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@xxxxxx>
---
This commit applies on top of cbfe360b14 (commit-reach: add
tips_reachable_from_bases(), 2023-03-20), which has recently been merged
to next.
commit-graph.c | 2 +-
t/t5328-commit-graph-64bit-time.sh | 9 +++++++++
2 files changed, 10 insertions(+), 1 deletion(-)
diff --git a/commit-graph.c b/commit-graph.c
index 172e679db1..b96509354e 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -1565,7 +1565,7 @@ static timestamp_t get_generation_from_graph_data(struct commit *c, void *data)
static void set_generation_v2(struct commit *c, timestamp_t t, void *data)
{
struct commit_graph_data *g = commit_graph_data_at(c);
- g->generation = (uint32_t)t;
+ g->generation = t;
}
static void compute_generation_numbers(struct write_commit_graph_context *ctx)
diff --git a/t/t5328-commit-graph-64bit-time.sh b/t/t5328-commit-graph-64bit-time.sh
index 093f0c067a..57e4d9c699 100755
--- a/t/t5328-commit-graph-64bit-time.sh
+++ b/t/t5328-commit-graph-64bit-time.sh
@@ -63,4 +63,13 @@ test_expect_success 'set up and verify repo with generation data overflow chunk'
graph_git_behavior 'overflow 2' repo left right
+test_expect_success 'single commit with generation data exceeding UINT32_MAX' '
+ git init repo-uint32-max &&
+ cd repo-uint32-max &&
+ test_commit --date "@4294967297 +0000" 1 &&
+ git commit-graph write --reachable &&
+ graph_read_expect 1 "generation_data" &&
+ git commit-graph verify
+'
+
test_done
--
2.40.0
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