[MSE][GStreamer] block data flow until source pad is exposed

https://bugs.webkit.org/show_bug.cgi?id=276251

Reviewed by Xabier Rodriguez-Calvar.

In WebKitMediaSrc, there could be a race between main thread (linking
typefind element, see gsturisourcebin.c:setup_typefind) and streaming
thread (pushing data). webKitMediaSrcLoop does wait for source pad to
linked. However, there is no guarantee that peer pad is already active
at that point. This may lead to data loss while peer the pad is being
activated.

This change blocks downstream flow during pad addition, allowing
urisourcebin to link/activate typefind sink pad.

See: #1355

Original patch by Eugene Mutavchi <Ievgen_Mutavchi@comcast.com>.

* Source/WebCore/platform/graphics/gstreamer/mse/WebKitMediaSourceGStreamer.cpp:
(webKitMediaSrcEmitStreams): Add blocking probes to the Streams Pads, which will be removed after the pad has been added to the element.

Canonical link: https://commits.webkit.org/280813@main

Author: emutavchi

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

@@ -37,6 +37,7 @@
 #include <wtf/MainThread.h>
 #include <wtf/MainThreadData.h>
 #include <wtf/RefPtr.h>
+#include <wtf/Scope.h>
 #include <wtf/glib/WTFGType.h>
 #include <wtf/text/AtomString.h>
 #include <wtf/text/AtomStringHash.h>
@@ -327,6 +328,13 @@ void webKitMediaSrcEmitStreams(WebKitMediaSrc* source, const Vector<RefPtr<Media
     gst_element_post_message(GST_ELEMENT(source), gst_message_new_stream_collection(GST_OBJECT(source), source->priv->collection.get()));
 
     for (const RefPtr<Stream>& stream: source->priv->streams.values()) {
+        // Block data flow until pad is exposed.
+        gulong blockId = gst_pad_add_probe(
+            GST_PAD(stream->pad.get()), GST_PAD_PROBE_TYPE_BLOCK_DOWNSTREAM,
+            [](GstPad*, GstPadProbeInfo*, gpointer) -> GstPadProbeReturn {
+                return GST_PAD_PROBE_OK;
+            }, nullptr, nullptr);
+
         if (!webkitGstCheckVersion(1, 20, 6)) {
             // Workaround: gst_element_add_pad() should already call gst_pad_set_active() if the element is PAUSED or
             // PLAYING. Unfortunately, as of GStreamer 1.18.2 it does so with the element lock taken, causing a deadlock
@@ -340,6 +348,7 @@ void webKitMediaSrcEmitStreams(WebKitMediaSrc* source, const Vector<RefPtr<Media
         }
         GST_DEBUG_OBJECT(source, "Adding pad '%s' for stream with name '%s'", GST_OBJECT_NAME(stream->pad.get()), stream->track->trackId().string().utf8().data());
         gst_element_add_pad(GST_ELEMENT(source), GST_PAD(stream->pad.get()));
+        gst_pad_remove_probe(GST_PAD(stream->pad.get()), blockId);
     }
     GST_DEBUG_OBJECT(source, "All pads added");
 }