player/Control: add "occupied" flag to skip REFRESH

Reduces main thread contention.  Avoids blocking the main thread in
"status" commands.

src/player/Control.cxx

@@ -44,6 +44,7 @@ PlayerControl::PlayerControl(PlayerListener &_listener,
 
 PlayerControl::~PlayerControl() noexcept
 {
+	assert(!occupied);
 }
 
 bool
@@ -155,7 +156,8 @@ PlayerControl::LockGetStatus() noexcept
 	player_status status;
 
 	const std::lock_guard<Mutex> protect(mutex);
-	SynchronousCommand(PlayerCommand::REFRESH);
+	if (!occupied)
+		SynchronousCommand(PlayerCommand::REFRESH);
 
 	status.state = state;
 

src/player/Control.hxx

@@ -185,6 +185,29 @@ struct PlayerControl final : AudioOutputClient {
 	 */
 	bool border_pause = false;
 
+	/**
+	 * If this flag is set, then the player thread is currently
+	 * occupied and will not be able to respond quickly to
+	 * commands (e.g. waiting for the decoder thread to finish
+	 * seeking).  This is used to skip #PlayerCommand::REFRESH to
+	 * avoid blocking the main thread.
+	 */
+	bool occupied = false;
+
+	struct ScopeOccupied {
+		PlayerControl &pc;
+
+		explicit ScopeOccupied(PlayerControl &_pc) noexcept:pc(_pc) {
+			assert(!pc.occupied);
+			pc.occupied = true;
+		}
+
+		~ScopeOccupied() noexcept {
+			assert(pc.occupied);
+			pc.occupied = false;
+		}
+	};
+
 	AudioFormat audio_format;
 	uint16_t bit_rate;
 

src/player/Thread.cxx

@@ -367,6 +367,8 @@ Player::StartDecoder(MusicPipe &_pipe) noexcept
 void
 Player::StopDecoder() noexcept
 {
+	const PlayerControl::ScopeOccupied occupied(pc);
+
 	dc.Stop();
 
 	if (dc.pipe != nullptr) {
@@ -624,6 +626,8 @@ Player::SeekDecoder() noexcept
 		}
 
 		try {
+			const PlayerControl::ScopeOccupied occupied(pc);
+
 			dc.Seek(where + start_time);
 		} catch (...) {
 			/* decoder failure */