rclone: Workaround for incorrect "not found" errors while listing files

rclone returns a "not found" error if an internal error occurs while
listing a folder. Ignoring this error lets restic erroneously think that
there are no files, which can cause `prune` to wipe the whole
repository.

changelog/unreleased/issue-4612

@@ -0,0 +1,11 @@
+Bugfix: Improve error handling for `rclone` backend
+
+Since restic 0.16.0, if rclone encountered an error while listing files,
+this could in rare circumstances cause restic to assume that there are no
+files. Although unlikely, this situation could result in data loss if it
+were to happen right when the `prune` command is listing existing snapshots.
+
+Error handling has now been improved to detect and work around this case.
+
+https://github.com/restic/restic/issues/4612
+https://github.com/restic/restic/pull/4618

internal/backend/rest/rest.go

@@ -328,8 +328,13 @@ func (b *Backend) List(ctx context.Context, t restic.FileType, fn func(restic.Fi
 	}
 
 	if resp.StatusCode == http.StatusNotFound {
-		// ignore missing directories
-		return nil
+		if !strings.HasPrefix(resp.Header.Get("Server"), "rclone/") {
+			// ignore missing directories, unless the server is rclone. rclone
+			// already ignores missing directories, but misuses "not found" to
+			// report certain internal errors, see
+			// https://github.com/rclone/rclone/pull/7550 for details.
+			return nil
+		}
 	}
 
 	if resp.StatusCode != http.StatusOK {