caps: When getting capsfeatures and none are there, store sysmem capsfeatures

... instead of returning a reference to a global instance. The caller might
want to change the global instance otherwise, which causes funny effects like
all global instances being changed and at the same time nothing in the caps
being changed.

As the caps might be immutable while we do this we have to do some magic
with atomic operations.

https://bugzilla.gnome.org/show_bug.cgi?id=723236

diff --git a/gst/gstcaps.c b/gst/gstcaps.c
index 179f518..9106f9f 100644 (file)
--- a/gst/gstcaps.c
+++ b/gst/gstcaps.c
@@ -114,8 +114,10 @@ typedef struct _GstCapsImpl
  * length check */
 #define gst_caps_get_structure_unchecked(caps, index) \
      (g_array_index (GST_CAPS_ARRAY (caps), GstCapsArrayElement, (index)).structure)
+#define gst_caps_get_features_storage_unchecked(caps, index) \
+     (&g_array_index (GST_CAPS_ARRAY (caps), GstCapsArrayElement, (index)).features)
 #define gst_caps_get_features_unchecked(caps, index) \
-     (g_array_index (GST_CAPS_ARRAY (caps), GstCapsArrayElement, (index)).features)
+     (g_atomic_pointer_get (gst_caps_get_features_storage_unchecked (caps, index)))
 /* quick way to append a structure without checking the args */
 #define gst_caps_append_structure_unchecked(caps, s, f) G_STMT_START{\
   GstCapsArrayElement __e={s, f};                                      \
@@ -856,8 +858,25 @@ gst_caps_get_features (const GstCaps * caps, guint index)
   g_return_val_if_fail (index < GST_CAPS_LEN (caps), NULL);
 
   features = gst_caps_get_features_unchecked (caps, index);
-  if (!features)
-    features = GST_CAPS_FEATURES_MEMORY_SYSTEM_MEMORY;
+  if (!features) {
+    GstCapsFeatures **storage;
+
+    /* We have to do some atomic pointer magic here as the caps
+     * might not be writable and someone else calls this function
+     * at the very same time */
+    features = gst_caps_features_copy (GST_CAPS_FEATURES_MEMORY_SYSTEM_MEMORY);
+    gst_caps_features_set_parent_refcount (features, &GST_CAPS_REFCOUNT (caps));
+
+    storage = gst_caps_get_features_storage_unchecked (caps, index);
+    if (!g_atomic_pointer_compare_and_exchange (storage, NULL, features)) {
+      /* Someone did the same we just tried in the meantime */
+      gst_caps_features_set_parent_refcount (features, NULL);
+      gst_caps_features_free (features);
+
+      features = gst_caps_get_features_unchecked (caps, index);
+      g_assert (features != NULL);
+    }
+  }
 
   return features;
 }
@@ -881,9 +900,10 @@ gst_caps_set_features (GstCaps * caps, guint index, GstCapsFeatures * features)
   g_return_if_fail (index <= gst_caps_get_size (caps));
   g_return_if_fail (IS_WRITABLE (caps));
 
-  storage = &gst_caps_get_features_unchecked (caps, index);
-  old = *storage;
-  *storage = features;
+  storage = gst_caps_get_features_storage_unchecked (caps, index);
+  /* Not much problem here as caps are writable */
+  old = g_atomic_pointer_get (storage);
+  g_atomic_pointer_set (storage, features);
 
   if (features)
     gst_caps_features_set_parent_refcount (features, &GST_CAPS_REFCOUNT (caps));