player/Thread: move mutex lock out of ActivateDecoder()

src/player/Thread.cxx

@@ -299,7 +299,7 @@ private:
 	 * yet, therefore we don't know the audio format yet.  To
 	 * finish decoder startup, call CheckDecoderStartup().
 	 *
-	 * The player lock is not held.
+	 * Caller must lock the mutex.
 	 */
 	void ActivateDecoder() noexcept;
 
@@ -419,9 +419,6 @@ Player::ActivateDecoder() noexcept
 
 	queued = false;
 
-	{
-		const std::lock_guard<Mutex> lock(pc.mutex);
-
 	pc.ClearTaggedSong();
 
 	song = std::exchange(pc.next_song, nullptr);
@@ -436,11 +433,13 @@ Player::ActivateDecoder() noexcept
 	pc.total_time = song->GetDuration();
 	pc.bit_rate = 0;
 	pc.audio_format.Clear();
-	}
 
+	{
 		/* call syncPlaylistWithQueue() in the main thread */
+		const ScopeUnlock unlock(pc.mutex);
 		pc.listener.OnPlayerSync();
 	}
+}
 
 /**
  * Returns the real duration of the song, comprising the duration
@@ -634,9 +633,9 @@ Player::SeekDecoder() noexcept
 
 		/* re-start the decoder */
 		StartDecoder(*pipe);
+		const std::lock_guard<Mutex> lock(pc.mutex);
 		ActivateDecoder();
 
-		const std::lock_guard<Mutex> lock(pc.mutex);
 		if (!WaitDecoderStartup())
 			return false;
 	} else {
@@ -966,7 +965,10 @@ Player::SongBorder() noexcept
 
 	pc.outputs.SongBorder();
 
+	{
+		const std::lock_guard<Mutex> lock(pc.mutex);
 		ActivateDecoder();
+	}
 
 	const bool border_pause = pc.LockApplyBorderPause();
 	if (border_pause) {
@@ -981,9 +983,9 @@ Player::Run() noexcept
 	pipe = new MusicPipe();
 
 	StartDecoder(*pipe);
+	pc.Lock();
 	ActivateDecoder();
 
-	pc.Lock();
 	pc.state = PlayerState::PLAY;
 
 	pc.CommandFinished();