ClassCastException in test TestRPC#testWrappedStopProxy

What happened:

In HCommon, test TestRPC#testWrappedStopProxy tries to cast org.apache.hadoop.ipc.WritableRpcEngine$Invoker to org.apache.hadoop.ipc.TestRPC$StoppedInvocationHandler and causes ClassCastException.

Buggy code:

In TestRPC.java:

@Test
public void testWrappedStopProxy() throws IOException {
  StoppedProtocol wrappedProxy = RPC.getProxy(StoppedProtocol.class,
      StoppedProtocol.versionID, null, conf);
  StoppedInvocationHandler invocationHandler = (StoppedInvocationHandler)    // <--- Here causes ClassCastException
      Proxy.getInvocationHandler(wrappedProxy);
  ...
} 

How to reproduce:

(1) Just directly run test TestRPC#testWrappedStopProxy.

You can use the reproduce.sh in the attachment to easily reproduce the bug.

We are happy to provide a patch if this issue is confirmed.