commit-graph: fix ordering bug in generation numbers

When computing the generation numbers for a commit-graph, we compute
the corrected commit dates and then check if their offsets from the
actual dates is too large to fit in the 32-bit Generation Data chunk.
However, there is a problem with this approach: if we have parsed the
generation data from the previous commit-graph, then we continue the
loop because the corrected commit date is already computed. This causes
an under-count in the number of overflow values.

It is incorrect to add an increment to num_generation_data_overflows
next to this 'continue' statement, because we might start
double-counting commits that are computed because of the depth-first
search walk from a commit with an earlier OID.

Instead, iterate over the full commit list at the end, checking the
offsets to see how many grow beyond the maximum value.

Create a new t5328-commit-graph-64-bit-time.sh test script to handle
special cases of testing 64-bit timestamps. This helps demonstrate this
bug in more cases. It still won't hit all potential cases until the next
change, which reenables reading generation numbers. Use the skip_all
trick from 0a2bfccb9c8 (t0051: use "skip_all" under !MINGW in
single-test file, 2022-02-04) to make the output clean when run on a
32-bit system.

Helped-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 39 insertions, 0 deletions

diff --git a/t/t5328-commit-graph-64bit-time.sh b/t/t5328-commit-graph-64bit-time.sh
new file mode 100755
index 0000000000..28114bcaf4
--- /dev/null
+++ b/t/t5328-commit-graph-64bit-time.sh
@@ -0,0 +1,39 @@
+#!/bin/sh
+
+test_description='commit graph with 64-bit timestamps'
+. ./test-lib.sh
+
+if ! test_have_prereq TIME_IS_64BIT || ! test_have_prereq TIME_T_IS_64BIT
+then
+	skip_all='skipping 64-bit timestamp tests'
+	test_done
+fi
+
+. "$TEST_DIRECTORY"/lib-commit-graph.sh
+
+UNIX_EPOCH_ZERO="@0 +0000"
+FUTURE_DATE="@4147483646 +0000"
+
+GIT_TEST_COMMIT_GRAPH_CHANGED_PATHS=0
+
+test_expect_success 'lower layers have overflow chunk' '
+	rm -f .git/objects/info/commit-graph &&
+	test_commit --date "$FUTURE_DATE" future-1 &&
+	test_commit --date "$UNIX_EPOCH_ZERO" old-1 &&
+	git commit-graph write --reachable &&
+	test_commit --date "$FUTURE_DATE" future-2 &&
+	test_commit --date "$UNIX_EPOCH_ZERO" old-2 &&
+	git commit-graph write --reachable --split=no-merge &&
+	test_commit extra &&
+	git commit-graph write --reachable --split=no-merge &&
+	git commit-graph write --reachable &&
+	graph_read_expect 5 "generation_data generation_data_overflow" &&
+	mv .git/objects/info/commit-graph commit-graph-upgraded &&
+	git commit-graph write --reachable &&
+	graph_read_expect 5 "generation_data generation_data_overflow" &&
+	test_cmp .git/objects/info/commit-graph commit-graph-upgraded
+'
+
+graph_git_behavior 'overflow' '' HEAD~2 HEAD
+
+test_done