[GSoC PATCH v2] backfill: error out when HEAD cannot be parsed

[Trieu Huynh <vikingtc4@gmail.com>]

From: Trieu Huynh <vikingtc4@gmail.com>

handle_revision_arg() returns non-zero on failure, but do_backfill()
ignores the return value. On an empty repo with no commits, HEAD is
unborn and handle_revision_arg() fails, but backfill silently
continues with an empty revision walk and exists with a zero return
code.

Check the return value and propagate the error, consistent with
how builtin/pack-objects.c handles handle_revision_arg() failures.

Add a test to verify that backfill on an empty repository fails
with a clear error message.

Signed-off-by: Trieu Huynh <vikingtc4@gmail.com>
---
Changes in v2:
- Update commit msg (Point out by Karthik Nayak <karthik.188@gmail.com>)
- Use test_grep instead of grep (Point out by Tian Yuchen <cat@malon.dev>)

 builtin/backfill.c  | 3 ++-
 t/t5620-backfill.sh | 6 ++++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/builtin/backfill.c b/builtin/backfill.c
index e9a33e81be..ca49e188df 100644
--- a/builtin/backfill.c
+++ b/builtin/backfill.c
@@ -92,7 +92,8 @@ static int do_backfill(struct backfill_context *ctx)
 	}
 
 	repo_init_revisions(ctx->repo, &revs, "");
-	handle_revision_arg("HEAD", &revs, 0, 0);
+	if (handle_revision_arg("HEAD", &revs, 0, 0))
+		return error(_("unable to parse HEAD revision"));
 
 	info.blobs = 1;
 	info.tags = info.commits = info.trees = 0;
diff --git a/t/t5620-backfill.sh b/t/t5620-backfill.sh
index 58c81556e7..066ee17d39 100755
--- a/t/t5620-backfill.sh
+++ b/t/t5620-backfill.sh
@@ -77,6 +77,12 @@ test_expect_success 'do partial clone 2, backfill min batch size' '
 	test_line_count = 0 revs2
 '
 
+test_expect_success 'backfill on empty repo fails gracefully' '
+	git init empty-repo &&
+	test_must_fail git -C empty-repo backfill 2>err &&
+	test_grep "unable to parse HEAD" err
+'
+
 test_expect_success 'backfill --sparse without sparse-checkout fails' '
 	git init not-sparse &&
 	test_must_fail git -C not-sparse backfill --sparse 2>err &&
-- 
2.43.0