[MSE][GStreamer] don't push samples while seeking

https://bugs.webkit.org/show_bug.cgi?id=295415

Reviewed by Alicia Boya Garcia.

MediaSource::waitForTarget completes asynchronously. Even when the
target time is already buffered, it still enqueues a task to compute
seek time on next event loop cycle. This can lead to SourceBuffer
providing media data for incorrect time, after GStreamer seek already
flushed the source. For example this happens if
TrackQueue::LowLevelHandler callback was posted just before the seek.

The proposed change implements SourceBufferPrivate::isSeeking() that
returns true until MSE seek is completed, effectivly blocking
SourceBufferPrivate::provideMediaData during the seek.

See: #1528

Original author: Eugene Mutavchi <Ievgen_Mutavchi@comcast.com>

* Source/WebCore/platform/graphics/gstreamer/mse/MediaPlayerPrivateGStreamerMSE.cpp:
(WebCore::MediaPlayerPrivateGStreamerMSE::doSeek): Signal the seek intent to the media source private.
* Source/WebCore/platform/graphics/gstreamer/mse/MediaSourcePrivateGStreamer.cpp:
(WebCore::MediaSourcePrivateGStreamer::willSeek): Forward the seek intent to all SourceBufferPrivates.
* Source/WebCore/platform/graphics/gstreamer/mse/MediaSourcePrivateGStreamer.h: Added willSeek().
* Source/WebCore/platform/graphics/gstreamer/mse/SourceBufferPrivateGStreamer.cpp:
(WebCore::SourceBufferPrivateGStreamer::willSeek): Set m_isSeeking to true.
(WebCore::SourceBufferPrivateGStreamer::isSeeking const): Return m_isSeeking value.
(WebCore::SourceBufferPrivateGStreamer::seekToTime): Unset m_isSeeking and call the default SourceBufferPrivate seekToTime() implementation.
* Source/WebCore/platform/graphics/gstreamer/mse/SourceBufferPrivateGStreamer.h: Added willSeek(), isSeeking(), seekToTime() and the m_isSeeking flag (to signal that we're waiting for samples from the multiplatform layer after a seek has been initiated; unset once the samples are ready for the platform-specific layer).

Canonical link: https://commits.webkit.org/298330@main

Author: emutavchi

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

@@ -269,6 +269,7 @@ bool MediaPlayerPrivateGStreamerMSE::doSeek(const SeekTarget& target, float rate
     // This will also add support for fastSeek once done (see webkit.org/b/260607)
     if (!m_mediaSourcePrivate)
         return false;
+    m_mediaSourcePrivate->willSeek();
     m_mediaSourcePrivate->waitForTarget(target)->whenSettled(RunLoop::current(), [this, weakThis = ThreadSafeWeakPtr { *this }](auto&& result) {
         RefPtr self = weakThis.get();
         if (!self || !result)

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

@@ -205,6 +205,12 @@ TrackID MediaSourcePrivateGStreamer::registerTrackId(TrackID preferredId)
     return assignedId;
 }
 
+void MediaSourcePrivateGStreamer::willSeek()
+{
+    for (auto* sourceBuffer : m_activeSourceBuffers)
+        downcast<SourceBufferPrivateGStreamer>(sourceBuffer)->willSeek();
+}
+
 bool MediaSourcePrivateGStreamer::unregisterTrackId(TrackID trackId)
 {
     ASSERT(isMainThread());

Source/WebCore/platform/graphics/gstreamer/mse/MediaSourcePrivateGStreamer.h

@@ -77,6 +77,8 @@ class MediaSourcePrivateGStreamer final : public MediaSourcePrivate
     TrackID registerTrackId(TrackID);
     bool unregisterTrackId(TrackID);
 
+    void willSeek();
+
 #if !RELEASE_LOG_DISABLED
     const Logger& logger() const final { return m_logger; }
     ASCIILiteral logClassName() const override { return "MediaSourcePrivateGStreamer"_s; }

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

@@ -403,6 +403,24 @@ size_t SourceBufferPrivateGStreamer::platformEvictionThreshold() const
     return evictionThreshold;
 }
 
+void SourceBufferPrivateGStreamer::willSeek()
+{
+    ALWAYS_LOG(LOGIDENTIFIER);
+    m_seeking = true;
+}
+
+bool SourceBufferPrivateGStreamer::isSeeking() const
+{
+    return m_seeking;
+}
+
+void SourceBufferPrivateGStreamer::seekToTime(const MediaTime& time)
+{
+    m_seeking = false;
+    // WebKit now has the samples to complete the seek and is about to enqueue them.
+    SourceBufferPrivate::seekToTime(time);
+}
+
 #undef GST_CAT_DEFAULT
 
 } // namespace WebCore

Source/WebCore/platform/graphics/gstreamer/mse/SourceBufferPrivateGStreamer.h

@@ -96,6 +96,10 @@ class SourceBufferPrivateGStreamer final : public SourceBufferPrivate, public Ca
     size_t platformMaximumBufferSize() const override;
     size_t platformEvictionThreshold() const final;
 
+    void willSeek();
+    bool isSeeking() const final;
+    void seekToTime(const MediaTime&) final;
+
 private:
     friend class AppendPipeline;
 
@@ -110,6 +114,10 @@ class SourceBufferPrivateGStreamer final : public SourceBufferPrivate, public Ca
     StdUnorderedMap<TrackID, RefPtr<MediaSourceTrackGStreamer>> m_tracks;
     std::optional<MediaPromise::Producer> m_appendPromise;
 
+    // Set while waiting for samples from the multiplatform layer after a seek has initiated.
+    // Unset once the samples are ready for the platform-specific layer.
+    bool m_seeking { false };
+
 #if !RELEASE_LOG_DISABLED
     Ref<const Logger> m_logger;
     const void* m_logIdentifier;