future.h

// Copyright 2025 The Pigweed Authors
//
// 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
//
//     https://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 <mutex>
#include <optional>
#include <type_traits>
 
#include "pw_assert/assert.h"
#include "pw_async2/context.h"
#include "pw_async2/poll.h"
#include "pw_containers/intrusive_list.h"
#include "pw_sync/interrupt_spin_lock.h"
 
namespace pw::async2::experimental {
 
template <typename Derived, typename T>
class Future {
 public:
  using value_type = T;
 
  Poll<value_type> Pend(Context& cx) {
    PW_ASSERT(!is_complete());
    Poll<value_type> poll = derived().DoPend(cx);
    if (poll.IsReady()) {
      derived().DoMarkComplete();
    }
    return poll;
  }
 
  bool is_complete() const { return derived().DoIsComplete(); }
 
 protected:
  constexpr Future() = default;
 
 private:
  Derived& derived() { return static_cast<Derived&>(*this); }
  const Derived& derived() const { return static_cast<const Derived&>(*this); }
};
 
namespace internal {
 
template <typename D, typename V>
std::true_type IsFuture(const volatile Future<D, V>*);
 
std::false_type IsFuture(...);
 
}  // namespace internal
 
template <typename T>
struct is_future : decltype(internal::IsFuture(std::declval<T*>())){};
 
template <typename T>
constexpr bool is_future_v =
    is_future<std::remove_cv_t<std::remove_reference_t<T>>>::value;
 
template <typename FutureType, typename Lock = sync::InterruptSpinLock>
class ListFutureProvider {
 public:
  constexpr ListFutureProvider() = default;
 
  ListFutureProvider(const ListFutureProvider&) = delete;
  ListFutureProvider& operator=(const ListFutureProvider&) = delete;
 
  void Push(FutureType& future) {
    std::lock_guard lock(lock_);
    futures_.push_back(future);
  }
 
  FutureType& Pop() {
    std::lock_guard lock(lock_);
    PW_ASSERT(!futures_.empty());
    FutureType& future = futures_.front();
    futures_.pop_front();
    return future;
  }
 
  bool empty() {
    std::lock_guard lock(lock_);
    return futures_.empty();
  }
 
  Lock& lock() { return lock_; }
 
 private:
  friend FutureType;
 
  template <typename, typename>
  friend class ListableFutureWithWaker;
 
  using LockType = Lock;
 
  IntrusiveList<FutureType> futures_;
  Lock lock_;
};
 
template <typename FutureType>
class SingleFutureProvider {
 public:
  constexpr SingleFutureProvider() = default;
 
  SingleFutureProvider(const SingleFutureProvider&) = delete;
  SingleFutureProvider& operator=(const SingleFutureProvider&) = delete;
 
  void Set(FutureType& future) {
    PW_ASSERT(!has_future());
    inner_.Push(future);
  }
 
  bool TrySet(FutureType& future) {
    if (has_future()) {
      return false;
    }
    inner_.Push(future);
    return true;
  }
 
  [[nodiscard]] FutureType& Take() { return inner_.Pop(); }
 
  bool has_future() { return !inner_.empty(); }
 
 private:
  template <typename, typename>
  friend class ListableFutureWithWaker;
  friend FutureType;
 
  ListFutureProvider<FutureType> inner_;
};
 
template <typename Derived, typename T>
class ListableFutureWithWaker
    : public Future<ListableFutureWithWaker<Derived, T>, T>,
      public IntrusiveList<Derived>::Item {
 public:
  ListableFutureWithWaker(const ListableFutureWithWaker&) = delete;
  ListableFutureWithWaker& operator=(const ListableFutureWithWaker&) = delete;
 
 protected:
  using Provider = ListFutureProvider<Derived>;
 
  enum MovedFromState { kMovedFrom };
 
  explicit ListableFutureWithWaker(MovedFromState)
      : provider_(nullptr), complete_(true) {}
 
  explicit ListableFutureWithWaker(Provider& provider) : provider_(&provider) {
    provider.Push(derived());
  }
  explicit ListableFutureWithWaker(SingleFutureProvider<Derived>& single)
      : ListableFutureWithWaker(single.inner_) {}
 
  ~ListableFutureWithWaker() {
    if (provider_ == nullptr) {
      return;
    }
    std::lock_guard guard(lock());
    if (!this->unlisted()) {
      this->unlist();
    }
  }
 
  void MoveFrom(ListableFutureWithWaker& other) {
    complete_ = std::exchange(other.complete_, true);
    provider_ = std::exchange(other.provider_, nullptr);
    waker_ = std::move(other.waker_);
 
    if (provider_) {
      std::lock_guard guard(lock());
      if (!other.unlisted()) {
        this->replace(other);
      }
    }
  }
 
  Provider& provider() {
    PW_ASSERT(provider_ != nullptr);
    return *provider_;
  }
 
  typename Provider::LockType& lock() { return provider().lock(); }
 
  void Wake() { std::move(waker_).Wake(); }
 
 private:
  using Base = Future<ListableFutureWithWaker<Derived, T>, T>;
 
  friend Base;
  friend Provider;
 
  Poll<T> DoPend(Context& cx) {
    static_assert(
        std::is_same_v<std::remove_extent_t<decltype(Derived::kWaitReason)>,
                       const char>,
        "kWaitReason must be a character array");
 
    Poll<T> poll = derived().DoPend(cx);
    if (poll.IsPending()) {
      PW_ASYNC_STORE_WAKER(cx, waker_, Derived::kWaitReason);
    }
    return poll;
  }
 
  void DoMarkComplete() { complete_ = true; }
  bool DoIsComplete() const { return complete_; }
 
  Derived& derived() { return static_cast<Derived&>(*this); }
 
  Provider* provider_;
  Waker waker_;
  bool complete_ = false;
};
 
}  // namespace pw::async2::experimental