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, 7 insertions, 3 deletions

diff --git a/commit-graph.c b/commit-graph.c
index 265c010122..a19bd96c2e 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -1556,12 +1556,16 @@ static void compute_generation_numbers(struct write_commit_graph_context *ctx)
 				if (current->date && current->date > max_corrected_commit_date)
 					max_corrected_commit_date = current->date - 1;
 				commit_graph_data_at(current)->generation = max_corrected_commit_date + 1;
-
-				if (commit_graph_data_at(current)->generation - current->date > GENERATION_NUMBER_V2_OFFSET_MAX)
-					ctx->num_generation_data_overflows++;
 			}
 		}
 	}
+
+	for (i = 0; i < ctx->commits.nr; i++) {
+		struct commit *c = ctx->commits.list[i];
+		timestamp_t offset = commit_graph_data_at(c)->generation - c->date;
+		if (offset > GENERATION_NUMBER_V2_OFFSET_MAX)
+			ctx->num_generation_data_overflows++;
+	}
 	stop_progress(&ctx->progress);
 }