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/*
* Copyright (C) 2017 The Android Open Source Project
*
* Licensed 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 com.android.dialer.common.concurrent;
import static com.google.common.util.concurrent.Futures.immediateCancelledFuture;
import static com.google.common.util.concurrent.Futures.immediateFuture;
import static com.google.common.util.concurrent.MoreExecutors.directExecutor;
import com.google.common.util.concurrent.AsyncCallable;
import com.google.common.util.concurrent.Futures;
import com.google.common.util.concurrent.ListenableFuture;
import com.google.common.util.concurrent.SettableFuture;
import java.util.concurrent.Callable;
import java.util.concurrent.Executor;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicReference;
/**
* Serializes execution of a set of operations. This class guarantees that a submitted callable will
* not be called before previously submitted callables have completed.
*/
public final class DialerFutureSerializer {
/** This reference acts as a pointer tracking the head of a linked list of ListenableFutures. */
private final AtomicReference<ListenableFuture<?>> ref =
new AtomicReference<>(immediateFuture(null));
/** Enqueues a task to run when the previous task (if any) completes. */
public <T> ListenableFuture<T> submit(final Callable<T> callable, Executor executor) {
return submitAsync(() -> immediateFuture(callable.call()), executor);
}
/**
* Enqueues a task to run when the previous task (if any) completes.
*
* <p>Cancellation does not propagate from the output future to the future returned from {@code
* callable}, but if the output future is cancelled before {@link AsyncCallable#call()} is
* invoked, {@link AsyncCallable#call()} will not be invoked.
*/
public <T> ListenableFuture<T> submitAsync(final AsyncCallable<T> callable, Executor executor) {
AtomicBoolean wasCancelled = new AtomicBoolean(false);
final AsyncCallable<T> task =
() -> {
if (wasCancelled.get()) {
return immediateCancelledFuture();
}
return callable.call();
};
/*
* Three futures are at play here:
* taskFuture is the future that comes from the callable.
* newFuture is the future we use to track the serialization of our task.
* oldFuture is the previous task's newFuture.
*
* newFuture is guaranteed to only complete once all tasks previously submitted to this instance
* once the futures returned from those submissions have completed.
*/
final SettableFuture<Object> newFuture = SettableFuture.create();
final ListenableFuture<?> oldFuture = ref.getAndSet(newFuture);
// Invoke our task once the previous future completes.
final ListenableFuture<T> taskFuture =
Futures.nonCancellationPropagating(
Futures.submitAsync(task, runnable -> oldFuture.addListener(runnable, executor)));
// newFuture's lifetime is determined by taskFuture, unless taskFuture is cancelled, in which
// case it falls back to oldFuture's. This is to ensure that if the future we return is
// cancelled, we don't begin execution of the next task until after oldFuture completes.
taskFuture.addListener(
() -> {
if (taskFuture.isCancelled()) {
// Since the value of oldFuture can only ever be immediateFuture(null) or setFuture of a
// future that eventually came from immediateFuture(null), this doesn't leak throwables
// or completion values.
wasCancelled.set(true);
newFuture.setFuture(oldFuture);
} else {
newFuture.set(null);
}
},
directExecutor());
return taskFuture;
}
}
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