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>
diff --git a/t/t5328-commit-graph-64bit-time.sh b/t/t5328-commit-graph-64bit-time.sh
index 093f0c0..57e4d9c 100755
--- a/t/t5328-commit-graph-64bit-time.sh
+++ b/t/t5328-commit-graph-64bit-time.sh
@@ -63,4 +63,13 @@
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
