/*
* Copyright (c) Facebook, Inc. and its affiliates.
*
* 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 <cassert>
#include <climits>
#include <utility>
#include <folly/Function.h>
#include <folly/Utility.h>
namespace folly {
namespace pushmi {
// derive from this for types that need to find operator|() overloads by ADL
struct folly_pipeorigin {};
} // namespace pushmi
using Func = Function<void()>;
namespace detail {
class ExecutorKeepAliveBase {
public:
// A dummy keep-alive is a keep-alive to an executor which does not support
// the keep-alive mechanism.
static constexpr uintptr_t kDummyFlag = uintptr_t(1) << 0;
// An alias keep-alive is a keep-alive to an executor to which there is
// known to be another keep-alive whose lifetime surrounds the lifetime of
// the alias.
static constexpr uintptr_t kAliasFlag = uintptr_t(1) << 1;
static constexpr uintptr_t kFlagMask = kDummyFlag | kAliasFlag;
static constexpr uintptr_t kExecutorMask = ~kFlagMask;
};
} // namespace detail
/// An Executor accepts units of work with add(), which should be
/// threadsafe.
class Executor {
// Workaround for a linkage problem with explicitly defaulted dtor t22914621
virtual ~Executor() {}
/// Enqueue a function to executed by this executor. This and all
/// variants must be threadsafe.
virtual void add(Func) = 0;
/// Enqueue a function with a given priority, where 0 is the medium priority
/// This is up to the implementation to enforce
virtual void addWithPriority(Func, int8_t priority);
virtual uint8_t getNumPriorities() const {
return 1;
}
static const int8_t LO_PRI = SCHAR_MIN;
static const int8_t MID_PRI = 0;
static const int8_t HI_PRI = SCHAR_MAX;
/**
* Executor::KeepAlive is a safe pointer to an Executor.
* For any Executor that supports KeepAlive functionality, Executor's
* destructor will block until all the KeepAlive objects associated with that
* Executor are destroyed.
* For Executors that don't support the KeepAlive funcionality, KeepAlive
* doesn't provide such protection.
* KeepAlive should *always* be used instead of Executor*. KeepAlive can be
* implicitly constructed from Executor*. getKeepAliveToken() helper method
* can be used to construct a KeepAlive in templated code if you need to
* preserve the original Executor type.
template <typename ExecutorT = Executor>
class KeepAlive : pushmi::folly_pipeorigin,
private detail::ExecutorKeepAliveBase {
using KeepAliveFunc = Function<void(KeepAlive&&)>;
KeepAlive() = default;
~KeepAlive() {
reset();
KeepAlive(KeepAlive&& other) noexcept
: storage_(std::exchange(other.storage_, 0)) {}
KeepAlive(const KeepAlive& other) noexcept
: KeepAlive(getKeepAliveToken(other.get())) {}
template <
typename OtherExecutor,
typename = typename std::enable_if<
std::is_convertible<OtherExecutor*, ExecutorT*>::value>::type>
/* implicit */ KeepAlive(KeepAlive<OtherExecutor>&& other) noexcept
: KeepAlive(other.get(), other.storage_ & kFlagMask) {
other.storage_ = 0;
/* implicit */ KeepAlive(const KeepAlive<OtherExecutor>& other) noexcept
/* implicit */ KeepAlive(ExecutorT* executor) {
*this = getKeepAliveToken(executor);
KeepAlive& operator=(KeepAlive&& other) {
storage_ = std::exchange(other.storage_, 0);
return *this;
KeepAlive& operator=(KeepAlive const& other) {
return operator=(folly::copy(other));
KeepAlive& operator=(KeepAlive<OtherExecutor>&& other) {
return *this = KeepAlive(std::move(other));
KeepAlive& operator=(const KeepAlive<OtherExecutor>& other) {
return *this = KeepAlive(other);
void reset() {
if (Executor* executor = get()) {
auto const flags = std::exchange(storage_, 0) & kFlagMask;
if (!(flags & (kDummyFlag | kAliasFlag))) {
executor->keepAliveRelease();
explicit operator bool() const {
return storage_;
ExecutorT* get() const {
return reinterpret_cast<ExecutorT*>(storage_ & kExecutorMask);
ExecutorT& operator*() const {
return *get();
ExecutorT* operator->() const {
return get();
KeepAlive copy() const {
return isKeepAliveDummy(*this) //
? makeKeepAliveDummy(get())
: getKeepAliveToken(get());
KeepAlive get_alias() const {
return KeepAlive(storage_ | kAliasFlag);
template <class KAF>
void add(KAF&& f) && {
static_assert(
is_invocable<KAF, KeepAlive&&>::value,
"Parameter to add must be void(KeepAlive&&)>");
auto ex = get();
ex->add([ka = std::move(*this), f = std::forward<KAF>(f)]() mutable {
f(std::move(ka));
});
private:
friend class Executor;
template <typename OtherExecutor>
friend class KeepAlive;
KeepAlive(ExecutorT* executor, uintptr_t flags) noexcept
: storage_(reinterpret_cast<uintptr_t>(executor) | flags) {
assert(executor);
assert(!(reinterpret_cast<uintptr_t>(executor) & ~kExecutorMask));
assert(!(flags & kExecutorMask));
explicit KeepAlive(uintptr_t storage) noexcept : storage_(storage) {}
// Combined storage for the executor pointer and for all flags.
uintptr_t storage_{reinterpret_cast<uintptr_t>(nullptr)};
template <typename ExecutorT>
static KeepAlive<ExecutorT> getKeepAliveToken(ExecutorT* executor) {
std::is_base_of<Executor, ExecutorT>::value,
"getKeepAliveToken only works for folly::Executor implementations.");
if (!executor) {
return {};
folly::Executor* executorPtr = executor;
if (executorPtr->keepAliveAcquire()) {
return makeKeepAlive<ExecutorT>(executor);
return makeKeepAliveDummy<ExecutorT>(executor);
static KeepAlive<ExecutorT> getKeepAliveToken(ExecutorT& executor) {
return getKeepAliveToken(&executor);
protected:
* Returns true if the KeepAlive is constructed from an executor that does
* not support the keep alive ref-counting functionality
static bool isKeepAliveDummy(const KeepAlive<ExecutorT>& keepAlive) {
return keepAlive.storage_ & KeepAlive<ExecutorT>::kDummyFlag;
// Acquire a keep alive token. Should return false if keep-alive mechanism
// is not supported.
virtual bool keepAliveAcquire();
// Release a keep alive token previously acquired by keepAliveAcquire().
// Will never be called if keepAliveAcquire() returns false.
virtual void keepAliveRelease();
static KeepAlive<ExecutorT> makeKeepAlive(ExecutorT* executor) {
"makeKeepAlive only works for folly::Executor implementations.");
return KeepAlive<ExecutorT>{executor, uintptr_t(0)};
static KeepAlive<ExecutorT> makeKeepAliveDummy(ExecutorT* executor) {
"makeKeepAliveDummy only works for folly::Executor implementations.");
return KeepAlive<ExecutorT>{executor, KeepAlive<ExecutorT>::kDummyFlag};
/// Returns a keep-alive token which guarantees that Executor will keep
/// processing tasks until the token is released (if supported by Executor).
/// KeepAlive always contains a valid pointer to an Executor.
Executor::KeepAlive<ExecutorT> getKeepAliveToken(ExecutorT* executor) {
return Executor::getKeepAliveToken(executor);
Executor::KeepAlive<ExecutorT> getKeepAliveToken(ExecutorT& executor) {
Executor::KeepAlive<ExecutorT> getKeepAliveToken(
Executor::KeepAlive<ExecutorT>& ka) {
return ka.copy();
} // namespace folly