Is GStreamer trying to execute a callback in another thread, according to this backtrace?
#0 0x00007ffff79a2f20 in MVM_panic ()
at //home/cbwood/Projects/rakudobrew/versions/moar-blead/install/lib/libmoar.so
#1 0x00007ffff79d6686 in ()
at //home/cbwood/Projects/rakudobrew/versions/moar-blead/install/lib/libmoar.so
#2 0x00007ffff79d6ff8 in callback_handler ()
at //home/cbwood/Projects/rakudobrew/versions/moar-blead/install/lib/libmoar.so
#3 0x00007ffff7add2c1 in dcCallback_x64_sysv ()
at //home/cbwood/Projects/rakudobrew/versions/moar-blead/install/lib/libmoar.so
#4 0x00007fffeeb6c81e in ffi_call_unix64 () at /usr/lib/x86_64-linux-gnu/libffi.so.6
#5 0x00007fffeeb6c1ef in ffi_call () at /usr/lib/x86_64-linux-gnu/libffi.so.6
#10 0x00007fffeecc7b6f in <emit signal ??? on instance 0x555568240080 [GstOggDemux]>
(instance=instance@entry=0x555568240080, signal_id=<optimized out>, detail=detail@entry=0)
at ../../../gobject/gsignal.c:3447
#6 0x00007fffeecab682 in g_cclosure_marshal_generic
(closure=0x555568317b40, return_gvalue=0x0, n_param_values=<optimized out>, param_values=<optimized out>, invocation_hint=<optimized out>, marshal_data=<optimized out>) at ../../../gobject/gclosure.c:1500
#7 0x00007fffeecaae8d in g_closure_invoke
(closure=0x555568317b40, return_value=0x0, n_param_values=2, param_values=0x7fffecf1a820, invocation_hint=0x7fffecf1a7a0) at ../../../gobject/gclosure.c:810
#8 0x00007fffeecbe555 in signal_emit_unlocked_R
--Type <RET> for more, q to quit, c to continue without paging--
(node=node@entry=0x555568476440, detail=detail@entry=0, instance=instance@entry=0x555568240080, emission_return=emission_return@entry=0x0, instance_and_params=instance_and_params@entry=0x7fffecf1a820)
at ../../../gobject/gsignal.c:3635
#9 0x00007fffeecc74ae in g_signal_emit_valist
(instance=<optimized out>, signal_id=<optimized out>, detail=<optimized out>, var_args=var_args@entry=0x7fffecf1a9f0) at ../../../gobject/gsignal.c:3391
#11 0x00007fffeed4f650 in gst_element_add_pad
(element=0x555568240080 [GstOggDemux|ogg-demuxer], pad=0x7fffd0008290 [GstOggPad|src_00004094])
at gstelement.c:711
#12 0x00007fffeea1b12d in () at /usr/lib/x86_64-linux-gnu/gstreamer-1.0/libgstogg.so
#13 0x00007fffeea202ba in () at /usr/lib/x86_64-linux-gnu/gstreamer-1.0/libgstogg.so
#14 0x00007fffeea249f5 in () at /usr/lib/x86_64-linux-gnu/gstreamer-1.0/libgstogg.so
#15 0x00007fffeeda2131 in gst_task_func (task=0x555568231290 [GstTask|ogg-demuxer:sink]) at gsttask.c:328
#16 0x00007fffeebe8243 in g_thread_pool_thread_proxy (data=<optimized out>)
at ../../../glib/gthreadpool.c:308
#17 0x00007fffeebe787d in g_thread_proxy (data=0x5555685a9280) at ../../../glib/gthread.c:805
#18 0x00007ffff7430182 in start_thread (arg=<optimized out>) at pthread_create.c:486
#19 0x00007ffff76b3b1f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95