We don't expect consumers to create asynchronous markers, so

don't allow them to set it in the packet event.

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

@@ -24,6 +24,7 @@ import org.bukkit.entity.Entity;
 import org.bukkit.entity.Player;
 import org.bukkit.plugin.Plugin;
 
+import com.comphenix.protocol.async.AsyncFilterManager;
 import com.comphenix.protocol.events.PacketContainer;
 import com.comphenix.protocol.events.PacketListener;
 import com.comphenix.protocol.injector.PacketConstructor;
@@ -122,4 +123,10 @@ public interface ProtocolManager extends PacketStream {
 	 * @return TRUE if it has, FALSE otherwise.
 	 */
 	public boolean isClosed();
+
+	/**
+	 * Retrieve the current asyncronous packet manager.
+	 * @return Asyncronous packet manager.
+	 */
+	public AsyncFilterManager getAsyncFilterManager();
 }

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

@@ -79,6 +79,7 @@ class PacketProcessingQueue extends AbstractConcurrentListenerMultimap<AsyncList
 				Collection<PrioritizedListener<AsyncListenerHandler>> list = getListener(packet.getPacketID());
 				AsyncMarker marker = packet.getAsyncMarker();
 				
+				// Yes, removing the marker will cause the chain to stop
 				if (list != null) {
 					Iterator<PrioritizedListener<AsyncListenerHandler>> iterator = list.iterator();
 					
@@ -89,7 +90,7 @@ class PacketProcessingQueue extends AbstractConcurrentListenerMultimap<AsyncList
 					}
 				}
 				
-				// The packet has no listeners. Just send it.
+				// The packet has no further listeners. Just send it.
 				sendingQueue.signalPacketUpdate(packet);
 				signalProcessingDone();
 				

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

@@ -42,7 +42,7 @@ class PacketSendingQueue {
 				AsyncMarker marker = current.getAsyncMarker();
 				
 				if (marker.isProcessed() || marker.hasExpired()) {
-					if (marker.isProcessed())
+					if (marker.isProcessed() && !current.isCancelled())
 						sendPacket(current);
 					
 					sendingQueue.poll();

ProtocolLib/src/com/comphenix/protocol/events/PacketEvent.java

@@ -60,6 +60,7 @@ public class PacketEvent extends EventObject implements Cancellable {
 		super(origial.source);
 		this.packet = origial.packet;
 		this.player = origial.player;
+		this.cancel = origial.cancel;
 		this.serverPacket = origial.serverPacket;
 		this.asyncMarker = asyncMarker;
 		this.asynchronous = true;
@@ -156,7 +157,7 @@ public class PacketEvent extends EventObject implements Cancellable {
 	/**
 	 * Retrieve the asynchronous marker.
 	 * <p>
-	 * If the packet is synchronous, this marker will be used to schedule an asynchronous event. In this
+	 * If the packet is synchronous, this marker will be used to schedule an asynchronous event. In the following
 	 * asynchronous event, the marker is used to correctly pass the packet around to the different threads.
 	 * <p>
 	 * Note that if there are no asynchronous events that can receive this packet, the marker is NULL.
@@ -166,19 +167,6 @@ public class PacketEvent extends EventObject implements Cancellable {
 		return asyncMarker;
 	}
 
-	/**
-	 * Set the asynchronous marker. 
-	 * <p>
-	 * If the marker is non-null at the end of an synchronous event processing, the packet will be scheduled
-	 * to be processed asynchronously with the given settings.
-	 * <p>
-	 * Note that if there are no asynchronous events that can receive this packet, the marker should be NULL. 
-	 * @param asyncMarker - the new asynchronous marker, or NULL.
-	 */
-	public void setAsyncMarker(AsyncMarker asyncMarker) {
-		this.asyncMarker = asyncMarker;
-	}
-
 	/**
 	 * Determine if the packet event has been executed asynchronously or not.
 	 * @return TRUE if this packet event is asynchronous, FALSE otherwise.

ProtocolLib/src/com/comphenix/protocol/injector/PacketFilterManager.java

@@ -137,10 +137,7 @@ public final class PacketFilterManager implements ProtocolManager {
 		}
 	}
 	
-	/**
+	@Override
-	 * Retrieve the current async packet filter manager.
-	 * @return Async filter manager.
-	 */
 	public AsyncFilterManager getAsyncFilterManager() {
 		return asyncFilterManager;
 	}
@@ -313,9 +310,11 @@ public final class PacketFilterManager implements ProtocolManager {
 		// Process synchronous events
 		packetListeners.invokePacketRecieving(logger, event);
 		
-		// To cancel the asynchronous processing, use the async marker
+		// To cancel asynchronous processing, use the async marker
 		if (!event.isCancelled() && !hasAsyncCancelled(event.getAsyncMarker())) {
 			asyncFilterManager.enqueueSyncPacket(event, event.getAsyncMarker());
+
+			// The above makes a copy of the event, so it's safe to cancel it
 			event.setCancelled(true);
 		}
 	}