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#pragma once
#if !defined(INCLUDE_FUTURE_INL_H)
#error "you should never include wait_group-inl.h directly"
#endif // INCLUDE_FUTURE_INL_H
#include "wait_policy.h"
#include <util/generic/maybe.h>
#include <util/generic/ptr.h>
#include <library/cpp/threading/future/core/future.h>
#include <util/system/spinlock.h>
#include <atomic>
#include <exception>
namespace NThreading {
namespace NWaitGroup::NImpl {
template <class WaitPolicy>
struct TState final : TAtomicRefCount<TState<WaitPolicy>> {
template <class T>
void Add(const TFuture<T>& future);
TFuture<void> Finish();
void TryPublish();
void Publish();
bool ShouldPublishByCount() const noexcept;
bool ShouldPublishByException() const noexcept;
TStateRef<WaitPolicy> SharedFromThis() noexcept {
return TStateRef<WaitPolicy>{this};
}
enum class EPhase {
Initial,
Publishing,
};
// initially we have one imaginary discovered future which we
// use for synchronization with ::Finish
std::atomic<ui64> Discovered{1};
std::atomic<ui64> Finished{0};
std::atomic<EPhase> Phase{EPhase::Initial};
TPromise<void> Subscribers = NewPromise();
mutable TAdaptiveLock Mut;
std::exception_ptr ExceptionInFlight;
void TrySetException(std::exception_ptr eptr) noexcept {
TGuard lock{Mut};
if (!ExceptionInFlight) {
ExceptionInFlight = std::move(eptr);
}
}
std::exception_ptr GetExceptionInFlight() const noexcept {
TGuard lock{Mut};
return ExceptionInFlight;
}
};
template <class WaitPolicy>
inline TFuture<void> TState<WaitPolicy>::Finish() {
Finished.fetch_add(1); // complete the imaginary future
// handle empty case explicitly:
if (Discovered.load() == 1) {
Y_ASSERT(Phase.load() == EPhase::Initial);
Publish();
} else {
TryPublish();
}
return Subscribers;
}
template <class WaitPolicy>
template <class T>
inline void TState<WaitPolicy>::Add(const TFuture<T>& future) {
future.EnsureInitialized();
Discovered.fetch_add(1);
// NoexceptSubscribe is needed to make ::Add exception-safe
future.NoexceptSubscribe([self = SharedFromThis()](auto&& future) {
try {
future.TryRethrow();
} catch (...) {
self->TrySetException(std::current_exception());
}
self->Finished.fetch_add(1);
self->TryPublish();
});
}
//
// ============================ PublishByCount ==================================
//
template <class WaitPolicy>
inline bool TState<WaitPolicy>::ShouldPublishByCount() const noexcept {
// - safety: a) If the future incremented ::Finished, and we observe the effect, then we will observe ::Discovered as incremented by its discovery later
// b) Every discovery of a future observes discovery of the imaginary future
// a, b => if finishedByNow == discoveredByNow, then every future discovered in [imaginary discovered, imaginary finished] is finished
//
// - liveness: a) TryPublish is called after each increment of ::Finished
// b) There is some last increment of ::Finished which follows all other operations with ::Finished and ::Discovered (provided that every future is eventually set)
// c) For each increment of ::Discovered there is an increment of ::Finished (provided that every future is eventually set)
// a, b c => some call to ShouldPublishByCount will always return true
//
// order of the following two operations is significant for the proof.
auto finishedByNow = Finished.load();
auto discoveredByNow = Discovered.load();
return finishedByNow == discoveredByNow;
}
template <>
inline bool TState<TWaitPolicy::TAny>::ShouldPublishByCount() const noexcept {
auto finishedByNow = Finished.load();
// note that the empty case is not handled here
return finishedByNow >= 2; // at least one non-imaginary
}
//
// ============================ PublishByException ==================================
//
template <>
inline bool TState<TWaitPolicy::TAny>::ShouldPublishByException() const noexcept {
// for TAny exceptions are handled by ShouldPublishByCount
return false;
}
template <>
inline bool TState<TWaitPolicy::TAll>::ShouldPublishByException() const noexcept {
return false;
}
template <>
inline bool TState<TWaitPolicy::TExceptionOrAll>::ShouldPublishByException() const noexcept {
return GetExceptionInFlight() != nullptr;
}
//
//
//
template <class WaitPolicy>
inline void TState<WaitPolicy>::TryPublish() {
// the order is insignificant (without proof)
bool shouldPublish = ShouldPublishByCount() || ShouldPublishByException();
if (shouldPublish) {
if (auto currentPhase = EPhase::Initial;
Phase.compare_exchange_strong(currentPhase, EPhase::Publishing)) {
Publish();
}
}
}
template <class WaitPolicy>
inline void TState<WaitPolicy>::Publish() {
auto eptr = GetExceptionInFlight();
// can potentially throw
if (eptr) {
Subscribers.SetException(std::move(eptr));
} else {
Subscribers.SetValue();
}
}
}
template <class WaitPolicy>
inline TWaitGroup<WaitPolicy>::TWaitGroup()
: State_{MakeIntrusive<NWaitGroup::NImpl::TState<WaitPolicy>>()}
{
}
template <class WaitPolicy>
template <class T>
inline TWaitGroup<WaitPolicy>& TWaitGroup<WaitPolicy>::Add(const TFuture<T>& future) {
State_->Add(future);
return *this;
}
template <class WaitPolicy>
inline TFuture<void> TWaitGroup<WaitPolicy>::Finish() && {
auto res = State_->Finish();
// just to prevent nasty bugs from use-after-move
State_.Reset();
return res;
}
}
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