@matheusbsbs / matheus-bagliano / commits / d3af1c193d

commit-graph: fix truncated generation numbers

In 80c928d947 (commit-graph: simplify compute_generation_numbers(), 2023-03-20), the code to compute generation numbers was simplified to use the same infrastructure as is used to compute topological levels. This refactoring introduced a bug where the generation numbers are truncated when they exceed UINT32_MAX because we explicitly cast the computed generation number to `uint32_t`. This is not required though: both the computed value and the field of `struct commit_graph_data` are of the same type `timestamp_t` already, so casting to `uint32_t` will cause truncation. This cast can cause us to miscompute generation data overflows: 1. Given a commit with no parents and committer date `UINT32_MAX + 1`. 2. We compute its generation number as `UINT32_MAX + 1`, but truncate it to `1`. 3. We calculate the generation offset via `$generation - $date`, which is thus `1 - (UINT32_MAX + 1)`. The computation underflows and we thus end up with an offset that is bigger than the maximum allowed offset. As a result, we'd be writing generation data overflow information into the commit-graph that is bogus and ultimately not even required. Fix this bug by removing the needless cast. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>

Patrick Steinhardt committed Mar 27, 2023 at 10:08 UTC d3af1c193d4d62668a9d7ea98e2ef3771ac4e65a

2 files changed +10 -1

commit-graph.c

+1 -1

@@ -1565,7 +1565,7 @@ static timestamp_t get_generation_from_graph_data(struct commit *c, void *data)

1565 static void set_generation_v2(struct commit *c, timestamp_t t, void *data)

1566 {

1567 struct commit_graph_data *g = commit_graph_data_at(c);

1568 - g->generation = (uint32_t)t;

1568 + g->generation = t;

1569 }

1570

1571 static void compute_generation_numbers(struct write_commit_graph_context *ctx)

t/t5328-commit-graph-64bit-time.sh

@@ -63,4 +63,13 @@ test_expect_success 'set up and verify repo with generation data overflow chunk'

64 graph_git_behavior 'overflow 2' repo left right

66 +test_expect_success 'single commit with generation data exceeding UINT32_MAX' '

67 + git init repo-uint32-max &&

68 + cd repo-uint32-max &&

69 + test_commit --date "@4294967297 +0000" 1 &&

70 + git commit-graph write --reachable &&

71 + graph_read_expect 1 "generation_data" &&

72 + git commit-graph verify

73 +'

74 +

75 test_done