Another diffcult to track down bug.

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

@@ -3,11 +3,17 @@ package com.comphenix.protocol.async;
 import java.io.IOException;
 import java.io.Serializable;
 import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.Method;
 import java.util.Iterator;
+import java.util.List;
+
+import net.minecraft.server.Packet;
 
 import com.comphenix.protocol.PacketStream;
 import com.comphenix.protocol.events.PacketEvent;
 import com.comphenix.protocol.injector.PrioritizedListener;
+import com.comphenix.protocol.reflect.FieldAccessException;
+import com.comphenix.protocol.reflect.FuzzyReflection;
 import com.google.common.primitives.Longs;
 
 /**
@@ -58,6 +64,10 @@ public class AsyncMarker implements Serializable, Comparable<AsyncMarker> {
 	
 	// Whether or not the asynchronous processing itself should be cancelled
 	private volatile boolean asyncCancelled;
+	
+	// Determine if Minecraft processes this packet asynchronously
+	private static Method isMinecraftAsync;
+	private static boolean alwaysSync;
 
 	/**
 	 * Create a container for asyncronous packets.
@@ -240,6 +250,49 @@ public class AsyncMarker implements Serializable, Comparable<AsyncMarker> {
 		}
 	}
 	
+	/**
+	 * Determine if Minecraft allows asynchronous processing of this packet.
+	 * @return TRUE if it does, FALSE otherwise.
+	 */
+	public boolean isMinecraftAsync(PacketEvent event) throws FieldAccessException {
+		
+		if (isMinecraftAsync == null) {
+			try {
+				isMinecraftAsync = FuzzyReflection.fromClass(Packet.class).getMethodByName("a_.*");
+			} catch (RuntimeException e) {
+				// This will occur in 1.2.5 (or possibly in later versions)
+				List<Method> methods = FuzzyReflection.fromClass(Packet.class).
+										getMethodListByParameters(boolean.class, new Class[] {});
+				
+				// Try to look for boolean methods
+				if (methods.size() == 2) {
+					isMinecraftAsync = methods.get(1);
+				} else if (methods.size() == 1) {
+					// We're in 1.2.5
+					alwaysSync = true;
+				} else {
+					System.err.println("Cannot determine asynchronous state of packets!");
+					alwaysSync = true;
+				}
+			}
+		}
+		
+		if (alwaysSync) {
+			return false;
+		} else {
+			try {
+				// Wrap exceptions 
+				return (Boolean) isMinecraftAsync.invoke(event.getPacket().getHandle());
+			} catch (IllegalArgumentException e) {
+				throw new FieldAccessException("Illegal argument", e);
+			} catch (IllegalAccessException e) {
+				throw new FieldAccessException("Unable to reflect method call 'a_', or: isAsyncPacket.", e);
+			} catch (InvocationTargetException e) {
+				throw new FieldAccessException("Minecraft error", e);
+			}
+		}
+	}
+	
 	@Override
 	public int compareTo(AsyncMarker o) {
 		if (o == null)

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

@@ -8,6 +8,7 @@ import java.util.Set;
 import java.util.concurrent.PriorityBlockingQueue;
 
 import com.comphenix.protocol.events.PacketEvent;
+import com.comphenix.protocol.reflect.FieldAccessException;
 import com.google.common.collect.ComparisonChain;
 
 /**
@@ -84,18 +85,30 @@ class PacketSendingQueue {
 	 * @param onMainThread - whether or not this is occuring on the main thread.
 	 */
 	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();
-			
+					
 			if (current != null) {
 				AsyncMarker marker = current.getAsyncMarker();
 				
+				// Abort if we're not on the main thread
+				if (synchronizeMain) {
+					try {
+						boolean wantAsync = marker.isMinecraftAsync(current);
+						boolean wantSync = !wantAsync;
+						
+						// Quit if we haven't fulfilled our promise
+						if ((onMainThread && wantAsync) || (!onMainThread && wantSync))
+							return;
+						
+					} catch (FieldAccessException e) {
+						e.printStackTrace();
+						return;
+					}
+				}
+				
 				if (marker.isProcessed() || marker.hasExpired()) {
 					if (marker.isProcessed() && !current.isCancelled()) {
 						sendPacket(current);

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

@@ -77,9 +77,14 @@ class ReadPacketModifier implements MethodInterceptor {
 		if (override.containsKey(thisObj)) {
 			Object overridenObject = override.get(thisObj);
 			
-			// Cancel EVERYTHING, including "processPacket"
-			if (overridenObject == null)
-				return null;
+			// This packet has been cancelled
+			if (overridenObject == null) {
+				// So, cancel all void methods
+				if (method.getReturnType().equals(Void.TYPE))
+					return null;
+				else // Revert to normal for everything else
+					overridenObject = thisObj;
+			}
 			
 			returnValue = proxy.invokeSuper(overridenObject, args);
 		} else {

ProtocolLib/src/com/comphenix/protocol/reflect/FuzzyReflection.java

@@ -19,8 +19,10 @@ package com.comphenix.protocol.reflect;
 
 import java.lang.reflect.Field;
 import java.lang.reflect.Method;
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.LinkedHashSet;
+import java.util.List;
 import java.util.Set;
 import java.util.regex.Pattern;
 
@@ -96,6 +98,7 @@ public class FuzzyReflection {
 	 * Retrieves a method by looking at its name.
 	 * @param nameRegex -  regular expression that will match method names.
 	 * @return The first method that satisfies the regular expression.
+	 * @throws RuntimeException If the method cannot be found.
 	 */
 	public Method getMethodByName(String nameRegex) {
 		
@@ -151,6 +154,26 @@ public class FuzzyReflection {
 		throw new RuntimeException("Unable to find " + name + " in " + source.getName());
 	}
 	
+	/**
+	 * Retrieves every method that has the given parameter types and return type.
+	 * @param returnType - return type of the method to find.
+	 * @param args - parameter types of the method to find.
+	 * @return Every method that satisfies the given constraints.
+	 */
+	public List<Method> getMethodListByParameters(Class<?> returnType, Class<?>[] args) {
+	
+		List<Method> methods = new ArrayList<Method>();
+		
+		// Find the correct method to call
+		for (Method method : getMethods()) {
+			if (method.getReturnType().equals(returnType) && Arrays.equals(method.getParameterTypes(), args)) {
+				methods.add(method);
+			}
+		}
+		
+		return methods;
+	}
+	
 	/**
 	 * Retrieves a field by name.
 	 * @param nameRegex - regular expression that will match a field name.