future_state.hpp

#pragma once
 
#include <atomic>
#include <exception>
 
#include <userver/engine/deadline.hpp>
#include <userver/engine/future_status.hpp>
#include <userver/engine/impl/context_accessor.hpp>
#include <userver/engine/impl/wait_list_fwd.hpp>
#include <userver/utils/result_store.hpp>
 
USERVER_NAMESPACE_BEGIN
 
namespace engine::impl {
 
class FutureStateBase : private engine::impl::ContextAccessor {
 public:
  bool IsReady() const noexcept final;
 
  [[nodiscard]] FutureStatus WaitUntil(Deadline deadline);
 
  void OnFutureCreated();
  bool IsFutureCreated() const noexcept;
 
  // Internal helper for WaitAny/WaitAll
  ContextAccessor* TryGetContextAccessor() noexcept { return this; }
 
 protected:
  class WaitStrategy;
 
  FutureStateBase() noexcept;
  ~FutureStateBase();
 
  void LockResultStore();
  void ReleaseResultStore();
  void WaitForResult();
 
 private:
  void AppendWaiter(impl::TaskContext& context) noexcept final;
  void RemoveWaiter(impl::TaskContext& context) noexcept final;
 
  FastPimplWaitListLight finish_waiters_;
  std::atomic<bool> is_ready_;
  std::atomic<bool> is_result_store_locked_;
  std::atomic<bool> is_future_created_;
};
 
template <typename T>
class FutureState final : public FutureStateBase {
 public:
  [[nodiscard]] T Get();
 
  void SetValue(const T& value);
  void SetValue(T&& value);
  void SetException(std::exception_ptr&& ex);
 
 private:
  void RethrowErrorResult() const override;
 
  utils::ResultStore<T> result_store_;
};
 
template <>
class FutureState<void> final : public FutureStateBase {
 public:
  void Get();
 
  void SetValue();
  void SetException(std::exception_ptr&& ex);
 
 private:
  void RethrowErrorResult() const override;
 
  utils::ResultStore<void> result_store_;
};
 
template <typename T>
T FutureState<T>::Get() {
  WaitForResult();
  return result_store_.Retrieve();
}
 
template <typename T>
void FutureState<T>::SetValue(const T& value) {
  LockResultStore();
  result_store_.SetValue(value);
  ReleaseResultStore();
}
 
template <typename T>
void FutureState<T>::SetValue(T&& value) {
  LockResultStore();
  result_store_.SetValue(std::move(value));
  ReleaseResultStore();
}
 
template <typename T>
void FutureState<T>::SetException(std::exception_ptr&& ex) {
  LockResultStore();
  result_store_.SetException(std::move(ex));
  ReleaseResultStore();
}
 
template <typename T>
void FutureState<T>::RethrowErrorResult() const {
  (void)result_store_.Get();
}
 
inline void FutureState<void>::Get() {
  WaitForResult();
  return result_store_.Retrieve();
}
 
inline void FutureState<void>::SetValue() {
  LockResultStore();
  result_store_.SetValue();
  ReleaseResultStore();
}
 
inline void FutureState<void>::SetException(std::exception_ptr&& ex) {
  LockResultStore();
  result_store_.SetException(std::move(ex));
  ReleaseResultStore();
}
 
inline void FutureState<void>::RethrowErrorResult() const {
  (void)result_store_.Get();
}
 
}  // namespace engine::impl
 
USERVER_NAMESPACE_END