Forward port GStreamer sticky events race fix (#77)

Applies
https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7632
on top of stable GStreamer. That merge request fixes a long standing
race condition prone to occur in MSE.

images/wkdev_sdk/jhbuild/patches/0004-pad-Never-push-sticky-events-in-response-to-a-FLUSH_.patch

@@ -0,0 +1,81 @@
+From 59b714edc36c6d5887d54741cd628f38efe9e30c Mon Sep 17 00:00:00 2001
+From: =?UTF-8?q?Alicia=20Boya=20Garc=C3=ADa?= <[email protected]>
+Date: Wed, 9 Oct 2024 13:35:33 -0400
+Subject: [PATCH] pad: Never push sticky events in response to a FLUSH_STOP
+
+FLUSH_STOP is meant to clear the flushing state of pads and elements
+downstream, not to process data. Hence, a FLUSH_STOP should not
+propagate sticky events. This is also consistent with how flushes are a
+special case for probes.
+
+Currently this is almost always the case, since a FLUSH_STOP is
+__usually__ preceded by a FLUSH_START, and events (sticky or not) are
+discarded while a pad has the FLUSHING flag active (set by FLUSH_START).
+
+However, it is currently assumed that a FLUSH_STOP not preceded by a
+FLUSH_START is correct behavior, and this will occur while autoplugging
+pipelines are constructed. This leaves us with an unhandled edge case!
+
+This patch explicitly disables sending sticky events when pushing a
+FLUSH_STOP, instead of relying on the flushing flag of the pad, which
+will break in the edge case of a FLUSH_STOP not preceded by a
+FLUSH_START.
+
+If sticky events are propagated in response to a FLUSH_STOP, the
+flushing thread can end up deadlocked in blocking code of a downstream
+pad, such as a blocking probe. Instead, those events should be
+propagated from the streaming thread of the pad when handling a
+non-flushing synchronized event or buffer.
+
+This fixes a deadlock found in WebKit with playbin3 when seeks occur
+before preroll, where the seeking thread ended up stuck in the blocking
+probe of playsink:
+https://github.com/WebPlatformForEmbedded/WPEWebKit/issues/1367
+
+Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7632>
+---
+ subprojects/gstreamer/gst/gstpad.c | 17 ++++++++++++++---
+ 1 file changed, 14 insertions(+), 3 deletions(-)
+
+diff --git a/subprojects/gstreamer/gst/gstpad.c b/subprojects/gstreamer/gst/gstpad.c
+index d8bda692b2..7c159d2816 100644
+--- a/subprojects/gstreamer/gst/gstpad.c
++++ b/subprojects/gstreamer/gst/gstpad.c
+@@ -5580,8 +5580,12 @@ gst_pad_push_event_unchecked (GstPad * pad, GstEvent * event,
+   PROBE_PUSH (pad, type | GST_PAD_PROBE_TYPE_PUSH, event, probe_stopped);
+
+   /* recheck sticky events because the probe might have cause a relink */
++  /* Note: FLUSH_STOP is a serialized event, but must not propagate sticky
++   * events. FLUSH_STOP is only targeted at removing the flushing state from
++   * pads and elements, and not actually pushing data/events. */
+   if (GST_PAD_HAS_PENDING_EVENTS (pad) && GST_PAD_IS_SRC (pad)
+-      && (GST_EVENT_IS_SERIALIZED (event))) {
++      && (GST_EVENT_IS_SERIALIZED (event))
++      && GST_EVENT_TYPE (event) != GST_EVENT_FLUSH_STOP) {
+     PushStickyData data = { GST_FLOW_OK, FALSE, event };
+     GST_OBJECT_FLAG_UNSET (pad, GST_PAD_FLAG_PENDING_EVENTS);
+
+@@ -5740,11 +5744,18 @@ gst_pad_push_event (GstPad * pad, GstEvent * event)
+         break;
+     }
+   }
+-  if (GST_PAD_IS_SRC (pad) && serialized) {
++  if (GST_PAD_IS_SRC (pad) && serialized
++      && GST_EVENT_TYPE (event) != GST_EVENT_FLUSH_STOP) {
+     /* All serialized events on the srcpad trigger push of sticky events.
+      *
+      * Note that we do not do this for non-serialized sticky events since it
+-     * could potentially block. */
++     * could potentially block.
++     *
++     * We must NOT propagate sticky events in response to FLUSH_STOP either, as
++     * FLUSH_STOP is only targeted at removing the flushing state from pads and
++     * elements, and not actually pushing data/events. This also makes it
++     * consistent with the way flush events are handled in "blocking" pad
++     * probes. */
+     res = (check_sticky (pad, event) == GST_FLOW_OK);
+   }
+   if (!serialized || !sticky) {
+-- 
+2.47.0
+

images/wkdev_sdk/jhbuild/webkit-sdk-deps.modules

@@ -113,6 +113,7 @@
       <patch file="0001-webrtcbin-create-and-associate-transceivers-earlier-.patch" strip="1"/>
       <patch file="0002-webrtcbin-reverse-direction-from-remote-media.patch" strip="1"/>
       <patch file="0003-webrtcbin-connect-output-stream-on-recv-transceivers.patch" strip="1"/>
+      <patch file="0004-pad-Never-push-sticky-events-in-response-to-a-FLUSH_.patch" strip="1"/>
     </branch>
     <dependencies>
       <dep package="openh264"/>