[PATCH 2/2] builtin/show-branch: detect empty reflogs

[Patrick Steinhardt <ps@xxxxxx>]

The `--reflog=n` option for git-show-branch(1) allows the user to list
the reflog of a specific branch, where `n` specifies how many reflog
entries to show. When there are less than `n` entries we handle this
gracefully and simply report less entries.

There is a special case though when the ref either has no reflog or when
its reflog is empty. In this case, we end up printing nothing at all
while returning successfully. This is rather confusing as there is no
indicator why we didn't print anything.

Adapt the behaviour so that we die instead of leaving no user visible
traces. This change in behaviour should be fine given that this case
used to segfault before the preceding commit.

Signed-off-by: Patrick Steinhardt <ps@xxxxxx>
---
 builtin/show-branch.c  |  2 ++
 t/t3202-show-branch.sh | 25 +++++++++++++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/builtin/show-branch.c b/builtin/show-branch.c
index b01ec761d2..8837415031 100644
--- a/builtin/show-branch.c
+++ b/builtin/show-branch.c
@@ -785,6 +785,8 @@ int cmd_show_branch(int ac, const char **av, const char *prefix)
 			if (read_ref_at(get_main_ref_store(the_repository),
 					ref, flags, 0, base + i, &oid, &logmsg,
 					&timestamp, &tz, NULL)) {
+				if (!i)
+					die(_("log for %s is empty"), ref);
 				reflog = i;
 				break;
 			}
diff --git a/t/t3202-show-branch.sh b/t/t3202-show-branch.sh
index 6a98b2df76..e5b74cc55f 100755
--- a/t/t3202-show-branch.sh
+++ b/t/t3202-show-branch.sh
@@ -264,4 +264,29 @@ test_expect_success 'error descriptions on orphan branch' '
 	test_branch_op_in_wt -c new-branch
 '
 
+test_expect_success 'show-branch --reflog with empty reflog' '
+	git checkout -B empty-reflog &&
+	git reflog expire --expire=now refs/heads/empty-reflog &&
+
+	cat >expect <<-EOF &&
+	fatal: log for refs/heads/empty-reflog is empty
+	EOF
+	test_must_fail git show-branch --reflog=1 empty-reflog 2>err &&
+	test_cmp expect err &&
+	test_must_fail git show-branch --reflog empty-reflog 2>err &&
+	test_cmp expect err
+'
+
+test_expect_success 'show-branch --reflog with missing reflog' '
+	git -c core.logAllRefUpdates=false checkout -B missing-reflog &&
+
+	cat >expect <<-EOF &&
+	fatal: log for refs/heads/missing-reflog is empty
+	EOF
+	test_must_fail git show-branch --reflog=1 missing-reflog 2>err &&
+	test_cmp expect err &&
+	test_must_fail git show-branch --reflog missing-reflog 2>err &&
+	test_cmp expect err
+'
+
 test_done
-- 
2.44.0-rc1

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