[GStreamer][WebRTC] Request video key-frame when connecting incoming streams

https://bugs.webkit.org/show_bug.cgi?id=273238

Reviewed by Xabier Rodriguez-Calvar.

In some cases of remote peer sending H.264, the resulting depayloaded bitstream might not contain
SPS/PPS informations required for decoding. Without these the parser would discard frames until the
stream can be parsed, potentially leading to delay in setting up the video decoder. To workaround
this issue, issue a key-frame request as soon as a new src pad is exposed on webrtcbin, and also
when adding video pads to the mediastreamsrc element.

* Source/WebCore/platform/mediastream/gstreamer/GStreamerIncomingTrackProcessor.cpp:
(WebCore::GStreamerIncomingTrackProcessor::incomingTrackProcessor):
* Source/WebCore/platform/mediastream/gstreamer/GStreamerMediaStreamSource.cpp:
(webkitMediaStreamSrcPadProbeCb):

Canonical link: https://commits.webkit.org/277980@main

Author: philn

Source/WebCore/platform/mediastream/gstreamer/GStreamerIncomingTrackProcessor.cpp

@@ -128,6 +128,9 @@ GRefPtr<GstElement> GStreamerIncomingTrackProcessor::incomingTrackProcessor()
     if (m_data.type == RealtimeMediaSource::Type::Audio)
         return createParser();
 
+    GST_DEBUG_OBJECT(m_bin.get(), "Requesting a key-frame");
+    gst_pad_send_event(m_pad.get(), gst_video_event_new_upstream_force_key_unit(GST_CLOCK_TIME_NONE, TRUE, 1));
+
     bool forceEarlyVideoDecoding = !g_strcmp0(g_getenv("WEBKIT_GST_WEBRTC_FORCE_EARLY_VIDEO_DECODING"), "1");
     GST_DEBUG_OBJECT(m_bin.get(), "Configuring for input caps: %" GST_PTR_FORMAT "%s", m_data.caps.get(), forceEarlyVideoDecoding ? " and early decoding" : "");
     if (!forceEarlyVideoDecoding) {

Source/WebCore/platform/mediastream/gstreamer/GStreamerMediaStreamSource.cpp

@@ -1057,6 +1057,11 @@ static GstPadProbeReturn webkitMediaStreamSrcPadProbeCb(GstPad* pad, GstPadProbe
             return GST_PAD_PROBE_REMOVE;
         }
 
+        if (data->sourceType == RealtimeMediaSource::Type::Video) {
+            GST_DEBUG_OBJECT(self, "Requesting a key-frame");
+            gst_pad_send_event(pad, gst_video_event_new_upstream_force_key_unit(GST_CLOCK_TIME_NONE, TRUE, 1));
+        }
+
         auto* streamStart = gst_event_new_stream_start(data->trackId.get());
         gst_event_set_group_id(streamStart, 1);
         gst_pad_push_event(pad, streamStart);