[GStreamer][MSE] Push STREAM_COLLECTION event after flushing

https://bugs.webkit.org/show_bug.cgi?id=262193

Reviewed by Alicia Boya Garcia <aboya@igalia.com>.

Flushing the playback pipeline when very first buffers are being pushed downstream causes
STREAM_COLLECTION event to be dropped before reaching decodebin3. As a result playback never
starts after flush (buffer push return not-linked). This happens on initial seek sometimes,
when it is triggered at the same time that first buffers push.

STREAM_COLLECTION is dropped by GstBaseParse element specificaly that is caching events and
sending them with first frame. Unfortunately flushing will drop all pending events if first
frame is not ready yet.

Original author: Andrzej Surdej (https://github.com/asurdej-comcast)

See: #1197

* Source/WebCore/platform/graphics/gstreamer/mse/WebKitMediaSourceGStreamer.cpp:
(webKitMediaSrcStreamFlush): Reset hasPushedStreamCollectionEvent at the end of the flush to force sending of the STREAM_COLLECTION event after that. Only for GStreamer versions that don't have a proper fix yet.

Canonical link: https://commits.webkit.org/268582@main

Author: asurdej-comcast

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

@@ -660,6 +660,18 @@ static void webKitMediaSrcStreamFlush(Stream* stream, bool isSeekingFlush)
             GST_DEBUG_OBJECT(stream->pad.get(), "StreamingMembers mutex taken, using it to set isFlushing = false.");
             streamingMembers->isFlushing = false;
             streamingMembers->doesNeedSegmentEvent = true;
+
+            if (!webkitGstCheckVersion(1, 22, 0)) {
+                // In older GST versions STREAM_COLLECTION event is delivered to decodebin3
+                // from parsebin src pad probe. On the way, this event is cached inside
+                // parser element (GstBaseParse) and pushed downstream with first frame.
+                // Flushing before first frame is handled by the parser,
+                // the event is dropped from GstBaseParse and never reaches decodebin3.
+                // GST 1.21.3 added STREAM_COLLECTION event handling on decodebin3 sink pad directly
+                // so this workaround is not needed anymore.
+                GST_DEBUG_OBJECT(stream->pad.get(), "Reset hasPushedStreamCollectionEvent");
+                streamingMembers->hasPushedStreamCollectionEvent = false;
+            }
         }
 
         GST_DEBUG_OBJECT(stream->pad.get(), "Sending FLUSH_STOP downstream (resetTime = %s).", boolForPrinting(isSeekingFlush));