// 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_