MusicChunk: add magic value IGNORE_REPLAY_GAIN

src/MusicChunk.hxx

@@ -80,6 +80,14 @@ struct MusicChunk {
 	ReplayGainInfo replay_gain_info;
 
 	/**
+	 * A magic value for #replay_gain_serial which omits updating
+	 * the #ReplayGainFilter.  This is used by "silence" chunks
+	 * (see PlayerThread::SendSilence()) so they don't affect the
+	 * replay gain.
+	 */
+	static constexpr unsigned IGNORE_REPLAY_GAIN = ~0u;
+
+	/**
 	 * A serial number for checking if replay gain info has
 	 * changed since the last chunk.  The magic value 0 indicates
 	 * that there is no replay gain info available.

src/output/Source.cxx

@@ -142,7 +142,8 @@ AudioOutputSource::GetChunkData(const MusicChunk &chunk,
 		replay_gain_filter_set_mode(*replay_gain_filter,
 					    replay_gain_mode);
 
-		if (chunk.replay_gain_serial != *replay_gain_serial_p) {
+		if (chunk.replay_gain_serial != *replay_gain_serial_p &&
+		    chunk.replay_gain_serial != MusicChunk::IGNORE_REPLAY_GAIN) {
 			replay_gain_filter_set_info(*replay_gain_filter,
 						    chunk.replay_gain_serial != 0
 						    ? &chunk.replay_gain_info

src/player/Thread.cxx

@@ -561,7 +561,7 @@ Player::SendSilence()
 	chunk->bit_rate = 0;
 	chunk->time = SignedSongTime::Negative(); /* undefined time stamp */
 	chunk->length = num_frames * frame_size;
-	chunk->replay_gain_serial = 0;
+	chunk->replay_gain_serial = MusicChunk::IGNORE_REPLAY_GAIN;
 	PcmSilence({chunk->data, chunk->length}, play_audio_format.format);
 
 	try {