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465 lines (368 loc) · 10.2 KB
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///////////////////////////////////////////////////////////////////////////////
// Copyright (c) Lewis Baker
// Licenced under MIT license. See LICENSE.txt for details.
///////////////////////////////////////////////////////////////////////////////
#ifndef CPPCORO_LAZY_TASK_HPP_INCLUDED
#define CPPCORO_LAZY_TASK_HPP_INCLUDED
#include <cppcoro/broken_promise.hpp>
#include <cppcoro/fmap.hpp>
#include <cppcoro/detail/continuation.hpp>
#include <atomic>
#include <exception>
#include <utility>
#include <type_traits>
#include <experimental/coroutine>
namespace cppcoro
{
template<typename T> class lazy_task;
namespace detail
{
class lazy_task_promise_base
{
public:
lazy_task_promise_base() noexcept
: m_continuation()
{}
auto initial_suspend() noexcept
{
return std::experimental::suspend_always{};
}
auto final_suspend() noexcept
{
struct awaitable
{
continuation m_continuation;
awaitable(continuation c) noexcept
: m_continuation(c)
{}
bool await_ready() const noexcept { return false; }
std::experimental::coroutine_handle<>
await_suspend([[maybe_unused]] std::experimental::coroutine_handle<> coroutine)
{
return m_continuation.tail_call_resume();
}
void await_resume() noexcept {}
};
return awaitable{ m_continuation };
}
void unhandled_exception() noexcept
{
m_exception = std::current_exception();
}
bool is_ready() const noexcept
{
return static_cast<bool>(m_continuation);
}
void set_continuation(continuation c)
{
m_continuation = c;
}
protected:
bool completed_with_unhandled_exception()
{
return m_exception != nullptr;
}
void rethrow_if_unhandled_exception()
{
if (m_exception != nullptr)
{
std::rethrow_exception(m_exception);
}
}
private:
continuation m_continuation;
std::exception_ptr m_exception;
};
template<typename T>
class lazy_task_promise : public lazy_task_promise_base
{
public:
lazy_task_promise() noexcept = default;
~lazy_task_promise()
{
if (is_ready() && !completed_with_unhandled_exception())
{
reinterpret_cast<T*>(&m_valueStorage)->~T();
}
}
lazy_task<T> get_return_object() noexcept;
template<
typename VALUE,
typename = std::enable_if_t<std::is_convertible_v<VALUE&&, T>>>
void return_value(VALUE&& value)
noexcept(std::is_nothrow_constructible_v<T, VALUE&&>)
{
new (&m_valueStorage) T(std::forward<VALUE>(value));
}
T& result() &
{
rethrow_if_unhandled_exception();
return *reinterpret_cast<T*>(&m_valueStorage);
}
T&& result() &&
{
rethrow_if_unhandled_exception();
return std::move(*reinterpret_cast<T*>(&m_valueStorage));
}
private:
// Not using std::aligned_storage here due to bug in MSVC 2015 Update 2
// that means it doesn't work for types with alignof(T) > 8.
// See MS-Connect bug #2658635.
alignas(T) char m_valueStorage[sizeof(T)];
};
template<>
class lazy_task_promise<void> : public lazy_task_promise_base
{
public:
lazy_task_promise() noexcept = default;
lazy_task<void> get_return_object() noexcept;
void return_void() noexcept
{}
void result()
{
rethrow_if_unhandled_exception();
}
};
template<typename T>
class lazy_task_promise<T&> : public lazy_task_promise_base
{
public:
lazy_task_promise() noexcept = default;
lazy_task<T&> get_return_object() noexcept;
void return_value(T& value) noexcept
{
m_value = std::addressof(value);
}
T& result()
{
rethrow_if_unhandled_exception();
return *m_value;
}
private:
T* m_value;
};
}
/// \brief
/// A lazy task represents an asynchronous operation that is not started
/// until it is first awaited.
///
/// When you call a coroutine that returns a lazy_task, the coroutine
/// simply captures any passed parameters and returns exeuction to the
/// caller. Execution of the coroutine body does not start until the
/// coroutine is first co_await'ed.
///
/// Comparison with task<T>
/// -----------------------
/// The lazy task has lower overhead than cppcoro::task<T> as it does not
/// require the use of atomic operations to synchronise potential races
/// between the awaiting coroutine suspending and the coroutine completing.
///
/// The awaiting coroutine is suspended prior to the lazy_task being started
/// which means that when the lazy_task completes it can unconditionally
/// resume the awaiter.
///
/// One limitation of this approach is that if the lazy_task completes
/// synchronously then, unless the compiler is able to perform tail-calls,
/// the awaiting coroutine will be resumed inside a nested stack-frame.
/// This call lead to stack-overflow if long chains of lazy_tasks complete
/// synchronously.
///
/// The task<T> type does not have this issue as the awaiting coroutine is
/// not suspended in the case that the task completes synchronously.
template<typename T = void>
class lazy_task
{
public:
using promise_type = detail::lazy_task_promise<T>;
using value_type = T;
private:
struct awaitable_base
{
std::experimental::coroutine_handle<promise_type> m_coroutine;
awaitable_base(std::experimental::coroutine_handle<promise_type> coroutine) noexcept
: m_coroutine(coroutine)
{}
bool await_ready() const noexcept
{
return !m_coroutine || m_coroutine.promise().is_ready();
}
auto await_suspend(std::experimental::coroutine_handle<> awaiter) noexcept
{
m_coroutine.promise().set_continuation(detail::continuation{ awaiter });
return m_coroutine;
}
};
public:
lazy_task() noexcept
: m_coroutine(nullptr)
{}
explicit lazy_task(std::experimental::coroutine_handle<promise_type> coroutine)
: m_coroutine(coroutine)
{}
lazy_task(lazy_task&& t) noexcept
: m_coroutine(t.m_coroutine)
{
t.m_coroutine = nullptr;
}
/// Disable copy construction/assignment.
lazy_task(const lazy_task&) = delete;
lazy_task& operator=(const lazy_task&) = delete;
/// Frees resources used by this task.
~lazy_task()
{
if (m_coroutine)
{
m_coroutine.destroy();
}
}
lazy_task& operator=(lazy_task&& other) noexcept
{
if (std::addressof(other) != this)
{
if (m_coroutine)
{
m_coroutine.destroy();
}
m_coroutine = other.m_coroutine;
other.m_coroutine = nullptr;
}
return *this;
}
/// \brief
/// Query if the task result is complete.
///
/// Awaiting a task that is ready will not block.
bool is_ready() const noexcept
{
return !m_coroutine || m_coroutine.promise().is_ready();
}
auto operator co_await() const & noexcept
{
struct awaitable : awaitable_base
{
using awaitable_base::awaitable_base;
decltype(auto) await_resume()
{
if (!this->m_coroutine)
{
throw broken_promise{};
}
return this->m_coroutine.promise().result();
}
};
return awaitable{ m_coroutine };
}
auto operator co_await() const && noexcept
{
struct awaitable : awaitable_base
{
using awaitable_base::awaitable_base;
decltype(auto) await_resume()
{
if (!this->m_coroutine)
{
throw broken_promise{};
}
return std::move(this->m_coroutine.promise()).result();
}
};
return awaitable{ m_coroutine };
}
/// \brief
/// Returns an awaitable that will await completion of the task without
/// attempting to retrieve the result.
auto when_ready() const noexcept
{
struct awaitable : awaitable_base
{
using awaitable_base::awaitable_base;
void await_resume() const noexcept {}
};
return awaitable{ m_coroutine };
}
// Internal helper method for when_all() implementation.
auto get_starter() const noexcept
{
class starter
{
public:
starter(std::experimental::coroutine_handle<promise_type> coroutine) noexcept
: m_coroutine(coroutine)
{}
void start(detail::continuation c) noexcept
{
if (m_coroutine && !m_coroutine.promise().is_ready())
{
m_coroutine.promise().set_continuation(c);
m_coroutine.resume();
}
else
{
c.resume();
}
}
private:
std::experimental::coroutine_handle<promise_type> m_coroutine;
};
return starter{ m_coroutine };
}
private:
std::experimental::coroutine_handle<promise_type> m_coroutine;
};
namespace detail
{
template<typename T>
lazy_task<T> lazy_task_promise<T>::get_return_object() noexcept
{
return lazy_task<T>{ std::experimental::coroutine_handle<lazy_task_promise>::from_promise(*this) };
}
inline lazy_task<void> lazy_task_promise<void>::get_return_object() noexcept
{
return lazy_task<void>{ std::experimental::coroutine_handle<lazy_task_promise>::from_promise(*this) };
}
template<typename T>
lazy_task<T&> lazy_task_promise<T&>::get_return_object() noexcept
{
return lazy_task<T&>{ std::experimental::coroutine_handle<lazy_task_promise>::from_promise(*this) };
}
}
// operator| for fmap_transform
namespace detail
{
template<typename T, typename FUNC>
lazy_task<std::result_of_t<FUNC&&(T&&)>> apply_fmap(lazy_task<T> t, FUNC func)
{
static_assert(
!std::is_reference_v<FUNC>,
"Passing by reference to lazy_task<T> coroutine is unsafe. "
"Use std::ref or std::cref to explicitly pass by reference.");
co_return std::invoke(std::move(func), co_await std::move(t));
}
template<typename FUNC>
lazy_task<std::result_of_t<FUNC&&()>> apply_fmap(lazy_task<> t, FUNC func)
{
static_assert(
!std::is_reference_v<FUNC>,
"Passing by reference to lazy_task<T> coroutine is unsafe. "
"Use std::ref or std::cref to explicitly pass by reference.");
co_await t;
co_return std::invoke(std::move(func));
}
}
template<typename T, typename FUNC>
auto operator|(lazy_task<T>&& t, fmap_transform<FUNC>&& transform)
{
return detail::apply_fmap(std::move(t), std::forward<FUNC>(transform.func));
}
template<typename T, typename FUNC>
auto operator|(lazy_task<T>&& t, fmap_transform<FUNC>& transform)
{
return detail::apply_fmap(std::move(t), transform.func);
}
template<typename T, typename FUNC>
auto operator|(lazy_task<T>&& t, const fmap_transform<FUNC>& transform)
{
return detail::apply_fmap(std::move(t), transform.func);
}
}
#endif
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