playbin(3): First allocation query from stateless decoder never make it to the video sink
While this works with software decoders, I notice that inside playbin and playbin3, the allocation query made by at least VA and V4L2 stateless decoders never make it to the videosink. Here's one way to reproduce it, one can set GST_PLUGIN_FEATURE_RANK=avdec_h264:9999
to force ffmpeg over VA to compare.
GST_DEBUG=videodecoder:5,2 gst-launch-1.0 playbin3 uri=https://download.blender.org/peach/bigbuckbunny_movies/big_buck_bunny_1080p_h264.mov flags=video 2>&1 | grep "didn't get downstream ALLOCATION hints"
The main different is that stateless decoders parse the stream headers immediatly, push caps and run allocation query before the first call to chain returns. I wonder if playbin does not depends on that first call to return to make this work properly.