1 /* Copyright (c) 2016. The SimGrid Team.
2 * All rights reserved. */
4 /* This program is free software; you can redistribute it and/or modify it
5 * under the terms of the license (GNU LGPL) which comes with this package. */
7 #ifndef SIMGRID_KERNEL_FUTURE_HPP
8 #define SIMGRID_KERNEL_FUTURE_HPP
14 #include <type_traits>
16 #include <boost/optional.hpp>
19 #include <xbt/functional.hpp>
20 #include <xbt/future.hpp>
25 // There are the public classes:
26 template<class T> class Future;
27 template<class T> class Promise;
29 // Those are implementation details:
30 enum class FutureStatus;
31 template<class T> class FutureState;
33 enum class FutureStatus {
40 struct is_future : std::false_type {};
42 struct is_future<Future<T>> : std::true_type {};
44 /** Bases stuff for all @ref simgrid::kernel::FutureState<T> */
45 class FutureStateBase {
48 FutureStateBase(FutureStateBase const&) = delete;
49 FutureStateBase& operator=(FutureStateBase const&) = delete;
51 XBT_PUBLIC(void) schedule(simgrid::xbt::Task<void()>&& job);
53 void set_exception(std::exception_ptr exception)
55 xbt_assert(exception_ == nullptr);
56 if (status_ != FutureStatus::not_ready)
57 throw std::future_error(std::future_errc::promise_already_satisfied);
58 exception_ = std::move(exception);
62 void set_continuation(simgrid::xbt::Task<void()>&& continuation)
64 xbt_assert(!continuation_);
66 case FutureStatus::done:
67 // This is not supposed to happen if continuation is set
69 xbt_die("Set continuation on finished future");
71 case FutureStatus::ready:
72 // The future is ready, execute the continuation directly.
73 // We might execute it from the event loop instead:
74 schedule(std::move(continuation));
76 case FutureStatus::not_ready:
77 // The future is not ready so we mast keep the continuation for
78 // executing it later:
79 continuation_ = std::move(continuation);
86 FutureStatus get_status() const
93 return status_ == FutureStatus::ready;
97 FutureStateBase() = default;
98 ~FutureStateBase() = default;
100 /** Set the future as ready and trigger the continuation */
103 status_ = FutureStatus::ready;
105 // We unregister the continuation before executing it.
106 // We need to do this becase the current implementation of the
107 // continuation has a shared_ptr to the FutureState.
108 auto continuation = std::move(continuation_);
109 this->schedule(std::move(continuation));
113 /** Set the future as done and raise an exception if any
115 * This does half the job of `.get()`.
119 if (status_ != FutureStatus::ready)
120 xbt_die("Deadlock: this future is not ready");
121 status_ = FutureStatus::done;
123 std::exception_ptr exception = std::move(exception_);
124 std::rethrow_exception(std::move(exception));
129 FutureStatus status_ = FutureStatus::not_ready;
130 std::exception_ptr exception_;
131 simgrid::xbt::Task<void()> continuation_;
134 /** Shared state for future and promises
136 * You are not expected to use them directly but to create them
137 * implicitely through a @ref simgrid::kernel::Promise.
138 * Alternatively kernel operations could inherit or contain FutureState
139 * if they are managed with @ref std::shared_ptr.
142 class FutureState : public FutureStateBase {
145 void set_value(T value)
147 if (this->get_status() != FutureStatus::not_ready)
148 throw std::future_error(std::future_errc::promise_already_satisfied);
149 value_ = std::move(value);
156 xbt_assert(this->value_);
157 auto result = std::move(this->value_.get());
158 this->value_ = boost::optional<T>();
159 return std::move(result);
163 boost::optional<T> value_;
167 class FutureState<T&> : public FutureStateBase {
169 void set_value(T& value)
171 if (this->get_status() != FutureStatus::not_ready)
172 throw std::future_error(std::future_errc::promise_already_satisfied);
180 xbt_assert(this->value_);
191 class FutureState<void> : public FutureStateBase {
195 if (this->get_status() != FutureStatus::not_ready)
196 throw std::future_error(std::future_errc::promise_already_satisfied);
207 void bindPromise(Promise<T> promise, Future<T> future)
209 struct PromiseBinder {
211 PromiseBinder(Promise<T> promise) : promise_(std::move(promise)) {}
212 void operator()(Future<T> future)
214 simgrid::xbt::setPromise(promise_, future);
219 future.then_(PromiseBinder(std::move(promise)));
222 template<class T> Future<T> unwrapFuture(Future<Future<T>> future);
224 /** Result of some (probably) asynchronous operation in the SimGrid kernel
226 * @ref simgrid::simix::Future and @ref simgrid::simix::Future provide an
227 * abstration for asynchronous stuff happening in the SimGrid kernel. They
228 * are based on C++1z futures.
230 * The future represents a value which will be available at some point when this
231 * asynchronous operaiont is finished. Alternatively, if this operations fails,
232 * the result of the operation might be an exception.
234 * As the operation is possibly no terminated yet, we cannot get the result
235 * yet. Moreover, as we cannot block in the SimGrid kernel we cannot wait for
236 * it. However, we can attach some code/callback/continuation which will be
237 * executed when the operation terminates.
239 * Example of the API (`simgrid::kernel::createProcess` does not exist):
241 * // Create a new process using the Worker code, this process returns
243 * simgrid::kernel::Future<std::string> future =
244 * simgrid::kernel::createProcess("worker42", host, Worker(42));
245 * // At this point, we just created the process so the result is not available.
246 * // However, we can attach some work do be done with this result:
247 * future.then([](simgrid::kernel::Future<std::string> result) {
248 * // This code is called when the operation is completed so the result is
251 * // Try to get value, this might throw an exception if the operation
252 * // failed (such as an exception throwed by the worker process):
253 * std::string value = result.get();
254 * XBT_INFO("Value: %s", value.c_str());
256 * catch(std::exception& e) {
257 * // This is an exception from the asynchronous operation:
258 * XBT_INFO("Error: %e", e.what());
263 * This is based on C++1z @ref std::future but with some differences:
265 * * there is no thread synchronization (atomic, mutex, condition variable,
266 * etc.) because everything happens in the SimGrid event loop;
268 * * it is purely asynchronous, you are expected to use `.then()`;
270 * * inside the `.then()`, `.get()` can be used;
272 * * `.get()` can only be used when `.is_ready()` (as everything happens in
273 * a single-thread, the future would be guaranted to deadlock if `.get()`
274 * is called when the future is not ready);
276 * * there is no future chaining support for now (`.then().then()`);
278 * * there is no sharing (`shared_future`) for now.
284 Future(std::shared_ptr<FutureState<T>> state): state_(std::move(state)) {}
287 Future(Future&) = delete;
288 Future& operator=(Future&) = delete;
289 Future(Future&& that) : state_(std::move(that.state_)) {}
290 Future& operator=(Future&& that)
292 state_ = std::move(that.state_);
296 /** Whether the future is valid:.
298 * A future which as been used (`.then` of `.get`) becomes invalid.
300 * We can use `.then` on a valid future.
304 return state_ != nullptr;
307 /** Whether the future is ready
309 * A future is ready when it has an associated value or exception.
311 * We can use `.get()` on ready futures.
313 bool is_ready() const
315 return state_ != nullptr && state_->is_ready();
318 /** Attach a continuation to this future
320 * This is like .then() but avoid the creation of a new future.
323 void then_(F continuation)
325 if (state_ == nullptr)
326 throw std::future_error(std::future_errc::no_state);
327 // Give shared-ownership to the continuation:
328 auto state = std::move(state_);
329 state->set_continuation(simgrid::xbt::makeTask(
330 std::move(continuation), state));
333 /** Attach a continuation to this future
335 * This version never does future unwrapping.
338 auto thenNoUnwrap(F continuation)
339 -> Future<decltype(continuation(std::move(*this)))>
341 typedef decltype(continuation(std::move(*this))) R;
342 if (state_ == nullptr)
343 throw std::future_error(std::future_errc::no_state);
344 auto state = std::move(state_);
345 // Create a new future...
347 Future<R> future = promise.get_future();
348 // ...and when the current future is ready...
349 state->set_continuation(simgrid::xbt::makeTask(
350 [](Promise<R> promise, std::shared_ptr<FutureState<T>> state, F continuation) {
351 // ...set the new future value by running the continuation.
352 Future<T> future(std::move(state));
353 simgrid::xbt::fulfillPromise(promise,[&]{
354 return continuation(std::move(future));
357 std::move(promise), state, std::move(continuation)));
358 return std::move(future);
361 /** Attach a continuation to this future
363 * The future must be valid in order to make this call.
364 * The continuation is executed when the future becomes ready.
365 * The future becomes invalid after this call.
367 * @param continuation This function is called with a ready future
368 * the future is ready
369 * @exception std::future_error no state is associated with the future
372 auto then(F continuation)
373 -> typename std::enable_if<
374 !is_future<decltype(continuation(std::move(*this)))>::value,
375 Future<decltype(continuation(std::move(*this)))>
378 return this->thenNoUnwrap(std::move(continuation));
381 /** Attach a continuation to this future (future chaining) */
383 auto then(F continuation)
384 -> typename std::enable_if<
385 is_future<decltype(continuation(std::move(*this)))>::value,
386 decltype(continuation(std::move(*this)))
389 return unwrapFuture(this->thenNoUnwap(std::move(continuation)));
392 /** Get the value from the future
394 * This is expected to be called
396 * The future must be valid and ready in order to make this call.
397 * @ref std::future blocks when the future is not ready but we are
398 * completely single-threaded so blocking would be a deadlock.
399 * After the call, the future becomes invalid.
401 * @return value of the future
402 * @exception any Exception from the future
403 * @exception std::future_error no state is associated with the future
407 if (state_ == nullptr)
408 throw std::future_error(std::future_errc::no_state);
409 std::shared_ptr<FutureState<T>> state = std::move(state_);
414 std::shared_ptr<FutureState<T>> state_;
418 Future<T> unwrapFuture(Future<Future<T>> future)
421 Future<T> result = promise.get_future();
422 bindPromise(std::move(promise), std::move(future));
423 return std::move(result);
426 /** Producer side of a @simgrid::kernel::Future
428 * A @ref Promise is connected to some `Future` and can be used to
431 * Similar to @ref std::promise
434 * // Create a promise and a future:
435 * auto promise = std::make_shared<simgrid::kernel::Promise<T>>();
436 * auto future = promise->get_future();
438 * SIMIX_timer_set(date, [promise] {
440 * int value = compute_the_value();
442 * throw std::logic_error("Bad value");
443 * // Whenever the operation is completed, we set the value
445 * promise.set_value(value);
448 * // If an error occured, we can set an exception which
449 * // will be throwed buy future.get():
450 * promise.set_exception(std::current_exception());
454 * // Return the future to the caller:
461 Promise() : state_(std::make_shared<FutureState<T>>()) {}
462 Promise(std::shared_ptr<FutureState<T>> state) : state_(std::move(state)) {}
465 Promise(Promise const&) = delete;
466 Promise& operator=(Promise const&) = delete;
467 Promise(Promise&& that) :
468 state_(std::move(that.state_)), future_get_(that.future_get_)
470 that.future_get_ = false;
473 Promise& operator=(Promise&& that)
475 this->state_ = std::move(that.state_);
476 this->future_get_ = that.future_get_;
477 that.future_get_ = false;
480 Future<T> get_future()
482 if (state_ == nullptr)
483 throw std::future_error(std::future_errc::no_state);
485 throw std::future_error(std::future_errc::future_already_retrieved);
487 return Future<T>(state_);
489 void set_value(T value)
491 if (state_ == nullptr)
492 throw std::future_error(std::future_errc::no_state);
493 state_->set_value(std::move(value));
495 void set_exception(std::exception_ptr exception)
497 if (state_ == nullptr)
498 throw std::future_error(std::future_errc::no_state);
499 state_->set_exception(std::move(exception));
503 if (state_ && state_->get_status() == FutureStatus::not_ready)
504 state_->set_exception(std::make_exception_ptr(
505 std::future_error(std::future_errc::broken_promise)));
509 std::shared_ptr<FutureState<T>> state_;
510 bool future_get_ = false;
514 class Promise<void> {
516 Promise() : state_(std::make_shared<FutureState<void>>()) {}
517 Promise(std::shared_ptr<FutureState<void>> state) : state_(std::move(state)) {}
520 if (state_ && state_->get_status() == FutureStatus::not_ready)
521 state_->set_exception(std::make_exception_ptr(
522 std::future_error(std::future_errc::broken_promise)));
526 Promise(Promise const&) = delete;
527 Promise& operator=(Promise const&) = delete;
528 Promise(Promise&& that) :
529 state_(std::move(that.state_)), future_get_(that.future_get_)
531 that.future_get_ = false;
533 Promise& operator=(Promise&& that)
535 this->state_ = std::move(that.state_);
536 this->future_get_ = that.future_get_;
537 that.future_get_ = false;
541 Future<void> get_future()
543 if (state_ == nullptr)
544 throw std::future_error(std::future_errc::no_state);
546 throw std::future_error(std::future_errc::future_already_retrieved);
548 return Future<void>(state_);
552 if (state_ == nullptr)
553 throw std::future_error(std::future_errc::no_state);
556 void set_exception(std::exception_ptr exception)
558 if (state_ == nullptr)
559 throw std::future_error(std::future_errc::no_state);
560 state_->set_exception(std::move(exception));
564 std::shared_ptr<FutureState<void>> state_;
565 bool future_get_ = false;