bftw: Don't immediately pin open directories

It is undesirable to close a directory that we haven't read yet to free
up cache capacity, but it's worse to fail to open the next directory
because too many upcoming directories are pinned.  This could happen
when sorting, because then we can't prioritize the already-opened ones.

src/bftw.c

@@ -659,8 +659,6 @@ static void bftw_file_set_dir(struct bftw_cache *cache, struct bftw_file *file,
 		file->fd = bfs_dirfd(dir);
 		bftw_cache_add(cache, file);
 	}
-
-	bftw_cache_pin(cache, file);
 }
 
 /** Free a bftw_file. */
@@ -1318,7 +1316,7 @@ static int bftw_opendir(struct bftw_state *state) {
 	struct bftw_file *file = state->file;
 	state->dir = file->dir;
 	if (state->dir) {
-		return 0;
+		goto pin;
 	}
 
 	if (bftw_build_path(state, NULL) != 0) {
@@ -1328,8 +1326,11 @@ static int bftw_opendir(struct bftw_state *state) {
 	state->dir = bftw_file_opendir(state, file, state->path);
 	if (!state->dir) {
 		state->direrror = errno;
+		return 0;
 	}
 
+pin:
+	bftw_cache_pin(&state->cache, file);
 	return 0;
 }
 
@@ -1577,7 +1578,7 @@ static int bftw_gc(struct bftw_state *state, enum bftw_gc_flags flags) {
 	int ret = 0;
 
 	struct bftw_file *file = state->file;
-	if (file && file->dir) {
+	if (file && state->dir) {
 		bftw_unpin_dir(state, file, true);
 	}
 	state->dir = NULL;