await.hpp

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/*
    Copyright 2015 Adobe
    Distributed under the Boost Software License, Version 1.0.
    (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
*/
 
/**************************************************************************************************/
 
#ifndef STLAB_CONCURRENCY_AWAIT_HPP
#define STLAB_CONCURRENCY_AWAIT_HPP

 
#include <stlab/config.hpp>
 
#include <chrono>
#include <condition_variable>
#include <exception>
#include <mutex>
#include <utility>
 
#include <stlab/concurrency/future.hpp>
#include <stlab/concurrency/immediate_executor.hpp>
#include <stlab/concurrency/ready_future.hpp>
#include <stlab/memory.hpp>
 
#include <stlab/concurrency/default_executor.hpp>
 
/**************************************************************************************************/
 
namespace stlab {
STLAB_VERSION_NAMESPACE_BEGIN()
 

 
/**************************************************************************************************/
 

template <class F>
auto invoke_waiting(F&& f) {
    if (!stlab_v2_default_executor_supports_blocking()) std::terminate();
    stlab_v2_notify_default_executor_before_waiting();
 
    return std::forward<F>(f)();
}
 
/**************************************************************************************************/

 
template <class T>
auto await(future<T>&& x) -> T {
    if (x.is_ready()) return std::move(x).get_ready(); // if ready, done
    if (!stlab_v2_default_executor_supports_blocking()) std::terminate();
 
    std::mutex m;
    std::condition_variable condition;
    future<T> result;
 
    auto hold = std::move(x).recover(immediate_executor, [&](future<T>&& r) {
        {
            std::unique_lock<std::mutex> lock{m};
            result = std::move(r);
            condition.notify_one(); // must notify under lock
        }
    });
    std::unique_lock<std::mutex> lock{m};
    invoke_waiting([&] { condition.wait(lock, [&] { return result.is_ready(); }); });
    return std::move(result).get_ready();
}

 
template <class T>
[[deprecated("implicit copy deprecated, use `await(std::move(f))` or `await(stlab::copy(f))`"
             " instead.")]]
auto await(const future<T>& x) -> T {
    return await(future<T>{x});
}
 
namespace detail {
 
template <class T>
struct blocking_get_guarded {
    bool _timed_out{false};
    future<T> _result;
    std::condition_variable _condition;
    std::mutex _mutex;
 
    template <class F>
    auto set(F&& r) -> future<T> {
        {
            std::unique_lock<std::mutex> lock{_mutex};
            if (_timed_out) return std::forward<F>(r);
            _result = std::forward<F>(r);
        }
        _condition.notify_one();
        /*
            A valid future<> result is required because the following reduction in
            recover() will attach a continuation - which will crash on an invalid
            future<>.
         */
        return make_exceptional_future<T>(
            std::make_exception_ptr(std::runtime_error("not an error")), immediate_executor);
    }
 
    auto wait_for(const std::chrono::nanoseconds& timeout) -> future<T> {
        std::unique_lock<std::mutex> lock{_mutex};
        _timed_out = !invoke_waiting(
            [&] { return _condition.wait_for(lock, timeout, [&] { return _result.valid(); }); });
        return _timed_out ? future<T>{} : std::move(_result);
    }
};
 
} // namespace detail

template <class T>
auto await_for(future<T>&& x, const std::chrono::nanoseconds& timeout) -> future<T> {
    if (x.is_ready() || !stlab_v2_default_executor_supports_blocking()) return std::move(x);
 
    auto p = std::make_shared<detail::blocking_get_guarded<T>>();
 
    auto hold = std::move(x).recover(immediate_executor, [_p = stlab::make_weak_ptr(p)](auto&& r) {
        if (auto p = _p.lock()) return p->set(std::forward<decltype(r)>(r));
        return std::forward<decltype(r)>(r);
    });
 
    auto result = p->wait_for(timeout);
    return result.valid() ? std::move(result) : std::move(hold);
}

 
template <class T>
[[deprecated("implicit copy deprecated, use `await_for(std::move(f), t)` or"
             " `await_for(stlab::copy(f), t)` instead.")]]
auto await_for(const future<T>& x, const std::chrono::nanoseconds& timeout) -> future<T> {
    return await_for(future<T>{x}, timeout);
}
 
/**************************************************************************************************/

 
STLAB_VERSION_NAMESPACE_END()
} // namespace stlab
 
/**************************************************************************************************/
 
#endif // STLAB_CONCURRENCY_AWAIT_HPP