public abstract IOStreamPair execContainer(ContainerExecContext ctx)
      throws ContainerExecutionException;
@Test
  public void testExecContainer() throws Exception {
    Container container = mock(Container.class);
    try {
      ContainerExecContext.Builder builder = new ContainerExecContext.Builder();
      builder.setUser("foo").setAppId("app1").setContainer(container);
      ContainerExecContext ctx = builder.build();
      containerExecutor.execContainer(ctx);
    } catch (Exception e) {
      // socket exception should be thrown immediately, without RPC retries.
      Assert.assertTrue(e instanceof ContainerExecutionException);
    }
  }