Found by Christoph Diehl's "Faulty" fuzzer, see bug 777067

Hit by webm-video reftests.

Not trivial to analyze why we're hitting this forbidden generic implementation, but we get this clue in the log: [Faulty] pickle field {int} of value: 9 changed to: 1 We don't have that many relevant enum types that go up to value 9. The only one I see is CompositableType: enum CompositableType { BUFFER_UNKNOWN, // the deprecated compositable types BUFFER_IMAGE_SINGLE, // image/canvas with a single texture, single buffered BUFFER_IMAGE_BUFFERED, // canvas, double buffered BUFFER_BRIDGE, // image bridge protocol BUFFER_CONTENT, // thebes layer interface, single buffering BUFFER_CONTENT_DIRECT, // thebes layer interface, double buffering BUFFER_CONTENT_INC, // thebes layer interface, only sends incremental // updates to a texture on the compositor side. BUFFER_TILED, // tiled thebes layer // the new compositable types COMPOSITABLE_IMAGE, // image with single buffering COMPOSITABLE_CONTENT_SINGLE, // thebes layer interface, single buffering COMPOSITABLE_CONTENT_DOUBLE, // thebes layer interface, double buffering BUFFER_COUNT }; So the original value 9 was COMPOSITABLE_CONTENT_SINGLE i.e. a ThebesLayer, and this is changed to 1 which is BUFFER_IMAGE_SINGLE. In other words a Content/ThebesLayer compositable is reinterpreted in place as an Image compositable, and then we call that ThebesLayer-specific method, UpdateThebes, on it. This will need some thinking to figure the right way to handle such bad enums gracefully. Classification: PLayerTransaction, bad compositable type, hard

Fixed by the landing of PLayerTransaction type checks before casting layers, bug 968833.