[PATCH v2] shallow: fix relative deepen on non-shallow repositories

["Samo Pogačnik" <samo_pogacnik@t-2.net>]

The commit "3ef68ff40e (shallow: handling fetch relative-deepen,
2026-02-15)" introduced a bug where using --deepen=<n> on a non-
shallow repository incorrectly treated the value as an absolute
depth, resulting in a shallow fetch and truncated history.

This patch prevents any modification when a relative deepen is
requested on a non-shallow repository.

A test is added to ensure that history is not changed when
--deepen is used on a non-shallow repository.

Reported-by: Owen Stephens <owen@owenstephens.co.uk>
Signed-off-by: Samo Pogačnik <samo_pogacnik@t-2.net>
---
Changes since v1:
- Fixed commit reference in the commit message.

 shallow.c                |  6 +++++-
 t/t5537-fetch-shallow.sh | 10 ++++++++++
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/shallow.c b/shallow.c
index a8ad92e303..610ff3d13b 100644
--- a/shallow.c
+++ b/shallow.c
@@ -245,7 +245,11 @@ struct commit_list *get_shallow_commits(struct object_array *heads,
 					int depth, int shallow_flag, int not_shallow_flag)
 {
 	if (shallows && deepen_relative) {
-		depth += get_shallows_depth(heads, shallows);
+		int cur_shallow_depth = get_shallows_depth(heads, shallows);
+		if (cur_shallow_depth)
+			depth += cur_shallow_depth;
+		else
+			return NULL;
 	}
 	return get_shallows_or_depth(heads, NULL, NULL,
 				     depth, shallow_flag, not_shallow_flag);
diff --git a/t/t5537-fetch-shallow.sh b/t/t5537-fetch-shallow.sh
index 6588ce6226..9982dd2aa6 100755
--- a/t/t5537-fetch-shallow.sh
+++ b/t/t5537-fetch-shallow.sh
@@ -251,6 +251,16 @@ test_expect_success '.git/shallow is edited by repack' '
 		origin "+refs/heads/*:refs/remotes/origin/*"
 '
 
+test_expect_success 'fetch --deepen does not truncate' '
+	git clone --no-local .git full-clone &&
+	git -C full-clone rev-parse --is-shallow-repository >expect &&
+	git -C full-clone log --oneline >>expect &&
+	git -C full-clone fetch --deepen=1 &&
+	git -C full-clone rev-parse --is-shallow-repository >actual &&
+	git -C full-clone log --oneline >>actual &&
+	test_cmp expect actual
+'
+
 . "$TEST_DIRECTORY"/lib-httpd.sh
 start_httpd
 
-- 
2.43.0