PresentsConnection does not check that its handler is non-null before

processing messages, so let's put a (logging) NOOP handler in place when we
disconnect it from the PresentsSession.

git-svn-id: svn+ssh://src.earth.threerings.net/narya/trunk@6629 542714f4-19e9-0310-aa3c-eee0fc999fb1

src/main/java/com/threerings/presents/server/PresentsSession.java

@@ -845,15 +845,15 @@ public class PresentsSession
             }
 
             // make damn sure we don't get any more messages from the old connection
-            _conn.setMessageHandler(null);
+            _conn.clearMessageHandler();
         }
 
         // keep a handle to the new connection
         _conn = conn;
 
-        // tell the connection to pass messages on to us (if we're setting a connection rather than
-        // clearing one out)
+        // if we're setting a connection rather than clearing one out...
         if (_conn != null) {
+            // tell the connection to pass messages on to us
             _conn.setMessageHandler(this);
 
             // configure any active custom class loader

src/main/java/com/threerings/presents/server/net/PresentsConnection.java

@@ -31,6 +31,8 @@ import java.nio.channels.SocketChannel;
 import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;
 
+import com.google.common.base.Preconditions;
+
 import com.threerings.io.FramedInputStream;
 import com.threerings.io.FramingOutputStream;
 import com.threerings.io.ObjectInputStream;
@@ -74,7 +76,20 @@ public class PresentsConnection extends Connection
      */
     public void setMessageHandler (MessageHandler handler)
     {
-        _handler = handler;
+        _handler = Preconditions.checkNotNull(handler);
+    }
+
+    /**
+     * Clears out our message handler, causing any subsequent messages to be dropped on arrival. A
+     * log message is recorded for the dropped messages.
+     */
+    public void clearMessageHandler ()
+    {
+        _handler = new MessageHandler() {
+            public void handleMessage (Message message) {
+                log.info("Dropping message from cleared connection", "conn", this, "msg", message);
+            }
+        };
     }
 
     /**