[PATCH] rebase -i: Fix possibly wrong onto hash in todo

["Antti Keränen" <detegr@rbx.email>]

'todo_list_write_to_file' may overwrite the static buffer, originating
from 'find_unique_abbrev', that was used to store the short commit hash
'c' for "# Rebase a..b onto c" message in the todo editor.

Fix by duplicating the string before usage, so subsequent calls to
'find_unique_abbrev' or other functions calling 'hash_to_hex_algop_r'
can't overwrite the buffer.

Found-by: Jussi Keränen <jussike@gmail.com>
Signed-off-by: Antti Keränen <detegr@rbx.email>
---
 sequencer.c                   |  7 ++++---
 t/t3404-rebase-interactive.sh | 13 +++++++++++++
 2 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/sequencer.c b/sequencer.c
index fd7701c88a..0679adb639 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -5178,13 +5178,12 @@ int complete_action(struct repository *r, struct replay_opts *opts, unsigned fla
 		    struct string_list *commands, unsigned autosquash,
 		    struct todo_list *todo_list)
 {
-	const char *shortonto, *todo_file = rebase_path_todo();
+	const char *todo_file = rebase_path_todo();
 	struct todo_list new_todo = TODO_LIST_INIT;
 	struct strbuf *buf = &todo_list->buf, buf2 = STRBUF_INIT;
 	struct object_id oid = onto->object.oid;
 	int res;
-
-	shortonto = find_unique_abbrev(&oid, DEFAULT_ABBREV);
+	char *shortonto;
 
 	if (buf->len == 0) {
 		struct todo_item *item = append_new_todo(todo_list);
@@ -5206,8 +5205,10 @@ int complete_action(struct repository *r, struct replay_opts *opts, unsigned fla
 		return error(_("nothing to do"));
 	}
 
+	shortonto = xstrdup(find_unique_abbrev(&oid, DEFAULT_ABBREV));
 	res = edit_todo_list(r, todo_list, &new_todo, shortrevisions,
 			     shortonto, flags);
+	free(shortonto);
 	if (res == -1)
 		return -1;
 	else if (res == -2) {
diff --git a/t/t3404-rebase-interactive.sh b/t/t3404-rebase-interactive.sh
index 4a7d21f898..09af16753c 100755
--- a/t/t3404-rebase-interactive.sh
+++ b/t/t3404-rebase-interactive.sh
@@ -1760,6 +1760,19 @@ test_expect_success 'correct error message for commit --amend after empty pick'
 	test_i18ngrep "middle of a rebase -- cannot amend." err
 '
 
+test_expect_success 'todo has correct onto hash' '
+	write_script dump-raw.sh <<-\EOF &&
+		cat "$1"
+	EOF
+	git checkout branch1 &&
+	(
+		test_set_editor "$(pwd)/dump-raw.sh" &&
+		git rebase -i HEAD~5 >actual
+	) &&
+	onto=$(git rev-parse --short HEAD~5) &&
+	test_i18ngrep "^# Rebase ..* onto $onto .*" actual
+'
+
 # This must be the last test in this file
 test_expect_success '$EDITOR and friends are unchanged' '
 	test_editor_unchanged
-- 
2.28.0