virajjasani commented on a change in pull request #2454: URL: https://github.com/apache/hbase/pull/2454#discussion_r496959173
########## File path: hbase-server/src/main/java/org/apache/hadoop/hbase/master/normalizer/RegionNormalizerWorkQueue.java ########## @@ -0,0 +1,244 @@ +/* + * 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.hadoop.hbase.master.normalizer; + +import java.util.Collection; +import java.util.Collections; +import java.util.Iterator; +import java.util.LinkedHashSet; +import java.util.Queue; +import java.util.Set; +import java.util.concurrent.BlockingQueue; +import java.util.concurrent.locks.Condition; +import java.util.concurrent.locks.ReentrantLock; +import org.apache.yetus.audience.InterfaceAudience; + +/** + * A specialized collection that holds pending work for the {@link RegionNormalizerWorker}. It is + * an ordered collection class that has the following properties: + * <ul> + * <li>Guarantees uniqueness of elements, as a {@link Set}.</li> + * <li>Consumers retrieve objects from the head, as a {@link Queue}, via {@link #take()}.</li> + * <li>Work is retrieved on a FIFO policy.</li> + * <li>Work retrieval blocks the calling thread until new work is available, as a + * {@link BlockingQueue}.</li> + * <li>Allows a producer to insert an item at the head of the queue, if desired.</li> + * </ul> + * Assumes low-frequency and low-parallelism concurrent access, so protects state using a + * simplistic synchronization strategy. + */ [email protected] +class RegionNormalizerWorkQueue<E> { + + /** Underlying storage structure that gives us the Set behavior and FIFO retrieval policy. */ + private LinkedHashSet<E> delegate; + + // the locking structure used here follows the example found in LinkedBlockingQueue. The + // difference is that our locks guard access to `delegate` rather than the head node. + + /** Lock held by take, poll, etc */ + private final ReentrantLock takeLock; + + /** Wait queue for waiting takes */ + private final Condition notEmpty; + + /** Lock held by put, offer, etc */ + private final ReentrantLock putLock; + + public RegionNormalizerWorkQueue() { + delegate = new LinkedHashSet<>(); + takeLock = new ReentrantLock(); + notEmpty = takeLock.newCondition(); + putLock = new ReentrantLock(); + } + + /** + * Signals a waiting take. Called only from put/offer (which do not + * otherwise ordinarily lock takeLock.) + */ + private void signalNotEmpty() { + final ReentrantLock takeLock = this.takeLock; + takeLock.lock(); + try { + notEmpty.signal(); + } finally { + takeLock.unlock(); + } + } + + /** + * Locks to prevent both puts and takes. + */ + private void fullyLock() { + putLock.lock(); + takeLock.lock(); + } + + /** + * Unlocks to allow both puts and takes. + */ + private void fullyUnlock() { + takeLock.unlock(); + putLock.unlock(); + } + + /** + * Inserts the specified element at the tail of the queue, if it's not already present. + * + * @param e the element to add + */ + public void put(E e) { + if (e == null) { + throw new NullPointerException(); + } + + putLock.lock(); + try { + delegate.add(e); + } finally { + putLock.unlock(); + } + + if (!delegate.isEmpty()) { + signalNotEmpty(); + } + } + + /** + * Inserts the specified element at the head of the queue. + * + * @param e the element to add + */ + public void putFirst(E e) { + if (e == null) { + throw new NullPointerException(); + } + putAllFirst(Collections.singleton(e)); + } + + /** + * Inserts the specified elements at the tail of the queue. Any elements already present in + * the queue are ignored. + * + * @param c the elements to add + */ + public void putAll(Collection<? extends E> c) { + if (c == null) { + throw new NullPointerException(); + } + + putLock.lock(); + try { + delegate.addAll(c); + } finally { + putLock.unlock(); + } + + if (!delegate.isEmpty()) { + signalNotEmpty(); + } + } + + /** + * Inserts the specified elements at the head of the queue. + * + * @param c the elements to add + */ + public void putAllFirst(Collection<? extends E> c) { + if (c == null) { + throw new NullPointerException(); + } + + fullyLock(); + try { + final LinkedHashSet<E> copy = new LinkedHashSet<>(c.size() + delegate.size()); + copy.addAll(c); + copy.addAll(delegate); + delegate = copy; + } finally { + fullyUnlock(); + } + + if (!delegate.isEmpty()) { + signalNotEmpty(); + } + } + + /** + * Retrieves and removes the head of this queue, waiting if necessary + * until an element becomes available. + * + * @return the head of this queue + * @throws InterruptedException if interrupted while waiting + */ + public E take() throws InterruptedException { + E x; + takeLock.lockInterruptibly(); + try { + while (delegate.isEmpty()) { + notEmpty.await(); + } + final Iterator<E> iter = delegate.iterator(); + x = iter.next(); + iter.remove(); Review comment: Agree, only `putLock` should be enough for `clear`. ---------------------------------------------------------------- This is an automated message from the Apache Git Service. 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