gstreamer: Decouple ownership of the GstPlay/AppSrc from signal callbacks

[tharkum]

The associated signal callbacks shouldn't capture strong or weak references of a owner GstPlay object with which they are interact otherwise it leads to leaked underlying media resources.

Note that upgrading the captured weak reference for a GstPlay object on another thread (the GstPlay streaming thread) is not safely because a GstPlay object could be destroyed on that thread after signal callback handling with mutex panic on the gst_play_finalize.

To solve the GstPlay ownership issue between client and Gstreamer was introduced the player shared state with minimal required states (metadata, pending playback rate, play state, ...).

Anyway the captured GstPlay weak reference will be used on sync source setup stage which is locking the client thread until a source will be ready.

For GstAppSrc and associated seek_data callback a capture of a weak reference will be used with explicit set empty callbacks on a player destruction.

Testing: Manually testing with the following WPT test
GST_DEBUG=3,gst-play*:7 ./mach test-wpt html/semantics/embedded-content/the-video-element/intrinsic_sizes.htm -r

Fixes (partially): #455

[tharkum]

@jdm < Please take a look

[tharkum]

There is crash under heavy testing scenario, so please postpone review

 0:30.53 pid:2649784    0: servoshell::backtrace::print
 0:30.53 pid:2649784    1: servoshell::crash_handler::install::handler
 0:30.57 pid:2649784    2: <unknown>
 0:30.57 pid:2649784    3: mozalloc_abort
 0:30.57 pid:2649784    4: abort
 0:30.57 pid:2649784    5: g_mutex_clear
 0:30.57 pid:2649784    6: <unknown>
 0:30.58 pid:2649784    7: g_object_unref
 0:30.58 pid:2649784    8: g_object_get
 0:30.58 pid:2649784    9: gst_play_get_position
 0:30.58 pid:2649784   10: <unknown>
 0:30.58 pid:2649784   11: g_cclosure_marshal_VOID__BOXEDv
 0:30.58 pid:2649784   12: g_signal_emit_valist
 0:30.58 pid:2649784   13: g_signal_emit
 0:30.58 pid:2649784   14: gst_bus_async_signal_func
 0:30.58 pid:2649784   15: <unknown>
 0:30.58 pid:2649784   16: g_main_context_dispatch
 0:30.58 pid:2649784   17: <unknown>
 0:30.58 pid:2649784   18: g_main_loop_run
 0:30.58 pid:2649784   19: <unknown>
 0:30.58 pid:2649784   20: <unknown>
 0:30.62 pid:2649784   21: start_thread
 0:30.62 pid:2649784              at ./nptl/pthread_create.c:442:8
 0:30.62 pid:2649784   22: __GI___clone3
 0:30.62 pid:2649784              at ./misc/../sysdeps/unix/sysv/linux/x86_64/clone3.S:81:0

[sdroege]

PlayerInner -> PlaySignalAdapter callbacks (strong -> weak refs)

Instead of going this way, you might want to use the gst::Bus on the Player instance directly instead of using the signals via callbacks. That makes it (in my experience) simpler to decouple ownership of values and avoid circular references.

[tharkum]

@jdm < Please take a look