Created attachment 180805 [details]
debug log

this issue showed up when trying to circumvent the issue 
https://bugzilla.gnome.org/show_bug.cgi?id=632788 (movies that contain multiple audio streams out of which some can be decoded by dvbaudiosink (mpeg hardware decoder sink) and some cannot - for example one AC3 and one vorbis stream)
by doing something like this in the code:

if ( m_gst_audiobin )
{
	GstElement *dvbaudiosink = gst_element_factory_make("dvbaudiosink", "dvbaudiosink");
	int ret = gst_bin_add(GST_BIN(m_gst_audiobin), dvbaudiosink);
	GstPad *dvbaudiosinkpad = gst_element_get_static_pad(dvbaudiosink, "sink");
	GstCaps *dvbaudiosinkcaps = gst_caps_copy(gst_pad_get_pad_template_caps (dvbaudiosinkpad));	
	GstCaps* decodebincaps = gst_caps_from_string("audio/x-vorbis; audio/x-flac; audio/x-wav; audio/x-raw-int");
	gst_caps_merge(decodebincaps, dvbaudiosinkcaps);
	eDebug("after merge: decodebincaps = %s", gst_caps_to_string(decodebincaps));
	GstPad *universal_audiosinkpad = gst_ghost_pad_new_no_target_from_template("sink", gst_pad_template_new("sink", GST_PAD_SINK, GST_PAD_ALWAYS, decodebincaps));
	gst_element_add_pad (m_gst_audiobin, universal_audiosinkpad);
//	GstElement *decodebin = gst_element_factory_make("decodebin", "decodebin");
//	GstElement *audioconvert = gst_element_factory_make("audioconvert", "audioconvert");
//	GstElement *alsasink = gst_element_factory_make("alsasink", "alsasink");
	GstElement *fakesink = gst_element_factory_make("fakesink", "audio_fakesink");
//	gst_bin_add_many(GST_BIN(m_gst_audiobin), dvbaudiosink, decodebin, audioconvert, alsasink, NULL);
	gst_bin_add(GST_BIN(m_gst_audiobin), fakesink);
// 	int ret = gst_element_link_many(decodebin, audioconvert, alsasink, NULL);
// 	gst_ghost_pad_set_target (GST_GHOST_PAD(universal_audiosinkpad), gst_element_get_static_pad(fakesink, "sink"));
	g_object_set (G_OBJECT (m_gst_playbin), "audio-sink", m_gst_audiobin, NULL);
	g_signal_connect (G_OBJECT (universal_audiosinkpad), "notify::caps", G_CALLBACK (gstGhostpadHasCAPS), this);
}


and then down below in the hascaps function doing this:

eDebug("gstGhostpadHasCAPS:: AUDIO signal::caps = %s", gst_caps_to_string(caps));

GstPad *dvbaudiosinkpad = gst_element_get_static_pad(gst_bin_get_by_name(GST_BIN(this->m_gst_audiobin), "dvbaudiosink"), "sink");
GstCaps *dvbaudiosinkcaps;
dvbaudiosinkcaps = gst_caps_copy(gst_pad_get_pad_template_caps (dvbaudiosinkpad));

eDebug("dvbaudiosink's caps = %s", gst_caps_to_string(dvbaudiosinkcaps));
int ret = gst_caps_is_subset(caps, dvbaudiosinkcaps);

eDebug("gst_caps_is_subset = %i", ret);

if ( ret == 1 )
{
	int r = gst_ghost_pad_set_target (GST_GHOST_PAD(pad), dvbaudiosinkpad);
	eDebug("setting ghostpad target to dvbaudiosinkpad@%p returns %i", dvbaudiosinkpad, r);
}
else
{
// 				GstPad *decodebinpad = gst_element_get_static_pad(gst_bin_get_by_name(GST_BIN(this->m_gst_audiobin), "decodebin"), "sink");
	GstPad *decodebinpad = gst_element_get_static_pad(gst_bin_get_by_name(GST_BIN(this->m_gst_audiobin), "audio_fakesink"), "sink");
	int r = gst_ghost_pad_set_target (GST_GHOST_PAD(pad), decodebinpad);
	eDebug("setting ghostpad target to decodebinpad@%p returns %i", decodebinpad, r);
}

the idea behind this is that any type of stream can be directed from playbin2 into one single sink pad and the audio bin will automatically determine whether the stream can be decoded by hardware or requires a software decoder and will set the target of the ghostbin accordingly. 

now to cut to the chase:
as per debug log, the custom audiobin with its seperate elements and the ghost are created correctly, and playbin2 also accepts the custom audio-sink property change. afterwards though, it ignores that and creates another dvbaudiosink element which it tries to use as a sink element. this leads to the problems described in 632788 because the sink can't handle the vorbis stream.

line 5297 shows that another dvbaudiosink is created
then starting at 9326 it goes into a loop with a message:
playbin2 gstplaybin2.c:1851:gst_play_bin_get_current_sink:<playbin> play_sink_get_sink() returned audio sink <dvbaudiosink0>, the originally set audio sink is <universal_audiobin>