DecoderControl: reduce the number of PlayerThread wakeups

src/DecoderControl.cxx

@@ -30,6 +30,7 @@ DecoderControl::DecoderControl(Mutex &_mutex, Cond &_client_cond)
 	:mutex(_mutex), client_cond(_client_cond),
 	 state(DecoderState::STOP),
 	 command(DecoderCommand::NONE),
+	 client_is_waiting(false),
 	 song(nullptr),
 	 replay_gain_db(0), replay_gain_prev_db(0) {}
 
@@ -41,6 +42,18 @@ DecoderControl::~DecoderControl()
 		song->Free();
 }
 
+void
+DecoderControl::WaitForDecoder()
+{
+	assert(!client_is_waiting);
+	client_is_waiting = true;
+
+	client_cond.wait(mutex);
+
+	assert(client_is_waiting);
+	client_is_waiting = false;
+}
+
 bool
 DecoderControl::IsCurrentSong(const Song &_song) const
 {

src/DecoderControl.hxx

@@ -97,6 +97,14 @@ struct DecoderControl {
 	Error error;
 
 	bool quit;
+
+	/**
+	 * Is the client currently waiting for the DecoderThread?  If
+	 * false, the DecoderThread may omit invoking Cond::signal(),
+	 * reducing the number of system calls.
+	 */
+	bool client_is_waiting;
+
 	bool seek_error;
 	bool seekable;
 	double seek_where;
@@ -193,10 +201,10 @@ struct DecoderControl {
 	 * Waits for a signal from the decoder thread.  This object
 	 * must be locked prior to calling this function.  This method
 	 * is only valid in the player thread.
+	 *
+	 * Caller must hold the lock.
 	 */
-	void WaitForDecoder() {
-		client_cond.wait(mutex);
-	}
+	void WaitForDecoder();
 
 	bool IsIdle() const {
 		return state == DecoderState::STOP ||

src/DecoderInternal.cxx

@@ -102,6 +102,7 @@ decoder_flush_chunk(Decoder &decoder)
 	decoder.chunk = nullptr;
 
 	dc.Lock();
-	dc.client_cond.signal();
+	if (dc.client_is_waiting)
+		dc.client_cond.signal();
 	dc.Unlock();
 }