2 * Copyright 2014-present Facebook, Inc.
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
8 * http://www.apache.org/licenses/LICENSE-2.0
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
19 #include <sys/types.h>
28 #include <folly/Exception.h>
29 #include <folly/FileUtil.h>
30 #include <folly/Likely.h>
31 #include <folly/ScopeGuard.h>
32 #include <folly/SpinLock.h>
33 #include <folly/io/async/DelayedDestruction.h>
34 #include <folly/io/async/EventBase.h>
35 #include <folly/io/async/EventHandler.h>
36 #include <folly/io/async/Request.h>
37 #include <folly/portability/Fcntl.h>
38 #include <folly/portability/Sockets.h>
39 #include <folly/portability/Unistd.h>
41 #include <glog/logging.h>
43 #if __linux__ && !__ANDROID__
44 #define FOLLY_HAVE_EVENTFD
45 #include <folly/io/async/EventFDWrapper.h>
51 * A producer-consumer queue for passing messages between EventBase threads.
53 * Messages can be added to the queue from any thread. Multiple consumers may
54 * listen to the queue from multiple EventBase threads.
56 * A NotificationQueue may not be destroyed while there are still consumers
57 * registered to receive events from the queue. It is the user's
58 * responsibility to ensure that all consumers are unregistered before the
61 * MessageT should be MoveConstructible (i.e., must support either a move
62 * constructor or a copy constructor, or both). Ideally it's move constructor
63 * (or copy constructor if no move constructor is provided) should never throw
64 * exceptions. If the constructor may throw, the consumers could end up
65 * spinning trying to move a message off the queue and failing, and then
68 template <typename MessageT>
69 class NotificationQueue {
72 * A callback interface for consuming messages from the queue as they arrive.
74 class Consumer : public DelayedDestruction, private EventHandler {
76 enum : uint16_t { kDefaultMaxReadAtOnce = 10 };
80 destroyedFlagPtr_(nullptr),
81 maxReadAtOnce_(kDefaultMaxReadAtOnce) {}
83 // create a consumer in-place, without the need to build new class
84 template <typename TCallback>
85 static std::unique_ptr<Consumer, DelayedDestruction::Destructor> make(
86 TCallback&& callback);
89 * messageAvailable() will be invoked whenever a new
90 * message is available from the pipe.
92 virtual void messageAvailable(MessageT&& message) noexcept = 0;
95 * Begin consuming messages from the specified queue.
97 * messageAvailable() will be called whenever a message is available. This
98 * consumer will continue to consume messages until stopConsuming() is
101 * A Consumer may only consume messages from a single NotificationQueue at
102 * a time. startConsuming() should not be called if this consumer is
105 void startConsuming(EventBase* eventBase, NotificationQueue* queue) {
106 init(eventBase, queue);
107 registerHandler(READ | PERSIST);
111 * Same as above but registers this event handler as internal so that it
112 * doesn't count towards the pending reader count for the IOLoop.
114 void startConsumingInternal(
115 EventBase* eventBase, NotificationQueue* queue) {
116 init(eventBase, queue);
117 registerInternalHandler(READ | PERSIST);
121 * Stop consuming messages.
123 * startConsuming() may be called again to resume consumption of messages
124 * at a later point in time.
126 void stopConsuming();
129 * Consume messages off the queue until it is empty. No messages may be
130 * added to the queue while it is draining, so that the process is bounded.
131 * To that end, putMessage/tryPutMessage will throw an std::runtime_error,
132 * and tryPutMessageNoThrow will return false.
134 * @returns true if the queue was drained, false otherwise. In practice,
135 * this will only fail if someone else is already draining the queue.
137 bool consumeUntilDrained(size_t* numConsumed = nullptr) noexcept;
140 * Get the NotificationQueue that this consumer is currently consuming
141 * messages from. Returns nullptr if the consumer is not currently
142 * consuming events from any queue.
144 NotificationQueue* getCurrentQueue() const {
149 * Set a limit on how many messages this consumer will read each iteration
150 * around the event loop.
152 * This helps rate-limit how much work the Consumer will do each event loop
153 * iteration, to prevent it from starving other event handlers.
155 * A limit of 0 means no limit will be enforced. If unset, the limit
156 * defaults to kDefaultMaxReadAtOnce (defined to 10 above).
158 void setMaxReadAtOnce(uint32_t maxAtOnce) {
159 maxReadAtOnce_ = maxAtOnce;
161 uint32_t getMaxReadAtOnce() const {
162 return maxReadAtOnce_;
165 EventBase* getEventBase() {
169 void handlerReady(uint16_t events) noexcept override;
173 void destroy() override;
175 ~Consumer() override {}
179 * Consume messages off the the queue until
180 * - the queue is empty (1), or
181 * - until the consumer is destroyed, or
182 * - until the consumer is uninstalled, or
183 * - an exception is thrown in the course of dequeueing, or
184 * - unless isDrain is true, until the maxReadAtOnce_ limit is hit
186 * (1) Well, maybe. See logic/comments around "wasEmpty" in implementation.
188 void consumeMessages(bool isDrain, size_t* numConsumed = nullptr) noexcept;
190 void setActive(bool active, bool shouldLock = false) {
196 queue_->spinlock_.lock();
198 if (!active_ && active) {
199 ++queue_->numActiveConsumers_;
200 } else if (active_ && !active) {
201 --queue_->numActiveConsumers_;
205 queue_->spinlock_.unlock();
208 void init(EventBase* eventBase, NotificationQueue* queue);
210 NotificationQueue* queue_;
211 bool* destroyedFlagPtr_;
212 uint32_t maxReadAtOnce_;
217 class SimpleConsumer {
219 explicit SimpleConsumer(NotificationQueue& queue) : queue_(queue) {
220 ++queue_.numConsumers_;
224 --queue_.numConsumers_;
228 return queue_.eventfd_ >= 0 ? queue_.eventfd_ : queue_.pipeFds_[0];
232 NotificationQueue& queue_;
237 #ifdef FOLLY_HAVE_EVENTFD
243 * Create a new NotificationQueue.
245 * If the maxSize parameter is specified, this sets the maximum queue size
246 * that will be enforced by tryPutMessage(). (This size is advisory, and may
247 * be exceeded if producers explicitly use putMessage() instead of
250 * The fdType parameter determines the type of file descriptor used
251 * internally to signal message availability. The default (eventfd) is
252 * preferable for performance and because it won't fail when the queue gets
253 * too long. It is not available on on older and non-linux kernels, however.
254 * In this case the code will fall back to using a pipe, the parameter is
255 * mostly for testing purposes.
257 explicit NotificationQueue(uint32_t maxSize = 0,
258 #ifdef FOLLY_HAVE_EVENTFD
259 FdType fdType = FdType::EVENTFD)
261 FdType fdType = FdType::PIPE)
265 advisoryMaxQueueSize_(maxSize),
266 pid_(pid_t(getpid())),
269 #ifdef FOLLY_HAVE_EVENTFD
270 if (fdType == FdType::EVENTFD) {
271 eventfd_ = eventfd(0, EFD_CLOEXEC | EFD_NONBLOCK);
272 if (eventfd_ == -1) {
273 if (errno == ENOSYS || errno == EINVAL) {
274 // eventfd not availalble
275 LOG(ERROR) << "failed to create eventfd for NotificationQueue: "
276 << errno << ", falling back to pipe mode (is your kernel "
278 fdType = FdType::PIPE;
281 folly::throwSystemError("Failed to create eventfd for "
282 "NotificationQueue", errno);
287 if (fdType == FdType::PIPE) {
288 if (pipe(pipeFds_)) {
289 folly::throwSystemError("Failed to create pipe for NotificationQueue",
293 // put both ends of the pipe into non-blocking mode
294 if (fcntl(pipeFds_[0], F_SETFL, O_RDONLY | O_NONBLOCK) != 0) {
295 folly::throwSystemError("failed to put NotificationQueue pipe read "
296 "endpoint into non-blocking mode", errno);
298 if (fcntl(pipeFds_[1], F_SETFL, O_WRONLY | O_NONBLOCK) != 0) {
299 folly::throwSystemError("failed to put NotificationQueue pipe write "
300 "endpoint into non-blocking mode", errno);
303 ::close(pipeFds_[0]);
304 ::close(pipeFds_[1]);
310 ~NotificationQueue() {
315 if (pipeFds_[0] >= 0) {
316 ::close(pipeFds_[0]);
319 if (pipeFds_[1] >= 0) {
320 ::close(pipeFds_[1]);
326 * Set the advisory maximum queue size.
328 * This maximum queue size affects calls to tryPutMessage(). Message
329 * producers can still use the putMessage() call to unconditionally put a
330 * message on the queue, ignoring the configured maximum queue size. This
331 * can cause the queue size to exceed the configured maximum.
333 void setMaxQueueSize(uint32_t max) {
334 advisoryMaxQueueSize_ = max;
338 * Attempt to put a message on the queue if the queue is not already full.
340 * If the queue is full, a std::overflow_error will be thrown. The
341 * setMaxQueueSize() function controls the maximum queue size.
343 * If the queue is currently draining, an std::runtime_error will be thrown.
345 * This method may contend briefly on a spinlock if many threads are
346 * concurrently accessing the queue, but for all intents and purposes it will
347 * immediately place the message on the queue and return.
349 * tryPutMessage() may throw std::bad_alloc if memory allocation fails, and
350 * may throw any other exception thrown by the MessageT move/copy
353 template <typename MessageTT>
354 void tryPutMessage(MessageTT&& message) {
355 putMessageImpl(std::forward<MessageTT>(message), advisoryMaxQueueSize_);
359 * No-throw versions of the above. Instead returns true on success, false on
362 * Only std::overflow_error (the common exception case) and std::runtime_error
363 * (which indicates that the queue is being drained) are prevented from being
364 * thrown. User code must still catch std::bad_alloc errors.
366 template <typename MessageTT>
367 bool tryPutMessageNoThrow(MessageTT&& message) {
368 return putMessageImpl(
369 std::forward<MessageTT>(message), advisoryMaxQueueSize_, false);
373 * Unconditionally put a message on the queue.
375 * This method is like tryPutMessage(), but ignores the maximum queue size
376 * and always puts the message on the queue, even if the maximum queue size
379 * putMessage() may throw
380 * - std::bad_alloc if memory allocation fails, and may
381 * - std::runtime_error if the queue is currently draining
382 * - any other exception thrown by the MessageT move/copy constructor.
384 template <typename MessageTT>
385 void putMessage(MessageTT&& message) {
386 putMessageImpl(std::forward<MessageTT>(message), 0);
390 * Put several messages on the queue.
392 template <typename InputIteratorT>
393 void putMessages(InputIteratorT first, InputIteratorT last) {
394 typedef typename std::iterator_traits<InputIteratorT>::iterator_category
396 putMessagesImpl(first, last, IterCategory());
400 * Try to immediately pull a message off of the queue, without blocking.
402 * If a message is immediately available, the result parameter will be
403 * updated to contain the message contents and true will be returned.
405 * If no message is available, false will be returned and result will be left
408 bool tryConsume(MessageT& result) {
409 SCOPE_EXIT { syncSignalAndQueue(); };
413 folly::SpinLockGuard g(spinlock_);
415 if (UNLIKELY(queue_.empty())) {
419 auto& data = queue_.front();
420 result = std::move(data.first);
421 RequestContext::setContext(std::move(data.second));
428 size_t size() const {
429 folly::SpinLockGuard g(spinlock_);
430 return queue_.size();
434 * Check that the NotificationQueue is being used from the correct process.
436 * If you create a NotificationQueue in one process, then fork, and try to
437 * send messages to the queue from the child process, you're going to have a
438 * bad time. Unfortunately users have (accidentally) run into this.
440 * Because we use an eventfd/pipe, the child process can actually signal the
441 * parent process that an event is ready. However, it can't put anything on
442 * the parent's queue, so the parent wakes up and finds an empty queue. This
443 * check ensures that we catch the problem in the misbehaving child process
444 * code, and crash before signalling the parent process.
446 void checkPid() const { CHECK_EQ(pid_, pid_t(getpid())); }
449 // Forbidden copy constructor and assignment operator
450 NotificationQueue(NotificationQueue const &) = delete;
451 NotificationQueue& operator=(NotificationQueue const &) = delete;
453 inline bool checkQueueSize(size_t maxSize, bool throws=true) const {
454 DCHECK(0 == spinlock_.try_lock());
455 if (maxSize > 0 && queue_.size() >= maxSize) {
457 throw std::overflow_error("unable to add message to NotificationQueue: "
465 inline bool checkDraining(bool throws=true) {
466 if (UNLIKELY(draining_ && throws)) {
467 throw std::runtime_error("queue is draining, cannot add message");
473 // TODO 10860938 Remove after figuring out crash
474 mutable std::atomic<int> eventBytes_{0};
475 mutable std::atomic<int> maxEventBytes_{0};
478 void ensureSignalLocked() const {
479 // semantics: empty fd == empty queue <=> !signal_
484 ssize_t bytes_written = 0;
485 size_t bytes_expected = 0;
489 // eventfd(2) dictates that we must write a 64-bit integer
491 bytes_expected = sizeof(signal);
492 bytes_written = ::write(eventfd_, &signal, bytes_expected);
495 bytes_expected = sizeof(signal);
496 bytes_written = ::write(pipeFds_[1], &signal, bytes_expected);
498 } while (bytes_written == -1 && errno == EINTR);
501 if (bytes_written > 0) {
502 eventBytes_ += bytes_written;
503 maxEventBytes_ = std::max((int)maxEventBytes_, (int)eventBytes_);
507 if (bytes_written == ssize_t(bytes_expected)) {
511 LOG(ERROR) << "NotificationQueue Write Error=" << errno
512 << " bytesInPipe=" << eventBytes_
513 << " maxInPipe=" << maxEventBytes_ << " queue=" << size();
515 folly::throwSystemError("failed to signal NotificationQueue after "
520 void drainSignalsLocked() {
521 ssize_t bytes_read = 0;
524 bytes_read = readNoInt(eventfd_, &message, sizeof(message));
525 CHECK(bytes_read != -1 || errno == EAGAIN);
527 // There should only be one byte in the pipe. To avoid potential leaks we still drain.
530 while ((result = readNoInt(pipeFds_[0], &message, sizeof(message))) != -1) {
531 bytes_read += result;
533 CHECK(result == -1 && errno == EAGAIN);
534 LOG_IF(ERROR, bytes_read > 1)
535 << "[NotificationQueue] Unexpected state while draining pipe: bytes_read="
536 << bytes_read << " bytes, expected <= 1";
538 LOG_IF(ERROR, (signal_ && bytes_read == 0) || (!signal_ && bytes_read > 0))
539 << "[NotificationQueue] Unexpected state while draining signals: signal_="
540 << signal_ << " bytes_read=" << bytes_read;
545 if (bytes_read > 0) {
546 eventBytes_ -= bytes_read;
551 void ensureSignal() const {
552 folly::SpinLockGuard g(spinlock_);
553 ensureSignalLocked();
556 void syncSignalAndQueue() {
557 folly::SpinLockGuard g(spinlock_);
559 if (queue_.empty()) {
560 drainSignalsLocked();
562 ensureSignalLocked();
566 template <typename MessageTT>
567 bool putMessageImpl(MessageTT&& message, size_t maxSize, bool throws = true) {
571 folly::SpinLockGuard g(spinlock_);
572 if (checkDraining(throws) || !checkQueueSize(maxSize, throws)) {
575 // We only need to signal an event if not all consumers are
577 if (numActiveConsumers_ < numConsumers_) {
581 std::forward<MessageTT>(message), RequestContext::saveContext());
583 ensureSignalLocked();
589 template <typename InputIteratorT>
590 void putMessagesImpl(InputIteratorT first, InputIteratorT last,
591 std::input_iterator_tag) {
596 folly::SpinLockGuard g(spinlock_);
598 while (first != last) {
599 queue_.emplace_back(*first, RequestContext::saveContext());
603 if (numActiveConsumers_ < numConsumers_) {
607 ensureSignalLocked();
612 mutable folly::SpinLock spinlock_;
613 mutable bool signal_{false};
615 int pipeFds_[2]; // to fallback to on older/non-linux systems
616 uint32_t advisoryMaxQueueSize_;
618 std::deque<std::pair<MessageT, std::shared_ptr<RequestContext>>> queue_;
619 int numConsumers_{0};
620 std::atomic<int> numActiveConsumers_{0};
621 bool draining_{false};
624 template <typename MessageT>
625 void NotificationQueue<MessageT>::Consumer::destroy() {
626 // If we are in the middle of a call to handlerReady(), destroyedFlagPtr_
627 // will be non-nullptr. Mark the value that it points to, so that
628 // handlerReady() will know the callback is destroyed, and that it cannot
629 // access any member variables anymore.
630 if (destroyedFlagPtr_) {
631 *destroyedFlagPtr_ = true;
634 DelayedDestruction::destroy();
637 template <typename MessageT>
638 void NotificationQueue<MessageT>::Consumer::handlerReady(uint16_t /*events*/)
640 consumeMessages(false);
643 template <typename MessageT>
644 void NotificationQueue<MessageT>::Consumer::consumeMessages(
645 bool isDrain, size_t* numConsumed) noexcept {
646 DestructorGuard dg(this);
647 uint32_t numProcessed = 0;
651 queue_->syncSignalAndQueue();
654 SCOPE_EXIT { setActive(false, /* shouldLock = */ true); };
656 if (numConsumed != nullptr) {
657 *numConsumed = numProcessed;
661 // Now pop the message off of the queue.
663 // We have to manually acquire and release the spinlock here, rather than
664 // using SpinLockHolder since the MessageT has to be constructed while
665 // holding the spinlock and available after we release it. SpinLockHolder
666 // unfortunately doesn't provide a release() method. (We can't construct
667 // MessageT first since we have no guarantee that MessageT has a default
669 queue_->spinlock_.lock();
673 if (UNLIKELY(queue_->queue_.empty())) {
674 // If there is no message, we've reached the end of the queue, return.
676 queue_->spinlock_.unlock();
680 // Pull a message off the queue.
681 auto& data = queue_->queue_.front();
683 MessageT msg(std::move(data.first));
684 RequestContextScopeGuard rctx(std::move(data.second));
685 queue_->queue_.pop_front();
687 // Check to see if the queue is empty now.
688 // We use this as an optimization to see if we should bother trying to
689 // loop again and read another message after invoking this callback.
690 bool wasEmpty = queue_->queue_.empty();
695 // Now unlock the spinlock before we invoke the callback.
696 queue_->spinlock_.unlock();
700 bool callbackDestroyed = false;
701 CHECK(destroyedFlagPtr_ == nullptr);
702 destroyedFlagPtr_ = &callbackDestroyed;
703 messageAvailable(std::move(msg));
704 destroyedFlagPtr_ = nullptr;
706 // If the callback was destroyed before it returned, we are done
707 if (callbackDestroyed) {
711 // If the callback is no longer installed, we are done.
712 if (queue_ == nullptr) {
716 // If we have hit maxReadAtOnce_, we are done.
718 if (!isDrain && maxReadAtOnce_ > 0 &&
719 numProcessed >= maxReadAtOnce_) {
723 // If the queue was empty before we invoked the callback, it's probable
724 // that it is still empty now. Just go ahead and return, rather than
725 // looping again and trying to re-read from the eventfd. (If a new
726 // message had in fact arrived while we were invoking the callback, we
727 // will simply be woken up the next time around the event loop and will
728 // process the message then.)
732 } catch (const std::exception&) {
733 // This catch block is really just to handle the case where the MessageT
734 // constructor throws. The messageAvailable() callback itself is
735 // declared as noexcept and should never throw.
737 // If the MessageT constructor does throw we try to handle it as best as
738 // we can, but we can't work miracles. We will just ignore the error for
739 // now and return. The next time around the event loop we will end up
740 // trying to read the message again. If MessageT continues to throw we
741 // will never make forward progress and will keep trying each time around
744 // Unlock the spinlock.
745 queue_->spinlock_.unlock();
753 template <typename MessageT>
754 void NotificationQueue<MessageT>::Consumer::init(
755 EventBase* eventBase,
756 NotificationQueue* queue) {
757 eventBase->dcheckIsInEventBaseThread();
758 assert(queue_ == nullptr);
759 assert(!isHandlerRegistered());
767 folly::SpinLockGuard g(queue_->spinlock_);
768 queue_->numConsumers_++;
770 queue_->ensureSignal();
772 if (queue_->eventfd_ >= 0) {
773 initHandler(eventBase, queue_->eventfd_);
775 initHandler(eventBase, queue_->pipeFds_[0]);
779 template <typename MessageT>
780 void NotificationQueue<MessageT>::Consumer::stopConsuming() {
781 if (queue_ == nullptr) {
782 assert(!isHandlerRegistered());
787 folly::SpinLockGuard g(queue_->spinlock_);
788 queue_->numConsumers_--;
792 assert(isHandlerRegistered());
798 template <typename MessageT>
799 bool NotificationQueue<MessageT>::Consumer::consumeUntilDrained(
800 size_t* numConsumed) noexcept {
801 DestructorGuard dg(this);
803 folly::SpinLockGuard g(queue_->spinlock_);
804 if (queue_->draining_) {
807 queue_->draining_ = true;
809 consumeMessages(true, numConsumed);
811 folly::SpinLockGuard g(queue_->spinlock_);
812 queue_->draining_ = false;
818 * Creates a NotificationQueue::Consumer wrapping a function object
819 * Modeled after AsyncTimeout::make
825 template <typename MessageT, typename TCallback>
826 struct notification_queue_consumer_wrapper
827 : public NotificationQueue<MessageT>::Consumer {
829 template <typename UCallback>
830 explicit notification_queue_consumer_wrapper(UCallback&& callback)
831 : callback_(std::forward<UCallback>(callback)) {}
833 // we are being stricter here and requiring noexcept for callback
834 void messageAvailable(MessageT&& message) noexcept override {
836 noexcept(std::declval<TCallback>()(std::forward<MessageT>(message))),
837 "callback must be declared noexcept, e.g.: `[]() noexcept {}`"
840 callback_(std::forward<MessageT>(message));
847 } // namespace detail
849 template <typename MessageT>
850 template <typename TCallback>
851 std::unique_ptr<typename NotificationQueue<MessageT>::Consumer,
852 DelayedDestruction::Destructor>
853 NotificationQueue<MessageT>::Consumer::make(TCallback&& callback) {
854 return std::unique_ptr<NotificationQueue<MessageT>::Consumer,
855 DelayedDestruction::Destructor>(
856 new detail::notification_queue_consumer_wrapper<
858 typename std::decay<TCallback>::type>(
859 std::forward<TCallback>(callback)));