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Apache Solr Core
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/*
 * 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.solr.util;

import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Executor;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.RejectedExecutionException;
import java.util.concurrent.Semaphore;

import org.apache.solr.common.util.ExecutorUtil;

public class OrderedExecutor implements Executor {
  private final ExecutorService delegate;
  private final SparseStripedLock sparseStripedLock;

  public OrderedExecutor(int numThreads, ExecutorService delegate) {
    this.delegate = delegate;
    this.sparseStripedLock = new SparseStripedLock<>(numThreads);
  }

  @Override
  public void execute(Runnable runnable) {
    execute(null, runnable);
  }

  /**
   * Execute the given command in the future.
   * If another command with same {@code lockId} is waiting in the queue or running,
   * this method will block until that command finish.
   * Therefore different commands with same {@code hash} will be executed in order of calling this method.
   *
   * If multiple caller are waiting for a command to finish, there are no guarantee that the earliest call will win.
   *
   * @param lockId of the {@code command}, if null then a random hash will be generated
   * @param command the runnable task
   *
   * @throws RejectedExecutionException if this task cannot be accepted for execution
   */
  public void execute(Integer lockId, Runnable command) {
    try {
      sparseStripedLock.add(lockId);
    } catch (InterruptedException e) {
      Thread.currentThread().interrupt();
      return;
    }

    try {
      if (delegate.isShutdown()) throw new RejectedExecutionException();

      delegate.execute(() -> {
        try {
          command.run();
        } finally {
          sparseStripedLock.remove(lockId);
        }
      });
    } catch (RejectedExecutionException e) {
      sparseStripedLock.remove(lockId);
      throw e;
    }
  }

  public void shutdownAndAwaitTermination() {
    ExecutorUtil.shutdownAndAwaitTermination(delegate);
  }

  /** A set of locks by a key {@code T}, kind of like Google Striped but the keys are sparse/lazy. */
  private static class SparseStripedLock {
    private ConcurrentHashMap map = new ConcurrentHashMap<>();
    private final Semaphore sizeSemaphore;

    SparseStripedLock(int maxSize) {
      this.sizeSemaphore = new Semaphore(maxSize);
    }

    public void add(T t) throws InterruptedException {
      if (t != null) {
        CountDownLatch myLock = new CountDownLatch(1);
        CountDownLatch existingLock = map.putIfAbsent(t, myLock);
        while (existingLock != null) {
          // wait for existing lock/permit to become available (see remove() below)
          existingLock.await();
          existingLock = map.putIfAbsent(t, myLock);
        }
        // myLock was successfully inserted
      }
      // won the lock
      sizeSemaphore.acquire();
    }

    public void remove(T t) {
      if (t != null) {
        // remove and signal to any "await"-ers
        map.remove(t).countDown();
      }
      sizeSemaphore.release();
    }
  }
}