rebase --update-refs: avoid unintended ref deletion

In b3b1a21d1a5 (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>

1 files changed, 6 insertions, 3 deletions

diff --git a/sequencer.c b/sequencer.c
index e658df7e8f..798a970296 100644
--- a/sequencer.c
+++ b/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);