folly/folly/fibers/Semaphore.cpp

/*
 * Copyright (c) Meta Platforms, Inc. and 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.
 */

#include <folly/fibers/Semaphore.h>

namespace folly {
namespace fibers {

bool Semaphore::signalSlow() { … }

void Semaphore::signal() { … }

bool Semaphore::waitSlow(Waiter& waiter) { … }

void Semaphore::wait() { … }

bool Semaphore::try_wait(Waiter& waiter) { … }

bool Semaphore::try_wait() { … }

#if FOLLY_HAS_COROUTINES

coro::Task<void> Semaphore::co_wait() {
  auto oldVal = tokens_.load(std::memory_order_acquire);
  do {
    while (oldVal == 0) {
      Waiter waiter;
      // If waitSlow fails it is because the token is non-zero by the time
      // the lock is taken, so we can just continue round the loop
      if (waitSlow(waiter)) {
        bool cancelled = false;
        {
          const auto& ct = co_await folly::coro::co_current_cancellation_token;
          folly::CancellationCallback cb{
              ct, [&] {
                {
                  auto waitListLock = waitList_.wlock();
                  auto& waitList = *waitListLock;

                  if (!waiter.hook_.is_linked()) {
                    // Already dequeued by signalSlow()
                    return;
                  }

                  cancelled = true;
                  waitList.erase(waitList.iterator_to(waiter));
                }

                waiter.baton.post();
              }};

          co_await waiter.baton;
        }

        // Check 'cancelled' flag only after deregistering the callback so we're
        // sure that we aren't reading it concurrently with a potential write
        // from a thread requesting cancellation.
        if (cancelled) {
          co_yield folly::coro::co_cancelled;
        }

        co_return;
      }
      oldVal = tokens_.load(std::memory_order_acquire);
    }
  } while (!tokens_.compare_exchange_weak(
      oldVal,
      oldVal - 1,
      std::memory_order_release,
      std::memory_order_acquire));
}

#endif

namespace {

class FutureWaiter final : public fibers::Baton::Waiter { … };

} // namespace

SemiFuture<Unit> Semaphore::future_wait() { … }

size_t Semaphore::getCapacity() const { … }

size_t Semaphore::getAvailableTokens() const { … }

} // namespace fibers
} // namespace folly