libstage: support caller-supplied stage memory

When !stage_conf_t.replace and stage_conf_t.stage is non-NULL,
stage_conf_t.stage will be used for the stage memory in stage_new()
instead of allocating some.

It's up to the caller to free this memory as needed, which may safely
be done from stage_ops_t.free_func() if needed.

Note that at this time there's no way for external callers to actually
know the stage_t size, as it's entirely opaque.  A future commit will
expose a padded variant of the struct, exposing its size appropriately
without making it completely public.

1 files changed, 11 insertions, 9 deletions

diff --git a/src/stage.c b/src/stage.c
index 7c07877..a714f1f 100644
--- a/src/stage.c
+++ b/src/stage.c
@@ -58,21 +58,23 @@ static stage_t * _stage_set_object(stage_t *stage, const char *name, const stage
 }
 
 
-/* allocate a stage, this purely creates a stage in isolation and assigns its associated name, object and functions */
-static stage_t * _stage_new(const char *name, const stage_ops_t *ops, void *object)
+/* Create a new stage, this purely creates a stage in isolation and assigns its associated name, object and functions.
+ * If stage is non-NULL, now allocation is performed and it's used instead.
+ */
+static stage_t * _stage_new(stage_t *stage, const char *name, const stage_ops_t *ops, void *object)
 {
-	stage_t	*stage;
+	if (!stage) {
+		stage = calloc(1, sizeof(stage_t));
+		if (!stage)
+			return NULL;
 
-	stage = calloc(1, sizeof(stage_t));
-	if (!stage)
-		return NULL;
+		stage->allocated = 1;
+	}
 
 	dll_init(&stage->layer);
 	for (int i = 0; i < STAGE_LAYERS_MAX; i++)
 		dll_init(&stage->layers[i]);
 
-	stage->allocated = 1;
-
 	return _stage_set_object(stage, name, ops, object);
 }
 
@@ -112,7 +114,7 @@ stage_t * stage_new(const stage_conf_t *conf, const stage_ops_t *ops, void *obje
 	if (conf->replace)
 		return stage_replace(conf->stage, conf->name, ops, object);
 
-	stage = _stage_new(conf->name, ops, object);
+	stage = _stage_new(conf->stage, conf->name, ops, object);
 	if (!stage)
 		return NULL;