fix: keep what a seek decoded at the new position

webrtc-java-media/src/main/cpp/src/media/MediaPlayer.cpp

@@ -156,25 +156,28 @@ namespace ffmpeg
 
 	void MediaPlayer::Seek(int64_t position_us)
 	{
-		{
-			std::lock_guard<std::mutex> lock(mutex_);
-
-			if (closing_) {
-				return;
-			}
-
-			seek_pending_ = true;
-			seek_position_us_ = position_us;
-			ended_ = false;
+		std::lock_guard<std::mutex> lock(mutex_);
 
-			command_.notify_all();
+		if (closing_) {
+			return;
 		}
 
+		seek_pending_ = true;
+		seek_position_us_ = position_us;
+		ended_ = false;
+
 		// The decode thread may be held up by a full queue, and it cannot act
 		// on the request until it gets out of that. Dropping what is queued
 		// both frees it and throws away what belongs to the old position; the
 		// thread flushes again once the seek has actually happened.
+		//
+		// Under the lock, as the thread takes the request only under it: a
+		// flush after that would throw away what it has decoded at the new
+		// position, and an audio source, decoded seconds ahead, would skip
+		// ahead or end at once.
 		pacer_->Flush(position_us + loop_offset_us_.load());
+
+		command_.notify_all();
 	}
 
 	void MediaPlayer::SetLooping(bool looping)

webrtc-java-media/src/test/java/dev/onvoid/webrtc/media/player/MediaPlayerTest.java

@@ -335,6 +335,11 @@ void positionFollowsABackwardSeekOfAudio() throws Exception {
 			// About 0.4 s into the source again, not held at where it was.
 			assertTrue(after < before / 2, "position after seeking back: " + after
 					+ " us, " + before + " us before it");
+			// And still playing: the source is decoded to its end long before
+			// it plays out, and a flush that came after the seek had happened
+			// threw away all of it, so that the player ended at once.
+			assertEquals(MediaPlayerState.PLAYING, playback.player.getState());
+			assertTrue(after > 0, "position after seeking back: " + after + " us");
 		}
 	}