[PATCH v2 5/5] commit-graph: avoid repeated mixed generation number warnings

[Taylor Blau <me@ttaylorr.com>]

When validating that a commit-graph has either all zero, or all non-zero
generation numbers, we emit a warning on both the rising and falling
edge of transitioning between the two.

So if we are unfortunate enough to see a commit-graph which has a
repeating sequence of zero, then non-zero generation numbers, we'll
generate many warnings that contain more or less the same information.

Avoid this by treating `generation_zero` as a bit-field, and warn under
the same conditions, but only do so once.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 commit-graph.c          | 17 ++++++++++-------
 t/t5318-commit-graph.sh | 14 ++++++++++++++
 2 files changed, 24 insertions(+), 7 deletions(-)

diff --git a/commit-graph.c b/commit-graph.c
index f7d9b31546..8d924b4509 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -2562,6 +2562,8 @@ static void graph_report(const char *fmt, ...)
 
 #define GENERATION_ZERO_EXISTS 1
 #define GENERATION_NUMBER_EXISTS 2
+#define GENERATION_NUMBER_BOTH_EXIST \
+	(GENERATION_ZERO_EXISTS | GENERATION_NUMBER_EXISTS)
 
 static int commit_graph_checksum_valid(struct commit_graph *g)
 {
@@ -2573,19 +2575,19 @@ static void verify_mixed_generation_numbers(struct commit_graph *g,
 					    int *generation_zero)
 {
 	if (commit_graph_generation_from_graph(graph_commit)) {
-		if (*generation_zero == GENERATION_ZERO_EXISTS)
+		if (*generation_zero & GENERATION_ZERO_EXISTS)
 			graph_report(_("commit-graph has non-zero generation "
 				       "number for commit %s, but zero "
 				       "elsewhere"),
 				     oid_to_hex(&graph_commit->object.oid));
-		*generation_zero = GENERATION_NUMBER_EXISTS;
+		*generation_zero |= GENERATION_NUMBER_EXISTS;
 	} else {
-		if (*generation_zero == GENERATION_NUMBER_EXISTS)
+		if (*generation_zero & GENERATION_NUMBER_EXISTS)
 			graph_report(_("commit-graph has generation number "
 				       "zero for commit %s, but non-zero "
 				       "elsewhere"),
 				     oid_to_hex(&graph_commit->object.oid));
-		*generation_zero = GENERATION_ZERO_EXISTS;
+		*generation_zero |= GENERATION_ZERO_EXISTS;
 	}
 }
 
@@ -2702,10 +2704,11 @@ static int verify_one_commit_graph(struct repository *r,
 			graph_report(_("commit-graph parent list for commit %s terminates early"),
 				     oid_to_hex(&cur_oid));
 
-		verify_mixed_generation_numbers(g, graph_commit,
-						&generation_zero);
+		if (generation_zero != GENERATION_NUMBER_BOTH_EXIST)
+			verify_mixed_generation_numbers(g, graph_commit,
+							&generation_zero);
 
-		if (generation_zero == GENERATION_ZERO_EXISTS)
+		if (generation_zero & GENERATION_ZERO_EXISTS)
 			continue;
 
 		/*
diff --git a/t/t5318-commit-graph.sh b/t/t5318-commit-graph.sh
index 8e96471b34..2626d41c94 100755
--- a/t/t5318-commit-graph.sh
+++ b/t/t5318-commit-graph.sh
@@ -628,6 +628,20 @@ test_expect_success 'detect mixed generation numbers (zero to non-zero)' '
 		"but zero elsewhere"
 '
 
+test_expect_success 'detect mixed generation numbers (flip-flop)' '
+	corrupt_graph_setup &&
+	for pos in \
+		$GRAPH_BYTE_COMMIT_GENERATION \
+		$GRAPH_BYTE_COMMIT_GENERATION_LAST
+	do
+		printf "\0\0\0\0" | dd of="full/$objdir/info/commit-graph" bs=1 \
+		seek="$pos" conv=notrunc || return 1
+	done &&
+
+	test_must_fail git -C full commit-graph verify 2>err &&
+	test 1 -eq "$(grep -c "generation number" err)"
+'
+
 test_expect_success 'git fsck (checks commit-graph when config set to true)' '
 	git -C full fsck &&
 	corrupt_graph_and_verify $GRAPH_BYTE_FOOTER "\00" \
-- 
2.42.0.rc0.30.gb82b15ebc8