// Copyright 2017 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.

//

// PerThreadSem is a low-level synchronization primitive controlling the

// runnability of a single thread, used internally by Mutex and CondVar.

//

// This is NOT a general-purpose synchronization mechanism, and should not be

// used directly by applications.  Applications should use Mutex and CondVar.

//

// The semantics of PerThreadSem are the same as that of a counting semaphore.

// Each thread maintains an abstract "count" value associated with its identity.

#ifndef ABSL_SYNCHRONIZATION_INTERNAL_PER_THREAD_SEM_H_

#define ABSL_SYNCHRONIZATION_INTERNAL_PER_THREAD_SEM_H_

#include <atomic>

#include "absl/base/internal/thread_identity.h"

#include "absl/synchronization/internal/create_thread_identity.h"

#include "absl/synchronization/internal/kernel_timeout.h"

namespace absl {

ABSL_NAMESPACE_BEGIN

class Mutex;

namespace synchronization_internal {

class PerThreadSem {

 public:

  PerThreadSem() = delete;

  PerThreadSem(const PerThreadSem&) = delete;

  PerThreadSem& operator=(const PerThreadSem&) = delete;

  // Routine invoked periodically (once a second) by a background thread.

  // Has no effect on user-visible state.

  static void Tick(base_internal::ThreadIdentity* identity);

  // ---------------------------------------------------------------------------

  // Routines used by autosizing threadpools to detect when threads are

  // blocked.  Each thread has a counter pointer, initially zero.  If non-zero,

  // the implementation atomically increments the counter when it blocks on a

  // semaphore, a decrements it again when it wakes.  This allows a threadpool

  // to keep track of how many of its threads are blocked.

  // SetThreadBlockedCounter() should be used only by threadpool

  // implementations.  GetThreadBlockedCounter() should be used by modules that

  // block threads; if the pointer returned is non-zero, the location should be

  // incremented before the thread blocks, and decremented after it wakes.

  static void SetThreadBlockedCounter(std::atomic<int> *counter);

  static std::atomic<int> *GetThreadBlockedCounter();

 private:

  // Create the PerThreadSem associated with "identity".  Initializes count=0.

  // REQUIRES: May only be called by ThreadIdentity.

  static inline void Init(base_internal::ThreadIdentity* identity);

  // Increments "identity"'s count.

  static inline void Post(base_internal::ThreadIdentity* identity);

  // Waits until either our count > 0 or t has expired.

  // If count > 0, decrements count and returns true.  Otherwise returns false.

  // !t.has_timeout() => Wait(t) will return true.

  static inline bool Wait(KernelTimeout t);

  // Permitted callers.

  friend class PerThreadSemTest;

  friend class absl::Mutex;

  friend void OneTimeInitThreadIdentity(absl::base_internal::ThreadIdentity*);

};

}  // namespace synchronization_internal

ABSL_NAMESPACE_END

}  // namespace absl

// In some build configurations we pass --detect-odr-violations to the

// gold linker.  This causes it to flag weak symbol overrides as ODR

// violations.  Because ODR only applies to C++ and not C,

// --detect-odr-violations ignores symbols not mangled with C++ names.

// By changing our extension points to be extern "C", we dodge this

// check.

extern "C" {

void ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemInit)(

    absl::base_internal::ThreadIdentity* identity);

void ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemPost)(

    absl::base_internal::ThreadIdentity* identity);

bool ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemWait)(

    absl::synchronization_internal::KernelTimeout t);

void ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemPoke)(

    absl::base_internal::ThreadIdentity* identity);

}  // extern "C"

void absl::synchronization_internal::PerThreadSem::Init(

    absl::base_internal::ThreadIdentity* identity) {

  ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemInit)(identity);

}

void absl::synchronization_internal::PerThreadSem::Post(

    absl::base_internal::ThreadIdentity* identity) {

  ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemPost)(identity);

}

bool absl::synchronization_internal::PerThreadSem::Wait(

    absl::synchronization_internal::KernelTimeout t) {

  return ABSL_INTERNAL_C_SYMBOL(AbslInternalPerThreadSemWait)(t);

}

#endif  // ABSL_SYNCHRONIZATION_INTERNAL_PER_THREAD_SEM_H_