private void closeCache(VM vm) {
    vm.invoke(new SerializableCallable() {
      @Override
      public Object call() throws Exception {
        closeCache();
        return null;
      }
    });
  }
Used for saving & restoring oldObserver without serialization
geode-core/src/distributedTest/java/org/apache/geode/cache/query/dunit/SelectStarQueryDUnitTest.java