sequencer: fix empty commit check when amending

This fixes a regression introduced in 356ee4659b ("sequencer: try to
commit without forking 'git commit'", 2017-11-24). When amending a
commit try_to_commit() was using the wrong parent when checking if the
commit would be empty. When amending we need to check against HEAD^ not
HEAD.

t3403 may not seem like the natural home for the new tests but a further
patch series will improve the advice printed by `git commit`. That
series will mutate these tests to check that the advice includes
suggesting `rebase --skip` to skip the fixup that would empty the
commit.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

2 files changed, 53 insertions, 5 deletions

diff --git a/sequencer.c b/sequencer.c
index da2decbd3a..f4f81cbddc 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -1351,11 +1351,27 @@ static int try_to_commit(struct repository *r,
 		goto out;
 	}
 
-	if (!(flags & ALLOW_EMPTY) && oideq(current_head ?
-					    get_commit_tree_oid(current_head) :
-					    the_hash_algo->empty_tree, &tree)) {
-		res = 1; /* run 'git commit' to display error message */
-		goto out;
+	if (!(flags & ALLOW_EMPTY)) {
+		struct commit *first_parent = current_head;
+
+		if (flags & AMEND_MSG) {
+			if (current_head->parents) {
+				first_parent = current_head->parents->item;
+				if (repo_parse_commit(r, first_parent)) {
+					res = error(_("could not parse HEAD commit"));
+					goto out;
+				}
+			} else {
+				first_parent = NULL;
+			}
+		}
+		if (oideq(first_parent
+			  ? get_commit_tree_oid(first_parent)
+			  : the_hash_algo->empty_tree,
+			  &tree)) {
+			res = 1; /* run 'git commit' to display error message */
+			goto out;
+		}
 	}
 
 	if (find_hook("prepare-commit-msg")) {
diff --git a/t/t3403-rebase-skip.sh b/t/t3403-rebase-skip.sh
index 1f5122b632..ee8a8dba52 100755
--- a/t/t3403-rebase-skip.sh
+++ b/t/t3403-rebase-skip.sh
@@ -7,6 +7,8 @@ test_description='git rebase --merge --skip tests'
 
 . ./test-lib.sh
 
+. "$TEST_DIRECTORY"/lib-rebase.sh
+
 # we assume the default git am -3 --skip strategy is tested independently
 # and always works :)
 
@@ -20,6 +22,13 @@ test_expect_success setup '
 	git commit -a -m "hello world" &&
 	echo goodbye >> hello &&
 	git commit -a -m "goodbye" &&
+	git tag goodbye &&
+
+	git checkout --detach &&
+	git checkout HEAD^ . &&
+	test_tick &&
+	git commit -m reverted-goodbye &&
+	git tag reverted-goodbye &&
 
 	git checkout -f skip-reference &&
 	echo moo > hello &&
@@ -76,4 +85,27 @@ test_expect_success 'moved back to branch correctly' '
 
 test_debug 'gitk --all & sleep 1'
 
+test_expect_success 'fixup that empties commit fails' '
+	test_when_finished "git rebase --abort" &&
+	(
+		set_fake_editor &&
+		test_must_fail env FAKE_LINES="1 fixup 2" git rebase -i \
+			goodbye^ reverted-goodbye
+	)
+'
+
+test_expect_success 'squash that empties commit fails' '
+	test_when_finished "git rebase --abort" &&
+	(
+		set_fake_editor &&
+		test_must_fail env FAKE_LINES="1 squash 2" git rebase -i \
+			goodbye^ reverted-goodbye
+	)
+'
+
+# Must be the last test in this file
+test_expect_success '$EDITOR and friends are unchanged' '
+	test_editor_unchanged
+'
+
 test_done