InlineTask.h

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/*
 * Copyright 2018-present Facebook, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
#pragma once

#include <folly/ScopeGuard.h>
#include <folly/Try.h>

#include <cassert>
#include <experimental/coroutine>
#include <utility>

namespace folly {
namespace coro {
namespace detail {

template <typename T>
class InlineTask;

class InlineTaskPromiseBase {
  struct FinalAwaiter {
    bool await_ready() noexcept {
      return false;
    }

    template <typename Promise>
    std::experimental::coroutine_handle<> await_suspend(
        std::experimental::coroutine_handle<Promise> h) noexcept {
      InlineTaskPromiseBase& promise = h.promise();
      return promise.continuation_;
    }

    void await_resume() noexcept {}
  };

 protected:
  InlineTaskPromiseBase() noexcept = default;

  InlineTaskPromiseBase(const InlineTaskPromiseBase&) = delete;
  InlineTaskPromiseBase(InlineTaskPromiseBase&&) = delete;
  InlineTaskPromiseBase& operator=(const InlineTaskPromiseBase&) = delete;
  InlineTaskPromiseBase& operator=(InlineTaskPromiseBase&&) = delete;

 public:
  std::experimental::suspend_always initial_suspend() noexcept {
    return {};
  }

  auto final_suspend() noexcept {
    return FinalAwaiter{};
  }

  void set_continuation(
      std::experimental::coroutine_handle<> continuation) noexcept {
    assert(!continuation_);
    continuation_ = continuation;
  }

 private:
  std::experimental::coroutine_handle<> continuation_;
};

template <typename T>
class InlineTaskPromise : public InlineTaskPromiseBase {
 public:
  static_assert(
      std::is_move_constructible<T>::value,
      "InlineTask<T> only supports types that are move-constructible.");
  static_assert(
      !std::is_rvalue_reference<T>::value,
      "InlineTask<T&&> is not supported");

  InlineTaskPromise() noexcept = default;

  ~InlineTaskPromise() = default;

  InlineTask<T> get_return_object() noexcept;

  template <
      typename Value,
      std::enable_if_t<std::is_convertible<Value&&, T>::value, int> = 0>
  void return_value(Value&& value) noexcept(
      std::is_nothrow_constructible<T, Value&&>::value) {
    result_.emplace(static_cast<Value&&>(value));
  }

  // Also provide non-template overload for T&& so that we can do
  // 'co_return {arg1, arg2}' as shorthand for 'co_return T{arg1, arg2}'.
  void return_value(T&& value) noexcept(
      std::is_nothrow_move_constructible<T>::value) {
    result_.emplace(static_cast<T&&>(value));
  }

  void unhandled_exception() noexcept {
    result_.emplaceException(
        folly::exception_wrapper::from_exception_ptr(std::current_exception()));
  }

  T result() {
    return std::move(result_).value();
  }

 private:
  // folly::Try<T> doesn't support storing reference types so we store a
  // std::reference_wrapper instead.
  using StorageType = std::conditional_t<
      std::is_lvalue_reference<T>::value,
      std::reference_wrapper<std::remove_reference_t<T>>,
      T>;

  folly::Try<StorageType> result_;
};

template <>
class InlineTaskPromise<void> : public InlineTaskPromiseBase {
 public:
  InlineTaskPromise() noexcept = default;

  InlineTask<void> get_return_object() noexcept;

  void return_void() noexcept {}

  void unhandled_exception() noexcept {
    result_.emplaceException(
        folly::exception_wrapper::from_exception_ptr(std::current_exception()));
  }

  void result() {
    return result_.value();
  }

 private:
  folly::Try<void> result_;
};

template <typename T>
class InlineTask {
 public:
  using promise_type = detail::InlineTaskPromise<T>;

 private:
  using handle_t = std::experimental::coroutine_handle<promise_type>;

 public:
  InlineTask(InlineTask&& other) noexcept
      : coro_(std::exchange(other.coro_, {})) {}

  ~InlineTask() {
    if (coro_) {
      coro_.destroy();
    }
  }

  class Awaiter {
   public:
    ~Awaiter() {
      if (coro_) {
        coro_.destroy();
      }
    }

    bool await_ready() noexcept {
      return false;
    }

    handle_t await_suspend(
        std::experimental::coroutine_handle<> awaitingCoroutine) noexcept {
      assert(coro_ && !coro_.done());
      coro_.promise().set_continuation(awaitingCoroutine);
      return coro_;
    }

    T await_resume() {
      auto destroyOnExit =
          folly::makeGuard([this] { std::exchange(coro_, {}).destroy(); });
      return coro_.promise().result();
    }

   private:
    friend class InlineTask<T>;
    explicit Awaiter(handle_t coro) noexcept : coro_(coro) {}
    handle_t coro_;
  };

  Awaiter operator co_await() && {
    assert(coro_ && !coro_.done());
    return Awaiter{std::exchange(coro_, {})};
  }

 private:
  friend class InlineTaskPromise<T>;
  explicit InlineTask(handle_t coro) noexcept : coro_(coro) {}
  handle_t coro_;
};

template <typename T>
inline InlineTask<T> InlineTaskPromise<T>::get_return_object() noexcept {
  return InlineTask<T>{
      std::experimental::coroutine_handle<InlineTaskPromise<T>>::from_promise(
          *this)};
}

inline InlineTask<void> InlineTaskPromise<void>::get_return_object() noexcept {
  return InlineTask<void>{std::experimental::coroutine_handle<
      InlineTaskPromise<void>>::from_promise(*this)};
}

} // namespace detail
} // namespace coro
} // namespace folly