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// Copyright 2023 The Abseil Authors.
//
// 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
//
// https://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.
//
#ifndef ABSL_SYNCHRONIZATION_INTERNAL_WAITER_BASE_H_
#define ABSL_SYNCHRONIZATION_INTERNAL_WAITER_BASE_H_
#include "absl/base/config.h"
#include "absl/base/internal/thread_identity.h"
#include "absl/synchronization/internal/kernel_timeout.h"
namespace absl {
ABSL_NAMESPACE_BEGIN
namespace synchronization_internal {
// `Waiter` is a platform specific semaphore implementation that `PerThreadSem`
// waits on to implement blocking in `absl::Mutex`. Implementations should
// inherit from `WaiterCrtp` and must implement `Wait()`, `Post()`, and `Poke()`
// as described in `WaiterBase`. `waiter.h` selects the implementation and uses
// static-dispatch for performance.
class WaiterBase {
public:
WaiterBase() = default;
// Not copyable or movable
WaiterBase(const WaiterBase&) = delete;
WaiterBase& operator=(const WaiterBase&) = delete;
// Blocks the calling thread until a matching call to `Post()` or
// `t` has passed. Returns `true` if woken (`Post()` called),
// `false` on timeout.
//
// bool Wait(KernelTimeout t);
// Restart the caller of `Wait()` as with a normal semaphore.
//
// void Post();
// If anyone is waiting, wake them up temporarily and cause them to
// call `MaybeBecomeIdle()`. They will then return to waiting for a
// `Post()` or timeout.
//
// void Poke();
// Returns the name of this implementation. Used only for debugging.
//
// static constexpr char kName[];
// How many periods to remain idle before releasing resources
#ifndef ABSL_HAVE_THREAD_SANITIZER
static constexpr int kIdlePeriods = 60;
#else
// Memory consumption under ThreadSanitizer is a serious concern,
// so we release resources sooner. The value of 1 leads to 1 to 2 second
// delay before marking a thread as idle.
static constexpr int kIdlePeriods = 1;
#endif
protected:
static void MaybeBecomeIdle();
};
template <typename T>
class WaiterCrtp : public WaiterBase {
public:
// Returns the Waiter associated with the identity.
static T* GetWaiter(base_internal::ThreadIdentity* identity) {
static_assert(
sizeof(T) <= sizeof(base_internal::ThreadIdentity::WaiterState),
"Insufficient space for Waiter");
return reinterpret_cast<T*>(identity->waiter_state.data);
}
};
} // namespace synchronization_internal
ABSL_NAMESPACE_END
} // namespace absl
#endif // ABSL_SYNCHRONIZATION_INTERNAL_WAITER_BASE_H_
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