message: Add gst_message_writable_structure()

Add gst_message_writable_structure() to be able to add extra fields to
messages (and be on par with GstEvent).

https://bugzilla.gnome.org/show_bug.cgi?id=792928

docs/gst/gstreamer-sections.txt

@@ -1638,6 +1638,7 @@ gst_message_ref
 gst_message_unref
 gst_message_copy
 gst_message_get_structure
+gst_message_writable_structure
 gst_message_make_writable
 gst_message_get_seqnum
 gst_message_set_seqnum

gst/gstmessage.c

@@ -1158,6 +1158,43 @@ gst_message_get_structure (GstMessage * message)
   return GST_MESSAGE_STRUCTURE (message);
 }
 
+/**
+ * gst_message_writable_structure:
+ * @message: The #GstMessage.
+ *
+ * Get a writable version of the structure.
+ *
+ * Returns: (transfer none): The structure of the message. The structure
+ * is still owned by the message, which means that you should not free
+ * it and that the pointer becomes invalid when you free the message.
+ * This function checks if @message is writable and will never return
+ * %NULL.
+ *
+ * MT safe.
+ *
+ * Since: 1.14
+ */
+GstStructure *
+gst_message_writable_structure (GstMessage * message)
+{
+  GstStructure *structure;
+
+  g_return_val_if_fail (GST_IS_MESSAGE (message), NULL);
+  g_return_val_if_fail (gst_message_is_writable (message), NULL);
+
+  structure = GST_MESSAGE_STRUCTURE (message);
+
+  if (structure == NULL) {
+    structure =
+        gst_structure_new_id_empty (gst_message_type_to_quark (GST_MESSAGE_TYPE
+            (message)));
+    gst_structure_set_parent_refcount (structure,
+        &message->mini_object.refcount);
+    GST_MESSAGE_STRUCTURE (message) = structure;
+  }
+  return structure;
+}
+
 /**
  * gst_message_has_name:
  * @message: The #GstMessage.

gst/gstmessage.h

@@ -449,6 +449,9 @@ GST_EXPORT
 const GstStructure *
                 gst_message_get_structure       (GstMessage *message);
 
+GST_EXPORT
+GstStructure *  gst_message_writable_structure  (GstMessage *message);
+
 GST_EXPORT
 gboolean        gst_message_has_name            (GstMessage *message, const gchar *name);
 

tests/check/gst/gstmessage.c

@@ -35,10 +35,18 @@ GST_START_TEST (test_parsing)
 
   /* GST_MESSAGE_EOS */
   {
+    GstStructure *s;
+
     message = gst_message_new_eos (NULL);
     fail_if (message == NULL);
     fail_unless (GST_MESSAGE_TYPE (message) == GST_MESSAGE_EOS);
     fail_unless (GST_MESSAGE_SRC (message) == NULL);
+
+    /* Add an extra field */
+    s = gst_message_writable_structure (message);
+    fail_if (s == NULL);
+    gst_structure_set (s, "eos-extra-field", G_TYPE_BOOLEAN, TRUE, NULL);
+
     gst_message_unref (message);
   }
   /* GST_MESSAGE_ERROR */

win32/common/libgstreamer.def

@@ -826,6 +826,7 @@ EXPORTS
 	gst_message_type_get_name
 	gst_message_type_get_type
 	gst_message_type_to_quark
+	gst_message_writable_structure
 	gst_meta_api_type_get_tags
 	gst_meta_api_type_has_tag
 	gst_meta_api_type_register