util/env_posix.cc [72:102]:
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class Limiter {
 public:
  // Limit maximum number of resources to |max_acquires|.
  Limiter(int max_acquires) : acquires_allowed_(max_acquires) {}

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

  // If another resource is available, acquire it and return true.
  // Else return false.
  bool Acquire() {
    int old_acquires_allowed =
        acquires_allowed_.fetch_sub(1, std::memory_order_relaxed);

    if (old_acquires_allowed > 0) return true;

    acquires_allowed_.fetch_add(1, std::memory_order_relaxed);
    return false;
  }

  // Release a resource acquired by a previous call to Acquire() that returned
  // true.
  void Release() { acquires_allowed_.fetch_add(1, std::memory_order_relaxed); }

 private:
  // The number of available resources.
  //
  // This is a counter and is not tied to the invariants of any other class, so
  // it can be operated on safely using std::memory_order_relaxed.
  std::atomic<int> acquires_allowed_;
};
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util/env_windows.cc [114:144]:
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class Limiter {
 public:
  // Limit maximum number of resources to |max_acquires|.
  Limiter(int max_acquires) : acquires_allowed_(max_acquires) {}

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

  // If another resource is available, acquire it and return true.
  // Else return false.
  bool Acquire() {
    int old_acquires_allowed =
        acquires_allowed_.fetch_sub(1, std::memory_order_relaxed);

    if (old_acquires_allowed > 0) return true;

    acquires_allowed_.fetch_add(1, std::memory_order_relaxed);
    return false;
  }

  // Release a resource acquired by a previous call to Acquire() that returned
  // true.
  void Release() { acquires_allowed_.fetch_add(1, std::memory_order_relaxed); }

 private:
  // The number of available resources.
  //
  // This is a counter and is not tied to the invariants of any other class, so
  // it can be operated on safely using std::memory_order_relaxed.
  std::atomic<int> acquires_allowed_;
};
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