unpack-trees: make dir an internal-only struct

Avoid accidental misuse or confusion over ownership by clearly making
unpack_trees_options.dir an internal-only variable.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

unpack-trees.c

@@ -1692,9 +1692,12 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 	static struct cache_entry *dfc;
 	struct pattern_list pl;
 	int free_pattern_list = 0;
+	struct dir_struct dir = DIR_INIT;
 
 	if (len > MAX_UNPACK_TREES)
 		die("unpack_trees takes at most %d trees", MAX_UNPACK_TREES);
+	if (o->dir)
+		BUG("o->dir is for internal use only");
 
 	trace_performance_enter();
 	trace2_region_enter("unpack_trees", "unpack_trees", the_repository);
@@ -1706,7 +1709,7 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 	}
 
 	if (!o->preserve_ignored) {
-		CALLOC_ARRAY(o->dir, 1);
+		o->dir = &dir;
 		o->dir->flags |= DIR_SHOW_IGNORED;
 		setup_standard_excludes(o->dir);
 	}
@@ -1874,7 +1877,7 @@ done:
 		clear_pattern_list(&pl);
 	if (o->dir) {
 		dir_clear(o->dir);
-		FREE_AND_NULL(o->dir);
+		o->dir = NULL;
 	}
 	trace2_region_leave("unpack_trees", "unpack_trees", the_repository);
 	trace_performance_leave("unpack_trees");

unpack-trees.h

@@ -67,7 +67,6 @@ struct unpack_trees_options {
 		     dry_run;
 	const char *prefix;
 	int cache_bottom;
-	struct dir_struct *dir;
 	struct pathspec *pathspec;
 	merge_fn_t fn;
 	const char *msgs[NB_UNPACK_TREES_WARNING_TYPES];
@@ -89,6 +88,7 @@ struct unpack_trees_options {
 	struct index_state result;
 
 	struct pattern_list *pl; /* for internal use */
+	struct dir_struct *dir; /* for internal use only */
 	struct checkout_metadata meta;
 };