rebase -i: stop overwriting ORIG_HEAD buffer

After rebasing, ORIG_HEAD is supposed to point to the old HEAD of the
rebased branch.  The code used find_unique_abbrev() to obtain the
object name of the old HEAD and wrote to both
.git/rebase-merge/orig-head (used by `rebase --abort` to go back to
the previous state) and to ORIG_HEAD.  The buffer find_unique_abbrev()
gives back is volatile, unfortunately, and was overwritten after the
former file is written but before ORIG_FILE is written, leaving an
incorrect object name in it.

Avoid relying on the volatile buffer of find_unique_abbrev(), and
instead supply our own buffer to keep the object name.

I think that all of the users of head_hash should actually be using
opts->orig_head instead as passing a string rather than a struct
object_id around is a hang over from the scripted implementation. This
patch just fixes the immediate bug and adds a regression test based on
Caspar's reproduction example[1]. The users will be converted to use
struct object_id and head_hash removed in the next few commits.

[1] https://lore.kernel.org/git/CAFzd1+7PDg2PZgKw7U0kdepdYuoML9wSN4kofmB_-8NHrbbrHg@mail.gmail.com

Reported-by: Caspar Duregger <herr.kaste@gmail.com>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rebase.c

@@ -270,15 +270,15 @@ static int edit_todo_file(unsigned flags)
 }
 
 static int get_revision_ranges(struct commit *upstream, struct commit *onto,
-			       struct object_id *orig_head, const char **head_hash,
+			       struct object_id *orig_head, char *head_hash,
 			       char **revisions, char **shortrevisions)
 {
 	struct commit *base_rev = upstream ? upstream : onto;
 	const char *shorthead;
 
-	*head_hash = find_unique_abbrev(orig_head, GIT_MAX_HEXSZ);
+	find_unique_abbrev_r(head_hash, orig_head, GIT_MAX_HEXSZ);
 	*revisions = xstrfmt("%s...%s", oid_to_hex(&base_rev->object.oid),
-						   *head_hash);
+						   head_hash);
 
 	shorthead = find_unique_abbrev(orig_head, DEFAULT_ABBREV);
 
@@ -327,7 +327,7 @@ static void split_exec_commands(const char *cmd, struct string_list *commands)
 static int do_interactive_rebase(struct rebase_options *opts, unsigned flags)
 {
 	int ret;
-	const char *head_hash = NULL;
+	char head_hash[GIT_MAX_HEXSZ];
 	char *revisions = NULL, *shortrevisions = NULL;
 	struct strvec make_script_args = STRVEC_INIT;
 	struct todo_list todo_list = TODO_LIST_INIT;
@@ -335,7 +335,7 @@ static int do_interactive_rebase(struct rebase_options *opts, unsigned flags)
 	struct string_list commands = STRING_LIST_INIT_DUP;
 
 	if (get_revision_ranges(opts->upstream, opts->onto, &opts->orig_head,
-				&head_hash, &revisions, &shortrevisions))
+				head_hash, &revisions, &shortrevisions))
 		return -1;
 
 	if (init_basic_state(&replay,

t/t3404-rebase-interactive.sh

@@ -1797,6 +1797,17 @@ test_expect_success 'todo has correct onto hash' '
 	test_i18ngrep "^# Rebase ..* onto $onto" actual
 '
 
+test_expect_success 'ORIG_HEAD is updated correctly' '
+	test_when_finished "git checkout master && git branch -D test-orig-head" &&
+	git checkout -b test-orig-head A &&
+	git commit --allow-empty -m A1 &&
+	git commit --allow-empty -m A2 &&
+	git commit --allow-empty -m A3 &&
+	git commit --allow-empty -m A4 &&
+	git rebase master &&
+	test_cmp_rev ORIG_HEAD test-orig-head@{1}
+'
+
 # This must be the last test in this file
 test_expect_success '$EDITOR and friends are unchanged' '
 	test_editor_unchanged