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The Apache Cassandra Project develops a highly scalable second-generation distributed database, bringing together Dynamo's fully distributed design and Bigtable's ColumnFamily-based data model.

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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.cassandra.net;

import java.util.Collection;
import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;

import io.netty.util.concurrent.Future;
import io.netty.util.concurrent.GenericFutureListener;
import io.netty.util.concurrent.GlobalEventExecutor;
import io.netty.util.concurrent.Promise;

/**
 * Netty's PromiseCombiner is not threadsafe, and we combine futures from multiple event executors.
 *
 * This class groups a number of Future into a single logical Future, by registering a listener to each that
 * decrements a shared counter; if any of them fail, the FutureCombiner is completed with the first cause,
 * but in all scenario only completes when all underlying future have completed (exceptionally or otherwise)
 *
 * This Future is always uncancellable.
 *
 * We extend FutureDelegate, and simply provide it an uncancellable Promise that will be completed by the listeners
 * registered to the input futures.
 */
class FutureCombiner extends FutureDelegate
{
    private volatile boolean failed;

    private volatile Throwable firstCause;
    private static final AtomicReferenceFieldUpdater firstCauseUpdater =
        AtomicReferenceFieldUpdater.newUpdater(FutureCombiner.class, Throwable.class, "firstCause");

    private volatile int waitingOn;
    private static final AtomicIntegerFieldUpdater waitingOnUpdater =
        AtomicIntegerFieldUpdater.newUpdater(FutureCombiner.class, "waitingOn");

    FutureCombiner(Collection> combine)
    {
        this(AsyncPromise.uncancellable(GlobalEventExecutor.INSTANCE), combine);
    }

    private FutureCombiner(Promise combined, Collection> combine)
    {
        super(combined);

        if (0 == (waitingOn = combine.size()))
            combined.trySuccess(null);

        GenericFutureListener> listener = result ->
        {
            if (!result.isSuccess())
            {
                firstCauseUpdater.compareAndSet(this, null, result.cause());
                failed = true;
            }

            if (0 == waitingOnUpdater.decrementAndGet(this))
            {
                if (failed)
                    combined.tryFailure(firstCause);
                else
                    combined.trySuccess(null);
            }
        };

        for (Future future : combine)
            future.addListener(listener);
    }
}