Commit f847fa7b34abdc7850b8739747e1d25eefafa5f2

lhchavez 2020-02-16T02:00:56

midx: Support multi-pack-index files in odb_pack.c This change adds support for reading multi-pack-index files from the packfile odb backend. This also makes git_pack_file objects open their backing failes lazily in more scenarios, since the multi-pack-index can avoid having to open them in some cases (yay!). This change also refreshes the documentation found in src/odb_pack.c to match the updated code. Part of: #5399

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
diff --git a/src/midx.c b/src/midx.c
index 00d7315..ee93b03 100644
--- a/src/midx.c
+++ b/src/midx.c
@@ -13,8 +13,6 @@
 #include "odb.h"
 #include "pack.h"
 
-#define GIT_MIDX_FILE_MODE 0444
-
 #define MIDX_SIGNATURE 0x4d494458 /* "MIDX" */
 #define MIDX_VERSION 1
 #define MIDX_OBJECT_ID_VERSION 1
@@ -116,7 +114,7 @@ static int midx_parse_oid_lookup(
 		return midx_error("missing OID Lookup chunk");
 	if (chunk_oid_lookup->length == 0)
 		return midx_error("empty OID Lookup chunk");
-	if (chunk_oid_lookup->length != idx->num_objects * 20)
+	if (chunk_oid_lookup->length != idx->num_objects * GIT_OID_RAWSZ)
 		return midx_error("OID Lookup chunk has wrong length");
 
 	idx->oid_lookup = oid = (git_oid *)(data + chunk_oid_lookup->offset);
@@ -183,7 +181,7 @@ int git_midx_parse(
 
 	GIT_ASSERT_ARG(idx);
 
-	if (size < sizeof(struct git_midx_header) + 20)
+	if (size < sizeof(struct git_midx_header) + GIT_OID_RAWSZ)
 		return midx_error("multi-pack index is too short");
 
 	hdr = ((struct git_midx_header *)data);
@@ -203,7 +201,7 @@ int git_midx_parse(
 	last_chunk_offset =
 			sizeof(struct git_midx_header) +
 			(1 + hdr->chunks) * 12;
-	trailer_offset = size - 20;
+	trailer_offset = size - GIT_OID_RAWSZ;
 	if (trailer_offset < last_chunk_offset)
 		return midx_error("wrong index size");
 	git_oid_cpy(&idx->checksum, (git_oid *)(data + trailer_offset));
@@ -309,6 +307,10 @@ int git_midx_open(
 	idx = git__calloc(1, sizeof(git_midx_file));
 	GIT_ERROR_CHECK_ALLOC(idx);
 
+	error = git_buf_sets(&idx->filename, path);
+	if (error < 0)
+		return error;
+
 	error = git_futils_mmap_ro(&idx->index_map, fd, 0, idx_size);
 	p_close(fd);
 	if (error < 0) {
@@ -325,6 +327,46 @@ int git_midx_open(
 	return 0;
 }
 
+bool git_midx_needs_refresh(
+		const git_midx_file *idx,
+		const char *path)
+{
+	git_file fd = -1;
+	struct stat st;
+	ssize_t bytes_read;
+	git_oid idx_checksum = {{0}};
+
+	/* TODO: properly open the file without access time using O_NOATIME */
+	fd = git_futils_open_ro(path);
+	if (fd < 0)
+		return true;
+
+	if (p_fstat(fd, &st) < 0) {
+		p_close(fd);
+		return true;
+	}
+
+	if (!S_ISREG(st.st_mode) ||
+	    !git__is_sizet(st.st_size) ||
+	    (size_t)st.st_size != idx->index_map.len) {
+		p_close(fd);
+		return true;
+	}
+
+	if (p_lseek(fd, -GIT_OID_RAWSZ, SEEK_END) < 0) {
+		p_close(fd);
+		return true;
+	}
+
+	bytes_read = p_read(fd, &idx_checksum, GIT_OID_RAWSZ);
+	p_close(fd);
+
+	if (bytes_read != GIT_OID_RAWSZ)
+		return true;
+
+	return git_oid_cmp(&idx_checksum, &idx->checksum) == 0;
+}
+
 int git_midx_entry_find(
 		git_midx_entry *e,
 		git_midx_file *idx,
@@ -343,7 +385,7 @@ int git_midx_entry_find(
 	hi = ntohl(idx->oid_fanout[(int)short_oid->id[0]]);
 	lo = ((short_oid->id[0] == 0x0) ? 0 : ntohl(idx->oid_fanout[(int)short_oid->id[0] - 1]));
 
-	pos = git_pack__lookup_sha1(idx->oid_lookup, 20, lo, hi, short_oid->id);
+	pos = git_pack__lookup_sha1(idx->oid_lookup, GIT_OID_RAWSZ, lo, hi, short_oid->id);
 
 	if (pos >= 0) {
 		/* An object matching exactly the oid was found */
@@ -399,6 +441,24 @@ int git_midx_entry_find(
 	return 0;
 }
 
+int git_midx_foreach_entry(
+		git_midx_file *idx,
+		git_odb_foreach_cb cb,
+		void *data)
+{
+	size_t i;
+	int error;
+
+	GIT_ASSERT_ARG(idx);
+
+	for (i = 0; i < idx->num_objects; ++i) {
+		if ((error = cb(&idx->oid_lookup[i], data)) != 0)
+			return git_error_set_after_callback(error);
+	}
+
+	return error;
+}
+
 int git_midx_close(git_midx_file *idx)
 {
 	GIT_ASSERT_ARG(idx);
@@ -416,6 +476,7 @@ void git_midx_free(git_midx_file *idx)
 	if (!idx)
 		return;
 
+	git_buf_dispose(&idx->filename);
 	git_midx_close(idx);
 	git__free(idx);
 }
diff --git a/src/midx.h b/src/midx.h
index 3b80295..543ff21 100644
--- a/src/midx.h
+++ b/src/midx.h
@@ -14,6 +14,7 @@
 
 #include "map.h"
 #include "mwindow.h"
+#include "odb.h"
 
 /*
  * A multi-pack-index file.
@@ -49,6 +50,9 @@ typedef struct git_midx_file {
 
 	/* The trailer of the file. Contains the SHA1-checksum of the whole file. */
 	git_oid checksum;
+
+	/* something like ".git/objects/pack/multi-pack-index". */
+	git_buf filename;
 } git_midx_file;
 
 /*
@@ -66,11 +70,18 @@ typedef struct git_midx_entry {
 int git_midx_open(
 		git_midx_file **idx_out,
 		const char *path);
+bool git_midx_needs_refresh(
+		const git_midx_file *idx,
+		const char *path);
 int git_midx_entry_find(
 		git_midx_entry *e,
 		git_midx_file *idx,
 		const git_oid *short_oid,
 		size_t len);
+int git_midx_foreach_entry(
+		git_midx_file *idx,
+		git_odb_foreach_cb cb,
+		void *data);
 int git_midx_close(git_midx_file *idx);
 void git_midx_free(git_midx_file *idx);
 
diff --git a/src/odb_pack.c b/src/odb_pack.c
index e4ad0f4..3df8a42 100644
--- a/src/odb_pack.c
+++ b/src/odb_pack.c
@@ -11,11 +11,12 @@
 #include "git2/repository.h"
 #include "git2/indexer.h"
 #include "git2/sys/odb_backend.h"
+#include "delta.h"
 #include "futils.h"
 #include "hash.h"
-#include "odb.h"
-#include "delta.h"
+#include "midx.h"
 #include "mwindow.h"
+#include "odb.h"
 #include "pack.h"
 
 #include "git2/odb_backend.h"
@@ -25,6 +26,8 @@
 
 struct pack_backend {
 	git_odb_backend parent;
+	git_midx_file *midx;
+	git_vector midx_packs;
 	git_vector packs;
 	struct git_pack_file *last_found;
 	char *pack_folder;
@@ -47,36 +50,43 @@ struct pack_writepack {
  *	Initialization of the Pack Backend
  *	--------------------------------------------------
  *
- *	# git_odb_backend_pack
- *	| Creates the pack backend structure, initializes the
- *	| callback pointers to our default read() and exist() methods,
- *	| and tries to preload all the known packfiles in the ODB.
+ * # git_odb_backend_pack
+ * | Creates the pack backend structure, initializes the
+ * | callback pointers to our default read() and exist() methods,
+ * | and tries to find the `pack` folder, if it exists. ODBs without a `pack`
+ * | folder are ignored altogether. If there is a `pack` folder, it tries to
+ * | preload all the known packfiles in the ODB.
  * |
- *	|-# packfile_load_all
- *	 | Tries to find the `pack` folder, if it exists. ODBs without
- *	 | a pack folder are ignored altogether. If there's a `pack` folder
- *	 | we run a `dirent` callback through every file in the pack folder
- *	 | to find our packfiles. The packfiles are then sorted according
- *	 | to a sorting callback.
- * 	 |
- *	 |-# packfile_load__cb
- *	 | | This callback is called from `dirent` with every single file
- *	 | | inside the pack folder. We find the packs by actually locating
- *	 | | their index (ends in ".idx"). From that index, we verify that
- *	 | | the corresponding packfile exists and is valid, and if so, we
- *	| | add it to the pack list.
- *	 | |
- *	 | |-# packfile_check
- *	 |		Make sure that there's a packfile to back this index, and store
- *	 |		some very basic information regarding the packfile itself,
- *	 |		such as the full path, the size, and the modification time.
- *	 |		We don't actually open the packfile to check for internal consistency.
- *	|
- *	|-# packfile_sort__cb
- *		Sort all the preloaded packs according to some specific criteria:
- *		we prioritize the "newer" packs because it's more likely they
- *		contain the objects we are looking for, and we prioritize local
- *		packs over remote ones.
+ * |-# pack_backend__refresh
+ *   | The `multi-pack-index` is loaded if it exists and is valid.
+ *   | Then we run a `dirent` callback through every file in the pack folder,
+ *   | even those present in `multi-pack-index`. The unindexed packfiles are
+ *   | then sorted according to a sorting callback.
+ *   |
+ *   |-# refresh_multi_pack_index
+ *   |   Detect the presence of the `multi-pack-index` file. If it needs to be
+ *   |   refreshed, frees the old copy and tries to load the new one, together
+ *   |   with all the packfiles it indexes. If the process fails, fall back to
+ *   |   the old behavior, as if the `multi-pack-index` file was not there.
+ *   |
+ *   |-# packfile_load__cb
+ *   | | This callback is called from `dirent` with every single file
+ *   | | inside the pack folder. We find the packs by actually locating
+ *   | | their index (ends in ".idx"). From that index, we verify that
+ *   | | the corresponding packfile exists and is valid, and if so, we
+ *   | | add it to the pack list.
+ *   | |
+ *   | # git_mwindow_get_pack
+ *   |   Make sure that there's a packfile to back this index, and store
+ *   |   some very basic information regarding the packfile itself,
+ *   |   such as the full path, the size, and the modification time.
+ *   |   We don't actually open the packfile to check for internal consistency.
+ *   |
+ *   |-# packfile_sort__cb
+ *       Sort all the preloaded packs according to some specific criteria:
+ *       we prioritize the "newer" packs because it's more likely they
+ *       contain the objects we are looking for, and we prioritize local
+ *       packs over remote ones.
  *
  *
  *
@@ -84,48 +94,66 @@ struct pack_writepack {
  *	A standard packed `exist` query for an OID
  *	--------------------------------------------------
  *
- * # pack_backend__exists
- * | Check if the given SHA1 oid exists in any of the packs
- * | that have been loaded for our ODB.
+ * # pack_backend__exists / pack_backend__exists_prefix
+ * | Check if the given SHA1 oid (or a SHA1 oid prefix) exists in any of the
+ * | packs that have been loaded for our ODB.
  * |
- * |-# pack_entry_find
- *	| Iterate through all the packs that have been preloaded
- *	| (starting by the pack where the latest object was found)
- *	| to try to find the OID in one of them.
- *	|
- *	|-# pack_entry_find1
- *		| Check the index of an individual pack to see if the SHA1
- *		| OID can be found. If we can find the offset to that SHA1
- *		| inside of the index, that means the object is contained
- *		| inside of the packfile and we can stop searching.
- *		| Before returning, we verify that the packfile behing the
- *		| index we are searching still exists on disk.
- *		|
- *		|-# pack_entry_find_offset
- *		| | Mmap the actual index file to disk if it hasn't been opened
- *		| | yet, and run a binary search through it to find the OID.
- *		| | See <http://book.git-scm.com/7_the_packfile.html> for specifics
- *		| | on the Packfile Index format and how do we find entries in it.
- *		| |
- *		| |-# pack_index_open
- *		|	| Guess the name of the index based on the full path to the
- *		|	| packfile, open it and verify its contents. Only if the index
- *		|	| has not been opened already.
- *		|	|
- *		|	|-# pack_index_check
- *		|		Mmap the index file and do a quick run through the header
- *		|		to guess the index version (right now we support v1 and v2),
- *		|		and to verify that the size of the index makes sense.
- *		|
- *		|-# packfile_open
- *			See `packfile_open` in Chapter 3
+ * |-# pack_entry_find / pack_entry_find_prefix
+ *   | If there is a multi-pack-index present, search the SHA1 oid in that
+ *   | index first. If it is not found there, iterate through all the unindexed
+ *   | packs that have been preloaded (starting by the pack where the latest
+ *   | object was found) to try to find the OID in one of them.
+ *   |
+ *   |-# git_midx_entry_find
+ *   |   Search for the SHA1 oid in the multi-pack-index. See
+ *   |   <https://github.com/git/git/blob/master/Documentation/technical/pack-format.txt>
+ *   |   for specifics on the multi-pack-index format and how do we find
+ *   |   entries in it.
+ *   |
+ *   |-# git_pack_entry_find
+ *     | Check the index of an individual unindexed pack to see if the SHA1
+ *     | OID can be found. If we can find the offset to that SHA1 inside of the
+ *     | index, that means the object is contained inside of the packfile and
+ *     | we can stop searching. Before returning, we verify that the
+ *     | packfile behing the index we are searching still exists on disk.
+ *     |
+ *     |-# pack_entry_find_offset
+ *       | Mmap the actual index file to disk if it hasn't been opened
+ *       | yet, and run a binary search through it to find the OID.
+ *       | See <https://github.com/git/git/blob/master/Documentation/technical/pack-format.txt>
+ *       | for specifics on the Packfile Index format and how do we find
+ *       | entries in it.
+ *       |
+ *       |-# pack_index_open
+ *         | Guess the name of the index based on the full path to the
+ *         | packfile, open it and verify its contents. Only if the index
+ *         | has not been opened already.
+ *         |
+ *         |-# pack_index_check
+ *             Mmap the index file and do a quick run through the header
+ *             to guess the index version (right now we support v1 and v2),
+ *             and to verify that the size of the index makes sense.
  *
  *
  *
  *	Chapter 3: The neverending story...
  *	A standard packed `lookup` query for an OID
  *	--------------------------------------------------
- *	TODO
+ *
+ * # pack_backend__read / pack_backend__read_prefix
+ * | Check if the given SHA1 oid (or a SHA1 oid prefix) exists in any of the
+ * | packs that have been loaded for our ODB. If it does, open the packfile and
+ * | read from it.
+ * |
+ * |-# git_packfile_unpack
+ *     Armed with a packfile and the offset within it, we can finally unpack
+ *     the object pointed at by the SHA1 oid. This involves mmapping part of
+ *     the `.pack` file, and uncompressing the object within it (if it is
+ *     stored in the undelfitied representation), or finding a base object and
+ *     applying some deltas to its uncompressed representation (if it is stored
+ *     in the deltified representation). See
+ *     <https://github.com/git/git/blob/master/Documentation/technical/pack-format.txt>
+ *     for specifics on the Packfile format and how do we read from it.
  *
  */
 
@@ -140,6 +168,8 @@ static int packfile_sort__cb(const void *a_, const void *b_);
 
 static int packfile_load__cb(void *_data, git_buf *path);
 
+static int packfile_byname_search_cmp(const void *path, const void *pack_entry);
+
 static int pack_entry_find(struct git_pack_entry *e,
 	struct pack_backend *backend, const git_oid *oid);
 
@@ -163,6 +193,14 @@ static int pack_entry_find_prefix(
  *
  ***********************************************************/
 
+static int packfile_byname_search_cmp(const void *path_, const void *p_)
+{
+	const git_buf *path = (const git_buf *)path_;
+	const struct git_pack_file *p = (const struct git_pack_file *)p_;
+
+	return strncmp(p->pack_name, git_buf_cstr(path), git_buf_len(path));
+}
+
 static int packfile_sort__cb(const void *a_, const void *b_)
 {
 	const struct git_pack_file *a = a_;
@@ -198,20 +236,20 @@ static int packfile_load__cb(void *data, git_buf *path)
 	struct pack_backend *backend = data;
 	struct git_pack_file *pack;
 	const char *path_str = git_buf_cstr(path);
-	size_t i, cmp_len = git_buf_len(path);
+	git_buf index_prefix = GIT_BUF_INIT;
+	size_t cmp_len = git_buf_len(path);
 	int error;
 
 	if (cmp_len <= strlen(".idx") || git__suffixcmp(path_str, ".idx") != 0)
 		return 0; /* not an index */
 
 	cmp_len -= strlen(".idx");
+	git_buf_attach_notowned(&index_prefix, path_str, cmp_len);
 
-	for (i = 0; i < backend->packs.length; ++i) {
-		struct git_pack_file *p = git_vector_get(&backend->packs, i);
-
-		if (strncmp(p->pack_name, path_str, cmp_len) == 0)
-			return 0;
-	}
+	if (git_vector_search2(NULL, &backend->midx_packs, packfile_byname_search_cmp, &index_prefix) == 0)
+		return 0;
+	if (git_vector_search2(NULL, &backend->packs, packfile_byname_search_cmp, &index_prefix) == 0)
+		return 0;
 
 	error = git_mwindow_get_pack(&pack, path->ptr);
 
@@ -228,22 +266,26 @@ static int packfile_load__cb(void *data, git_buf *path)
 
 }
 
-static int pack_entry_find_inner(
-	struct git_pack_entry *e,
-	struct pack_backend *backend,
-	const git_oid *oid,
-	struct git_pack_file *last_found)
+static int pack_entry_find(struct git_pack_entry *e, struct pack_backend *backend, const git_oid *oid)
 {
+	struct git_pack_file *last_found = backend->last_found, *p;
+	git_midx_entry midx_entry;
 	size_t i;
 
+	if (backend->midx &&
+		git_midx_entry_find(&midx_entry, backend->midx, oid, GIT_OID_HEXSZ) == 0 &&
+		midx_entry.pack_index < git_vector_length(&backend->midx_packs)) {
+		e->offset = midx_entry.offset;
+		git_oid_cpy(&e->sha1, &midx_entry.sha1);
+		e->p = git_vector_get(&backend->midx_packs, midx_entry.pack_index);
+		return 0;
+	}
+
 	if (last_found &&
 		git_pack_entry_find(e, last_found, oid, GIT_OID_HEXSZ) == 0)
 		return 0;
 
-	for (i = 0; i < backend->packs.length; ++i) {
-		struct git_pack_file *p;
-
-		p = git_vector_get(&backend->packs, i);
+	git_vector_foreach(&backend->packs, i, p) {
 		if (p == last_found)
 			continue;
 
@@ -253,20 +295,6 @@ static int pack_entry_find_inner(
 		}
 	}
 
-	return -1;
-}
-
-static int pack_entry_find(struct git_pack_entry *e, struct pack_backend *backend, const git_oid *oid)
-{
-	struct git_pack_file *last_found = backend->last_found;
-
-	if (backend->last_found &&
-		git_pack_entry_find(e, backend->last_found, oid, GIT_OID_HEXSZ) == 0)
-		return 0;
-
-	if (!pack_entry_find_inner(e, backend, oid, last_found))
-		return 0;
-
 	return git_odb__error_notfound(
 		"failed to find pack entry", oid, GIT_OID_HEXSZ);
 }
@@ -281,22 +309,35 @@ static int pack_entry_find_prefix(
 	size_t i;
 	git_oid found_full_oid = {{0}};
 	bool found = false;
-	struct git_pack_file *last_found = backend->last_found;
+	struct git_pack_file *last_found = backend->last_found, *p;
+	git_midx_entry midx_entry;
+
+	if (backend->midx) {
+		error = git_midx_entry_find(&midx_entry, backend->midx, short_oid, len);
+		if (error == GIT_EAMBIGUOUS)
+			return error;
+		if (!error && midx_entry.pack_index < git_vector_length(&backend->midx_packs)) {
+			e->offset = midx_entry.offset;
+			git_oid_cpy(&e->sha1, &midx_entry.sha1);
+			e->p = git_vector_get(&backend->midx_packs, midx_entry.pack_index);
+			git_oid_cpy(&found_full_oid, &e->sha1);
+			found = true;
+		}
+	}
 
 	if (last_found) {
 		error = git_pack_entry_find(e, last_found, short_oid, len);
 		if (error == GIT_EAMBIGUOUS)
 			return error;
 		if (!error) {
+			if (found && git_oid_cmp(&e->sha1, &found_full_oid))
+				return git_odb__error_ambiguous("found multiple pack entries");
 			git_oid_cpy(&found_full_oid, &e->sha1);
 			found = true;
 		}
 	}
 
-	for (i = 0; i < backend->packs.length; ++i) {
-		struct git_pack_file *p;
-
-		p = git_vector_get(&backend->packs, i);
+	git_vector_foreach(&backend->packs, i, p) {
 		if (p == last_found)
 			continue;
 
@@ -319,6 +360,141 @@ static int pack_entry_find_prefix(
 		return 0;
 }
 
+/***********************************************************
+ *
+ * MULTI-PACK-INDEX SUPPORT
+ *
+ * Functions needed to support the multi-pack-index.
+ *
+ ***********************************************************/
+
+/*
+ * Remove the multi-pack-index, and move all midx_packs to packs.
+ */
+static int remove_multi_pack_index(struct pack_backend *backend)
+{
+	size_t i, j = git_vector_length(&backend->packs);
+	struct pack_backend *p;
+	int error = git_vector_size_hint(
+			&backend->packs,
+			j + git_vector_length(&backend->midx_packs));
+	if (error < 0)
+		return error;
+
+	git_vector_foreach(&backend->midx_packs, i, p)
+		git_vector_set(NULL, &backend->packs, j++, p);
+	git_vector_clear(&backend->midx_packs);
+
+	git_midx_free(backend->midx);
+	backend->midx = NULL;
+
+	return 0;
+}
+
+/*
+ * Loads a single .pack file referred to by the multi-pack-index. These must
+ * match the order in which they are declared in the multi-pack-index file,
+ * since these files are referred to by their index.
+ */
+static int process_multi_pack_index_pack(
+		struct pack_backend *backend,
+		size_t i,
+		const char *packfile_name)
+{
+	int error;
+	size_t cmp_len = strlen(packfile_name);
+	struct git_pack_file *pack;
+	size_t found_position;
+	git_buf pack_path = GIT_BUF_INIT, index_prefix = GIT_BUF_INIT;
+
+	error = git_buf_joinpath(&pack_path, backend->pack_folder, packfile_name);
+	if (error < 0)
+		return error;
+
+	/* This is ensured by midx__parse_packfile_name() */
+	if (cmp_len <= strlen(".idx") || git__suffixcmp(git_buf_cstr(&pack_path), ".idx") != 0)
+		return git_odb__error_notfound("midx file contained a non-index", NULL, 0);
+
+	cmp_len -= strlen(".idx");
+	git_buf_attach_notowned(&index_prefix, git_buf_cstr(&pack_path), cmp_len);
+
+	if (git_vector_search2(&found_position, &backend->packs, packfile_byname_search_cmp, &index_prefix) == 0) {
+		/* Pack was found in the packs list. Moving it to the midx_packs list. */
+		git_buf_dispose(&pack_path);
+		git_vector_set(NULL, &backend->midx_packs, i, git_vector_get(&backend->packs, found_position));
+		git_vector_remove(&backend->packs, found_position);
+		return 0;
+	}
+
+	/* Pack was not found. Allocate a new one. */
+	error = git_mwindow_get_pack(&pack, git_buf_cstr(&pack_path));
+	git_buf_dispose(&pack_path);
+	if (error < 0)
+		return error;
+
+	git_vector_set(NULL, &backend->midx_packs, i, pack);
+	return 0;
+}
+
+/*
+ * Reads the multi-pack-index. If this fails for whatever reason, the
+ * multi-pack-index object is freed, and all the packfiles that are related to
+ * it are moved to the unindexed packfiles vector.
+ */
+static int refresh_multi_pack_index(struct pack_backend *backend)
+{
+	int error;
+	git_buf midx_path = GIT_BUF_INIT;
+	const char *packfile_name;
+	size_t i;
+
+	error = git_buf_joinpath(&midx_path, backend->pack_folder, "multi-pack-index");
+	if (error < 0)
+		return error;
+
+	/*
+	 * Check whether the multi-pack-index has changed. If it has, close any
+	 * old multi-pack-index and move all the packfiles to the unindexed
+	 * packs. This is done to prevent losing any open packfiles in case
+	 * refreshing the new multi-pack-index fails, or the file is deleted.
+	 */
+	if (backend->midx) {
+		if (!git_midx_needs_refresh(backend->midx, git_buf_cstr(&midx_path))) {
+			git_buf_dispose(&midx_path);
+			return 0;
+		}
+		error = remove_multi_pack_index(backend);
+		if (error < 0) {
+			git_buf_dispose(&midx_path);
+			return error;
+		}
+	}
+
+	error = git_midx_open(&backend->midx, git_buf_cstr(&midx_path));
+	git_buf_dispose(&midx_path);
+	if (error < 0)
+		return error;
+
+	git_vector_resize_to(&backend->midx_packs, git_vector_length(&backend->midx->packfile_names));
+
+	git_vector_foreach(&backend->midx->packfile_names, i, packfile_name) {
+		error = process_multi_pack_index_pack(backend, i, packfile_name);
+		if (error < 0) {
+			/*
+			 * Something failed during reading multi-pack-index.
+			 * Restore the state of backend as if the
+			 * multi-pack-index was never there, and move all
+			 * packfiles that have been processed so far to the
+			 * unindexed packs.
+			 */
+			git_vector_resize_to(&backend->midx_packs, i);
+			remove_multi_pack_index(backend);
+			return error;
+		}
+	}
+
+	return 0;
+}
 
 /***********************************************************
  *
@@ -340,9 +516,16 @@ static int pack_backend__refresh(git_odb_backend *backend_)
 	if (p_stat(backend->pack_folder, &st) < 0 || !S_ISDIR(st.st_mode))
 		return git_odb__error_notfound("failed to refresh packfiles", NULL, 0);
 
-	git_buf_sets(&path, backend->pack_folder);
+	if (refresh_multi_pack_index(backend) < 0) {
+		/*
+		 * It is okay if this fails. We will just not use the
+		 * multi-pack-index in this case.
+		 */
+		git_error_clear();
+	}
 
 	/* reload all packs */
+	git_buf_sets(&path, backend->pack_folder);
 	error = git_path_direach(&path, 0, packfile_load__cb, backend);
 
 	git_buf_dispose(&path);
@@ -478,9 +661,11 @@ static int pack_backend__foreach(git_odb_backend *_backend, git_odb_foreach_cb c
 	backend = (struct pack_backend *)_backend;
 
 	/* Make sure we know about the packfiles */
-	if ((error = pack_backend__refresh(_backend)) < 0)
+	if ((error = pack_backend__refresh(_backend)) != 0)
 		return error;
 
+	if (backend->midx && (error = git_midx_foreach_entry(backend->midx, cb, data)) != 0)
+		return error;
 	git_vector_foreach(&backend->packs, i, p) {
 		if ((error = git_pack_foreach_entry(p, cb, data)) != 0)
 			return error;
@@ -562,6 +747,7 @@ static int pack_backend__writepack(struct git_odb_writepack **out,
 static void pack_backend__free(git_odb_backend *_backend)
 {
 	struct pack_backend *backend;
+	struct git_pack_file *p;
 	size_t i;
 
 	if (!_backend)
@@ -569,11 +755,13 @@ static void pack_backend__free(git_odb_backend *_backend)
 
 	backend = (struct pack_backend *)_backend;
 
-	for (i = 0; i < backend->packs.length; ++i) {
-		struct git_pack_file *p = git_vector_get(&backend->packs, i);
+	git_vector_foreach(&backend->midx_packs, i, p)
+		git_mwindow_put_pack(p);
+	git_vector_foreach(&backend->packs, i, p)
 		git_mwindow_put_pack(p);
-	}
 
+	git_midx_free(backend->midx);
+	git_vector_free(&backend->midx_packs);
 	git_vector_free(&backend->packs);
 	git__free(backend->pack_folder);
 	git__free(backend);
@@ -584,7 +772,12 @@ static int pack_backend__alloc(struct pack_backend **out, size_t initial_size)
 	struct pack_backend *backend = git__calloc(1, sizeof(struct pack_backend));
 	GIT_ERROR_CHECK_ALLOC(backend);
 
+	if (git_vector_init(&backend->midx_packs, 0, NULL) < 0) {
+		git__free(backend);
+		return -1;
+	}
 	if (git_vector_init(&backend->packs, initial_size, packfile_sort__cb) < 0) {
+		git_vector_free(&backend->midx_packs);
 		git__free(backend);
 		return -1;
 	}
diff --git a/src/pack.c b/src/pack.c
index 982ad17..f81bc91 100644
--- a/src/pack.c
+++ b/src/pack.c
@@ -481,6 +481,9 @@ int git_packfile_resolve_header(
 	off64_t base_offset;
 	int error;
 
+	if (p->mwf.fd == -1 && (error = packfile_open(p)) < 0)
+		return error;
+
 	error = git_packfile_unpack_header(&size, &type, &p->mwf, &w_curs, &curpos);
 	if (error < 0)
 		return error;
@@ -631,6 +634,9 @@ int git_packfile_unpack(
 	size_t stack_size = 0, elem_pos, alloclen;
 	git_object_t base_type;
 
+	if (p->mwf.fd == -1 && (error = packfile_open(p)) < 0)
+		return error;
+
 	/*
 	 * TODO: optionally check the CRC on the packfile
 	 */
diff --git a/tests/pack/midx.c b/tests/pack/midx.c
index 1f47d95..92d9ae2 100644
--- a/tests/pack/midx.c
+++ b/tests/pack/midx.c
@@ -27,3 +27,19 @@ void test_pack_midx__parse(void)
 	git_repository_free(repo);
 	git_buf_dispose(&midx_path);
 }
+
+void test_pack_midx__lookup(void)
+{
+	git_repository *repo;
+	git_commit *commit;
+	git_oid id;
+
+	cl_git_pass(git_repository_open(&repo, cl_fixture("testrepo.git")));
+
+	cl_git_pass(git_oid_fromstr(&id, "5001298e0c09ad9c34e4249bc5801c75e9754fa5"));
+	cl_git_pass(git_commit_lookup_prefix(&commit, repo, &id, GIT_OID_HEXSZ));
+	cl_assert_equal_s(git_commit_message(commit), "packed commit one\n");
+
+	git_commit_free(commit);
+	git_repository_free(repo);
+}