[MSE][GStreamer] Make fake preroll asynchronous

https://bugs.webkit.org/show_bug.cgi?id=295289

Reviewed by Alicia Boya Garcia.

With the current implementation the whole seek flow as executed in a
single main loop cycle. As a result, the JS app had no chance to spot
that video.seeking attribute is ever set to true. Also some apps listen
to video.onseeking and expects that video.seeking attribute to be true
in such case. That wasn't a valid assumption.

Also, the behaviour isn't the same in <video> elements that are
audio-only as in <audio> elements.

This happens on the Amlogic platform, that doesn't have asynchronous
state changes on its audio sink. Spotify is broken there because of this
problem.

See:
#1232
#1527

This patch calls didPreroll() also for <video> elements that are
audio-only. Also makes that call async by putting it on the HTML media
element task queue to make sure it is executed after dispatching
'seeking' event to JS so the app has a chance to notice that
HTMLmedia.seeking attribute is 'true'.

Original author: Andrzej Surdej <101130014+asurdej-comcast@users.noreply.github.com>

* Source/WebCore/platform/graphics/gstreamer/mse/MediaPlayerPrivateGStreamerMSE.cpp:
(WebCore::MediaPlayerPrivateGStreamerMSE::doSeek): Find the actual final audio sink. Call didPreroll() asynchronously for any media element having only audio.

Canonical link: https://commits.webkit.org/298239@main

Author: asurdej-comcast

Source/WebCore/platform/graphics/gstreamer/mse/MediaPlayerPrivateGStreamerMSE.cpp

@@ -278,20 +278,41 @@ bool MediaPlayerPrivateGStreamerMSE::doSeek(const SeekTarget& target, float rate
             m_mediaSourcePrivate->seekToTime(*result);
 
         auto player = m_player.get();
-        if (player && !player->isVideoPlayer() && m_audioSink) {
-            gboolean audioSinkPerformsAsyncStateChanges;
-            g_object_get(m_audioSink.get(), "async", &audioSinkPerformsAsyncStateChanges, nullptr);
+        if (player && !hasVideo() && m_audioSink) {
+            gboolean audioSinkPerformsAsyncStateChanges = true;
+
+            GRefPtr<GstElement> sink = m_audioSink;
+            while (GST_IS_BIN(sink.get())) {
+                GUniquePtr<GstIterator> iter(gst_bin_iterate_sinks(GST_BIN_CAST(sink.get())));
+                GValue value = G_VALUE_INIT;
+                auto result = gst_iterator_next(iter.get(), &value);
+                ASSERT_UNUSED(result, result == GST_ITERATOR_OK);
+                sink = GST_ELEMENT(g_value_get_object(&value));
+                g_value_unset(&value);
+            }
+            if (gstObjectHasProperty(sink.get(), "async"))
+                g_object_get(sink.get(), "async", &audioSinkPerformsAsyncStateChanges, nullptr);
             if (!audioSinkPerformsAsyncStateChanges) {
                 // If audio-only pipeline's sink is not performing async state changes
                 // we must simulate preroll right away as otherwise nothing will trigger it.
-                bool mustPreventPositionReset = m_isWaitingForPreroll && m_isSeeking;
-                if (mustPreventPositionReset)
-                    m_cachedPosition = currentTime();
-                didPreroll();
-                if (mustPreventPositionReset) {
-                    propagateReadyStateToPlayer();
-                    invalidateCachedPosition();
-                }
+
+                // Post this on HTML media element queue so it will be executed
+                // synchonously with media events (e.g. seeking). This will ensure
+                // that HTML element attributes (like HTMLmedia.seeking) are not reseted
+                // before app receives "seeking" event
+                player->queueTaskOnEventLoop([this, weakThis = ThreadSafeWeakPtr { *this }] {
+                    RefPtr self = weakThis.get();
+                    if (!self)
+                        return;
+                    bool mustPreventPositionReset = m_isWaitingForPreroll && m_isSeeking;
+                    if (mustPreventPositionReset)
+                        m_cachedPosition = currentTime();
+                    didPreroll();
+                    if (mustPreventPositionReset) {
+                        propagateReadyStateToPlayer();
+                        invalidateCachedPosition();
+                    }
+                });
             }
         }
     });