123 lines
3.2 KiB
C++
123 lines
3.2 KiB
C++
/*
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* Copyright (c) Facebook, Inc. and its affiliates.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#pragma once
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#include <folly/IntrusiveList.h>
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#include <folly/Synchronized.h>
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#include <folly/experimental/coro/Task.h>
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#include <folly/fibers/Baton.h>
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#include <folly/futures/Future.h>
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#include <deque>
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namespace folly {
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namespace fibers {
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/*
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* Fiber-compatible semaphore. Will safely block fibers that wait when no
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* tokens are available and wake fibers when signalled.
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*/
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class Semaphore {
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public:
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explicit Semaphore(size_t tokenCount)
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: capacity_(tokenCount), tokens_(int64_t(capacity_)) {}
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Semaphore(const Semaphore&) = delete;
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Semaphore(Semaphore&&) = delete;
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Semaphore& operator=(const Semaphore&) = delete;
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Semaphore& operator=(Semaphore&&) = delete;
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/*
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* Release a token in the semaphore. Signal the waiter if necessary.
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*/
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void signal();
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/*
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* Wait for capacity in the semaphore.
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*/
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void wait();
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struct Waiter {
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Waiter() noexcept {}
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// The baton will be signalled when this waiter acquires the semaphore.
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Baton baton;
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private:
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friend Semaphore;
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folly::SafeIntrusiveListHook hook_;
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};
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/**
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* Try to wait on the semaphore.
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* Return true on success.
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* On failure, the passed waiter is enqueued, its baton will be posted once
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* semaphore has capacity. Caller is responsible to wait then signal.
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*/
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bool try_wait(Waiter& waiter);
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/**
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* If the semaphore has capacity, removes a token and returns true. Otherwise
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* returns false and leaves the semaphore unchanged.
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*/
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bool try_wait();
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#if FOLLY_HAS_COROUTINES
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/*
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* Wait for capacity in the semaphore.
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*
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* Note that this wait-operation can be cancelled by requesting cancellation
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* on the awaiting coroutine's associated CancellationToken.
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* If the operation is successfully cancelled then it will complete with
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* an error of type folly::OperationCancelled.
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*
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* Note that requesting cancellation of the operation will only have an
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* effect if the operation does not complete synchronously (ie. was not
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* already in a signalled state).
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*
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* If the semaphore was already in a signalled state prior to awaiting the
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* returned Task then the operation will complete successfully regardless
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* of whether cancellation was requested.
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*/
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coro::Task<void> co_wait();
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#endif
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/*
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* Wait for capacity in the semaphore.
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*/
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SemiFuture<Unit> future_wait();
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size_t getCapacity() const;
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private:
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bool waitSlow(Waiter& waiter);
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bool signalSlow();
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size_t capacity_;
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// Atomic counter
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std::atomic<int64_t> tokens_;
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using WaiterList = folly::SafeIntrusiveList<Waiter, &Waiter::hook_>;
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folly::Synchronized<WaiterList> waitList_;
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};
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} // namespace fibers
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} // namespace folly
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