Client packets are processed on the server, so they must be

synchronized with the main thread.

ProtocolLib/src/com/comphenix/protocol/ProtocolLibrary.java

@@ -44,12 +44,11 @@ public class ProtocolLibrary extends JavaPlugin {
 	// Structure compiler
 	private BackgroundCompiler backgroundCompiler;
 	
-	// Used to (mostly) clean up packets that have expired
+	// Used to clean up server packets that have expired. 
+	// But mostly required to simulate recieving client packets.
 	private int asyncPacketTask = -1;
-	
+	private int tickCounter = 0;
-	// Number of ticks between each cleanup. We don't need to do this often,
+	private static final int ASYNC_PACKET_DELAY = 1;
-	// as it's only indeeded to detected timeouts.
-	private static final int ASYNC_PACKET_DELAY = 10;
 	
 	@Override
 	public void onLoad() {
@@ -100,7 +99,9 @@ public class ProtocolLibrary extends JavaPlugin {
 				@Override
 				public void run() {
 					AsyncFilterManager manager = (AsyncFilterManager) protocolManager.getAsynchronousManager();
-					manager.sendProcessedPackets();
+					
+					// We KNOW we're on the main thread at the moment
+					manager.sendProcessedPackets(tickCounter++, true);
 				}
 			}, ASYNC_PACKET_DELAY, ASYNC_PACKET_DELAY);
 		

ProtocolLib/src/com/comphenix/protocol/async/AsyncFilterManager.java

@@ -42,8 +42,10 @@ public class AsyncFilterManager implements AsynchronousManager {
 	private volatile boolean cleaningUp;
 	
 	public AsyncFilterManager(Logger logger, PacketStream packetStream) {
-		this.serverQueue = new PacketSendingQueue();
+		this.serverQueue = new PacketSendingQueue(false);
-		this.clientQueue = new PacketSendingQueue();
+		// Client packets must be synchronized
+		this.clientQueue = new PacketSendingQueue(true); 
+		
 		this.serverProcessingQueue = new PacketProcessingQueue(serverQueue);
 		this.clientProcessingQueue = new PacketProcessingQueue(clientQueue);
 		this.packetStream = packetStream;
@@ -88,6 +90,7 @@ public class AsyncFilterManager implements AsynchronousManager {
 	void unregisterAsyncHandlerInternal(AsyncListenerHandler handler) {
 		
 		PacketListener listener = handler.getAsyncListener();
+		boolean synchronusOK = onMainThread();
 		
 		// Just remove it from the queue(s)
 		if (hasValidWhitelist(listener.getSendingWhitelist())) {
@@ -95,16 +98,24 @@ public class AsyncFilterManager implements AsynchronousManager {
 			
 			// We're already taking care of this, so don't do anything
 			if (!cleaningUp)
-				serverQueue.signalPacketUpdate(removed);
+				serverQueue.signalPacketUpdate(removed, synchronusOK);
 		}
 		if (hasValidWhitelist(listener.getReceivingWhitelist())) {
 			List<Integer> removed = clientProcessingQueue.removeListener(handler, listener.getReceivingWhitelist());
 			
 			if (!cleaningUp)
-				clientQueue.signalPacketUpdate(removed);
+				clientQueue.signalPacketUpdate(removed, synchronusOK);
 		}
 	}
 	
+	/**
+	 * Determine if we're running on the main thread.
+	 * @return TRUE if we are, FALSE otherwise.
+	 */
+	private boolean onMainThread() {
+		return Thread.currentThread().getId() == mainThread.getId();
+	}
+	
 	@Override
 	public void unregisterAsyncHandlers(Plugin plugin) {
 		unregisterAsyncHandlers(serverProcessingQueue, plugin);
@@ -196,7 +207,7 @@ public class AsyncFilterManager implements AsynchronousManager {
 	 * @param packet - packet to signal.
 	 */
 	public void signalPacketUpdate(PacketEvent packet) {
-		getSendingQueue(packet).signalPacketUpdate(packet);
+		getSendingQueue(packet).signalPacketUpdate(packet, onMainThread());
 	}
 
 	/**
@@ -228,8 +239,13 @@ public class AsyncFilterManager implements AsynchronousManager {
 	/**
 	 * Send any due packets, or clean up packets that have expired.
 	 */
-	public void sendProcessedPackets() {
+	public void sendProcessedPackets(int tickCounter, boolean onMainThread) {
-		clientQueue.trySendPackets();
+		
-		serverQueue.trySendPackets();
+		// The server queue is unlikely to need checking that often
+		if (tickCounter % 10 == 0) {
+			serverQueue.trySendPackets(onMainThread);
+		}
+
+		clientQueue.trySendPackets(onMainThread);
 	}
 }

ProtocolLib/src/com/comphenix/protocol/async/PacketSendingQueue.java

@@ -20,8 +20,12 @@ class PacketSendingQueue {
 	
 	private PriorityBlockingQueue<PacketEvent> sendingQueue;
 	
-	public PacketSendingQueue() {
+	// Whether or not packet transmission can only occur on the main thread
-		sendingQueue = new PriorityBlockingQueue<PacketEvent>(INITIAL_CAPACITY, new Comparator<PacketEvent>() {
+	private final boolean synchronizeMain;
+	
+	public PacketSendingQueue(boolean synchronizeMain) {
+		this.synchronizeMain = synchronizeMain;
+		this.sendingQueue = new PriorityBlockingQueue<PacketEvent>(INITIAL_CAPACITY, new Comparator<PacketEvent>() {
 			// Compare using the async marker
 			@Override
 			public int compare(PacketEvent o1, PacketEvent o2) {
@@ -43,13 +47,13 @@ class PacketSendingQueue {
 	/**
 	 * Invoked when one of the packets have finished processing.
 	 */
-	public synchronized void signalPacketUpdate(PacketEvent packetUpdated) {
+	public synchronized void signalPacketUpdate(PacketEvent packetUpdated, boolean onMainThread) {
 		// Mark this packet as finished
 		packetUpdated.getAsyncMarker().setProcessed(true);
-		trySendPackets();
+		trySendPackets(onMainThread);
 	}
 
-	public synchronized void signalPacketUpdate(List<Integer> packetsRemoved) {
+	public synchronized void signalPacketUpdate(List<Integer> packetsRemoved, boolean onMainThread) {
 		
 		Set<Integer> lookup = new HashSet<Integer>(packetsRemoved);
 		
@@ -60,14 +64,19 @@ class PacketSendingQueue {
 			}
 		}
 		
-		// This is likely to have changed the situation a bit 
+		// This is likely to have changed the situation a bit
-		trySendPackets();
+		trySendPackets(onMainThread);
 	}
 	
 	/**
 	 * Attempt to send any remaining packets.
 	 */
-	public synchronized void trySendPackets() {
+	public void trySendPackets(boolean onMainThread) {
+		
+		// Abort if we're not on the main thread
+		if (synchronizeMain && !onMainThread)
+			return;
+		
 		// Transmit as many packets as we can
 		while (true) {
 			PacketEvent current = sendingQueue.peek();
@@ -92,7 +101,7 @@ class PacketSendingQueue {
 	/**
 	 * Send every packet, regardless of the processing state.
 	 */
-	public synchronized void forceSend() {
+	private void forceSend() {
 		while (true) {
 			PacketEvent current = sendingQueue.poll();
 			
@@ -104,6 +113,14 @@ class PacketSendingQueue {
 		}
 	}
 	
+	/**
+	 * Whether or not the packet transmission must synchronize with the main thread.
+	 * @return TRUE if it must, FALSE otherwise.
+	 */
+	public boolean isSynchronizeMain() {
+		return synchronizeMain;
+	}
+
 	/**
 	 * Transmit a packet, if it hasn't already.
 	 * @param event - the packet to transmit.
@@ -128,6 +145,7 @@ class PacketSendingQueue {
 	 * Automatically transmits every delayed packet.
 	 */
 	public void cleanupAll() {
+		// Note that the cleanup itself will always occur on the main thread
 		forceSend();
 	}
 }