/*
- * Copyright 2014 Facebook, Inc.
+ * Copyright 2016 Facebook, Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
#include <stdexcept>
#include <vector>
+#include <folly/Executor.h>
+#include <folly/Function.h>
+#include <folly/MicroSpinLock.h>
#include <folly/Optional.h>
-#include <folly/SmallLocks.h>
-
-#include <folly/futures/Try.h>
-#include <folly/futures/Promise.h>
#include <folly/futures/Future.h>
-#include <folly/Executor.h>
+#include <folly/futures/Promise.h>
+#include <folly/futures/Try.h>
#include <folly/futures/detail/FSM.h>
#include <folly/io/async/Request.h>
namespace folly { namespace detail {
-// As of GCC 4.8.1, the std::function in libstdc++ optimizes only for pointers
-// to functions, using a helper avoids a call to malloc.
-template<typename T>
-void empty_callback(Try<T>&&) { }
-
-enum class State {
- Waiting,
- Interruptible,
- Interrupted,
+/*
+ OnlyCallback
+ / \
+ Start Armed - Done
+ \ /
+ OnlyResult
+
+This state machine is fairly self-explanatory. The most important bit is
+that the callback is only executed on the transition from Armed to Done,
+and that transition can happen immediately after transitioning from Only*
+to Armed, if it is active (the usual case).
+*/
+enum class State : uint8_t {
+ Start,
+ OnlyResult,
+ OnlyCallback,
+ Armed,
Done,
};
-/** The shared state object for Future and Promise. */
+/// The shared state object for Future and Promise.
+/// Some methods must only be called by either the Future thread or the
+/// Promise thread. The Future thread is the thread that currently "owns" the
+/// Future and its callback-related operations, and the Promise thread is
+/// likewise the thread that currently "owns" the Promise and its
+/// result-related operations. Also, Futures own interruption, Promises own
+/// interrupt handlers. Unfortunately, there are things that users can do to
+/// break this, and we can't detect that. However if they follow move
+/// semantics religiously wrt threading, they should be ok.
+///
+/// It's worth pointing out that Futures and/or Promises can and usually will
+/// migrate between threads, though this usually happens within the API code.
+/// For example, an async operation will probably make a Promise, grab its
+/// Future, then move the Promise into another thread that will eventually
+/// fulfill it. With executors and via, this gets slightly more complicated at
+/// first blush, but it's the same principle. In general, as long as the user
+/// doesn't access a Future or Promise object from more than one thread at a
+/// time there won't be any problems.
template<typename T>
-class Core : protected FSM<State> {
+class Core {
+ static_assert(!std::is_void<T>::value,
+ "void futures are not supported. Use Unit instead.");
public:
- // This must be heap-constructed. There's probably a way to enforce that in
- // code but since this is just internal detail code and I don't know how
- // off-hand, I'm punting.
- Core() : FSM<State>(State::Waiting) {}
+ /// This must be heap-constructed. There's probably a way to enforce that in
+ /// code but since this is just internal detail code and I don't know how
+ /// off-hand, I'm punting.
+ Core() : result_(), fsm_(State::Start), attached_(2) {}
+
+ explicit Core(Try<T>&& t)
+ : result_(std::move(t)),
+ fsm_(State::OnlyResult),
+ attached_(1) {}
+
~Core() {
- assert(calledBack_);
- assert(detached_ == 2);
+ DCHECK(attached_ == 0);
}
// not copyable
Core(Core&&) noexcept = delete;
Core& operator=(Core&&) = delete;
+ // Core is assumed to be convertible only if the type is convertible
+ // and the size is the same. This is a compromise for the complexity
+ // of having to make Core truly have a conversion constructor which
+ // would cause various other problems.
+ // If we made Core move constructible then we would need to update the
+ // Promise and Future with the location of the new Core. This is complex
+ // and may be inefficient.
+ // Core should only be modified so that for size(T) == size(U),
+ // sizeof(Core<T>) == size(Core<U>).
+ // This assumption is used as a proxy to make sure that
+ // the members of Core<T> and Core<U> line up so that we can use a
+ // reinterpret cast.
+ template <
+ class U,
+ typename = typename std::enable_if<std::is_convertible<U, T>::value &&
+ sizeof(U) == sizeof(T)>::type>
+ static Core<T>* convert(Core<U>* from) {
+ return reinterpret_cast<Core<T>*>(from);
+ }
+
+ /// May call from any thread
+ bool hasResult() const {
+ switch (fsm_.getState()) {
+ case State::OnlyResult:
+ case State::Armed:
+ case State::Done:
+ assert(!!result_);
+ return true;
+
+ default:
+ return false;
+ }
+ }
+
+ /// May call from any thread
+ bool ready() const {
+ return hasResult();
+ }
+
+ /// May call from any thread
Try<T>& getTry() {
if (ready()) {
return *result_;
}
}
+ /// Call only from Future thread.
template <typename F>
- void setCallback(F func) {
+ void setCallback(F&& func) {
+ bool transitionToArmed = false;
auto setCallback_ = [&]{
- if (callback_) {
- throw std::logic_error("setCallback called twice");
- }
-
context_ = RequestContext::saveContext();
- callback_ = std::move(func);
+ callback_ = std::forward<F>(func);
};
- FSM_START
- case State::Waiting:
- case State::Interruptible:
- case State::Interrupted:
- FSM_UPDATE(state, setCallback_);
+ FSM_START(fsm_)
+ case State::Start:
+ FSM_UPDATE(fsm_, State::OnlyCallback, setCallback_);
break;
- case State::Done:
- FSM_UPDATE2(State::Done,
- setCallback_,
- [&]{ maybeCallback(); });
+ case State::OnlyResult:
+ FSM_UPDATE(fsm_, State::Armed, setCallback_);
+ transitionToArmed = true;
break;
+
+ case State::OnlyCallback:
+ case State::Armed:
+ case State::Done:
+ throw std::logic_error("setCallback called twice");
FSM_END
+
+ // we could always call this, it is an optimization to only call it when
+ // it might be needed.
+ if (transitionToArmed) {
+ maybeCallback();
+ }
}
+ /// Call only from Promise thread
void setResult(Try<T>&& t) {
- FSM_START
- case State::Waiting:
- case State::Interruptible:
- case State::Interrupted:
- FSM_UPDATE2(State::Done,
- [&]{ result_ = std::move(t); },
- [&]{ maybeCallback(); });
+ bool transitionToArmed = false;
+ auto setResult_ = [&]{ result_ = std::move(t); };
+ FSM_START(fsm_)
+ case State::Start:
+ FSM_UPDATE(fsm_, State::OnlyResult, setResult_);
break;
+ case State::OnlyCallback:
+ FSM_UPDATE(fsm_, State::Armed, setResult_);
+ transitionToArmed = true;
+ break;
+
+ case State::OnlyResult:
+ case State::Armed:
case State::Done:
throw std::logic_error("setResult called twice");
FSM_END
- }
- bool ready() const {
- return getState() == State::Done;
+ if (transitionToArmed) {
+ maybeCallback();
+ }
}
- // Called by a destructing Future
+ /// Called by a destructing Future (in the Future thread, by definition)
void detachFuture() {
- if (!callback_) {
- setCallback(empty_callback<T>);
- }
activate();
detachOne();
}
- // Called by a destructing Promise
+ /// Called by a destructing Promise (in the Promise thread, by definition)
void detachPromise() {
- if (!ready()) {
- setResult(Try<T>(exception_wrapper(BrokenPromise())));
+ // detachPromise() and setResult() should never be called in parallel
+ // so we don't need to protect this.
+ if (UNLIKELY(!result_)) {
+ setResult(Try<T>(exception_wrapper(BrokenPromise(typeid(T).name()))));
}
detachOne();
}
+ /// May call from any thread
void deactivate() {
- active_ = false;
+ active_.store(false, std::memory_order_release);
}
+ /// May call from any thread
void activate() {
- active_ = true;
- if (ready()) {
- maybeCallback();
- }
+ active_.store(true, std::memory_order_release);
+ maybeCallback();
}
- bool isActive() { return active_; }
+ /// May call from any thread
+ bool isActive() { return active_.load(std::memory_order_acquire); }
- void setExecutor(Executor* x) {
+ /// Call only from Future thread
+ void setExecutor(Executor* x, int8_t priority = Executor::MID_PRI) {
+ if (!executorLock_.try_lock()) {
+ executorLock_.lock();
+ }
executor_ = x;
+ priority_ = priority;
+ executorLock_.unlock();
}
- void raise(exception_wrapper const& e) {
- FSM_START
- case State::Interruptible:
- FSM_UPDATE2(State::Interrupted,
- [&]{ interrupt_ = folly::make_unique<exception_wrapper>(e); },
- [&]{ interruptHandler_(*interrupt_); });
- break;
+ void setExecutorNoLock(Executor* x, int8_t priority = Executor::MID_PRI) {
+ executor_ = x;
+ priority_ = priority;
+ }
- case State::Waiting:
- case State::Interrupted:
- FSM_UPDATE(State::Interrupted,
- [&]{ interrupt_ = folly::make_unique<exception_wrapper>(e); });
- break;
+ Executor* getExecutor() {
+ return executor_;
+ }
- case State::Done:
- FSM_BREAK
- FSM_END
+ /// Call only from Future thread
+ void raise(exception_wrapper e) {
+ if (!interruptLock_.try_lock()) {
+ interruptLock_.lock();
+ }
+ if (!interrupt_ && !hasResult()) {
+ interrupt_ = folly::make_unique<exception_wrapper>(std::move(e));
+ if (interruptHandler_) {
+ interruptHandler_(*interrupt_);
+ }
+ }
+ interruptLock_.unlock();
}
- void setInterruptHandler(std::function<void(exception_wrapper const&)> fn) {
- FSM_START
- case State::Waiting:
- case State::Interruptible:
- FSM_UPDATE(State::Interruptible,
- [&]{ interruptHandler_ = std::move(fn); });
- break;
+ std::function<void(exception_wrapper const&)> getInterruptHandler() {
+ if (!interruptHandlerSet_.load(std::memory_order_acquire)) {
+ return nullptr;
+ }
+ if (!interruptLock_.try_lock()) {
+ interruptLock_.lock();
+ }
+ auto handler = interruptHandler_;
+ interruptLock_.unlock();
+ return handler;
+ }
- case State::Interrupted:
+ /// Call only from Promise thread
+ void setInterruptHandler(std::function<void(exception_wrapper const&)> fn) {
+ if (!interruptLock_.try_lock()) {
+ interruptLock_.lock();
+ }
+ if (!hasResult()) {
+ if (interrupt_) {
fn(*interrupt_);
+ } else {
+ setInterruptHandlerNoLock(std::move(fn));
+ }
+ }
+ interruptLock_.unlock();
+ }
+
+ void setInterruptHandlerNoLock(
+ std::function<void(exception_wrapper const&)> fn) {
+ interruptHandlerSet_.store(true, std::memory_order_relaxed);
+ interruptHandler_ = std::move(fn);
+ }
+
+ protected:
+ void maybeCallback() {
+ FSM_START(fsm_)
+ case State::Armed:
+ if (active_.load(std::memory_order_acquire)) {
+ FSM_UPDATE2(fsm_, State::Done, []{}, [this]{ this->doCallback(); });
+ }
FSM_BREAK
- case State::Done:
+ default:
FSM_BREAK
FSM_END
}
- private:
- void maybeCallback() {
- assert(ready());
- if (isActive() && callback_) {
- if (!calledBack_.exchange(true)) {
- // TODO(5306911) we should probably try/catch
- Executor* x = executor_;
+ void doCallback() {
+ Executor* x = executor_;
+ int8_t priority;
+ if (x) {
+ if (!executorLock_.try_lock()) {
+ executorLock_.lock();
+ }
+ x = executor_;
+ priority = priority_;
+ executorLock_.unlock();
+ }
- RequestContext::setContext(context_);
- if (x) {
- MoveWrapper<std::function<void(Try<T>&&)>> cb(std::move(callback_));
- MoveWrapper<folly::Optional<Try<T>>> val(std::move(result_));
- x->add([cb, val]() mutable { (*cb)(std::move(**val)); });
+ // keep Core alive until callback did its thing
+ ++attached_;
+
+ if (x) {
+ try {
+ if (LIKELY(x->getNumPriorities() == 1)) {
+ x->add([this]() mutable {
+ SCOPE_EXIT { detachOne(); };
+ RequestContext::setContext(context_);
+ SCOPE_EXIT { callback_ = {}; };
+ callback_(std::move(*result_));
+ });
} else {
- callback_(std::move(*result_));
+ x->addWithPriority([this]() mutable {
+ SCOPE_EXIT { detachOne(); };
+ RequestContext::setContext(context_);
+ SCOPE_EXIT { callback_ = {}; };
+ callback_(std::move(*result_));
+ }, priority);
}
+ } catch (...) {
+ --attached_; // Account for extra ++attached_ before try
+ RequestContext::setContext(context_);
+ result_ = Try<T>(exception_wrapper(std::current_exception()));
+ SCOPE_EXIT { callback_ = {}; };
+ callback_(std::move(*result_));
}
+ } else {
+ SCOPE_EXIT { detachOne(); };
+ RequestContext::setContext(context_);
+ SCOPE_EXIT { callback_ = {}; };
+ callback_(std::move(*result_));
}
}
void detachOne() {
- auto d = ++detached_;
- assert(d >= 1);
- assert(d <= 2);
- if (d == 2) {
- // we should have already executed the callback with the value
- assert(calledBack_);
+ auto a = --attached_;
+ assert(a >= 0);
+ assert(a <= 2);
+ if (a == 0) {
delete this;
}
}
+ // Core should only be modified so that for size(T) == size(U),
+ // sizeof(Core<T>) == size(Core<U>).
+ // See Core::convert for details.
+
+ folly::Function<
+ void(Try<T>&&),
+ folly::FunctionMoveCtor::MAY_THROW,
+ 8 * sizeof(void*)>
+ callback_;
+ // place result_ next to increase the likelihood that the value will be
+ // contained entirely in one cache line
folly::Optional<Try<T>> result_;
- std::function<void(Try<T>&&)> callback_;
- std::shared_ptr<RequestContext> context_{nullptr};
- std::atomic<bool> calledBack_ {false};
- std::atomic<unsigned char> detached_ {0};
+ FSM<State> fsm_;
+ std::atomic<unsigned char> attached_;
std::atomic<bool> active_ {true};
- std::atomic<Executor*> executor_ {nullptr};
- std::unique_ptr<exception_wrapper> interrupt_;
- std::function<void(exception_wrapper const&)> interruptHandler_;
+ std::atomic<bool> interruptHandlerSet_ {false};
+ folly::MicroSpinLock interruptLock_ {0};
+ folly::MicroSpinLock executorLock_ {0};
+ int8_t priority_ {-1};
+ Executor* executor_ {nullptr};
+ std::shared_ptr<RequestContext> context_ {nullptr};
+ std::unique_ptr<exception_wrapper> interrupt_ {};
+ std::function<void(exception_wrapper const&)> interruptHandler_ {nullptr};
};
template <typename... Ts>
-struct VariadicContext {
- VariadicContext() : total(0), count(0) {}
- Promise<std::tuple<Try<Ts>... > > p;
- std::tuple<Try<Ts>... > results;
- size_t total;
- std::atomic<size_t> count;
+struct CollectAllVariadicContext {
+ CollectAllVariadicContext() {}
+ template <typename T, size_t I>
+ inline void setPartialResult(Try<T>& t) {
+ std::get<I>(results) = std::move(t);
+ }
+ ~CollectAllVariadicContext() {
+ p.setValue(std::move(results));
+ }
+ Promise<std::tuple<Try<Ts>...>> p;
+ std::tuple<Try<Ts>...> results;
typedef Future<std::tuple<Try<Ts>...>> type;
};
-template <typename... Ts, typename THead, typename... Fs>
-typename std::enable_if<sizeof...(Fs) == 0, void>::type
-whenAllVariadicHelper(VariadicContext<Ts...> *ctx, THead&& head, Fs&&... tail) {
- head.setCallback_([ctx](Try<typename THead::value_type>&& t) {
- std::get<sizeof...(Ts) - sizeof...(Fs) - 1>(ctx->results) = std::move(t);
- if (++ctx->count == ctx->total) {
- ctx->p.setValue(std::move(ctx->results));
- delete ctx;
+template <typename... Ts>
+struct CollectVariadicContext {
+ CollectVariadicContext() {}
+ template <typename T, size_t I>
+ inline void setPartialResult(Try<T>& t) {
+ if (t.hasException()) {
+ if (!threw.exchange(true)) {
+ p.setException(std::move(t.exception()));
+ }
+ } else if (!threw) {
+ std::get<I>(results) = std::move(t);
+ }
+ }
+ ~CollectVariadicContext() {
+ if (!threw.exchange(true)) {
+ p.setValue(unwrap(std::move(results)));
}
- });
+ }
+ Promise<std::tuple<Ts...>> p;
+ std::tuple<folly::Try<Ts>...> results;
+ std::atomic<bool> threw {false};
+ typedef Future<std::tuple<Ts...>> type;
+
+ private:
+ template <typename... Ts2>
+ static std::tuple<Ts...> unwrap(std::tuple<folly::Try<Ts>...>&& o,
+ Ts2&&... ts2) {
+ static_assert(sizeof...(ts2) <
+ std::tuple_size<std::tuple<folly::Try<Ts>...>>::value,
+ "Non-templated unwrap should be used instead");
+ assert(std::get<sizeof...(ts2)>(o).hasValue());
+
+ return unwrap(std::move(o),
+ std::forward<Ts2>(ts2)...,
+ std::move(*std::get<sizeof...(ts2)>(o)));
+ }
+
+ static std::tuple<Ts...> unwrap(std::tuple<folly::Try<Ts>...>&& /* o */,
+ Ts&&... ts) {
+ return std::tuple<Ts...>(std::forward<Ts>(ts)...);
+ }
+};
+
+template <template <typename...> class T, typename... Ts>
+void collectVariadicHelper(const std::shared_ptr<T<Ts...>>& /* ctx */) {
+ // base case
}
-template <typename... Ts, typename THead, typename... Fs>
-typename std::enable_if<sizeof...(Fs) != 0, void>::type
-whenAllVariadicHelper(VariadicContext<Ts...> *ctx, THead&& head, Fs&&... tail) {
+template <template <typename ...> class T, typename... Ts,
+ typename THead, typename... TTail>
+void collectVariadicHelper(const std::shared_ptr<T<Ts...>>& ctx,
+ THead&& head, TTail&&... tail) {
head.setCallback_([ctx](Try<typename THead::value_type>&& t) {
- std::get<sizeof...(Ts) - sizeof...(Fs) - 1>(ctx->results) = std::move(t);
- if (++ctx->count == ctx->total) {
- ctx->p.setValue(std::move(ctx->results));
- delete ctx;
- }
+ ctx->template setPartialResult<typename THead::value_type,
+ sizeof...(Ts) - sizeof...(TTail) - 1>(t);
});
// template tail-recursion
- whenAllVariadicHelper(ctx, std::forward<Fs>(tail)...);
+ collectVariadicHelper(ctx, std::forward<TTail>(tail)...);
}
-template <typename T>
-struct WhenAllContext {
- WhenAllContext() : count(0) {}
- Promise<std::vector<Try<T> > > p;
- std::vector<Try<T> > results;
- std::atomic<size_t> count;
-};
-
-template <typename T>
-struct WhenAnyContext {
- explicit WhenAnyContext(size_t n) : done(false), ref_count(n) {};
- Promise<std::pair<size_t, Try<T>>> p;
- std::atomic<bool> done;
- std::atomic<size_t> ref_count;
- void decref() {
- if (--ref_count == 0) {
- delete this;
- }
- }
-};
-
}} // folly::detail
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