_error_createf
      (SVN_ERR_FS_GENERAL, NULL,
       _("Rep contents are too large: "
         "got %s, limit is %s"),
       apr_psprintf(pool, "%" SVN_FILESIZE_T_FMT, contents_size),
       apr_psprintf(pool, "%" APR_SIZE_T_FMT, SVN_MAX_OBJECT_SIZE));
  else
    str->len = (apr_size_t) contents_size;

  data = apr_palloc(pool, str->len);
  str->data = data;
  len = str->len;
  SVN_ERR(rep_read_range(fs, rep_key, 0, data, &len, trail, pool));

  /* Paranoia. */
  if (len != str->len)
    return svn_error_createf
      (SVN_ERR_FS_CORRUPT, NULL,
       _("Failure reading representation '%s'"), rep_key);

  /* Just the standard paranoia. */
  {
    representation_t *rep;
    svn_checksum_t *checksum, *rep_checksum;

    SVN_ERR(svn_fs_bdb__read_rep(&rep, fs, rep_key, trail, pool));
    rep_checksum = rep->sha1_checksum ? rep->sha1_checksum : rep->md5_checksum;
    SVN_ERR(svn_checksum(&checksum, rep_checksum->kind, str->data, str->len,
                         pool));

    if (! svn_checksum_match(checksum, rep_checksum))
      return svn_error_create(SVN_ERR_FS_CORRUPT,
                svn_checksum_mismatch_err(rep_checksum, checksum, pool,
                            _("Checksum mismatch on representation '%s'"),
                            rep_key),
                NULL);
  }

  return SVN_NO_ERROR;
}


struct read_rep_args
{
  struct rep_read_baton *rb;   /* The data source.             */
  char *buf;                   /* Where to put what we read.   */
  apr_size_t *len;             /* How much to read / was read. */
};


/* BATON is of type `read_rep_args':

   Read into BATON->rb->buf the *(BATON->len) bytes starting at
   BATON->rb->offset from the data represented at BATON->rb->rep_key
   in BATON->rb->fs, as part of TRAIL.

   Afterwards, *(BATON->len) is the number of bytes actually read, and
   BATON->rb->offset is incremented by that amount.

   If BATON->rb->rep_key is null, this is assumed to mean the file's
   contents have no representation, i.e., the file has no contents.
   In that case, if BATON->rb->offset > 0, return the error
   SVN_ERR_FS_FILE_CONTENTS_CHANGED, else just set *(BATON->len) to
   zero and return.  */
static svn_error_t *
txn_body_read_rep(void *baton, trail_t *trail)
{
  struct read_rep_args *args = baton;

  if (args->rb->rep_key)
    {
      SVN_ERR(rep_read_range(args->rb->fs,
                             args->rb->rep_key,
                             args->rb->offset,
                             args->buf,
                             args->len,
                             trail,
                             args->rb->scratch_pool));

      args->rb->offset += *(args->len);

      /* We calculate the checksum just once, the moment we see the
       * last byte of data.  But we can't assume there was a short
       * read.  The caller may have known the length of the data and
       * requested exactly that amount, so there would never be a
       * short read.  (That's why the read baton has to know the
       * length of the data in advance.)
       *
       * On the other hand, some callers invoke the stream reader in a
       * loop whose termination condition is that the read returned
       * zero bytes of data -- which usually results in the read
       * function being called one more time *after* the call that got
       * a short read (indicating end-of-stream).
       *
       * The conditions below ensure that we compare checksums even
       * when there is no short read associated with the last byte of
       * data, while also ensuring that it's harmless to repeatedly
       * read 0 bytes from the stream.
       */
      if (! args->rb->checksum_finalized)
        {
          SVN_ERR(svn_checksum_update(args->rb->md5_checksum_ctx, args->buf,
                                      *(args->len)));
          SVN_ERR(svn_checksum_update(args->rb->sha1_checksum_ctx, args->buf,
                                      *(args->len)));

          if (args->rb->offset == args->rb->size)
            {
              representation_t *rep;

              SVN_ERR(svn_checksum_final(&args->rb->md5_checksum,
                                         args->rb->md5_checksum_ctx,
                                         trail->pool));
              SVN_ERR(svn_checksum_final(&args->rb->sha1_checksum,
                                         args->rb->sha1_checksum_ctx,
                                         trail->pool));
              args->rb->checksum_finalized = TRUE;

              SVN_ERR(svn_fs_bdb__read_rep(&rep, args->rb->fs,
                                           args->rb->rep_key,
                                           trail, trail->pool));

              if (rep->md5_checksum
                  && (! svn_checksum_match(rep->md5_checksum,
                                           args->rb->md5_checksum)))
                return svn_error_create(SVN_ERR_FS_CORRUPT,
                        svn_checksum_mismatch_err(rep->md5_checksum,
                             args->rb->md5_checksum, trail->pool,
                             _("MD5 checksum mismatch on representation '%s'"),
                             args->rb->rep_key),
                        NULL);

              if (rep->sha1_checksum
                  && (! svn_checksum_match(rep->sha1_checksum,
                                           args->rb->sha1_checksum)))
                return svn_error_createf(SVN_ERR_FS_CORRUPT,
                        svn_checksum_mismatch_err(rep->sha1_checksum,
                            args->rb->sha1_checksum, trail->pool,
                            _("SHA1 checksum mismatch on representation '%s'"),
                            args->rb->rep_key),
                        NULL);
            }
        }
    }
  else if (args->rb->offset > 0)
    {
      return
        svn_error_create
        (SVN_ERR_FS_REP_CHANGED, NULL,
         _("Null rep, but offset past zero already"));
    }
  else
    *(args->len) = 0;

  return SVN_NO_ERROR;
}


static svn_error_t *
rep_read_contents(void *baton, char *buf, apr_size_t *len)
{
  struct rep_read_baton *rb = baton;
  struct read_rep_args args;

  /* Clear the scratch pool of the results of previous invocations. */
  svn_pool_clear(rb->scratch_pool);

  args.rb = rb;
  args.buf = buf;
  args.len = len;

  /* If we got a trail, use it; else make one. */
  if (rb->trail)
    SVN_ERR(txn_body_read_rep(&args, rb->trail));
  else
    {
      /* In the case of reading from the db, any returned data should
         live in our pre-allocated buffer, so the whole operation can
         happen within a single malloc/free cycle.  This prevents us
         from creating millions of unnecessary trail subpools when
         reading a big file.  */
      SVN_ERR(svn_fs_base__retry_txn(rb->fs,
                                     txn_body_read_rep,
                                     &args,
                                     TRUE,
                                     rb->scratch_pool));
    }
  return SVN_NO_ERROR;
}


/** Writing. **/


struct rep_write_baton
{
  /* The FS in which we're writing. */
  svn_fs_t *fs;

  /* The representation skel whose contents we want to write. */
  const char *rep_key;

  /* The transaction id under which this write action will take
     place. */
  const char *txn_id;

  /* If present, do the write as part of this trail, and use trail's
     pool.  Otherwise, see `pool' below.  */
  trail_t *trail;

  /* SHA1 and MD5 checksums.  Initialized when the baton is created,
     updated as we write data, and finalized and stored when the
     stream is closed. */
  svn_checksum_ctx_t *md5_checksum_ctx;
  svn_checksum_t *md5_checksum;
  svn_checksum_ctx_t *sha1_checksum_ctx;
  svn_checksum_t *sha1_checksum;
  svn_boolean_t finalized;

  /* Used for temporary allocations, iff `trail' (above) is null.  */
  apr_pool_t *pool;

};


static struct rep_write_baton *
rep_write_get_baton(svn_fs_t *fs,
                    const char *rep_key,
                    const char *txn_id,
                    trail_t *trail,
                    apr_pool_t *pool)
{
  struct rep_write_baton *b;

  b = apr_pcalloc(pool, sizeof(*b));
  b->md5_checksum_ctx = svn_checksum_ctx_create(svn_checksum_md5, pool);
  b->sha1_checksum_ctx = svn_checksum_ctx_create(svn_checksum_sha1, pool);
  b->fs = fs;
  b->trail = trail;
  b->pool = pool;
  b->rep_key = rep_key;
  b->txn_id = txn_id;
  return b;
}



/* Write LEN bytes from BUF into the end of the string represented via
   REP_KEY in FS, as part of TRAIL.  If the representation is not
   mutable, return the error SVN_FS_REP_NOT_MUTABLE. */
static svn_error_t *
rep_write(svn_fs_t *fs,
          const char *rep_key,
          const char *buf,
          apr_size_t len,
          const char *txn_id,
          trail_t *trail,
          apr_pool_t *pool)
{
  representation_t *rep;

  SVN_ERR(svn_fs_bdb__read_rep(&rep, fs, rep_key, trail, pool));

  if (! rep_is_mutable(rep, txn_id))
    return svn_error_createf
      (SVN_ERR_FS_REP_NOT_MUTABLE, NULL,
       _("Rep '%s' is not mutable"), rep_key);

  if (rep->kind == rep_kind_fulltext)
    {
      SVN_ERR(svn_fs_bdb__string_append
              (fs, &(rep->contents.fulltext.string_key), len, buf,
               trail, pool));
    }
  else if (rep->kind == rep_kind_delta)
    {
      /* There should never be a case when we have a mutable
         non-fulltext rep.  The only code that creates mutable reps is
         in this file, and it creates them fulltext. */
      return svn_error_createf
        (SVN_ERR_FS_CORRUPT, NULL,
         _("Rep '%s' both mutable and non-fulltext"), rep_key);
    }
  else /* unknown kind */
    return UNKNOWN_NODE_KIND(rep_key);

  return SVN_NO_ERROR;
}


struct write_rep_args
{
  struct rep_write_baton *wb;   /* Destination.       */
  const char *buf;              /* Data.              */
  apr_size_t len;               /* How much to write. */
};


/* BATON is of type `write_rep_args':
   Append onto BATON->wb->rep_key's contents BATON->len bytes of
   data from BATON->wb->buf, in BATON->rb->fs, as part of TRAIL.

   If the representation is not mutable, return the error
   SVN_FS_REP_NOT_MUTABLE.  */
static svn_error_t *
txn_body_write_rep(void *baton, trail_t *trail)
{
  struct write_rep_args *args = baton;

  SVN_ERR(rep_write(args->wb->fs,
                    args->wb->rep_key,
                    args->buf,
                    args->len,
                    args->wb->txn_id,
                    trail,
                    trail->pool));
  SVN_ERR(svn_checksum_update(args->wb->md5_checksum_ctx,
                              args->buf, args->len));
  SVN_ERR(svn_checksum_update(args->wb->sha1_checksum_ctx,
                              args->buf, args->len));
  return SVN_NO_ERROR;
}


static svn_error_t *
rep_write_contents(void *baton,
                   const char *buf,
                   apr_size_t *len)
{
  struct rep_write_baton *wb = baton;
  struct write_rep_args args;

  /* We toss LEN's indirectness because if not all the bytes are
     written, it's an error, so we wouldn't be reporting anything back
     through *LEN anyway. */
  args.wb = wb;
  args.buf = buf;
  args.len = *len;

  /* If we got a trail, use it; else make one. */
  if (wb->trail)
    SVN_ERR(txn_body_write_rep(&args, wb->trail));
  else
    {
      /* In the case of simply writing the rep to the db, we're
         *certain* that there's no data coming back to us that needs
         to be preserved... so the whole operation can happen within a
         single malloc/free cycle.  This prevents us from creating
         millions of unnecessary trail subpools when writing a big
         file. */
      SVN_ERR(svn_fs_base__retry_txn(wb->fs,
                                     txn_body_write_rep,
                                     &args,
                                     TRUE,
                                     wb->pool));
    }

  return SVN_NO_ERROR;
}


/* Helper for rep_write_close_contents(); see that doc string for
   more.  BATON is of type `struct rep_write_baton'. */
static svn_error_t *
txn_body_write_close_rep(void *baton, trail_t *trail)
{
  struct rep_write_baton *wb = baton;
  representation_t *rep;

  SVN_ERR(svn_fs_bdb__read_rep(&rep, wb->fs, wb->rep_key,
                               trail, trail->pool));
  rep->md5_checksum = svn_checksum_dup(wb->md5_checksum, trail->pool);
  rep->sha1_checksum = svn_checksum_dup(wb->sha1_checksum, trail->pool);
  return svn_fs_bdb__write_rep(wb->fs, wb->rep_key, rep,
                               trail, trail->pool);
}


/* BATON is of type `struct rep_write_baton'.
 *
 * Finalize BATON->md5_context and store the resulting digest under
 * BATON->rep_key.
 */
static svn_error_t *
rep_write_close_contents(void *baton)
{
  struct rep_write_baton *wb = baton;

  /* ### Thought: if we fixed apr-util MD5 contexts to allow repeated
     digestification, then we wouldn't need a stream close function at
     all -- instead, we could update the stored checksum each time a
     write occurred, which would have the added advantage of making
     interleaving reads and writes work.  Currently, they'd fail with
     a checksum mismatch, it just happens that our code never tries to
     do that anyway. */

  if (! wb->finalized)
    {
      SVN_ERR(svn_checksum_final(&wb->md5_checksum, wb->md5_checksum_ctx,
                                 wb->pool));
      SVN_ERR(svn_checksum_final(&wb->sha1_checksum, wb->sha1_checksum_ctx,
                                 wb->pool));
      wb->finalized = TRUE;
    }

  /* If we got a trail, use it; else make one. */
  if (wb->trail)
    return txn_body_write_close_rep(wb, wb->trail);
  else
    /* We need to keep our trail pool around this time so the
       checksums we've calculated survive. */
    return svn_fs_base__retry_txn(wb->fs, txn_body_write_close_rep,
                                  wb, FALSE, wb->pool);
}


/** Public read and write stream constructors. **/

svn_error_t *
svn_fs_base__rep_contents_read_stream(svn_stream_t **rs_p,
                                      svn_fs_t *fs,
                                      const char *rep_key,
                                      svn_boolean_t use_trail_for_reads,
                                      trail_t *trail,
                                      apr_pool_t *pool)
{
  struct rep_read_baton *rb;

  SVN_ERR(rep_read_get_baton(&rb, fs, rep_key, use_trail_for_reads,
                             trail, pool));
  *rs_p = svn_stream_create(rb, pool);
  svn_stream_set_read2(*rs_p, NULL /* only full read support */,
                       rep_read_contents);

  return SVN_NO_ERROR;
}


/* Clear the contents of REP_KEY, so that it represents the empty
   string, as part of TRAIL.  TXN_ID is the id of the Subversion
   transaction under which this occurs.  If REP_KEY is not mutable,
   return the error SVN_ERR_FS_REP_NOT_MUTABLE.  */
static svn_error_t *
rep_contents_clear(svn_fs_t *fs,
                   const char *rep_key,
                   const char *txn_id,
                   trail_t *trail,
                   apr_pool_t *pool)
{
  representation_t *rep;
  const char *str_key;

  SVN_ERR(svn_fs_bdb__read_rep(&rep, fs, rep_key, trail, pool));

  /* Make sure it's mutable. */
  if (! rep_is_mutable(rep, txn_id))
    return svn_error_createf
      (SVN_ERR_FS_REP_NOT_MUTABLE, NULL,
       _("Rep '%s' is not mutable"), rep_key);

  SVN_ERR_ASSERT(rep->kind == rep_kind_fulltext);

  /* If rep has no string, just return success.  Else, clear the
     underlying string.  */
  str_key = rep->contents.fulltext.string_key;
  if (str_key && *str_key)
    {
      SVN_ERR(svn_fs_bdb__string_clear(fs, str_key, trail, pool));
      rep->md5_checksum = NULL;
      rep->sha1_checksum = NULL;
      SVN_ERR(svn_fs_bdb__write_rep(fs, rep_key, rep, trail, pool));
    }
  return SVN_NO_ERROR;
}


svn_error_t *
svn_fs_base__rep_contents_write_stream(svn_stream_t **ws_p,
                                       svn_fs_t *fs,
                                       const char *rep_key,
                                       const char *txn_id,
                                       svn_boolean_t use_trail_for_writes,
                                       trail_t *trail,
                                       apr_pool_t *pool)
{
  struct rep_write_baton *wb;

  /* Clear the current rep contents (free mutability check!). */
  SVN_ERR(rep_contents_clear(fs, rep_key, txn_id, trail, pool));

  /* Now, generate the write baton and stream. */
  wb = rep_write_get_baton(fs, rep_key, txn_id,
                           use_trail_for_writes ? trail : NULL, pool);
  *ws_p = svn_stream_create(wb, pool);
  svn_stream_set_write(*ws_p, rep_write_contents);
  svn_stream_set_close(*ws_p, rep_write_close_contents);

  return SVN_NO_ERROR;
}



/*** Deltified storage. ***/

/* Baton for svn_write_fn_t write_string_set(). */
struct write_svndiff_strings_baton
{
  /* The fs where lives the string we're writing. */
  svn_fs_t *fs;

  /* The key of the string we're writing to.  Typically this is
     initialized to NULL, so svn_fs_base__string_append() can fill in a
     value. */
  const char *key;

  /* The amount of txdelta data written to the current
     string-in-progress. */
  apr_size_t size;

  /* The amount of svndiff header information we've written thus far
     to the strings table. */
  apr_size_t header_read;

  /* The version number of the svndiff data written.  ### You'd better
     not count on this being populated after the first chunk is sent
     through the interface, since it lives at the 4th byte of the
     stream. */
  apr_byte_t version;

  /* The trail we're writing in. */
  trail_t *trail;

};


/* Function of type `svn_write_fn_t', for writing to a collection of
   strings; BATON is `struct write_svndiff_strings_baton *'.

   On the first call, BATON->key is null.  A new string key in
   BATON->fs is chosen and stored in BATON->key; each call appends
   *LEN bytes from DATA onto the string.  *LEN is never changed; if
   the write fails to write all *LEN bytes, an error is returned.
   BATON->size is used to track the total amount of data written via
   this handler, and must be reset by the caller to 0 when appropriate.  */
static svn_error_t *
write_svndiff_strings(void *baton, const char *data, apr_size_t *len)
{
  struct write_svndiff_strings_baton *wb = baton;
  const char *buf = data;
  apr_size_t nheader = 0;

  /* If we haven't stripped all the header information from this
     stream yet, keep stripping.  If someone sends a first window
     through here that's shorter than 4 bytes long, this will probably
     cause a nuclear reactor meltdown somewhere in the American
     midwest.  */
  if (wb->header_read < 4)
    {
      nheader = 4 - wb->header_read;
      *len -= nheader;
      buf += nheader;
      wb->header_read += nheader;

      /* If we have *now* read the full 4-byte header, check that
         least byte for the version number of the svndiff format. */
      if (wb->header_read == 4)
        wb->version = *(buf - 1);
    }

  /* Append to the current string we're writing (or create a new one
     if WB->key is NULL). */
  SVN_ERR(svn_fs_bdb__string_append(wb->fs, &(wb->key), *len,
                                    buf, wb->trail, wb->trail->pool));

  /* Make sure we (still) have a key. */
  if (wb->key == NULL)
    return svn_error_create(SVN_ERR_FS_GENERAL, NULL,
                            _("Failed to get new string key"));

  /* Restore *LEN to the value it *would* have been were it not for
     header stripping. */
  *len += nheader;

  /* Increment our running total of bytes written to this string. */
  wb->size += *len;

  return SVN_NO_ERROR;
}


typedef struct window_write_t
{
  const char *key; /* string key for this window */
  apr_size_t svndiff_len; /* amount of svndiff data written to the string */
  svn_filesize_t text_off; /* offset of fulltext represented by this window */
  apr_size_t text_len; /* amount of fulltext data represented by this window */

} window_write_t;


svn_error_t *
svn_fs_base__rep_deltify(svn_fs_t *fs,
                         const char *target,
                         const char *source,
                         trail_t *trail,
                         apr_pool_t *pool)
{
  base_fs_data_t *bfd = fs->fsap_data;
  svn_stream_t *source_stream; /* stream to read the source */
  svn_stream_t *target_stream; /* stream to read the target */
  svn_txdelta_stream_t *txdelta_stream; /* stream to read delta windows  */

  /* window-y things, and an array to track them */
  window_write_t *ww;
  apr_array_header_t *windows;

  /* stream to write new (deltified) target data and its baton */
  svn_stream_t *new_target_stream;
  struct write_svndiff_strings_baton new_target_baton;

  /* window handler/baton for writing to above stream */
  svn_txdelta_window_handler_t new_target_handler;
  void *new_target_handler_baton;

  /* yes, we do windows */
  svn_txdelta_window_t *window;

  /* The current offset into the fulltext that our window is about to
     write.  This doubles, after all windows are written, as the
     total size of the svndiff data for the deltification process. */
  svn_filesize_t tview_off = 0;

  /* The total amount of diff data written while deltifying. */
  svn_filesize_t diffsize = 0;

  /* TARGET's original string keys */
  apr_array_header_t *orig_str_keys;

  /* The checksums for the representation's fulltext contents. */
  svn_checksum_t *rep_md5_checksum;
  svn_checksum_t *rep_sha1_checksum;

  /* MD5 digest */
  const unsigned char *digest;

  /* pool for holding the windows */
  apr_pool_t *wpool;

  /* Paranoia: never allow a rep to be deltified against itself,
     because then there would be no fulltext reachable in the delta
     chain, and badness would ensue.  */
  if (strcmp(target, source) == 0)
    return svn_error_createf
      (SVN_ERR_FS_CORRUPT, NULL,
       _("Attempt to deltify '%s' against itself"),
       target);

  /* Set up a handler for the svndiff data, which will write each
     window to its own string in the `strings' table. */
  new_target_baton.fs = fs;
  new_target_baton.trail = trail;
  new_target_baton.header_read = FALSE;
  new_target_stream = svn_stream_create(&new_target_baton, pool);
  svn_stream_set_write(new_target_stream, write_svndiff_strings);

  /* Get streams to our source and target text data. */
  SVN_ERR(svn_fs_base__rep_contents_read_stream(&source_stream, fs, source,
                                                TRUE, trail, pool));
  SVN_ERR(svn_fs_base__rep_contents_read_stream(&target_stream, fs, target,
                                                TRUE, trail, pool));

  /* Setup a stream to convert the textdelta data into svndiff windows. */
  svn_txdelta2(&txdelta_stream, source_stream, target_stream, TRUE, pool);

  if (bfd->format >= SVN_FS_BASE__MIN_SVNDIFF1_FORMAT)
    svn_txdelta_to_svndiff3(&new_target_handler, &new_target_handler_baton,
                            new_target_stream, 1,
                            SVN_DELTA_COMPRESSION_LEVEL_DEFAULT, pool);
  else
    svn_txdelta_to_svndiff3(&new_target_handler, &new_target_handler_baton,
                            new_target_stream, 0,
                            SVN_DELTA_COMPRESSION_LEVEL_DEFAULT, pool);

  /* subpool for the windows */
  wpool = svn_pool_create(pool);

  /* Now, loop, manufacturing and dispatching windows of svndiff data. */
  windows = apr_array_make(pool, 1, sizeof(ww));
  do
    {
      /* Reset some baton variables. */
      new_target_baton.size = 0;
      new_target_baton.key = NULL;

      /* Free the window. */
      svn_pool_clear(wpool);

      /* Fetch the next window of txdelta data. */
      SVN_ERR(svn_txdelta_next_window(&window, txdelta_stream, wpool));

      /* Send off this package to be written as svndiff data. */
      SVN_ERR(new_target_handler(window, new_target_handler_baton));
      if (window)
        {
          /* Add a new window description to our array. */
          ww = apr_pcalloc(pool, sizeof(*ww));
          ww->key = new_target_baton.key;
          ww->svndiff_len = new_target_baton.size;
          ww->text_off = tview_off;
          ww->text_len = window->tview_len;
          APR_ARRAY_PUSH(windows, window_write_t *) = ww;

          /* Update our recordkeeping variables. */
          tview_off += window->tview_len;
          diffsize += ww->svndiff_len;
        }

    } while (window);

  svn_pool_destroy(wpool);

  /* Having processed all the windows, we can query the MD5 digest
     from the stream.  */
  digest = svn_txdelta_md5_digest(txdelta_stream);
  if (! digest)
    return svn_error_createf
      (SVN_ERR_DELTA_MD5_CHECKSUM_ABSENT, NULL,
       _("Failed to calculate MD5 digest for '%s'"),
       source);

  /* Construct a list of the strings used by the old representation so
     that we can delete them later.  While we are here, if the old
     representation was a fulltext, check to make sure the delta we're
     replacing it with is actually smaller.  (Don't perform this check
     if we're replacing a delta; in that case, we're going for a time
     optimization, not a space optimization.)  */
  {
    representation_t *old_rep;
    const char *str_key;

    SVN_ERR(svn_fs_bdb__read_rep(&old_rep, fs, target, trail, pool));
    if (old_rep->kind == rep_kind_fulltext)
      {
        svn_filesize_t old_size = 0;

        str_key = old_rep->contents.fulltext.string_key;
        SVN_ERR(svn_fs_bdb__string_size(&old_size, fs, str_key,
                                        trail, pool));
        orig_str_keys = apr_array_make(pool, 1, sizeof(str_key));
        APR_ARRAY_PUSH(orig_str_keys, const char *) = str_key;

        /* If the new data is NOT an space optimization, destroy the
           string(s) we created, and get outta here. */
        if (diffsize >= old_size)
          {
            int i;
            for (i = 0; i < windows->nelts; i++)
              {
                ww = APR_ARRAY_IDX(windows, i, window_write_t *);
                SVN_ERR(svn_fs_bdb__string_delete(fs, ww->key, trail, pool));
              }
            return SVN_NO_ERROR;
          }
      }
    else if (old_rep->kind == rep_kind_delta)
      SVN_ERR(delta_string_keys(&orig_str_keys, old_rep, pool));
    else /* unknown kind */
      return UNKNOWN_NODE_KIND(target);

    /* Save the checksums, since the new rep needs them. */
    rep_md5_checksum = svn_checksum_dup(old_rep->md5_checksum, pool);
    rep_sha1_checksum = svn_checksum_dup(old_rep->sha1_checksum, pool);
  }

  /* Hook the new strings we wrote into the rest of the filesystem by
     building a new representation to replace our old one. */
  {
    representation_t new_rep;
    rep_delta_chunk_t *chunk;
    apr_array_header_t *chunks;
    int i;

    new_rep.kind = rep_kind_delta;
    new_rep.txn_id = NULL;

    /* Migrate the old rep's checksums to the new rep. */
    new_rep.md5_checksum = svn_checksum_dup(rep_md5_checksum, pool);
    new_rep.sha1_checksum = svn_checksum_dup(rep_sha1_checksum, pool);

    chunks = apr_array_make(pool, windows->nelts, sizeof(chunk));

    /* Loop through the windows we wrote, creating and adding new
       chunks to the representation. */
    for (i = 0; i < windows->nelts; i++)
      {
        ww = APR_ARRAY_IDX(windows, i, window_write_t *);

        /* Allocate a chunk and its window */
        chunk = apr_palloc(pool, sizeof(*chunk));
        chunk->offset = ww->text_off;

        /* Populate the window */
        chunk->version = new_target_baton.version;
        chunk->string_key = ww->key;
        chunk->size = ww->text_len;
        chunk->rep_key = source;

        /* Add this chunk to the array. */
        APR_ARRAY_PUSH(chunks, rep_delta_chunk_t *) = chunk;
      }

    /* Put the chunks array into the representation. */
    new_rep.contents.delta.chunks = chunks;

    /* Write out the new representation. */
    SVN_ERR(svn_fs_bdb__write_rep(fs, target, &new_rep, trail, pool));

    /* Delete the original pre-deltified strings. */
    SVN_ERR(delete_strings(orig_str_keys, fs, trail, pool));
  }

  return SVN_NO_ERROR;
}