@leroysheep / rag-system / commits / ff21343a59

pack-bitmap-write: sort pseudo-merge commit lookup table in pack order

The pseudo-merge commit lookup table stores each commit's position in the pack- or pseudo-pack order, and is used to perform a binary search in order to determine which pseudo-merge(s) a given commit belongs to. However, the table was previously sorted in lexical order (via `oid_array_sort()`), causing the binary search to fail. While this causes pseudo-merge bitmaps to be de-facto broken for fill-in traversal, there are a couple of important points to keep in mind: * Pseudo-merge application during the initial phases of a bitmap-based traversal are applied via `cascade_pseudo_merges_1()`. This function enumerates the known pseudo-merges and determines if its parents are a subset of the traversal roots. This is a different path than the fill-in traversal, where we are looking for any pseudo-merges which may be satisfied after visiting some commit along an object walk, which involves the aforementioned (broken) binary search. As a consequence, any pseudo-merges we apply at this stage are done so correctly. * While this bug makes applying pseudo-merges during fill-in traversal effectively broken, it does not produce wrong results. Instead of applying the *wrong* pseudo-merge, we will simply fail to find satisfied pseudo-merges, leaving the traversal to use the existing fill-in routines. Fix this by sorting the table by bit position before writing, matching the order that the reader's binary search expects. This does produce a change the on-disk format insofar as the actual code now complies with the documented format (for more details, refer to: Documentation/technical/bitmap-format.adoc). Given that this never worked in the first place, such a change should be OK to perform. If an out-of-tree implementation of pseudo-merges happened to generate bitmaps that comply with the documented format, they will continue to be read and interpreted as normal. Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>

Taylor Blau committed May 11, 2026 at 20:46 UTC ff21343a597cfa5d6cc5ea463f5941644d9bf754

2 files changed +21 -2

pack-bitmap-write.c

+20 -1

index 8338d7217e..86ed6a5d78 100644 --- a/pack-bitmap-write.c +++ b/pack-bitmap-write.c @@ -819,6 +819,20 @@ static void write_selected_commits_v1(struct bitmap_writer *writer, } } +static int pseudo_merge_commit_pos_cmp(const void *_va, const void *_vb, + void *_data) +{ + struct bitmap_writer *writer = _data; + uint32_t pos_a = find_object_pos(writer, _va, NULL); + uint32_t pos_b = find_object_pos(writer, _vb, NULL); + + if (pos_a < pos_b) + return -1; + if (pos_a > pos_b) + return 1; + return 0; +} + static void write_pseudo_merges(struct bitmap_writer *writer, struct hashfile *f) { @@ -876,7 +890,12 @@ static void write_pseudo_merges(struct bitmap_writer *writer, oid_array_append(&commits, &kh_key(writer->pseudo_merge_commits, i)); } - oid_array_sort(&commits); + /* + * Sort the commits by their bit position so that the lookup + * table can be binary searched by the reader (see + * find_pseudo_merge()). + */ + QSORT_S(commits.oid, commits.nr, pseudo_merge_commit_pos_cmp, writer); /* write lookup table (non-extended) */ for (i = 0; i < commits.nr; i++) {

t/t5333-pseudo-merge-bitmaps.sh

+1 -1

index 0e9638c31c..3d7a766812 100755 --- a/t/t5333-pseudo-merge-bitmaps.sh +++ b/t/t5333-pseudo-merge-bitmaps.sh @@ -462,7 +462,7 @@ test_expect_success 'use pseudo-merge in boundary traversal' ' ) ' -test_expect_failure 'apply pseudo-merges during fill-in traversal' ' +test_expect_success 'apply pseudo-merges during fill-in traversal' ' test_when_finished "rm -fr pseudo-merge-fill-in-traversal" && git init pseudo-merge-fill-in-traversal && (