2 * Copyright 2017 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.
23 #include <type_traits>
26 #include <folly/Optional.h>
27 #include <folly/Portability.h>
28 #include <folly/ScopeGuard.h>
29 #include <folly/Try.h>
30 #include <folly/Utility.h>
31 #include <folly/executors/DrivableExecutor.h>
32 #include <folly/futures/FutureException.h>
33 #include <folly/futures/Promise.h>
34 #include <folly/futures/detail/Types.h>
36 // boring predeclarations and details
37 #include <folly/futures/Future-pre.h>
39 // not-boring helpers, e.g. all in folly::futures, makeFuture variants, etc.
40 // Needs to be included after Future-pre.h and before Future-inl.h
41 #include <folly/futures/helpers.h>
58 /// Construct a Future from a value (perfect forwarding)
61 typename = typename std::enable_if<
62 !isFuture<typename std::decay<T2>::type>::value &&
63 !isSemiFuture<typename std::decay<T2>::type>::value>::type>
64 /* implicit */ FutureBase(T2&& val);
66 template <class T2 = T>
67 /* implicit */ FutureBase(
68 typename std::enable_if<std::is_same<Unit, T2>::value>::type*);
72 typename std::enable_if<std::is_constructible<T, Args&&...>::value, int>::
74 explicit FutureBase(in_place_t, Args&&... args);
76 FutureBase(FutureBase<T> const&) = delete;
77 FutureBase(SemiFuture<T>&&) noexcept;
78 FutureBase(Future<T>&&) noexcept;
81 FutureBase(Future<T> const&) = delete;
82 FutureBase(SemiFuture<T> const&) = delete;
86 /// Returns a reference to the result, with a reference category and const-
87 /// qualification equivalent to the reference category and const-qualification
90 /// If moved-from, throws NoState.
92 /// If !isReady(), throws FutureNotReady.
94 /// If an exception has been captured, throws that exception.
96 T const& value() const&;
98 T const&& value() const&&;
100 /** True when the result (or exception) is ready. */
101 bool isReady() const;
103 /// sugar for getTry().hasValue()
106 /// sugar for getTry().hasException()
109 /** A reference to the Try of the value */
112 /// If the promise has been fulfilled, return an Optional with the Try<T>.
113 /// Otherwise return an empty Optional.
114 /// Note that this moves the Try<T> out.
115 Optional<Try<T>> poll();
117 /// This is not the method you're looking for.
119 /// This needs to be public because it's used by make* and when*, and it's
120 /// not worth listing all those and their fancy template signatures as
121 /// friends. But it's not for public consumption.
123 void setCallback_(F&& func);
126 return core_->isActive();
130 void raise(E&& exception) {
131 raise(make_exception_wrapper<typename std::remove_reference<E>::type>(
132 std::forward<E>(exception)));
135 /// Raise an interrupt. If the promise holder has an interrupt
136 /// handler it will be called and potentially stop asynchronous work from
137 /// being done. This is advisory only - a promise holder may not set an
138 /// interrupt handler, or may do anything including ignore. But, if you know
139 /// your future supports this the most likely result is stopping or
140 /// preventing the asynchronous operation (if in time), and the promise
141 /// holder setting an exception on the future. (That may happen
142 /// asynchronously, of course.)
143 void raise(exception_wrapper interrupt);
146 raise(FutureCancellation());
150 friend class Promise<T>;
152 friend class SemiFuture;
156 using corePtr = futures::detail::Core<T>*;
158 // shared core state object
161 explicit FutureBase(corePtr obj) : core_(obj) {}
163 explicit FutureBase(futures::detail::EmptyConstruct) noexcept;
167 void throwIfInvalid() const;
169 template <class FutureType>
170 void assign(FutureType&) noexcept;
172 Executor* getExecutor() {
173 return core_->getExecutor();
176 void setExecutor(Executor* x, int8_t priority = Executor::MID_PRI) {
177 core_->setExecutor(x, priority);
180 // Variant: returns a value
181 // e.g. f.then([](Try<T> t){ return t.value(); });
182 template <typename F, typename R, bool isTry, typename... Args>
183 typename std::enable_if<!R::ReturnsFuture::value, typename R::Return>::type
184 thenImplementation(F&& func, futures::detail::argResult<isTry, F, Args...>);
186 // Variant: returns a Future
187 // e.g. f.then([](Try<T> t){ return makeFuture<T>(t); });
188 template <typename F, typename R, bool isTry, typename... Args>
189 typename std::enable_if<R::ReturnsFuture::value, typename R::Return>::type
190 thenImplementation(F&& func, futures::detail::argResult<isTry, F, Args...>);
192 } // namespace detail
193 } // namespace futures
196 class SemiFuture : private futures::detail::FutureBase<T> {
198 using Base = futures::detail::FutureBase<T>;
199 using DeferredExecutor = futures::detail::DeferredExecutor;
202 static SemiFuture<T> makeEmpty(); // equivalent to moved-from
204 // Export public interface of FutureBase
205 // FutureBase is inherited privately to avoid subclasses being cast to
206 // a FutureBase pointer
207 using typename Base::value_type;
209 /// Construct a Future from a value (perfect forwarding)
212 typename = typename std::enable_if<
213 !isFuture<typename std::decay<T2>::type>::value &&
214 !isSemiFuture<typename std::decay<T2>::type>::value>::type>
215 /* implicit */ SemiFuture(T2&& val) : Base(std::forward<T2>(val)) {}
217 template <class T2 = T>
218 /* implicit */ SemiFuture(
219 typename std::enable_if<std::is_same<Unit, T2>::value>::type* p = nullptr)
224 typename std::enable_if<std::is_constructible<T, Args&&...>::value, int>::
226 explicit SemiFuture(in_place_t, Args&&... args)
227 : Base(in_place, std::forward<Args>(args)...) {}
229 SemiFuture(SemiFuture<T> const&) = delete;
231 SemiFuture(SemiFuture<T>&&) noexcept;
232 // safe move-constructabilty from Future
233 /* implicit */ SemiFuture(Future<T>&&) noexcept;
237 using Base::hasException;
238 using Base::hasValue;
239 using Base::isActive;
243 using Base::setCallback_;
246 SemiFuture& operator=(SemiFuture const&) = delete;
247 SemiFuture& operator=(SemiFuture&&) noexcept;
248 SemiFuture& operator=(Future<T>&&) noexcept;
250 /// Block until the future is fulfilled. Returns the value (moved out), or
251 /// throws the exception. The future must not already have a callback.
254 /// Block until the future is fulfilled, or until timed out. Returns the
255 /// value (moved out), or throws the exception (which might be a TimedOut
257 T get(Duration dur) &&;
259 /// Block until this Future is complete. Returns a reference to this Future.
260 SemiFuture<T>& wait() &;
262 /// Overload of wait() for rvalue Futures
263 SemiFuture<T>&& wait() &&;
265 /// Block until this Future is complete or until the given Duration passes.
266 /// Returns a reference to this Future
267 SemiFuture<T>& wait(Duration) &;
269 /// Overload of wait(Duration) for rvalue Futures
270 SemiFuture<T>&& wait(Duration) &&;
272 /// Returns an inactive Future which will call back on the other side of
273 /// executor (when it is activated).
275 /// NB remember that Futures activate when they destruct. This is good,
276 /// it means that this will work:
278 /// f.via(e).then(a).then(b);
280 /// a and b will execute in the same context (the far side of e), because
281 /// the Future (temporary variable) created by via(e) does not call back
282 /// until it destructs, which is after then(a) and then(b) have been wired
285 /// But this is still racy:
287 /// f = f.via(e).then(a);
289 // The ref-qualifier allows for `this` to be moved out so we
290 // don't get access-after-free situations in chaining.
291 // https://akrzemi1.wordpress.com/2014/06/02/ref-qualifiers/
292 inline Future<T> via(
294 int8_t priority = Executor::MID_PRI) &&;
297 * Defer work to run on the consumer of the future.
298 * This work will be run eithe ron an executor that the caller sets on the
299 * SemiFuture, or inline with the call to .get().
300 * NB: This is a custom method because boost-blocking executors is a
301 * special-case for work deferral in folly. With more general boost-blocking
302 * support all executors would boost block and we would simply use some form
303 * of driveable executor here.
305 template <typename F>
306 SemiFuture<typename futures::detail::callableResult<T, F>::Return::value_type>
309 // Public as for setCallback_
310 // Ensure that a boostable executor performs work to chain deferred work
315 friend class Promise<T>;
317 friend class futures::detail::FutureBase;
319 friend class SemiFuture;
321 using typename Base::corePtr;
322 using Base::setExecutor;
323 using Base::throwIfInvalid;
326 friend SemiFuture<T2> makeSemiFuture(Try<T2>&&);
328 explicit SemiFuture(corePtr obj) : Base(obj) {}
330 explicit SemiFuture(futures::detail::EmptyConstruct) noexcept
331 : Base(futures::detail::EmptyConstruct{}) {}
335 class Future : private futures::detail::FutureBase<T> {
337 using Base = futures::detail::FutureBase<T>;
340 // Export public interface of FutureBase
341 // FutureBase is inherited privately to avoid subclasses being cast to
342 // a FutureBase pointer
343 using typename Base::value_type;
345 /// Construct a Future from a value (perfect forwarding)
348 typename = typename std::enable_if<
349 !isFuture<typename std::decay<T2>::type>::value &&
350 !isSemiFuture<typename std::decay<T2>::type>::value>::type>
351 /* implicit */ Future(T2&& val) : Base(std::forward<T2>(val)) {}
353 template <class T2 = T>
354 /* implicit */ Future(
355 typename std::enable_if<std::is_same<Unit, T2>::value>::type* p = nullptr)
360 typename std::enable_if<std::is_constructible<T, Args&&...>::value, int>::
362 explicit Future(in_place_t, Args&&... args)
363 : Base(in_place, std::forward<Args>(args)...) {}
365 Future(Future<T> const&) = delete;
367 Future(Future<T>&&) noexcept;
372 typename std::enable_if<
373 !std::is_same<T, typename std::decay<T2>::type>::value &&
374 std::is_constructible<T, T2&&>::value &&
375 std::is_convertible<T2&&, T>::value,
377 /* implicit */ Future(Future<T2>&&);
380 typename std::enable_if<
381 !std::is_same<T, typename std::decay<T2>::type>::value &&
382 std::is_constructible<T, T2&&>::value &&
383 !std::is_convertible<T2&&, T>::value,
385 explicit Future(Future<T2>&&);
388 typename std::enable_if<
389 !std::is_same<T, typename std::decay<T2>::type>::value &&
390 std::is_constructible<T, T2&&>::value,
392 Future& operator=(Future<T2>&&);
396 using Base::hasException;
397 using Base::hasValue;
398 using Base::isActive;
402 using Base::setCallback_;
405 static Future<T> makeEmpty(); // equivalent to moved-from
408 Future& operator=(Future const&) = delete;
411 Future& operator=(Future&&) noexcept;
413 /// Call e->drive() repeatedly until the future is fulfilled. Examples
414 /// of DrivableExecutor include EventBase and ManualExecutor. Returns a
415 /// reference to the Try of the value.
416 Try<T>& getTryVia(DrivableExecutor* e);
418 /// Call e->drive() repeatedly until the future is fulfilled. Examples
419 /// of DrivableExecutor include EventBase and ManualExecutor. Returns the
420 /// value (moved out), or throws the exception.
421 T getVia(DrivableExecutor* e);
423 /// Unwraps the case of a Future<Future<T>> instance, and returns a simple
424 /// Future<T> instance.
425 template <class F = T>
427 enable_if<isFuture<F>::value, Future<typename isFuture<T>::Inner>>::type
430 /// Returns an inactive Future which will call back on the other side of
431 /// executor (when it is activated).
433 /// NB remember that Futures activate when they destruct. This is good,
434 /// it means that this will work:
436 /// f.via(e).then(a).then(b);
438 /// a and b will execute in the same context (the far side of e), because
439 /// the Future (temporary variable) created by via(e) does not call back
440 /// until it destructs, which is after then(a) and then(b) have been wired
443 /// But this is still racy:
445 /// f = f.via(e).then(a);
447 // The ref-qualifier allows for `this` to be moved out so we
448 // don't get access-after-free situations in chaining.
449 // https://akrzemi1.wordpress.com/2014/06/02/ref-qualifiers/
450 inline Future<T> via(
452 int8_t priority = Executor::MID_PRI) &&;
454 /// This variant creates a new future, where the ref-qualifier && version
455 /// moves `this` out. This one is less efficient but avoids confusing users
456 /// when "return f.via(x);" fails.
457 inline Future<T> via(
459 int8_t priority = Executor::MID_PRI) &;
461 /** When this Future has completed, execute func which is a function that
471 Func shall return either another Future or a value.
473 A Future for the return type of func is returned.
475 Future<string> f2 = f1.then([](Try<T>&&) { return string("foo"); });
477 The Future given to the functor is ready, and the functor may call
478 value(), which may rethrow if this has captured an exception. If func
479 throws, the exception will be captured in the Future that is returned.
481 template <typename F, typename R = futures::detail::callableResult<T, F>>
482 typename R::Return then(F&& func) {
483 return this->template thenImplementation<F, R>(
484 std::forward<F>(func), typename R::Arg());
487 /// Variant where func is an member function
489 /// struct Worker { R doWork(Try<T>); }
492 /// Future<R> f2 = f1.then(&Worker::doWork, w);
494 /// This is just sugar for
496 /// f1.then(std::bind(&Worker::doWork, w));
497 template <typename R, typename Caller, typename... Args>
498 Future<typename isFuture<R>::Inner> then(
499 R (Caller::*func)(Args...),
502 /// Execute the callback via the given Executor. The executor doesn't stick.
506 /// f.via(x).then(b).then(c)
510 /// f.then(x, b).then(c)
512 /// In the former both b and c execute via x. In the latter, only b executes
513 /// via x, and c executes via the same executor (if any) that f had.
514 template <class Executor, class Arg, class... Args>
515 auto then(Executor* x, Arg&& arg, Args&&... args) {
516 auto oldX = this->getExecutor();
517 this->setExecutor(x);
518 return this->then(std::forward<Arg>(arg), std::forward<Args>(args)...)
522 /// Convenience method for ignoring the value and creating a Future<Unit>.
523 /// Exceptions still propagate.
524 /// This function is identical to .unit().
527 /// Convenience method for ignoring the value and creating a Future<Unit>.
528 /// Exceptions still propagate.
529 /// This function is identical to parameterless .then().
530 Future<Unit> unit() {
534 /// Set an error callback for this Future. The callback should take a single
535 /// argument of the type that you want to catch, and should return a value of
536 /// the same type as this Future, or a Future of that type (see overload
537 /// below). For instance,
541 /// throw std::runtime_error("oh no!");
544 /// .onError([] (std::runtime_error& e) {
545 /// LOG(INFO) << "std::runtime_error: " << e.what();
546 /// return -1; // or makeFuture<int>(-1)
549 typename std::enable_if<
550 !futures::detail::callableWith<F, exception_wrapper>::value &&
551 !futures::detail::callableWith<F, exception_wrapper&>::value &&
552 !futures::detail::Extract<F>::ReturnsFuture::value,
556 /// Overload of onError where the error callback returns a Future<T>
558 typename std::enable_if<
559 !futures::detail::callableWith<F, exception_wrapper>::value &&
560 !futures::detail::callableWith<F, exception_wrapper&>::value &&
561 futures::detail::Extract<F>::ReturnsFuture::value,
565 /// Overload of onError that takes exception_wrapper and returns Future<T>
567 typename std::enable_if<
568 futures::detail::callableWith<F, exception_wrapper>::value &&
569 futures::detail::Extract<F>::ReturnsFuture::value,
573 /// Overload of onError that takes exception_wrapper and returns T
575 typename std::enable_if<
576 futures::detail::callableWith<F, exception_wrapper>::value &&
577 !futures::detail::Extract<F>::ReturnsFuture::value,
581 /// func is like std::function<void()> and is executed unconditionally, and
582 /// the value/exception is passed through to the resulting Future.
583 /// func shouldn't throw, but if it does it will be captured and propagated,
584 /// and discard any value/exception that this Future has obtained.
586 Future<T> ensure(F&& func);
588 /// Like onError, but for timeouts. example:
590 /// Future<int> f = makeFuture<int>(42)
591 /// .delayed(long_time)
592 /// .onTimeout(short_time,
593 /// []() -> int{ return -1; });
597 /// Future<int> f = makeFuture<int>(42)
598 /// .delayed(long_time)
599 /// .onTimeout(short_time,
600 /// []() { return makeFuture<int>(some_exception); });
602 Future<T> onTimeout(Duration, F&& func, Timekeeper* = nullptr);
604 /// A Future's callback is executed when all three of these conditions have
605 /// become true: it has a value (set by the Promise), it has a callback (set
606 /// by then), and it is active (active by default).
608 /// Inactive Futures will activate upon destruction.
609 FOLLY_DEPRECATED("do not use") Future<T>& activate() & {
610 this->core_->activate();
613 FOLLY_DEPRECATED("do not use") Future<T>& deactivate() & {
614 this->core_->deactivate();
617 FOLLY_DEPRECATED("do not use") Future<T> activate() && {
618 this->core_->activate();
619 return std::move(*this);
621 FOLLY_DEPRECATED("do not use") Future<T> deactivate() && {
622 this->core_->deactivate();
623 return std::move(*this);
626 /// Throw TimedOut if this Future does not complete within the given
627 /// duration from now. The optional Timeekeeper is as with futures::sleep().
628 Future<T> within(Duration, Timekeeper* = nullptr);
630 /// Throw the given exception if this Future does not complete within the
631 /// given duration from now. The optional Timeekeeper is as with
632 /// futures::sleep().
634 Future<T> within(Duration, E exception, Timekeeper* = nullptr);
636 /// Delay the completion of this Future for at least this duration from
637 /// now. The optional Timekeeper is as with futures::sleep().
638 Future<T> delayed(Duration, Timekeeper* = nullptr);
640 /// Block until the future is fulfilled. Returns the value (moved out), or
641 /// throws the exception. The future must not already have a callback.
644 /// Block until the future is fulfilled, or until timed out. Returns the
645 /// value (moved out), or throws the exception (which might be a TimedOut
649 /// Block until this Future is complete. Returns a reference to this Future.
652 /// Overload of wait() for rvalue Futures
653 Future<T>&& wait() &&;
655 /// Block until this Future is complete or until the given Duration passes.
656 /// Returns a reference to this Future
657 Future<T>& wait(Duration) &;
659 /// Overload of wait(Duration) for rvalue Futures
660 Future<T>&& wait(Duration) &&;
662 /// Call e->drive() repeatedly until the future is fulfilled. Examples
663 /// of DrivableExecutor include EventBase and ManualExecutor. Returns a
664 /// reference to this Future so that you can chain calls if desired.
665 /// value (moved out), or throws the exception.
666 Future<T>& waitVia(DrivableExecutor* e) &;
668 /// Overload of waitVia() for rvalue Futures
669 Future<T>&& waitVia(DrivableExecutor* e) &&;
671 /// If the value in this Future is equal to the given Future, when they have
672 /// both completed, the value of the resulting Future<bool> will be true. It
673 /// will be false otherwise (including when one or both Futures have an
675 Future<bool> willEqual(Future<T>&);
677 /// predicate behaves like std::function<bool(T const&)>
678 /// If the predicate does not obtain with the value, the result
679 /// is a folly::PredicateDoesNotObtain exception
681 Future<T> filter(F&& predicate);
683 /// Like reduce, but works on a Future<std::vector<T / Try<T>>>, for example
684 /// the result of collect or collectAll
685 template <class I, class F>
686 Future<I> reduce(I&& initial, F&& func);
688 /// Create a Future chain from a sequence of callbacks. i.e.
690 /// f.then(a).then(b).then(c)
692 /// where f is a Future<A> and the result of the chain is a Future<D>
695 /// f.thenMulti(a, b, c);
696 template <class Callback, class... Callbacks>
697 auto thenMulti(Callback&& fn, Callbacks&&... fns) {
698 // thenMulti with two callbacks is just then(a).thenMulti(b, ...)
699 return then(std::forward<Callback>(fn))
700 .thenMulti(std::forward<Callbacks>(fns)...);
703 template <class Callback>
704 auto thenMulti(Callback&& fn) {
705 // thenMulti with one callback is just a then
706 return then(std::forward<Callback>(fn));
709 /// Create a Future chain from a sequence of callbacks. i.e.
711 /// f.via(executor).then(a).then(b).then(c).via(oldExecutor)
713 /// where f is a Future<A> and the result of the chain is a Future<D>
716 /// f.thenMultiWithExecutor(executor, a, b, c);
717 template <class Callback, class... Callbacks>
718 auto thenMultiWithExecutor(Executor* x, Callback&& fn, Callbacks&&... fns) {
719 // thenMultiExecutor with two callbacks is
720 // via(x).then(a).thenMulti(b, ...).via(oldX)
721 auto oldX = this->getExecutor();
722 this->setExecutor(x);
723 return then(std::forward<Callback>(fn))
724 .thenMulti(std::forward<Callbacks>(fns)...)
728 template <class Callback>
729 auto thenMultiWithExecutor(Executor* x, Callback&& fn) {
730 // thenMulti with one callback is just a then with an executor
731 return then(x, std::forward<Callback>(fn));
734 // Convert this Future to a SemiFuture to safely export from a library
735 // without exposing a continuation interface
736 SemiFuture<T> semi() {
737 return SemiFuture<T>{std::move(*this)};
741 friend class Promise<T>;
743 friend class futures::detail::FutureBase;
747 friend class SemiFuture;
749 using Base::setExecutor;
750 using Base::throwIfInvalid;
751 using typename Base::corePtr;
753 explicit Future(corePtr obj) : Base(obj) {}
755 explicit Future(futures::detail::EmptyConstruct) noexcept
756 : Base(futures::detail::EmptyConstruct{}) {}
759 friend Future<T2> makeFuture(Try<T2>&&);
761 /// Repeat the given future (i.e., the computation it contains)
764 /// thunk behaves like std::function<Future<T2>(void)>
766 friend Future<Unit> times(int n, F&& thunk);
768 /// Carry out the computation contained in the given future if
769 /// the predicate holds.
771 /// thunk behaves like std::function<Future<T2>(void)>
773 friend Future<Unit> when(bool p, F&& thunk);
775 /// Carry out the computation contained in the given future if
776 /// while the predicate continues to hold.
778 /// thunk behaves like std::function<Future<T2>(void)>
780 /// predicate behaves like std::function<bool(void)>
781 template <class P, class F>
782 friend Future<Unit> whileDo(P&& predicate, F&& thunk);
787 #include <folly/futures/Future-inl.h>