cassandra-trunk/test/unit/org/apache/cassandra/utils/RecomputingSupplierTest.java
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 * 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
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 * Unless required by applicable law or agreed to in writing, software
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package org.apache.cassandra.utils;

import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.locks.LockSupport;

import org.junit.Assert;
import org.junit.Test;

import org.apache.cassandra.Util;

public class RecomputingSupplierTest
{
    // This test case verifies that recomputing supplier never returns out of order values during concurrent updates and
    // eventually returns the most recent value.
    @Test
    public void recomputingSupplierTest() throws Throwable
    {
        ThreadPoolExecutor executor = (ThreadPoolExecutor) Executors.newFixedThreadPool(10);
        ExecutorService testExecutor = Executors.newFixedThreadPool(10);
        AtomicReference<Throwable> thrown = new AtomicReference<>();
        final AtomicLong counter = new AtomicLong(0);

        final RecomputingSupplier<Long> supplier = new RecomputingSupplier<>(() -> {
            try
            {
                long v = counter.incrementAndGet();
                LockSupport.parkNanos(1);
                // Make sure that the value still hasn't changed
                Assert.assertEquals(v, counter.get());
                return v;
            }
            catch (Throwable e)
            {
                thrown.set(e);
                throw new RuntimeException(e);
            }
        }, executor);

        for (int i = 0; i < 5; i++)
        {
            testExecutor.submit(() -> {
                try
                {
                    while (!Thread.interrupted() && !testExecutor.isShutdown())
                        supplier.recompute();
                }
                catch (Throwable e)
                {
                    thrown.set(e);
                }
            });
        }

        AtomicLong lastSeen = new AtomicLong(0);
        for (int i = 0; i < 5; i++)
        {
            testExecutor.submit(() -> {
                while (!Thread.interrupted() && !testExecutor.isShutdown())
                {
                    try
                    {
                        long seenBeforeGet = lastSeen.get();
                        Long v = supplier.get(5, TimeUnit.SECONDS);
                        if (v != null)
                        {
                            lastSeen.accumulateAndGet(v, Math::max);
                            Assert.assertTrue(String.format("Seen %d out of order. Last seen value %d", v, seenBeforeGet),
                                              v >= seenBeforeGet);
                        }
                    }
                    catch (Throwable e)
                    {
                        thrown.set(e);
                    }
                }
            });
        }

        Util.spinAssertEquals(true, () -> counter.get() > 1000, 30);
        testExecutor.shutdown();
        Assert.assertTrue(testExecutor.awaitTermination(30, TimeUnit.SECONDS));

        if (thrown.get() != null)
            throw new AssertionError(supplier.toString(), thrown.get());

        Util.spinAssertEquals(true, () -> {
            try
            {
                return supplier.get(1, TimeUnit.SECONDS) == counter.get();
            }
            catch (InterruptedException | ExecutionException | TimeoutException e)
            {
                e.printStackTrace();
                return false;
            }
        }, 10);

        executor.shutdown();
        Assert.assertTrue(executor.awaitTermination(30, TimeUnit.SECONDS));

        if (thrown.get() != null)
            throw new AssertionError(supplier.toString(), thrown.get());
    }

    @Test
    public void throwingSupplier() throws InterruptedException, TimeoutException
    {
        ThreadPoolExecutor executor = (ThreadPoolExecutor) Executors.newFixedThreadPool(1);


        final RecomputingSupplier<Long> supplier = new RecomputingSupplier<>(() -> {
            throw new RuntimeException();
        }, executor);

        supplier.recompute();

        try
        {
            supplier.get(10, TimeUnit.SECONDS);
            Assert.fail("Should have thrown");
        }
        catch (ExecutionException t)
        {
            // ignore
        }
        finally
        {
            executor.shutdown();
            executor.awaitTermination(10, TimeUnit.SECONDS);
        }
    }
}
