Coverage Report

Created: 2024-09-23 06:29

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