[GStreamer] Sometimes progressive playback is not resumed after seek

https://bugs.webkit.org/show_bug.cgi?id=300041

Reviewed by Philippe Normand.

Sometimes, in a downstream multimedia player, regular video playback
remains paused after seek instead of resuming automatically. It will
play normally when play() or another seek operation are manually
triggered.

Log analys shows that in the failing case the browser missed the state
transition to 'HaveCurrentData', and application can't resume playback
after seek. It was missed because it happened the gstreamer pipeline
remains in state transition for too long and can't process buffering
events. Normally it goes: seek -> Paused pipeline -> buffering
(HaveCurrentData) -> finish_buffering(HaveEnoughData) -> start_playback.
However, in the failing case, seek takes too long (~2sec) to set the
PAUSED state on the pipeline and we are missing the
buffering(HaveCurrentData) stage.

See: #1561

This patch returns the player readyState to HaveCurrentData and the
networkState to Loading when a seek is being processed. The states will
reach their final values as soon as the async state transition caused by
the seek finishes.

Original author: Volodymyr Ogorodnik (https://github.com/volodymyr-ogorodnik-red)

* Source/WebCore/platform/graphics/gstreamer/MediaPlayerPrivateGStreamer.cpp:
(WebCore::MediaPlayerPrivateGStreamer::updateStates): Reset readyState and networkState to HaveCurrentData and Loading while the player is in an async state transition (in a seek) and buffering.

Canonical link: https://commits.webkit.org/300933@main

Author: eocanha

Source/WebCore/platform/graphics/gstreamer/MediaPlayerPrivateGStreamer.cpp

@@ -2686,6 +2686,13 @@ void MediaPlayerPrivateGStreamer::updateStates()
         GST_DEBUG_OBJECT(pipeline(), "Async: State: %s, pending: %s", gst_element_state_get_name(m_currentState), gst_element_state_get_name(pending));
         // Change in progress.
 
+        if (m_currentState == GST_STATE_PAUSED && m_isBuffering) {
+            // If we are buffering, we could miss HaveCurrentData state if buffering finished before the transition completes.
+            GST_DEBUG_OBJECT(pipeline(), "Async: [Buffering] still buffering, so force HaveCurrentData.");
+            m_readyState = MediaPlayer::ReadyState::HaveCurrentData;
+            m_networkState = MediaPlayer::NetworkState::Loading;
+        }
+
         // Delay the m_isBuffering change by returning it to its previous value. Without this, the false --> true change
         // would go unnoticed by the code that should trigger a pause.
         if (m_wasBuffering != m_isBuffering && !m_isPaused && m_playbackRate) {