object_array: add and use `object_array_pop()`

In a couple of places, we pop objects off an object array `foo` by
decreasing `foo.nr`. We access `foo.nr` in many places, but most if not
all other times we do so read-only, e.g., as we iterate over the array.
But when we change `foo.nr` behind the array's back, it feels a bit
nasty and looks like it might leak memory.

Leaks happen if the popped element has an allocated `name` or `path`.
At the moment, that is not the case. Still, 1) the object array might
gain more fields that want to be freed, 2) a code path where we pop
might start using names or paths, 3) one of these code paths might be
copied to somewhere where we do, and 4) using a dedicated function for
popping is conceptually cleaner.

Introduce and use `object_array_pop()` instead. Release memory in the
new function. Document that popping an object leaves the associated
elements in limbo.

The converted places were identified by grepping for "\.nr\>" and
looking for "--".

Make the new function return NULL on an empty array. This is consistent
with `pop_commit()` and allows the following:

	while ((o = object_array_pop(&foo)) != NULL) {
		// do something
	}

But as noted above, we don't need to go out of our way to avoid reading
`foo.nr`. This is probably more readable:

	while (foo.nr) {
		... o = object_array_pop(&foo);
		// do something
	}

The name of `object_array_pop()` does not quite align with
`add_object_array()`. That is unfortunate. On the other hand, it matches
`object_array_clear()`. Arguably it's `add_...` that is the odd one out,
since it reads like it's used to "add" an "object array". For that
reason, side with `object_array_clear()`.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fast-export.c

@@ -634,11 +634,10 @@ static void handle_tail(struct object_array *commits, struct rev_info *revs)
 {
 	struct commit *commit;
 	while (commits->nr) {
-		commit = (struct commit *)commits->objects[commits->nr - 1].item;
+		commit = (struct commit *)object_array_pop(commits);
 		if (has_unshown_parent(commit))
 			return;
 		handle_commit(commit, revs);
-		commits->nr--;
 	}
 }
 

builtin/fsck.c

@@ -181,12 +181,7 @@ static int traverse_reachable(void)
 	if (show_progress)
 		progress = start_progress_delay(_("Checking connectivity"), 0, 0, 2);
 	while (pending.nr) {
-		struct object_array_entry *entry;
-		struct object *obj;
-
-		entry = pending.objects + --pending.nr;
-		obj = entry->item;
-		result |= traverse_one_object(obj);
+		result |= traverse_one_object(object_array_pop(&pending));
 		display_progress(progress, ++nr);
 	}
 	stop_progress(&progress);

builtin/reflog.c

@@ -126,7 +126,7 @@ static int commit_is_complete(struct commit *commit)
 		struct commit *c;
 		struct commit_list *parent;
 
-		c = (struct commit *)study.objects[--study.nr].item;
+		c = (struct commit *)object_array_pop(&study);
 		if (!c->object.parsed && !parse_object(&c->object.oid))
 			c->object.flags |= INCOMPLETE;
 

object.c

@@ -353,6 +353,19 @@ static void object_array_release_entry(struct object_array_entry *ent)
 	free(ent->path);
 }
 
+struct object *object_array_pop(struct object_array *array)
+{
+	struct object *ret;
+
+	if (!array->nr)
+		return NULL;
+
+	ret = array->objects[array->nr - 1].item;
+	object_array_release_entry(&array->objects[array->nr - 1]);
+	array->nr--;
+	return ret;
+}
+
 void object_array_filter(struct object_array *array,
 			 object_array_each_func_t want, void *cb_data)
 {

object.h

@@ -116,6 +116,14 @@ int object_list_contains(struct object_list *list, struct object *obj);
 void add_object_array(struct object *obj, const char *name, struct object_array *array);
 void add_object_array_with_path(struct object *obj, const char *name, struct object_array *array, unsigned mode, const char *path);
 
+/*
+ * Returns NULL if the array is empty. Otherwise, returns the last object
+ * after removing its entry from the array. Other resources associated
+ * with that object are left in an unspecified state and should not be
+ * examined.
+ */
+struct object *object_array_pop(struct object_array *array);
+
 typedef int (*object_array_each_func_t)(struct object_array_entry *, void *);
 
 /*

shallow.c

@@ -99,7 +99,7 @@ struct commit_list *get_shallow_commits(struct object_array *heads, int depth,
 				cur_depth = 0;
 			} else {
 				commit = (struct commit *)
-					stack.objects[--stack.nr].item;
+					object_array_pop(&stack);
 				cur_depth = *(int *)commit->util;
 			}
 		}