1 /* Copyright (c) 2016-2022. 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/promise.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) const;
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(not 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 must 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 because 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 xbt_assert(status_ == FutureStatus::ready, "Deadlock: this future is not ready");
120 status_ = FutureStatus::done;
122 std::exception_ptr exception = std::move(exception_);
123 exception_ = nullptr;
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 * implicitly through a @ref simgrid::kernel::Promise.
138 * Alternatively kernel operations could inherit or contain FutureState
139 * if they are managed with std::shared_ptr.
142 class FutureState : public FutureStateBase {
144 void set_value(T value)
146 if (this->get_status() != FutureStatus::not_ready)
147 throw std::future_error(std::future_errc::promise_already_satisfied);
148 value_ = std::move(value);
155 xbt_assert(this->value_);
156 auto result = std::move(this->value_.get());
157 this->value_ = boost::optional<T>();
162 boost::optional<T> value_;
166 class FutureState<T&> : public FutureStateBase {
168 void set_value(T& value)
170 if (this->get_status() != FutureStatus::not_ready)
171 throw std::future_error(std::future_errc::promise_already_satisfied);
179 xbt_assert(this->value_);
190 class FutureState<void> : public FutureStateBase {
194 if (this->get_status() != FutureStatus::not_ready)
195 throw std::future_error(std::future_errc::promise_already_satisfied);
205 template <class T> void bind_promise(Promise<T>&& promise, Future<T> future)
207 class PromiseBinder {
209 explicit PromiseBinder(Promise<T>&& promise) : promise_(std::move(promise)) {}
210 void operator()(Future<T> future) { simgrid::xbt::set_promise(promise_, future); }
215 future.then_(PromiseBinder(std::move(promise)));
218 template <class T> Future<T> unwrap_future(Future<Future<T>> future);
220 /** Result of some (probably) asynchronous operation in the SimGrid kernel
222 * @ref simgrid::simix::Future and @ref simgrid::simix::Future provide an
223 * abstraction for asynchronous stuff happening in the SimGrid kernel. They
224 * are based on C++1z futures.
226 * The future represents a value which will be available at some point when this
227 * asynchronous operation is finished. Alternatively, if this operations fails,
228 * the result of the operation might be an exception.
230 * As the operation is possibly no terminated yet, we cannot get the result
231 * yet. Moreover, as we cannot block in the SimGrid kernel we cannot wait for
232 * it. However, we can attach some code/callback/continuation which will be
233 * executed when the operation terminates.
235 * Example of the API (`simgrid::kernel::createProcess` does not exist):
237 * // Create a new process using the Worker code, this process returns
239 * simgrid::kernel::Future<std::string> future =
240 * simgrid::kernel::createProcess("worker42", host, Worker(42));
241 * // At this point, we just created the process so the result is not available.
242 * // However, we can attach some work do be done with this result:
243 * future.then([](simgrid::kernel::Future<std::string> result) {
244 * // This code is called when the operation is completed so the result is
247 * // Try to get value, this might throw an exception if the operation
248 * // failed (such as an exception thrown by the worker process):
249 * std::string value = result.get();
250 * XBT_INFO("Value: %s", value.c_str());
252 * catch(std::exception& e) {
253 * // This is an exception from the asynchronous operation:
254 * XBT_INFO("Error: %e", e.what());
259 * This is based on C++1z std::future but with some differences:
261 * * there is no thread synchronization (atomic, mutex, condition variable,
262 * etc.) because everything happens in the SimGrid event loop;
264 * * it is purely asynchronous, you are expected to use `.then()`;
266 * * inside the `.then()`, `.get()` can be used;
268 * * `.get()` can only be used when `.is_ready()` (as everything happens in
269 * a single-thread, the future would be guaranteed to deadlock if `.get()`
270 * is called when the future is not ready);
272 * * there is no future chaining support for now (`.then().then()`);
274 * * there is no sharing (`shared_future`) for now.
280 explicit Future(std::shared_ptr<FutureState<T>> state) : state_(std::move(state)) {}
283 Future(Future&) = delete;
284 Future& operator=(const Future&) = delete;
285 Future(Future&&) noexcept = default;
286 Future& operator=(Future&&) noexcept = default;
288 /** Whether the future is valid:.
290 * A future which as been used (`.then` of `.get`) becomes invalid.
292 * We can use `.then` on a valid future.
296 return state_ != nullptr;
299 /** Whether the future is ready
301 * A future is ready when it has an associated value or exception.
303 * We can use `.get()` on ready futures.
305 bool is_ready() const
307 return state_ != nullptr && state_->is_ready();
310 /** Attach a continuation to this future
312 * This is like .then() but avoid the creation of a new future.
315 void then_(F continuation)
317 if (state_ == nullptr)
318 throw std::future_error(std::future_errc::no_state);
319 // Give shared-ownership to the continuation:
320 auto state = std::move(state_);
321 state->set_continuation(simgrid::xbt::make_task(std::move(continuation), state));
324 /** Attach a continuation to this future
326 * This version never does future unwrapping.
328 template <class F> auto then_no_unwrap(F continuation) -> Future<decltype(continuation(std::move(*this)))>
330 using R = decltype(continuation(std::move(*this)));
331 if (state_ == nullptr)
332 throw std::future_error(std::future_errc::no_state);
333 auto state = std::move(state_);
334 // Create a new future...
336 Future<R> future = promise.get_future();
337 // ...and when the current future is ready...
338 state->set_continuation(simgrid::xbt::make_task(
339 [](Promise<R> promise, std::shared_ptr<FutureState<T>> state, F continuation) {
340 // ...set the new future value by running the continuation.
341 Future<T> future(std::move(state));
342 simgrid::xbt::fulfill_promise(promise, [&continuation, &future] { return continuation(std::move(future)); });
344 std::move(promise), state, std::move(continuation)));
348 /** Attach a continuation to this future
350 * The future must be valid in order to make this call.
351 * The continuation is executed when the future becomes ready.
352 * The future becomes invalid after this call.
354 * @param continuation This function is called with a ready future
355 * the future is ready
356 * @exception std::future_error no state is associated with the future
359 auto then(F continuation) -> typename std::enable_if_t<not is_future<decltype(continuation(std::move(*this)))>::value,
360 Future<decltype(continuation(std::move(*this)))>>
362 return this->then_no_unwrap(std::move(continuation));
365 /** Attach a continuation to this future (future chaining) */
367 auto then(F continuation) -> typename std::enable_if_t<is_future<decltype(continuation(std::move(*this)))>::value,
368 decltype(continuation(std::move(*this)))>
370 return unwrap_future(this->then_no_unwrap(std::move(continuation)));
373 /** Get the value from the future
375 * The future must be valid and ready in order to make this call.
376 * std::future blocks when the future is not ready but we are
377 * completely single-threaded so blocking would be a deadlock.
378 * After the call, the future becomes invalid.
380 * @return value of the future
381 * @exception any Exception from the future
382 * @exception std::future_error no state is associated with the future
386 if (state_ == nullptr)
387 throw std::future_error(std::future_errc::no_state);
388 std::shared_ptr<FutureState<T>> state = std::move(state_);
393 std::shared_ptr<FutureState<T>> state_;
396 template <class T> Future<T> unwrap_future(Future<Future<T>> future)
399 Future<T> result = promise.get_future();
400 bind_promise(std::move(promise), std::move(future));
404 /** Producer side of a @ref simgrid::kernel::Future
406 * A @ref Promise is connected to some `Future` and can be used to
409 * Similar to std::promise
412 * // Create a promise and a future:
413 * auto promise = std::make_shared<simgrid::kernel::Promise<T>>();
414 * auto future = promise->get_future();
416 * simgrid::kernel::timer::Timer::set(date, [promise] {
418 * int value = compute_the_value();
420 * throw std::logic_error("Bad value");
421 * // Whenever the operation is completed, we set the value
423 * promise.set_value(value);
426 * // If an error occurred, we can set an exception which
427 * // will be thrown by future.get():
428 * promise.set_exception(std::current_exception());
432 * // Return the future to the caller:
440 explicit Promise(std::shared_ptr<FutureState<T>> state) : state_(std::move(state)) {}
443 Promise(Promise const&) = delete;
444 Promise& operator=(Promise const&) = delete;
445 Promise(Promise&& that) noexcept : state_(std::move(that.state_)) { std::swap(future_get_, that.future_get_); }
447 Promise& operator=(Promise&& that) noexcept
449 this->state_ = std::move(that.state_);
450 this->future_get_ = that.future_get_;
451 that.future_get_ = false;
454 Future<T> get_future()
456 if (state_ == nullptr)
457 throw std::future_error(std::future_errc::no_state);
459 throw std::future_error(std::future_errc::future_already_retrieved);
461 return Future<T>(state_);
463 void set_value(T value)
465 if (state_ == nullptr)
466 throw std::future_error(std::future_errc::no_state);
467 state_->set_value(std::move(value));
469 void set_exception(std::exception_ptr exception)
471 if (state_ == nullptr)
472 throw std::future_error(std::future_errc::no_state);
473 state_->set_exception(std::move(exception));
477 if (state_ && state_->get_status() == FutureStatus::not_ready)
478 state_->set_exception(std::make_exception_ptr(
479 std::future_error(std::future_errc::broken_promise)));
483 std::shared_ptr<FutureState<T>> state_ = std::make_shared<FutureState<T>>();
484 bool future_get_ = false;
488 class Promise<void> {
491 explicit Promise(std::shared_ptr<FutureState<void>> state) : state_(std::move(state)) {}
494 if (state_ && state_->get_status() == FutureStatus::not_ready)
495 state_->set_exception(std::make_exception_ptr(
496 std::future_error(std::future_errc::broken_promise)));
500 Promise(Promise const&) = delete;
501 Promise& operator=(Promise const&) = delete;
502 Promise(Promise&& that) noexcept : state_(std::move(that.state_)) { std::swap(future_get_, that.future_get_); }
503 Promise& operator=(Promise&& that) noexcept
505 this->state_ = std::move(that.state_);
506 this->future_get_ = that.future_get_;
507 that.future_get_ = false;
511 Future<void> get_future()
513 if (state_ == nullptr)
514 throw std::future_error(std::future_errc::no_state);
516 throw std::future_error(std::future_errc::future_already_retrieved);
518 return Future<void>(state_);
520 void set_value() const
522 if (state_ == nullptr)
523 throw std::future_error(std::future_errc::no_state);
526 void set_exception(std::exception_ptr exception) const
528 if (state_ == nullptr)
529 throw std::future_error(std::future_errc::no_state);
530 state_->set_exception(std::move(exception));
534 std::shared_ptr<FutureState<void>> state_ = std::make_shared<FutureState<void>>();
535 bool future_get_ = false;