[POOL-315] GenericObjectPool close() does not wait for the current eviction task

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 2.4.3
• Assignee: Unassigned
• Reporter: Buğra Gedik
• Created: 18/Oct/16 13:03
• Updated: 29/Oct/17 16:07
• Resolved: 02/Nov/16 20:54

Description

The close method is implemented as follows:

public void close() {
        if(!this.isClosed()) {
            Object var1 = this.closeLock;
            synchronized(this.closeLock) {
                if(!this.isClosed()) {
                    this.startEvictor(-1L);
                    this.closed = true;
                    this.clear();
                    this.jmxUnregister();
                    this.idleObjects.interuptTakeWaiters();
                }
            }
        }
    }

The line this.startEvictor(-1L); calls EvictionTimer.cancel(this.evictor); from BaseGenericObjectPool and that in turn calls the following method in EvictionTimer:

    static synchronized void cancel(TimerTask task) {
        task.cancel();
        --_usageCount;
        if(_usageCount == 0) {
            _timer.cancel();
            _timer = null;
        }
    }

Here, _timer is a java.util.TimerTask. If you look at the documentation of it, you'll see that it does not block on the currently executing task. Even though task.cancel() is called, there is no code to wait for it to complete. The end result is that, even if you close the pool, there may still be an eviction thread running. Yes, it will eventually go away, but in some rare cases it takes a bit of time to go away.

When running code under Tomcat, this results in the "commons-pool-EvictionTimer" thread (created in the class EvictionTimer) to be reported as leaking, despite the pool being closed. This happens rarely, since most of the time the timer thread goes away before Tomcat checks for leaking threads.

In my opinion a fix can be put into startEvictor in BaseGenericObjectPool:

    final void startEvictor(long delay) {
        synchronized (evictionLock) {
            if (null != evictor) {
                EvictionTimer.cancel(evictor);
                // HERE: evictor.waitForCompletion();
                evictor = null;
                evictionIterator = null;
            }
            if (delay > 0) {
                evictor = new Evictor();
                EvictionTimer.schedule(evictor, delay, delay);
            }
        }
    }

Here is an example that illustrates the problem:

import static org.junit.Assert.assertFalse;

import org.apache.commons.pool2.PooledObject;
import org.apache.commons.pool2.PooledObjectFactory;
import org.apache.commons.pool2.impl.DefaultPooledObject;
import org.apache.commons.pool2.impl.GenericObjectPool;
import org.apache.commons.pool2.impl.GenericObjectPoolConfig;
import org.junit.Test;

public class PoolTest
{
    private static final CharSequence COMMONS_POOL_EVICTIONS_TIMER_THREAD_NAME =
        "commons-pool-EvictionTimer";
    private static final long EVICTION_PERIOD_IN_MILLIS = 100;

    private static class Foo
    {
    }

    private static class PooledFooFactory implements PooledObjectFactory<Foo>
    {
        private static final long VALIDATION_WAIT_IN_MILLIS = 1000;

        @Override
        public PooledObject<Foo> makeObject() throws Exception
        {
            return new DefaultPooledObject<>(new Foo());
        }

        @Override
        public void destroyObject(
            PooledObject<Foo> pooledObject) throws Exception
        {
        }

        @Override
        public boolean validateObject(
            PooledObject<Foo> pooledObject)
        {
            try
            {
                Thread.sleep(VALIDATION_WAIT_IN_MILLIS);
            }
            catch (final InterruptedException e)
            {
                Thread.interrupted();
            }
            return false;
        }

        @Override
        public void activateObject(
            PooledObject<Foo> pooledObject) throws Exception
        {
        }

        @Override
        public void passivateObject(
            PooledObject<Foo> pooledObject) throws Exception
        {
        }
    }

    @Test
    public void testPool() throws Exception
    {
        final GenericObjectPoolConfig poolConfig =
            new GenericObjectPoolConfig();
        poolConfig.setTestWhileIdle(true /* testWhileIdle */);
        final PooledFooFactory pooledFooFactory = new PooledFooFactory();
        GenericObjectPool<Foo> pool = null;
        try
        {
            pool = new GenericObjectPool<>(pooledFooFactory, poolConfig);
            pool.setTimeBetweenEvictionRunsMillis(EVICTION_PERIOD_IN_MILLIS);
            pool.addObject();
            try
            {
                Thread.sleep(EVICTION_PERIOD_IN_MILLIS);
            }
            catch (final InterruptedException e)
            {
               Thread.interrupted();
            }
        }
        finally
        {
            if (pool != null)
            {
                pool.close();
            }
        }
        final Thread[] threads = new Thread[Thread.activeCount()];
        Thread.enumerate(threads);
        for (final Thread thread : threads)
        {
            if (thread == null)
            {
                continue;
            }
            assertFalse(thread.getName()
                .contains(COMMONS_POOL_EVICTIONS_TIMER_THREAD_NAME));
        }
    }
}