@matheusbsbs / matheus-bagliano / commits / 1fdc383c5c

commit-graph: use generation v2 only if entire chain does

Since there are released versions of Git that understand generation numbers in the commit-graph's CDAT chunk but do not understand the GDAT chunk, the following scenario is possible: 1. "New" Git writes a commit-graph with the GDAT chunk. 2. "Old" Git writes a split commit-graph on top without a GDAT chunk. If each layer of split commit-graph is treated independently, as it was the case before this commit, with Git inspecting only the current layer for chunk_generation_data pointer, commits in the lower layer (one with GDAT) whould have corrected commit date as their generation number, while commits in the upper layer would have topological levels as their generation. Corrected commit dates usually have much larger values than topological levels. This means that if we take two commits, one from the upper layer, and one reachable from it in the lower layer, then the expectation that the generation of a parent is smaller than the generation of a child would be violated. It is difficult to expose this issue in a test. Since we _start_ with artificially low generation numbers, any commit walk that prioritizes generation numbers will walk all of the commits with high generation number before walking the commits with low generation number. In all the cases I tried, the commit-graph layers themselves "protect" any incorrect behavior since none of the commits in the lower layer can reach the commits in the upper layer. This issue would manifest itself as a performance problem in this case, especially with something like "git log --graph" since the low generation numbers would cause the in-degree queue to walk all of the commits in the lower layer before allowing the topo-order queue to write anything to output (depending on the size of the upper layer). Therefore, When writing the new layer in split commit-graph, we write a GDAT chunk only if the topmost layer has a GDAT chunk. This guarantees that if a layer has GDAT chunk, all lower layers must have a GDAT chunk as well. Rewriting layers follows similar approach: if the topmost layer below the set of layers being rewritten (in the split commit-graph chain) exists, and it does not contain GDAT chunk, then the result of rewrite does not have GDAT chunks either. Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Abhishek Kumar <abhishekkumar8222@gmail.com> Reviewed-by: Taylor Blau <me@ttaylorr.com> Reviewed-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>

Abhishek Kumar committed Jan 16, 2021 at 18:11 UTC 1fdc383c5c87b6f2bcacddd07d5d45dd04c2d146
3 files changed +210 -2
commit-graph.c
+28 -2
@@ -614,6 +614,21 @@ static struct commit_graph *load_commit_graph_chain(struct repository *r,
614 return graph_chain;
615 }
616
617 +static void validate_mixed_generation_chain(struct commit_graph *g)
618 +{
619 + int read_generation_data;
620 +
621 + if (!g)
622 + return;
623 +
624 + read_generation_data = !!g->chunk_generation_data;
625 +
626 + while (g) {
627 + g->read_generation_data = read_generation_data;
628 + g = g->base_graph;
629 + }
630 +}
631 +
632 struct commit_graph *read_commit_graph_one(struct repository *r,
633 struct object_directory *odb)
634 {
@@ -622,6 +637,8 @@ struct commit_graph *read_commit_graph_one(struct repository *r,
637 if (!g)
638 g = load_commit_graph_chain(r, odb);
639
640 + validate_mixed_generation_chain(g);
641 +
642 return g;
643 }
644
@@ -791,7 +808,7 @@ static void fill_commit_graph_info(struct commit *item, struct commit_graph *g,
808 date_low = get_be32(commit_data + g->hash_len + 12);
809 item->date = (timestamp_t)((date_high << 32) | date_low);
810
794 - if (g->chunk_generation_data) {
811 + if (g->read_generation_data) {
812 offset = (timestamp_t)get_be32(g->chunk_generation_data + sizeof(uint32_t) * lex_index);
813
814 if (offset & CORRECTED_COMMIT_DATE_OFFSET_OVERFLOW) {
@@ -2019,6 +2036,13 @@ static void split_graph_merge_strategy(struct write_commit_graph_context *ctx)
2036 if (i < ctx->num_commit_graphs_after)
2037 ctx->commit_graph_hash_after[i] = xstrdup(oid_to_hex(&g->oid));
2038
2039 + /*
2040 + * If the topmost remaining layer has generation data chunk, the
2041 + * resultant layer also has generation data chunk.
2042 + */
2043 + if (i == ctx->num_commit_graphs_after - 2)
2044 + ctx->write_generation_data = !!g->chunk_generation_data;
2045 +
2046 i--;
2047 g = g->base_graph;
2048 }
@@ -2343,6 +2367,8 @@ int write_commit_graph(struct object_directory *odb,
2367 } else
2368 ctx->num_commit_graphs_after = 1;
2369
2370 + validate_mixed_generation_chain(ctx->r->objects->commit_graph);
2371 +
2372 compute_generation_numbers(ctx);
2373
2374 if (ctx->changed_paths)
@@ -2541,7 +2567,7 @@ int verify_commit_graph(struct repository *r, struct commit_graph *g, int flags)
2567 * also GENERATION_NUMBER_V1_MAX. Decrement to avoid extra logic
2568 * in the following condition.
2569 */
2544 - if (!g->chunk_generation_data && max_generation == GENERATION_NUMBER_V1_MAX)
2570 + if (!g->read_generation_data && max_generation == GENERATION_NUMBER_V1_MAX)
2571 max_generation--;
2572
2573 generation = commit_graph_generation(graph_commit);
commit-graph.h
+1
@@ -64,6 +64,7 @@ struct commit_graph {
64 struct object_directory *odb;
65
66 uint32_t num_commits_in_base;
67 + unsigned int read_generation_data;
68 struct commit_graph *base_graph;
69
70 const uint32_t *chunk_oid_fanout;
t/t5324-split-commit-graph.sh
+181
@@ -453,4 +453,185 @@ test_expect_success 'prevent regression for duplicate commits across layers' '
453 git -C dup commit-graph verify
454 '
455
456 +NUM_FIRST_LAYER_COMMITS=64
457 +NUM_SECOND_LAYER_COMMITS=16
458 +NUM_THIRD_LAYER_COMMITS=7
459 +NUM_FOURTH_LAYER_COMMITS=8
460 +NUM_FIFTH_LAYER_COMMITS=16
461 +SECOND_LAYER_SEQUENCE_START=$(($NUM_FIRST_LAYER_COMMITS + 1))
462 +SECOND_LAYER_SEQUENCE_END=$(($SECOND_LAYER_SEQUENCE_START + $NUM_SECOND_LAYER_COMMITS - 1))
463 +THIRD_LAYER_SEQUENCE_START=$(($SECOND_LAYER_SEQUENCE_END + 1))
464 +THIRD_LAYER_SEQUENCE_END=$(($THIRD_LAYER_SEQUENCE_START + $NUM_THIRD_LAYER_COMMITS - 1))
465 +FOURTH_LAYER_SEQUENCE_START=$(($THIRD_LAYER_SEQUENCE_END + 1))
466 +FOURTH_LAYER_SEQUENCE_END=$(($FOURTH_LAYER_SEQUENCE_START + $NUM_FOURTH_LAYER_COMMITS - 1))
467 +FIFTH_LAYER_SEQUENCE_START=$(($FOURTH_LAYER_SEQUENCE_END + 1))
468 +FIFTH_LAYER_SEQUENCE_END=$(($FIFTH_LAYER_SEQUENCE_START + $NUM_FIFTH_LAYER_COMMITS - 1))
469 +
470 +# Current split graph chain:
471 +#
472 +# 16 commits (No GDAT)
473 +# ------------------------
474 +# 64 commits (GDAT)
475 +#
476 +test_expect_success 'setup repo for mixed generation commit-graph-chain' '
477 + graphdir=".git/objects/info/commit-graphs" &&
478 + test_oid_cache <<-EOF &&
479 + oid_version sha1:1
480 + oid_version sha256:2
481 + EOF
482 + git init mixed &&
483 + (
484 + cd mixed &&
485 + git config core.commitGraph true &&
486 + git config gc.writeCommitGraph false &&
487 + for i in $(test_seq $NUM_FIRST_LAYER_COMMITS)
488 + do
489 + test_commit $i &&
490 + git branch commits/$i || return 1
491 + done &&
492 + git commit-graph write --reachable --split &&
493 + graph_read_expect $NUM_FIRST_LAYER_COMMITS &&
494 + test_line_count = 1 $graphdir/commit-graph-chain &&
495 + for i in $(test_seq $SECOND_LAYER_SEQUENCE_START $SECOND_LAYER_SEQUENCE_END)
496 + do
497 + test_commit $i &&
498 + git branch commits/$i || return 1
499 + done &&
500 + GIT_TEST_COMMIT_GRAPH_NO_GDAT=1 git commit-graph write --reachable --split=no-merge &&
501 + test_line_count = 2 $graphdir/commit-graph-chain &&
502 + test-tool read-graph >output &&
503 + cat >expect <<-EOF &&
504 + header: 43475048 1 $(test_oid oid_version) 4 1
505 + num_commits: $NUM_SECOND_LAYER_COMMITS
506 + chunks: oid_fanout oid_lookup commit_metadata
507 + EOF
508 + test_cmp expect output &&
509 + git commit-graph verify &&
510 + cat $graphdir/commit-graph-chain
511 + )
512 +'
513 +
514 +# The new layer will be added without generation data chunk as it was not
515 +# present on the layer underneath it.
516 +#
517 +# 7 commits (No GDAT)
518 +# ------------------------
519 +# 16 commits (No GDAT)
520 +# ------------------------
521 +# 64 commits (GDAT)
522 +#
523 +test_expect_success 'do not write generation data chunk if not present on existing tip' '
524 + git clone mixed mixed-no-gdat &&
525 + (
526 + cd mixed-no-gdat &&
527 + for i in $(test_seq $THIRD_LAYER_SEQUENCE_START $THIRD_LAYER_SEQUENCE_END)
528 + do
529 + test_commit $i &&
530 + git branch commits/$i || return 1
531 + done &&
532 + git commit-graph write --reachable --split=no-merge &&
533 + test_line_count = 3 $graphdir/commit-graph-chain &&
534 + test-tool read-graph >output &&
535 + cat >expect <<-EOF &&
536 + header: 43475048 1 $(test_oid oid_version) 4 2
537 + num_commits: $NUM_THIRD_LAYER_COMMITS
538 + chunks: oid_fanout oid_lookup commit_metadata
539 + EOF
540 + test_cmp expect output &&
541 + git commit-graph verify
542 + )
543 +'
544 +
545 +# Number of commits in each layer of the split-commit graph before merge:
546 +#
547 +# 8 commits (No GDAT)
548 +# ------------------------
549 +# 7 commits (No GDAT)
550 +# ------------------------
551 +# 16 commits (No GDAT)
552 +# ------------------------
553 +# 64 commits (GDAT)
554 +#
555 +# The top two layers are merged and do not have generation data chunk as layer below them does
556 +# not have generation data chunk.
557 +#
558 +# 15 commits (No GDAT)
559 +# ------------------------
560 +# 16 commits (No GDAT)
561 +# ------------------------
562 +# 64 commits (GDAT)
563 +#
564 +test_expect_success 'do not write generation data chunk if the topmost remaining layer does not have generation data chunk' '
565 + git clone mixed-no-gdat mixed-merge-no-gdat &&
566 + (
567 + cd mixed-merge-no-gdat &&
568 + for i in $(test_seq $FOURTH_LAYER_SEQUENCE_START $FOURTH_LAYER_SEQUENCE_END)
569 + do
570 + test_commit $i &&
571 + git branch commits/$i || return 1
572 + done &&
573 + git commit-graph write --reachable --split --size-multiple 1 &&
574 + test_line_count = 3 $graphdir/commit-graph-chain &&
575 + test-tool read-graph >output &&
576 + cat >expect <<-EOF &&
577 + header: 43475048 1 $(test_oid oid_version) 4 2
578 + num_commits: $(($NUM_THIRD_LAYER_COMMITS + $NUM_FOURTH_LAYER_COMMITS))
579 + chunks: oid_fanout oid_lookup commit_metadata
580 + EOF
581 + test_cmp expect output &&
582 + git commit-graph verify
583 + )
584 +'
585 +
586 +# Number of commits in each layer of the split-commit graph before merge:
587 +#
588 +# 16 commits (No GDAT)
589 +# ------------------------
590 +# 15 commits (No GDAT)
591 +# ------------------------
592 +# 16 commits (No GDAT)
593 +# ------------------------
594 +# 64 commits (GDAT)
595 +#
596 +# The top three layers are merged and has generation data chunk as the topmost remaining layer
597 +# has generation data chunk.
598 +#
599 +# 47 commits (GDAT)
600 +# ------------------------
601 +# 64 commits (GDAT)
602 +#
603 +test_expect_success 'write generation data chunk if topmost remaining layer has generation data chunk' '
604 + git clone mixed-merge-no-gdat mixed-merge-gdat &&
605 + (
606 + cd mixed-merge-gdat &&
607 + for i in $(test_seq $FIFTH_LAYER_SEQUENCE_START $FIFTH_LAYER_SEQUENCE_END)
608 + do
609 + test_commit $i &&
610 + git branch commits/$i || return 1
611 + done &&
612 + git commit-graph write --reachable --split --size-multiple 1 &&
613 + test_line_count = 2 $graphdir/commit-graph-chain &&
614 + test-tool read-graph >output &&
615 + cat >expect <<-EOF &&
616 + header: 43475048 1 $(test_oid oid_version) 5 1
617 + num_commits: $(($NUM_SECOND_LAYER_COMMITS + $NUM_THIRD_LAYER_COMMITS + $NUM_FOURTH_LAYER_COMMITS + $NUM_FIFTH_LAYER_COMMITS))
618 + chunks: oid_fanout oid_lookup commit_metadata generation_data
619 + EOF
620 + test_cmp expect output
621 + )
622 +'
623 +
624 +test_expect_success 'write generation data chunk when commit-graph chain is replaced' '
625 + git clone mixed mixed-replace &&
626 + (
627 + cd mixed-replace &&
628 + git commit-graph write --reachable --split=replace &&
629 + test_path_is_file $graphdir/commit-graph-chain &&
630 + test_line_count = 1 $graphdir/commit-graph-chain &&
631 + verify_chain_files_exist $graphdir &&
632 + graph_read_expect $(($NUM_FIRST_LAYER_COMMITS + $NUM_SECOND_LAYER_COMMITS)) &&
633 + git commit-graph verify
634 + )
635 +'
636 +
637 test_done