Asynchronous client packets are handled twice. FIXES 118

This causes wierd artifacts such as double placing of half-slabs and so
on, as Minecraft will process the packets again (except chat packets). 
We correct this by uncancelling the asynchronous packet, and then 
cancelling it again.

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

@@ -669,9 +669,12 @@ public final class PacketFilterManager implements ProtocolManager, ListenerInvok
 			incrementPhases(GamePhase.PLAYING);
 		
 		Object mcPacket = packet.getHandle();
+		boolean cancelled = packetInjector.isCancelled(mcPacket);
 		
 		// Make sure the packet isn't cancelled
-		packetInjector.undoCancel(packet.getID(), mcPacket);
+		if (cancelled) {
+			packetInjector.setCancelled(mcPacket, false);
+		}
 		
 		if (filters) {
 			byte[] data = NetworkMarker.getByteBuffer(marker);
@@ -691,6 +694,11 @@ public final class PacketFilterManager implements ProtocolManager, ListenerInvok
 		}
 		
 		playerInjection.recieveClientPacket(sender, mcPacket);
+		
+		// Let it stay cancelled
+		if (cancelled) {
+			packetInjector.setCancelled(mcPacket, true);
+		}
 	}
 	
 	@Override

ProtocolLib/src/main/java/com/comphenix/protocol/injector/packet/PacketInjector.java

@@ -14,12 +14,19 @@ import com.comphenix.protocol.events.PacketEvent;
  */
 public interface PacketInjector {
 	/**
-	 * Undo a packet cancel.
-	 * @param id - the id of the packet.
-	 * @param packet - packet to uncancel.
+	 * Determine if a packet is cancelled or not.
+	 * @param packet - the packet to check.
+	 * @return TRUE if it is, FALSE otherwise.
 	 */
-	public abstract void undoCancel(Integer id, Object packet);
-
+	public abstract boolean isCancelled(Object packet);
+	
+	/**
+	 * Set whether or not a packet is cancelled.
+	 * @param packet - the packet to set.
+	 * @param cancelled - TRUE to cancel the packet, FALSE otherwise.
+	 */
+	public abstract void setCancelled(Object packet, boolean cancelled);
+	
 	/**
 	 * Start intercepting packets with the given packet ID.
 	 * @param packetID - the ID of the packets to start intercepting.

ProtocolLib/src/main/java/com/comphenix/protocol/injector/packet/ProxyPacketInjector.java

@@ -176,15 +176,16 @@ class ProxyPacketInjector implements PacketInjector {
 		initialize();
 	}
 	
-	/**
-	 * Undo a packet cancel.
-	 * @param id - the id of the packet.
-	 * @param packet - packet to uncancel.
-	 */
 	@Override
-	public void undoCancel(Integer id, Object packet) {
-		// See if this packet has been cancelled before
-		if (ReadPacketModifier.hasCancelled(packet)) {
+	public boolean isCancelled(Object packet) {
+		return ReadPacketModifier.isCancelled(packet);
+	}
+	
+	@Override
+	public void setCancelled(Object packet, boolean cancelled) {
+		if (cancelled) {
+			ReadPacketModifier.setOverride(packet, null);
+		} else {
 			ReadPacketModifier.removeOverride(packet);
 		}
 	}

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

@@ -74,16 +74,27 @@ class ReadPacketModifier implements MethodInterceptor {
 	public static Object getOverride(Object packet) {
 		return override.get(packet);
 	}
+	
+	/**
+	 * Set the packet instance to delegate to instead, or mark the packet as cancelled.
+	 * <p>
+	 * To undo a override, use {@link #removeOverride(Object)}.
+	 * @param packet - the packet.
+	 * @param override - the override method. NULL to cancel this packet.
+	 */
+	public static void setOverride(Object packet, Object overridePacket) {
+		override.put(packet, overridePacket != null ? overridePacket : CANCEL_MARKER);
+	}
 
 	/**
 	 * Determine if the given packet has been cancelled before.
 	 * @param packet - the packet to check.
 	 * @return TRUE if it has been cancelled, FALSE otherwise.
 	 */
-	public static boolean hasCancelled(Object packet) {
+	public static boolean isCancelled(Object packet) {
 		return getOverride(packet) == CANCEL_MARKER;
 	}
-
+	
 	@Override
 	public Object intercept(Object thisObj, Method method, Object[] args, MethodProxy proxy) throws Throwable {
 		// Atomic retrieval
@@ -105,7 +116,7 @@ class ReadPacketModifier implements MethodInterceptor {
 		
 		if (overridenObject != null) {
 			// This packet has been cancelled
-			if (overridenObject == CANCEL_MARKER) {
+			if (overridenObject == CANCEL_MARKER) {				
 				// So, cancel all void methods
 				if (method.getReturnType().equals(Void.TYPE))
 					return null;

ProtocolLib/src/main/java/com/comphenix/protocol/injector/spigot/DummyPacketInjector.java

@@ -28,8 +28,14 @@ class DummyPacketInjector implements PacketInjector {
 	}
 
 	@Override
-	public void undoCancel(Integer id, Object packet) {
-		// Do nothing yet
+	public boolean isCancelled(Object packet) {
+		// No, it's never cancelled
+		return false;
+	}
+	
+	@Override
+	public void setCancelled(Object packet, boolean cancelled) {
+		throw new UnsupportedOperationException();
 	}
 	
 	@Override

ProtocolLib/src/main/java/com/comphenix/protocol/injector/spigot/SpigotPacketInjector.java

@@ -38,6 +38,7 @@ import com.comphenix.protocol.reflect.FuzzyReflection;
 import com.comphenix.protocol.reflect.MethodInfo;
 import com.comphenix.protocol.reflect.fuzzy.FuzzyMethodContract;
 import com.comphenix.protocol.utility.MinecraftReflection;
+
 import com.google.common.collect.MapMaker;
 import com.google.common.collect.Maps;