From 737e7dcc396352966dbc075f62aed2b5b16ae257 Mon Sep 17 00:00:00 2001
From: Ganesh prasad Sahu <GaneshPrasad_Sahu@comcast.com>
Date: Mon, 15 Jun 2026 04:54:58 +0000
Subject: [PATCH] =?UTF-8?q?Queue=20frames=20till=20Video=20Frame=20Observe?=
 =?UTF-8?q?r=20is=20ready=20https://bugs.webkit.org/show=5Fbug.cgi=3Fid=3D?=
 =?UTF-8?q?317111=20Reviewed=20by:=20E.=20Oca=C3=B1a=20Gonz=C3=A1lez?=
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Problem : WebRTC playback doesnot recover from a black screen and the issue is seen Intermittently

 Cause: The first video frame(s) (containing SPS/PPS/IDR) being dropped
 because libwebrtc starts delivering decoded frames before the downstream
 GStreamer pipeline's InternalSource registers as a VideoFrameObserver.

 Without first having the SPS/PPS and the first sync frame , the decoder
 can not decode any subsequent delta frames.

 In Following code snippet from mediastream/gstreamer/GStreamerMediaStreamSource.cpp,
 frames received while m_isObserving is false might be dropped.

void videoFrameAvailable(VideoFrame& videoFrame, VideoFrameTimeMetadata) final
{
       return;

    updateFirstVideoSampleSeenFlag();

Change : While this patch doesnot address the root cause of delayed start of
 VideoFrameObserver, add a small frame buffer to hold frames
 (currently max size set to 30 frames) that arrive before any observer registers.
 deliver the frames when observer is registered
Note: From tests it is observed that only the first frame or 2 is actually needed to be queued.

* Source/WebCore/platform/mediastream/RealtimeMediaSource.cpp:
  (RealtimeMediaSource::videoFrameAvailable) : Queue frames till
observer is ready
* Source/WebCore/platform/mediastream/RealtimeMediaSource.h:
  (struct PendingVideoFrame) : Added a buffer to hold the frames
---
 .../mediastream/RealtimeMediaSource.cpp       | 19 +++++++++++++++++++
 .../mediastream/RealtimeMediaSource.h         |  7 +++++++
 2 files changed, 26 insertions(+)

diff --git a/Source/WebCore/platform/mediastream/RealtimeMediaSource.cpp b/Source/WebCore/platform/mediastream/RealtimeMediaSource.cpp
index 5ebc2b98ca639..935b8f8ee2bea 100644
--- a/Source/WebCore/platform/mediastream/RealtimeMediaSource.cpp
+++ b/Source/WebCore/platform/mediastream/RealtimeMediaSource.cpp
@@ -218,6 +218,25 @@ void RealtimeMediaSource::videoFrameAvailable(VideoFrame& videoFrame, VideoFrame
     updateHasStartedProducingData();
 
     Locker locker { m_videoFrameObserversLock };
+    if (m_videoFrameObservers.isEmpty()) {
+        if (m_pendingVideoFrames.size() < maxPendingVideoFramesBeforeAddTrack) {
+            m_pendingVideoFrames.append(PendingVideoFrame { &videoFrame, metadata });
+        }
+        else {
+            WTFLogAlways("RealtimeMediaSource: Dropping video frame (queue is full) %zu frames", m_pendingVideoFrames.size());
+        }
+        return;
+    }
+    if (!m_pendingVideoFrames.isEmpty()) {
+        WTFLogAlways("RealtimeMediaSource: Delivering %zu queued frame(s) (pipeline ready)", m_pendingVideoFrames.size());
+        for (auto& pending : m_pendingVideoFrames) {
+            if (pending.frame) {
+                for (auto* obs : m_videoFrameObservers)
+                    obs->videoFrameAvailable(*pending.frame, pending.metadata);
+            }
+        }
+        m_pendingVideoFrames.clear();
+    }
     for (auto* observer : m_videoFrameObservers)
         observer->videoFrameAvailable(videoFrame, metadata);
 }
diff --git a/Source/WebCore/platform/mediastream/RealtimeMediaSource.h b/Source/WebCore/platform/mediastream/RealtimeMediaSource.h
index 091aa0ead2afe..ffc3ad1007dc8 100644
--- a/Source/WebCore/platform/mediastream/RealtimeMediaSource.h
+++ b/Source/WebCore/platform/mediastream/RealtimeMediaSource.h
@@ -300,6 +300,13 @@ class WEBCORE_EXPORT RealtimeMediaSource
     mutable Lock m_videoFrameObserversLock;
     HashSet<VideoFrameObserver*> m_videoFrameObservers WTF_GUARDED_BY_LOCK(m_videoFrameObserversLock);
 
+    struct PendingVideoFrame {
+        RefPtr<VideoFrame> frame;
+        VideoFrameTimeMetadata metadata;
+    };
+    static constexpr size_t maxPendingVideoFramesBeforeAddTrack = 30;
+    Vector<PendingVideoFrame> m_pendingVideoFrames WTF_GUARDED_BY_LOCK(m_videoFrameObserversLock);
+
     // Set on the main thread from constraints.
     IntSize m_size;
     // Set on sample generation thread.