decodebin/vaapi: ranking vaapi breaks plugging

Submitted by Marc Leeman @den_erpel

Link to original bug (#761405)

Description

Installing gstreamer-vaapi is enough to break auto plugging behaviour.

Because vaapi modules require specific hardware (e.g. QuickSync) and a specific environment (DISPLAY set with running X Server); installing these modules break systems that depends on autoplugging.

Consider a simple application that does transcoding in the background, while using vaapi to decode video if an X server is running.

Due to the higher ranking (PRIMARY+2), the vaapi modules are always picked over the generic sofware based solution.

See the att'd simple example.

barco@EMS-005008087c8a:~$ gst-launch-1.0 uridecodebin uri=file:///home/barco/some-file.mkv ! fakesink
Setting pipeline to PAUSED ...
Pipeline is PREROLLING ...
Stream with high frequencies VQ coding
libva info: VA-API version 0.38.0
libva info: va_getDriverName() returns -1
libva error: va_getDriverName() failed with unknown libva error,driver_name=(null)
libva info: VA-API version 0.38.0
libva info: va_getDriverName() returns -1
libva error: va_getDriverName() failed with unknown libva error,driver_name=(null)
libva info: VA-API version 0.38.0
libva info: va_getDriverName() returns 1
libva error: va_getDriverName() failed with operation failed,driver_name=i965
ERROR: from element /GstPipeline:pipeline0/GstURIDecodeBin:uridecodebin0/GstDecodeBin:decodebin0/GstVaapiDecodeBin:vaapidecodebin0/GstVaapiDecode:vaapidecode: Could not initialize supporting library.
Additional debug info:
gstvideodecoder.c(2571): gst_video_decoder_change_state (): /GstPipeline:pipeline0/GstURIDecodeBin:uridecodebin0/GstDecodeBin:decodebin0/GstVaapiDecodeBin:vaapidecodebin0/GstVaapiDecode:vaapidecode:
Failed to open decoder
ERROR: pipeline doesn't want to preroll.
Setting pipeline to NULL ...
libva info: VA-API version 0.38.0
libva info: va_getDriverName() returns -1
libva error: va_getDriverName() failed with unknown libva error,driver_name=(null)
libva info: VA-API version 0.38.0
libva info: va_getDriverName() returns -1
libva error: va_getDriverName() failed with unknown libva error,driver_name=(null)
libva info: VA-API version 0.38.0
libva info: va_getDriverName() returns 1
libva error: va_getDriverName() failed with operation failed,driver_name=i965
Freeing pipeline ...

The current workaround is lowering the ranking of the hardware based elements:
We might need to resort to (in the app):

GstPluginFeature *feature;
GstRegistry *registry = gst_registry_get();

feature = gst_registry_lookup_feature (registry, "vaapidecodebin");
if (feature != NULL) {
gst_plugin_feature_set_rank (feature, GST_RANK_NONE);
gst_object_unref (feature);
}

and so forth :-(

Though this is obviously not a viable solution when new modules are installed that were not yet considered.