rebase --update-refs: avoid unintended ref deletion

In b3b1a21d1a (sequencer: rewrite update-refs as user edits todo list,
2022-07-19), the 'todo_list_filter_update_refs()' step was added to handle
the removal of 'update-ref' lines from a 'rebase-todo'. Specifically, it
removes potential ref updates from the "update refs state" if a ref does not
have a corresponding 'update-ref' line.

However, because 'write_update_refs_state()' will not update the state if
the 'refs_to_oids' list was empty, removing *all* 'update-ref' lines will
result in the state remaining unchanged from how it was initialized (with
all refs' "after" OID being null). Then, when the ref update is applied, all
refs will be updated to null and consequently deleted.

To fix this, delete the 'update-refs' state file when 'refs_to_oids' is
empty. Additionally, add a tests covering "all update-ref lines removed"
cases.

Reported-by: herr.kaste <herr.kaste@gmail.com>
Helped-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Helped-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

sequencer.c

@@ -4130,11 +4130,14 @@ static int write_update_refs_state(struct string_list *refs_to_oids)
 	struct string_list_item *item;
 	char *path;
 
-	if (!refs_to_oids->nr)
-		return 0;
-
 	path = rebase_path_update_refs(the_repository->gitdir);
 
+	if (!refs_to_oids->nr) {
+		if (unlink(path) && errno != ENOENT)
+			result = error_errno(_("could not unlink: %s"), path);
+		goto cleanup;
+	}
+
 	if (safe_create_leading_directories(path)) {
 		result = error(_("unable to create leading directories of %s"),
 			       path);

t/t3404-rebase-interactive.sh

@@ -1964,6 +1964,113 @@ test_expect_success 'respect user edits to update-ref steps' '
 	test_cmp_rev HEAD refs/heads/no-conflict-branch
 '
 
+test_expect_success '--update-refs: all update-ref lines removed' '
+	git checkout -b test-refs-not-removed no-conflict-branch &&
+	git branch -f base HEAD~4 &&
+	git branch -f first HEAD~3 &&
+	git branch -f second HEAD~3 &&
+	git branch -f third HEAD~1 &&
+	git branch -f tip &&
+
+	test_commit test-refs-not-removed &&
+	git commit --amend --fixup first &&
+
+	git rev-parse first second third tip no-conflict-branch >expect-oids &&
+
+	(
+		set_cat_todo_editor &&
+		test_must_fail git rebase -i --update-refs base >todo.raw &&
+		sed -e "/^update-ref/d" <todo.raw >todo
+	) &&
+	(
+		set_replace_editor todo &&
+		git rebase -i --update-refs base
+	) &&
+
+	# Ensure refs are not deleted and their OIDs have not changed
+	git rev-parse first second third tip no-conflict-branch >actual-oids &&
+	test_cmp expect-oids actual-oids
+'
+
+test_expect_success '--update-refs: all update-ref lines removed, then some re-added' '
+	git checkout -b test-refs-not-removed2 no-conflict-branch &&
+	git branch -f base HEAD~4 &&
+	git branch -f first HEAD~3 &&
+	git branch -f second HEAD~3 &&
+	git branch -f third HEAD~1 &&
+	git branch -f tip &&
+
+	test_commit test-refs-not-removed2 &&
+	git commit --amend --fixup first &&
+
+	git rev-parse first second third >expect-oids &&
+
+	(
+		set_cat_todo_editor &&
+		test_must_fail git rebase -i \
+			--autosquash --update-refs \
+			base >todo.raw &&
+		sed -e "/^update-ref/d" <todo.raw >todo
+	) &&
+
+	# Add a break to the end of the todo so we can edit later
+	echo "break" >>todo &&
+
+	(
+		set_replace_editor todo &&
+		git rebase -i --autosquash --update-refs base &&
+		echo "update-ref refs/heads/tip" >todo &&
+		git rebase --edit-todo &&
+		git rebase --continue
+	) &&
+
+	# Ensure first/second/third are unchanged, but tip is updated
+	git rev-parse first second third >actual-oids &&
+	test_cmp expect-oids actual-oids &&
+	test_cmp_rev HEAD tip
+'
+
+test_expect_success '--update-refs: --edit-todo with no update-ref lines' '
+	git checkout -b test-refs-not-removed3 no-conflict-branch &&
+	git branch -f base HEAD~4 &&
+	git branch -f first HEAD~3 &&
+	git branch -f second HEAD~3 &&
+	git branch -f third HEAD~1 &&
+	git branch -f tip &&
+
+	test_commit test-refs-not-removed3 &&
+	git commit --amend --fixup first &&
+
+	git rev-parse first second third tip no-conflict-branch >expect-oids &&
+
+	(
+		set_cat_todo_editor &&
+		test_must_fail git rebase -i \
+			--autosquash --update-refs \
+			base >todo.raw &&
+		sed -e "/^update-ref/d" <todo.raw >todo
+	) &&
+
+	# Add a break to the beginning of the todo so we can resume with no
+	# update-ref lines
+	echo "break" >todo.new &&
+	cat todo >>todo.new &&
+
+	(
+		set_replace_editor todo.new &&
+		git rebase -i --autosquash --update-refs base &&
+
+		# Make no changes when editing so update-refs is still empty
+		cat todo >todo.new &&
+		git rebase --edit-todo &&
+		git rebase --continue
+	) &&
+
+	# Ensure refs are not deleted and their OIDs have not changed
+	git rev-parse first second third tip no-conflict-branch >actual-oids &&
+	test_cmp expect-oids actual-oids
+'
+
 test_expect_success '--update-refs: check failed ref update' '
 	git checkout -B update-refs-error no-conflict-branch &&
 	git branch -f base HEAD~4 &&