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}