// Licensed to the Apache Software Foundation (ASF) under one
// or more contributor license agreements.  See the NOTICE file
// distributed with this work for additional information
// regarding copyright ownership.  The ASF licenses this file
// to you 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
//
//   http://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.

#pragma once

#include <memory>

#include "arrow/util/macros.h"
#include "arrow/util/visibility.h"

namespace arrow {
namespace util {

/// A wrapper around std::mutex since we can't use it directly in
/// public headers due to C++/CLI.
/// https://docs.microsoft.com/en-us/cpp/standard-library/mutex#remarks
class ARROW_EXPORT Mutex {
 public:
  Mutex();
  Mutex(Mutex&&) = default;
  Mutex& operator=(Mutex&&) = default;

  /// A Guard is falsy if a lock could not be acquired.
  class ARROW_EXPORT Guard {
   public:
    Guard() : locked_(NULLPTR, [](Mutex* mutex) {}) {}
    Guard(Guard&&) = default;
    Guard& operator=(Guard&&) = default;

    explicit operator bool() const { return bool(locked_); }

    void Unlock() { locked_.reset(); }

   private:
    explicit Guard(Mutex* locked);

    std::unique_ptr<Mutex, void (*)(Mutex*)> locked_;
    friend Mutex;
  };

  Guard TryLock();
  Guard Lock();

 private:
  struct Impl;
  std::unique_ptr<Impl, void (*)(Impl*)> impl_;
};

#ifndef _WIN32
/// Return a pointer to a process-wide, process-specific Mutex that can be used
/// at any point in a child process.  NULL is returned when called in the parent.
///
/// The rule is to first check that getpid() corresponds to the parent process pid
/// and, if not, call this function to lock any after-fork reinitialization code.
/// Like this:
///
///   std::atomic<pid_t> pid{getpid()};
///   ...
///   if (pid.load() != getpid()) {
///     // In child process
///     auto lock = GlobalForkSafeMutex()->Lock();
///     if (pid.load() != getpid()) {
///       // Reinitialize internal structures after fork
///       ...
///       pid.store(getpid());
ARROW_EXPORT
Mutex* GlobalForkSafeMutex();
#endif

}  // namespace util
}  // namespace arrow