exec/rpc/src/main/java/org/apache/drill/exec/rpc/ResettableBarrier.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.drill.exec.rpc;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;

/**
 * Modified implementation of countdown latch that allows a barrier to be unilaterally opened and closed.  All others simply wait when it is closed.  Is initialized in a closed state.
 */
public class ResettableBarrier {
  static final org.slf4j.Logger logger = org.slf4j.LoggerFactory.getLogger(ResettableBarrier.class);

  private final InternalSynchronizer sync = new InternalSynchronizer();

  public ResettableBarrier() {
  }

  private static final class InternalSynchronizer extends AbstractQueuedSynchronizer {

    private InternalSynchronizer() {
      setState(1);
    }

    @Override
    protected int tryAcquireShared(int acquires) {
      assert acquires == 1;
      return (getState() == 0) ? 1 : -1;
    }

    @Override
    protected boolean tryReleaseShared(int releases) {
      assert releases == 1;

      while(true) {
        int c = getState();
        if (c == 0) {
          return false;
        }
        int nextc = c - 1;
        if (compareAndSetState(c, nextc)) {
          return nextc == 0;
        }
      }
    }

    protected void reset() {
      setState(1);
    }

  }

  public void await() throws InterruptedException {
//    logger.debug("awaiting barrier interruptibly.");
    sync.acquireSharedInterruptibly(1);
  }

  public boolean await(long timeout, TimeUnit unit) throws InterruptedException {
//    logger.debug("awaiting barrier with timeout {}.", timeout);
    return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
  }

  public void openBarrier() {
//    logger.debug("opening barrier.");
    sync.releaseShared(1);
  }

  public void closeBarrier() {
//    logger.debug("closing barrier.");
    sync.reset();
  }

}