001/*
002 * Copyright (C) 2018 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
005 * in compliance with the License. You may obtain a copy of the License at
006 *
007 * http://www.apache.org/licenses/LICENSE-2.0
008 *
009 * Unless required by applicable law or agreed to in writing, software distributed under the License
010 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
011 * or implied. See the License for the specific language governing permissions and limitations under
012 * the License.
013 */
014
015package com.google.common.util.concurrent;
016
017import static com.google.common.base.Preconditions.checkNotNull;
018import static com.google.common.base.Preconditions.checkState;
019import static com.google.common.util.concurrent.ExecutionSequencer.RunningState.CANCELLED;
020import static com.google.common.util.concurrent.ExecutionSequencer.RunningState.NOT_RUN;
021import static com.google.common.util.concurrent.ExecutionSequencer.RunningState.STARTED;
022import static com.google.common.util.concurrent.Futures.immediateCancelledFuture;
023import static com.google.common.util.concurrent.Futures.immediateFuture;
024import static com.google.common.util.concurrent.Futures.immediateVoidFuture;
025import static com.google.common.util.concurrent.MoreExecutors.directExecutor;
026import static java.util.Objects.requireNonNull;
027
028import java.util.concurrent.Callable;
029import java.util.concurrent.Executor;
030import java.util.concurrent.atomic.AtomicReference;
031import javax.annotation.CheckForNull;
032import org.checkerframework.checker.nullness.qual.Nullable;
033
034/**
035 * Serializes execution of tasks, somewhat like an "asynchronous {@code synchronized} block." Each
036 * {@linkplain #submit enqueued} callable will not be submitted to its associated executor until the
037 * previous callable has returned -- and, if the previous callable was an {@link AsyncCallable}, not
038 * until the {@code Future} it returned is {@linkplain Future#isDone done} (successful, failed, or
039 * cancelled).
040 *
041 * <p>This class serializes execution of <i>submitted</i> tasks but not any <i>listeners</i> of
042 * those tasks.
043 *
044 * <p>Submitted tasks have a happens-before order as defined in the Java Language Specification.
045 * Tasks execute with the same happens-before order that the function calls to {@link #submit} and
046 * {@link #submitAsync} that submitted those tasks had.
047 *
048 * <p>This class has limited support for cancellation and other "early completions":
049 *
050 * <ul>
051 *   <li>While calls to {@code submit} and {@code submitAsync} return a {@code Future} that can be
052 *       cancelled, cancellation never propagates to a task that has started to run -- neither to
053 *       the callable itself nor to any {@code Future} returned by an {@code AsyncCallable}.
054 *       (However, cancellation can prevent an <i>unstarted</i> task from running.) Therefore, the
055 *       next task will wait for any running callable (or pending {@code Future} returned by an
056 *       {@code AsyncCallable}) to complete, without interrupting it (and without calling {@code
057 *       cancel} on the {@code Future}). So beware: <i>Even if you cancel every precededing {@code
058 *       Future} returned by this class, the next task may still have to wait.</i>.
059 *   <li>Once an {@code AsyncCallable} returns a {@code Future}, this class considers that task to
060 *       be "done" as soon as <i>that</i> {@code Future} completes in any way. Notably, a {@code
061 *       Future} is "completed" even if it is cancelled while its underlying work continues on a
062 *       thread, an RPC, etc. The {@code Future} is also "completed" if it fails "early" -- for
063 *       example, if the deadline expires on a {@code Future} returned from {@link
064 *       Futures#withTimeout} while the {@code Future} it wraps continues its underlying work. So
065 *       beware: <i>Your {@code AsyncCallable} should not complete its {@code Future} until it is
066 *       safe for the next task to start.</i>
067 * </ul>
068 *
069 * <p>This class is similar to {@link MoreExecutors#newSequentialExecutor}. This class is different
070 * in a few ways:
071 *
072 * <ul>
073 *   <li>Each task may be associated with a different executor.
074 *   <li>Tasks may be of type {@code AsyncCallable}.
075 *   <li>Running tasks <i>cannot</i> be interrupted. (Note that {@code newSequentialExecutor} does
076 *       not return {@code Future} objects, so it doesn't support interruption directly, either.
077 *       However, utilities that <i>use</i> that executor have the ability to interrupt tasks
078 *       running on it. This class, by contrast, does not expose an {@code Executor} API.)
079 * </ul>
080 *
081 * <p>If you don't need the features of this class, you may prefer {@code newSequentialExecutor} for
082 * its simplicity and ability to accommodate interruption.
083 *
084 * @since 26.0
085 */
086@ElementTypesAreNonnullByDefault
087public final class ExecutionSequencer {
088
089  private ExecutionSequencer() {}
090
091  /** Creates a new instance. */
092  public static ExecutionSequencer create() {
093    return new ExecutionSequencer();
094  }
095
096  /** This reference acts as a pointer tracking the head of a linked list of ListenableFutures. */
097  private final AtomicReference<ListenableFuture<@Nullable Void>> ref =
098      new AtomicReference<>(immediateVoidFuture());
099
100  private ThreadConfinedTaskQueue latestTaskQueue = new ThreadConfinedTaskQueue();
101
102  /**
103   * This object is unsafely published, but avoids problematic races by relying exclusively on the
104   * identity equality of its Thread field so that the task field is only accessed by a single
105   * thread.
106   */
107  private static final class ThreadConfinedTaskQueue {
108    /**
109     * This field is only used for identity comparisons with the current thread. Field assignments
110     * are atomic, but do not provide happens-before ordering; however:
111     *
112     * <ul>
113     *   <li>If this field's value == currentThread, we know that it's up to date, because write
114     *       operations in a thread always happen-before subsequent read operations in the same
115     *       thread
116     *   <li>If this field's value == null because of unsafe publication, we know that it isn't the
117     *       object associated with our thread, because if it was the publication wouldn't have been
118     *       unsafe and we'd have seen our thread as the value. This state is also why a new
119     *       ThreadConfinedTaskQueue object must be created for each inline execution, because
120     *       observing a null thread does not mean the object is safe to reuse.
121     *   <li>If this field's value is some other thread object, we know that it's not our thread.
122     *   <li>If this field's value == null because it originally belonged to another thread and that
123     *       thread cleared it, we still know that it's not associated with our thread
124     *   <li>If this field's value == null because it was associated with our thread and was
125     *       cleared, we know that we're not executing inline any more
126     * </ul>
127     *
128     * All the states where thread != currentThread are identical for our purposes, and so even
129     * though it's racy, we don't care which of those values we get, so no need to synchronize.
130     */
131    @CheckForNull Thread thread;
132    /** Only used by the thread associated with this object */
133    @CheckForNull Runnable nextTask;
134    /** Only used by the thread associated with this object */
135    @CheckForNull Executor nextExecutor;
136  }
137
138  /**
139   * Enqueues a task to run when the previous task (if any) completes.
140   *
141   * <p>Cancellation does not propagate from the output future to a callable that has begun to
142   * execute, but if the output future is cancelled before {@link Callable#call()} is invoked,
143   * {@link Callable#call()} will not be invoked.
144   */
145  public <T extends @Nullable Object> ListenableFuture<T> submit(
146      Callable<T> callable, Executor executor) {
147    checkNotNull(callable);
148    checkNotNull(executor);
149    return submitAsync(
150        new AsyncCallable<T>() {
151          @Override
152          public ListenableFuture<T> call() throws Exception {
153            return immediateFuture(callable.call());
154          }
155
156          @Override
157          public String toString() {
158            return callable.toString();
159          }
160        },
161        executor);
162  }
163
164  /**
165   * Enqueues a task to run when the previous task (if any) completes.
166   *
167   * <p>Cancellation does not propagate from the output future to the future returned from {@code
168   * callable} or a callable that has begun to execute, but if the output future is cancelled before
169   * {@link AsyncCallable#call()} is invoked, {@link AsyncCallable#call()} will not be invoked.
170   */
171  public <T extends @Nullable Object> ListenableFuture<T> submitAsync(
172      AsyncCallable<T> callable, Executor executor) {
173    checkNotNull(callable);
174    checkNotNull(executor);
175    TaskNonReentrantExecutor taskExecutor = new TaskNonReentrantExecutor(executor, this);
176    AsyncCallable<T> task =
177        new AsyncCallable<T>() {
178          @Override
179          public ListenableFuture<T> call() throws Exception {
180            if (!taskExecutor.trySetStarted()) {
181              return immediateCancelledFuture();
182            }
183            return callable.call();
184          }
185
186          @Override
187          public String toString() {
188            return callable.toString();
189          }
190        };
191    /*
192     * Four futures are at play here:
193     * taskFuture is the future tracking the result of the callable.
194     * newFuture is a future that completes after this and all prior tasks are done.
195     * oldFuture is the previous task's newFuture.
196     * outputFuture is the future we return to the caller, a nonCancellationPropagating taskFuture.
197     *
198     * newFuture is guaranteed to only complete once all tasks previously submitted to this instance
199     * have completed - namely after oldFuture is done, and taskFuture has either completed or been
200     * cancelled before the callable started execution.
201     */
202    SettableFuture<@Nullable Void> newFuture = SettableFuture.create();
203
204    ListenableFuture<@Nullable Void> oldFuture = ref.getAndSet(newFuture);
205
206    // Invoke our task once the previous future completes.
207    TrustedListenableFutureTask<T> taskFuture = TrustedListenableFutureTask.create(task);
208    oldFuture.addListener(taskFuture, taskExecutor);
209
210    ListenableFuture<T> outputFuture = Futures.nonCancellationPropagating(taskFuture);
211
212    // newFuture's lifetime is determined by taskFuture, which can't complete before oldFuture
213    // unless taskFuture is cancelled, in which case it falls back to oldFuture. This ensures that
214    // if the future we return is cancelled, we don't begin execution of the next task until after
215    // oldFuture completes.
216    Runnable listener =
217        () -> {
218          if (taskFuture.isDone()) {
219            // Since the value of oldFuture can only ever be immediateFuture(null) or setFuture of
220            // a future that eventually came from immediateFuture(null), this doesn't leak
221            // throwables or completion values.
222            newFuture.setFuture(oldFuture);
223          } else if (outputFuture.isCancelled() && taskExecutor.trySetCancelled()) {
224            // If this CAS succeeds, we know that the provided callable will never be invoked,
225            // so when oldFuture completes it is safe to allow the next submitted task to
226            // proceed. Doing this immediately here lets the next task run without waiting for
227            // the cancelled task's executor to run the noop AsyncCallable.
228            //
229            // ---
230            //
231            // If the CAS fails, the provided callable already started running (or it is about
232            // to). Our contract promises:
233            //
234            // 1. not to execute a new callable until the old one has returned
235            //
236            // If we were to cancel taskFuture, that would let the next task start while the old
237            // one is still running.
238            //
239            // Now, maybe we could tweak our implementation to not start the next task until the
240            // callable actually completes. (We could detect completion in our wrapper
241            // `AsyncCallable task`.) However, our contract also promises:
242            //
243            // 2. not to cancel any Future the user returned from an AsyncCallable
244            //
245            // We promise this because, once we cancel that Future, we would no longer be able to
246            // tell when any underlying work it is doing is done. Thus, we might start a new task
247            // while that underlying work is still running.
248            //
249            // So that is why we cancel only in the case of CAS success.
250            taskFuture.cancel(false);
251          }
252        };
253    // Adding the listener to both futures guarantees that newFuture will aways be set. Adding to
254    // taskFuture guarantees completion if the callable is invoked, and adding to outputFuture
255    // propagates cancellation if the callable has not yet been invoked.
256    outputFuture.addListener(listener, directExecutor());
257    taskFuture.addListener(listener, directExecutor());
258
259    return outputFuture;
260  }
261
262  enum RunningState {
263    NOT_RUN,
264    CANCELLED,
265    STARTED,
266  }
267
268  /**
269   * This class helps avoid a StackOverflowError when large numbers of tasks are submitted with
270   * {@link MoreExecutors#directExecutor}. Normally, when the first future completes, all the other
271   * tasks would be called recursively. Here, we detect that the delegate executor is executing
272   * inline, and maintain a queue to dispatch tasks iteratively. There is one instance of this class
273   * per call to submit() or submitAsync(), and each instance supports only one call to execute().
274   *
275   * <p>This class would certainly be simpler and easier to reason about if it were built with
276   * ThreadLocal; however, ThreadLocal is not well optimized for the case where the ThreadLocal is
277   * non-static, and is initialized/removed frequently - this causes churn in the Thread specific
278   * hashmaps. Using a static ThreadLocal to avoid that overhead would mean that different
279   * ExecutionSequencer objects interfere with each other, which would be undesirable, in addition
280   * to increasing the memory footprint of every thread that interacted with it. In order to release
281   * entries in thread-specific maps when the ThreadLocal object itself is no longer referenced,
282   * ThreadLocal is usually implemented with a WeakReference, which can have negative performance
283   * properties; for example, calling WeakReference.get() on Android will block during an
284   * otherwise-concurrent GC cycle.
285   */
286  private static final class TaskNonReentrantExecutor extends AtomicReference<RunningState>
287      implements Executor, Runnable {
288
289    /**
290     * Used to update and read the latestTaskQueue field. Set to null once the runnable has been run
291     * or queued.
292     */
293    @CheckForNull ExecutionSequencer sequencer;
294
295    /**
296     * Executor the task was set to run on. Set to null when the task has been queued, run, or
297     * cancelled.
298     */
299    @CheckForNull Executor delegate;
300
301    /**
302     * Set before calling delegate.execute(); set to null once run, so that it can be GCed; this
303     * object may live on after, if submitAsync returns an incomplete future.
304     */
305    @CheckForNull Runnable task;
306
307    /** Thread that called execute(). Set in execute, cleared when delegate.execute() returns. */
308    @CheckForNull Thread submitting;
309
310    private TaskNonReentrantExecutor(Executor delegate, ExecutionSequencer sequencer) {
311      super(NOT_RUN);
312      this.delegate = delegate;
313      this.sequencer = sequencer;
314    }
315
316    @Override
317    public void execute(Runnable task) {
318      // If this operation was successfully cancelled already, calling the runnable will be a noop.
319      // This also avoids a race where if outputFuture is cancelled, it will call taskFuture.cancel,
320      // which will call newFuture.setFuture(oldFuture), to allow the next task in the queue to run
321      // without waiting for the user's executor to run our submitted Runnable. However, this can
322      // interact poorly with the reentrancy-avoiding behavior of this executor - when the operation
323      // before the cancelled future completes, it will synchronously complete both the newFuture
324      // from the cancelled operation and its own. This can cause one runnable to queue two tasks,
325      // breaking the invariant this method relies on to iteratively run the next task after the
326      // previous one completes.
327      if (get() == RunningState.CANCELLED) {
328        delegate = null;
329        sequencer = null;
330        return;
331      }
332      submitting = Thread.currentThread();
333
334      try {
335        /*
336         * requireNonNull is safe because we don't null out `sequencer` except:
337         *
338         * - above, where we return (in which case we never get here)
339         *
340         * - in `run`, which can't run until this Runnable is submitted to an executor, which
341         *   doesn't happen until below. (And this Executor -- yes, the object is both a Runnable
342         *   and an Executor -- is used for only a single `execute` call.)
343         */
344        ThreadConfinedTaskQueue submittingTaskQueue = requireNonNull(sequencer).latestTaskQueue;
345        if (submittingTaskQueue.thread == submitting) {
346          sequencer = null;
347          // Submit from inside a reentrant submit. We don't know if this one will be reentrant (and
348          // can't know without submitting something to the executor) so queue to run iteratively.
349          // Task must be null, since each execution on this executor can only produce one more
350          // execution.
351          checkState(submittingTaskQueue.nextTask == null);
352          submittingTaskQueue.nextTask = task;
353          // requireNonNull(delegate) is safe for reasons similar to requireNonNull(sequencer).
354          submittingTaskQueue.nextExecutor = requireNonNull(delegate);
355          delegate = null;
356        } else {
357          // requireNonNull(delegate) is safe for reasons similar to requireNonNull(sequencer).
358          Executor localDelegate = requireNonNull(delegate);
359          delegate = null;
360          this.task = task;
361          localDelegate.execute(this);
362        }
363      } finally {
364        // Important to null this out here - if we did *not* execute inline, we might still
365        // run() on the same thread that called execute() - such as in a thread pool, and think
366        // that it was happening inline. As a side benefit, avoids holding on to the Thread object
367        // longer than necessary.
368        submitting = null;
369      }
370    }
371
372    @SuppressWarnings("ShortCircuitBoolean")
373    @Override
374    public void run() {
375      Thread currentThread = Thread.currentThread();
376      if (currentThread != submitting) {
377        /*
378         * requireNonNull is safe because we set `task` before submitting this Runnable to an
379         * Executor, and we don't null it out until here.
380         */
381        Runnable localTask = requireNonNull(task);
382        task = null;
383        localTask.run();
384        return;
385      }
386      // Executor called reentrantly! Make sure that further calls don't overflow stack. Further
387      // reentrant calls will see that their current thread is the same as the one set in
388      // latestTaskQueue, and queue rather than calling execute() directly.
389      ThreadConfinedTaskQueue executingTaskQueue = new ThreadConfinedTaskQueue();
390      executingTaskQueue.thread = currentThread;
391      /*
392       * requireNonNull is safe because we don't null out `sequencer` except:
393       *
394       * - after the requireNonNull call below. (And this object has its Runnable.run override
395       *   called only once, just as it has its Executor.execute override called only once.)
396       *
397       * - if we return immediately from `execute` (in which case we never get here)
398       *
399       * - in the "reentrant submit" case of `execute` (in which case we must have started running a
400       *   user task -- which means that we already got past this code (or else we exited early
401       *   above))
402       */
403      // Unconditionally set; there is no risk of throwing away a queued task from another thread,
404      // because in order for the current task to run on this executor the previous task must have
405      // already started execution. Because each task on a TaskNonReentrantExecutor can only produce
406      // one execute() call to another instance from the same ExecutionSequencer, we know by
407      // induction that the task that launched this one must not have added any other runnables to
408      // that thread's queue, and thus we cannot be replacing a TaskAndThread object that would
409      // otherwise have another task queued on to it. Note the exception to this, cancellation, is
410      // specially handled in execute() - execute() calls triggered by cancellation are no-ops, and
411      // thus don't count.
412      requireNonNull(sequencer).latestTaskQueue = executingTaskQueue;
413      sequencer = null;
414      try {
415        // requireNonNull is safe, as discussed above.
416        Runnable localTask = requireNonNull(task);
417        task = null;
418        localTask.run();
419        // Now check if our task attempted to reentrantly execute the next task.
420        Runnable queuedTask;
421        Executor queuedExecutor;
422        // Intentionally using non-short-circuit operator
423        while ((queuedTask = executingTaskQueue.nextTask) != null
424            && (queuedExecutor = executingTaskQueue.nextExecutor) != null) {
425          executingTaskQueue.nextTask = null;
426          executingTaskQueue.nextExecutor = null;
427          queuedExecutor.execute(queuedTask);
428        }
429      } finally {
430        // Null out the thread field, so that we don't leak a reference to Thread, and so that
431        // future `thread == currentThread()` calls from this thread don't incorrectly queue instead
432        // of executing. Don't null out the latestTaskQueue field, because the work done here
433        // may have scheduled more operations on another thread, and if those operations then
434        // trigger reentrant calls that thread will have updated the latestTaskQueue field, and
435        // we'd be interfering with their operation.
436        executingTaskQueue.thread = null;
437      }
438    }
439
440    private boolean trySetStarted() {
441      return compareAndSet(NOT_RUN, STARTED);
442    }
443
444    private boolean trySetCancelled() {
445      return compareAndSet(NOT_RUN, CANCELLED);
446    }
447  }
448}