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/Try.h>
29 #include <folly/Utility.h>
30 #include <folly/futures/DrivableExecutor.h>
31 #include <folly/futures/FutureException.h>
32 #include <folly/futures/Promise.h>
33 #include <folly/futures/detail/Types.h>
35 // boring predeclarations and details
36 #include <folly/futures/Future-pre.h>
38 // not-boring helpers, e.g. all in folly::futures, makeFuture variants, etc.
39 // Needs to be included after Future-pre.h and before Future-inl.h
40 #include <folly/futures/helpers.h>
49 static Future<T> makeEmpty(); // equivalent to moved-from
52 Future(Future const&) = delete;
53 Future& operator=(Future const&) = delete;
56 Future(Future&&) noexcept;
57 Future& operator=(Future&&) noexcept;
62 typename std::enable_if<
63 !std::is_same<T, typename std::decay<T2>::type>::value &&
64 std::is_constructible<T, T2&&>::value &&
65 std::is_convertible<T2&&, T>::value,
67 /* implicit */ Future(Future<T2>&&);
70 typename std::enable_if<
71 !std::is_same<T, typename std::decay<T2>::type>::value &&
72 std::is_constructible<T, T2&&>::value &&
73 !std::is_convertible<T2&&, T>::value,
75 explicit Future(Future<T2>&&);
78 typename std::enable_if<
79 !std::is_same<T, typename std::decay<T2>::type>::value &&
80 std::is_constructible<T, T2&&>::value,
82 Future& operator=(Future<T2>&&);
84 /// Construct a Future from a value (perfect forwarding)
85 template <class T2 = T, typename =
86 typename std::enable_if<
87 !isFuture<typename std::decay<T2>::type>::value>::type>
88 /* implicit */ Future(T2&& val);
90 template <class T2 = T>
91 /* implicit */ Future(
92 typename std::enable_if<std::is_same<Unit, T2>::value>::type* = nullptr);
96 typename std::enable_if<std::is_constructible<T, Args&&...>::value, int>::
98 explicit Future(in_place_t, Args&&... args);
102 /** Return the reference to result. Should not be called if !isReady().
103 Will rethrow the exception if an exception has been
106 typename std::add_lvalue_reference<T>::type
108 typename std::add_lvalue_reference<const T>::type
111 /// Returns an inactive Future which will call back on the other side of
112 /// executor (when it is activated).
114 /// NB remember that Futures activate when they destruct. This is good,
115 /// it means that this will work:
117 /// f.via(e).then(a).then(b);
119 /// a and b will execute in the same context (the far side of e), because
120 /// the Future (temporary variable) created by via(e) does not call back
121 /// until it destructs, which is after then(a) and then(b) have been wired
124 /// But this is still racy:
126 /// f = f.via(e).then(a);
128 // The ref-qualifier allows for `this` to be moved out so we
129 // don't get access-after-free situations in chaining.
130 // https://akrzemi1.wordpress.com/2014/06/02/ref-qualifiers/
131 inline Future<T> via(
133 int8_t priority = Executor::MID_PRI) &&;
135 /// This variant creates a new future, where the ref-qualifier && version
136 /// moves `this` out. This one is less efficient but avoids confusing users
137 /// when "return f.via(x);" fails.
138 inline Future<T> via(
140 int8_t priority = Executor::MID_PRI) &;
142 /** True when the result (or exception) is ready. */
143 bool isReady() const;
145 /// sugar for getTry().hasValue()
148 /// sugar for getTry().hasException()
151 /** A reference to the Try of the value */
154 /// Call e->drive() repeatedly until the future is fulfilled. Examples
155 /// of DrivableExecutor include EventBase and ManualExecutor. Returns a
156 /// reference to the Try of the value.
157 Try<T>& getTryVia(DrivableExecutor* e);
159 /// If the promise has been fulfilled, return an Optional with the Try<T>.
160 /// Otherwise return an empty Optional.
161 /// Note that this moves the Try<T> out.
162 Optional<Try<T>> poll();
164 /// Block until the future is fulfilled. Returns the value (moved out), or
165 /// throws the exception. The future must not already have a callback.
168 /// Block until the future is fulfilled, or until timed out. Returns the
169 /// value (moved out), or throws the exception (which might be a TimedOut
173 /// Call e->drive() repeatedly until the future is fulfilled. Examples
174 /// of DrivableExecutor include EventBase and ManualExecutor. Returns the
175 /// value (moved out), or throws the exception.
176 T getVia(DrivableExecutor* e);
178 /// Unwraps the case of a Future<Future<T>> instance, and returns a simple
179 /// Future<T> instance.
180 template <class F = T>
181 typename std::enable_if<isFuture<F>::value,
182 Future<typename isFuture<T>::Inner>>::type
185 /** When this Future has completed, execute func which is a function that
195 Func shall return either another Future or a value.
197 A Future for the return type of func is returned.
199 Future<string> f2 = f1.then([](Try<T>&&) { return string("foo"); });
201 The Future given to the functor is ready, and the functor may call
202 value(), which may rethrow if this has captured an exception. If func
203 throws, the exception will be captured in the Future that is returned.
205 // gcc 4.8 requires that we cast function reference types to function pointer
206 // types. Fore more details see the comment on FunctionReferenceToPointer
208 // gcc versions 4.9 and above (as well as clang) do not require this hack.
209 // For those, the FF tenplate parameter can be removed and occurences of FF
214 typename futures::detail::FunctionReferenceToPointer<F>::type,
215 typename R = futures::detail::callableResult<T, FF>>
216 typename R::Return then(F&& func) {
217 typedef typename R::Arg Arguments;
218 return thenImplementation<FF, R>(std::forward<FF>(func), Arguments());
221 /// Variant where func is an member function
223 /// struct Worker { R doWork(Try<T>); }
226 /// Future<R> f2 = f1.then(&Worker::doWork, w);
228 /// This is just sugar for
230 /// f1.then(std::bind(&Worker::doWork, w));
231 template <typename R, typename Caller, typename... Args>
232 Future<typename isFuture<R>::Inner>
233 then(R(Caller::*func)(Args...), Caller *instance);
235 /// Execute the callback via the given Executor. The executor doesn't stick.
239 /// f.via(x).then(b).then(c)
243 /// f.then(x, b).then(c)
245 /// In the former both b and c execute via x. In the latter, only b executes
246 /// via x, and c executes via the same executor (if any) that f had.
247 template <class Executor, class Arg, class... Args>
248 auto then(Executor* x, Arg&& arg, Args&&... args) {
249 auto oldX = getExecutor();
251 return this->then(std::forward<Arg>(arg), std::forward<Args>(args)...)
255 /// Convenience method for ignoring the value and creating a Future<Unit>.
256 /// Exceptions still propagate.
259 /// Set an error callback for this Future. The callback should take a single
260 /// argument of the type that you want to catch, and should return a value of
261 /// the same type as this Future, or a Future of that type (see overload
262 /// below). For instance,
266 /// throw std::runtime_error("oh no!");
269 /// .onError([] (std::runtime_error& e) {
270 /// LOG(INFO) << "std::runtime_error: " << e.what();
271 /// return -1; // or makeFuture<int>(-1)
274 typename std::enable_if<
275 !futures::detail::callableWith<F, exception_wrapper>::value &&
276 !futures::detail::Extract<F>::ReturnsFuture::value,
280 /// Overload of onError where the error callback returns a Future<T>
282 typename std::enable_if<
283 !futures::detail::callableWith<F, exception_wrapper>::value &&
284 futures::detail::Extract<F>::ReturnsFuture::value,
288 /// Overload of onError that takes exception_wrapper and returns Future<T>
290 typename std::enable_if<
291 futures::detail::callableWith<F, exception_wrapper>::value &&
292 futures::detail::Extract<F>::ReturnsFuture::value,
296 /// Overload of onError that takes exception_wrapper and returns T
298 typename std::enable_if<
299 futures::detail::callableWith<F, exception_wrapper>::value &&
300 !futures::detail::Extract<F>::ReturnsFuture::value,
304 /// func is like std::function<void()> and is executed unconditionally, and
305 /// the value/exception is passed through to the resulting Future.
306 /// func shouldn't throw, but if it does it will be captured and propagated,
307 /// and discard any value/exception that this Future has obtained.
309 Future<T> ensure(F&& func);
311 /// Like onError, but for timeouts. example:
313 /// Future<int> f = makeFuture<int>(42)
314 /// .delayed(long_time)
315 /// .onTimeout(short_time,
316 /// []() -> int{ return -1; });
320 /// Future<int> f = makeFuture<int>(42)
321 /// .delayed(long_time)
322 /// .onTimeout(short_time,
323 /// []() { return makeFuture<int>(some_exception); });
325 Future<T> onTimeout(Duration, F&& func, Timekeeper* = nullptr);
327 /// This is not the method you're looking for.
329 /// This needs to be public because it's used by make* and when*, and it's
330 /// not worth listing all those and their fancy template signatures as
331 /// friends. But it's not for public consumption.
333 void setCallback_(F&& func);
335 /// A Future's callback is executed when all three of these conditions have
336 /// become true: it has a value (set by the Promise), it has a callback (set
337 /// by then), and it is active (active by default).
339 /// Inactive Futures will activate upon destruction.
340 FOLLY_DEPRECATED("do not use") Future<T>& activate() & {
344 FOLLY_DEPRECATED("do not use") Future<T>& deactivate() & {
348 FOLLY_DEPRECATED("do not use") Future<T> activate() && {
350 return std::move(*this);
352 FOLLY_DEPRECATED("do not use") Future<T> deactivate() && {
354 return std::move(*this);
358 return core_->isActive();
362 void raise(E&& exception) {
363 raise(make_exception_wrapper<typename std::remove_reference<E>::type>(
364 std::forward<E>(exception)));
367 /// Raise an interrupt. If the promise holder has an interrupt
368 /// handler it will be called and potentially stop asynchronous work from
369 /// being done. This is advisory only - a promise holder may not set an
370 /// interrupt handler, or may do anything including ignore. But, if you know
371 /// your future supports this the most likely result is stopping or
372 /// preventing the asynchronous operation (if in time), and the promise
373 /// holder setting an exception on the future. (That may happen
374 /// asynchronously, of course.)
375 void raise(exception_wrapper interrupt);
378 raise(FutureCancellation());
381 /// Throw TimedOut if this Future does not complete within the given
382 /// duration from now. The optional Timeekeeper is as with futures::sleep().
383 Future<T> within(Duration, Timekeeper* = nullptr);
385 /// Throw the given exception if this Future does not complete within the
386 /// given duration from now. The optional Timeekeeper is as with
387 /// futures::sleep().
389 Future<T> within(Duration, E exception, Timekeeper* = nullptr);
391 /// Delay the completion of this Future for at least this duration from
392 /// now. The optional Timekeeper is as with futures::sleep().
393 Future<T> delayed(Duration, Timekeeper* = nullptr);
395 /// Block until this Future is complete. Returns a reference to this Future.
398 /// Overload of wait() for rvalue Futures
399 Future<T>&& wait() &&;
401 /// Block until this Future is complete or until the given Duration passes.
402 /// Returns a reference to this Future
403 Future<T>& wait(Duration) &;
405 /// Overload of wait(Duration) for rvalue Futures
406 Future<T>&& wait(Duration) &&;
408 /// Call e->drive() repeatedly until the future is fulfilled. Examples
409 /// of DrivableExecutor include EventBase and ManualExecutor. Returns a
410 /// reference to this Future so that you can chain calls if desired.
411 /// value (moved out), or throws the exception.
412 Future<T>& waitVia(DrivableExecutor* e) &;
414 /// Overload of waitVia() for rvalue Futures
415 Future<T>&& waitVia(DrivableExecutor* e) &&;
417 /// If the value in this Future is equal to the given Future, when they have
418 /// both completed, the value of the resulting Future<bool> will be true. It
419 /// will be false otherwise (including when one or both Futures have an
421 Future<bool> willEqual(Future<T>&);
423 /// predicate behaves like std::function<bool(T const&)>
424 /// If the predicate does not obtain with the value, the result
425 /// is a folly::PredicateDoesNotObtain exception
427 Future<T> filter(F&& predicate);
429 /// Like reduce, but works on a Future<std::vector<T / Try<T>>>, for example
430 /// the result of collect or collectAll
431 template <class I, class F>
432 Future<I> reduce(I&& initial, F&& func);
434 /// Create a Future chain from a sequence of callbacks. i.e.
436 /// f.then(a).then(b).then(c)
438 /// where f is a Future<A> and the result of the chain is a Future<D>
441 /// f.thenMulti(a, b, c);
442 template <class Callback, class... Callbacks>
443 auto thenMulti(Callback&& fn, Callbacks&&... fns) {
444 // thenMulti with two callbacks is just then(a).thenMulti(b, ...)
445 return then(std::forward<Callback>(fn))
446 .thenMulti(std::forward<Callbacks>(fns)...);
449 template <class Callback>
450 auto thenMulti(Callback&& fn) {
451 // thenMulti with one callback is just a then
452 return then(std::forward<Callback>(fn));
455 /// Create a Future chain from a sequence of callbacks. i.e.
457 /// f.via(executor).then(a).then(b).then(c).via(oldExecutor)
459 /// where f is a Future<A> and the result of the chain is a Future<D>
462 /// f.thenMultiWithExecutor(executor, a, b, c);
463 template <class Callback, class... Callbacks>
464 auto thenMultiWithExecutor(Executor* x, Callback&& fn, Callbacks&&... fns) {
465 // thenMultiExecutor with two callbacks is
466 // via(x).then(a).thenMulti(b, ...).via(oldX)
467 auto oldX = getExecutor();
469 return then(std::forward<Callback>(fn))
470 .thenMulti(std::forward<Callbacks>(fns)...)
474 template <class Callback>
475 auto thenMultiWithExecutor(Executor* x, Callback&& fn) {
476 // thenMulti with one callback is just a then with an executor
477 return then(x, std::forward<Callback>(fn));
480 /// Discard a result, but propagate an exception.
481 Future<Unit> unit() {
482 return then([]{ return Unit{}; });
486 typedef futures::detail::Core<T>* corePtr;
488 // shared core state object
492 Future(corePtr obj) : core_(obj) {}
494 explicit Future(futures::detail::EmptyConstruct) noexcept;
498 void throwIfInvalid() const;
500 friend class Promise<T>;
501 template <class> friend class Future;
504 friend Future<T2> makeFuture(Try<T2>&&);
506 /// Repeat the given future (i.e., the computation it contains)
509 /// thunk behaves like std::function<Future<T2>(void)>
511 friend Future<Unit> times(int n, F&& thunk);
513 /// Carry out the computation contained in the given future if
514 /// the predicate holds.
516 /// thunk behaves like std::function<Future<T2>(void)>
518 friend Future<Unit> when(bool p, F&& thunk);
520 /// Carry out the computation contained in the given future if
521 /// while the predicate continues to hold.
523 /// thunk behaves like std::function<Future<T2>(void)>
525 /// predicate behaves like std::function<bool(void)>
526 template <class P, class F>
527 friend Future<Unit> whileDo(P&& predicate, F&& thunk);
529 // Variant: returns a value
530 // e.g. f.then([](Try<T> t){ return t.value(); });
531 template <typename F, typename R, bool isTry, typename... Args>
532 typename std::enable_if<!R::ReturnsFuture::value, typename R::Return>::type
533 thenImplementation(F&& func, futures::detail::argResult<isTry, F, Args...>);
535 // Variant: returns a Future
536 // e.g. f.then([](Try<T> t){ return makeFuture<T>(t); });
537 template <typename F, typename R, bool isTry, typename... Args>
538 typename std::enable_if<R::ReturnsFuture::value, typename R::Return>::type
539 thenImplementation(F&& func, futures::detail::argResult<isTry, F, Args...>);
541 Executor* getExecutor() { return core_->getExecutor(); }
542 void setExecutor(Executor* x, int8_t priority = Executor::MID_PRI) {
543 core_->setExecutor(x, priority);
549 #include <folly/futures/Future-inl.h>