shared_mutex

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// <shared_mutex> -*- C++ -*-

// Copyright (C) 2013-2018 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.

// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

/** @file include/shared_mutex
 *  This is a Standard C++ Library header.
 */

#ifndef _GLIBCXX_SHARED_MUTEX
#define _GLIBCXX_SHARED_MUTEX 1

#pragma GCC system_header

#if __cplusplus >= 201402L

#include <bits/c++config.h>
#include <condition_variable>
#include <bits/functexcept.h>

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

  /**
   * @ingroup mutexes
   * @{
   */

#ifdef _GLIBCXX_USE_C99_STDINT_TR1
#ifdef _GLIBCXX_HAS_GTHREADS

#if __cplusplus >= 201703L
#define __cpp_lib_shared_mutex 201505
  class shared_mutex;
#endif

#define __cpp_lib_shared_timed_mutex 201402
  class shared_timed_mutex;

#if _GLIBCXX_USE_PTHREAD_RWLOCK_T
  /// A shared mutex type implemented using pthread_rwlock_t.
  class __shared_mutex_pthread
  {
    friend class shared_timed_mutex;

#ifdef PTHREAD_RWLOCK_INITIALIZER
    pthread_rwlock_t    _M_rwlock = PTHREAD_RWLOCK_INITIALIZER;

  public:
    __shared_mutex_pthread() = default;
    ~__shared_mutex_pthread() = default;
#else
    pthread_rwlock_t    _M_rwlock;

  public:
    __shared_mutex_pthread()
    {
      int __ret = pthread_rwlock_init(&_M_rwlock, NULL);
      if (__ret == ENOMEM)
        __throw_bad_alloc();
      else if (__ret == EAGAIN)
        __throw_system_error(int(errc::resource_unavailable_try_again));
      else if (__ret == EPERM)
        __throw_system_error(int(errc::operation_not_permitted));
      // Errors not handled: EBUSY, EINVAL
      __glibcxx_assert(__ret == 0);
    }

    ~__shared_mutex_pthread()
    {
      int __ret __attribute((__unused__)) = pthread_rwlock_destroy(&_M_rwlock);
      // Errors not handled: EBUSY, EINVAL
      __glibcxx_assert(__ret == 0);
    }
#endif

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

    void
    lock()
    {
      int __ret = pthread_rwlock_wrlock(&_M_rwlock);
      if (__ret == EDEADLK)
        __throw_system_error(int(errc::resource_deadlock_would_occur));
      // Errors not handled: EINVAL
      __glibcxx_assert(__ret == 0);
    }

    bool
    try_lock()
    {
      int __ret = pthread_rwlock_trywrlock(&_M_rwlock);
      if (__ret == EBUSY) return false;
      // Errors not handled: EINVAL
      __glibcxx_assert(__ret == 0);
      return true;
    }

    void
    unlock()
    {
      int __ret __attribute((__unused__)) = pthread_rwlock_unlock(&_M_rwlock);
      // Errors not handled: EPERM, EBUSY, EINVAL
      __glibcxx_assert(__ret == 0);
    }

    // Shared ownership

    void
    lock_shared()
    {
      int __ret;
      // We retry if we exceeded the maximum number of read locks supported by
      // the POSIX implementation; this can result in busy-waiting, but this
      // is okay based on the current specification of forward progress
      // guarantees by the standard.
      do
        __ret = pthread_rwlock_rdlock(&_M_rwlock);
      while (__ret == EAGAIN);
      if (__ret == EDEADLK)
        __throw_system_error(int(errc::resource_deadlock_would_occur));
      // Errors not handled: EINVAL
      __glibcxx_assert(__ret == 0);
    }

    bool
    try_lock_shared()
    {
      int __ret = pthread_rwlock_tryrdlock(&_M_rwlock);
      // If the maximum number of read locks has been exceeded, we just fail
      // to acquire the lock.  Unlike for lock(), we are not allowed to throw
      // an exception.
      if (__ret == EBUSY || __ret == EAGAIN) return false;
      // Errors not handled: EINVAL
      __glibcxx_assert(__ret == 0);
      return true;
    }

    void
    unlock_shared()
    {
      unlock();
    }

    void* native_handle() { return &_M_rwlock; }
  };
#endif

#if ! (_GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK)
  /// A shared mutex type implemented using std::condition_variable.
  class __shared_mutex_cv
  {
    friend class shared_timed_mutex;

    // Based on Howard Hinnant's reference implementation from N2406.

    // The high bit of _M_state is the write-entered flag which is set to
    // indicate a writer has taken the lock or is queuing to take the lock.
    // The remaining bits are the count of reader locks.
    //
    // To take a reader lock, block on gate1 while the write-entered flag is
    // set or the maximum number of reader locks is held, then increment the
    // reader lock count.
    // To release, decrement the count, then if the write-entered flag is set
    // and the count is zero then signal gate2 to wake a queued writer,
    // otherwise if the maximum number of reader locks was held signal gate1
    // to wake a reader.
    //
    // To take a writer lock, block on gate1 while the write-entered flag is
    // set, then set the write-entered flag to start queueing, then block on
    // gate2 while the number of reader locks is non-zero.
    // To release, unset the write-entered flag and signal gate1 to wake all
    // blocked readers and writers.
    //
    // This means that when no reader locks are held readers and writers get
    // equal priority. When one or more reader locks is held a writer gets
    // priority and no more reader locks can be taken while the writer is
    // queued.

    // Only locked when accessing _M_state or waiting on condition variables.
    mutex               _M_mut;
    // Used to block while write-entered is set or reader count at maximum.
    condition_variable  _M_gate1;
    // Used to block queued writers while reader count is non-zero.
    condition_variable  _M_gate2;
    // The write-entered flag and reader count.
    unsigned            _M_state;

    static constexpr unsigned _S_write_entered
      = 1U << (sizeof(unsigned)*__CHAR_BIT__ - 1);
    static constexpr unsigned _S_max_readers = ~_S_write_entered;

    // Test whether the write-entered flag is set. _M_mut must be locked.
    bool _M_write_entered() const { return _M_state & _S_write_entered; }

    // The number of reader locks currently held. _M_mut must be locked.
    unsigned _M_readers() const { return _M_state & _S_max_readers; }

  public:
    __shared_mutex_cv() : _M_state(0) {}

    ~__shared_mutex_cv()
    {
      __glibcxx_assert( _M_state == 0 );
    }

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

    // Exclusive ownership

    void
    lock()
    {
      unique_lock<mutex> __lk(_M_mut);
      // Wait until we can set the write-entered flag.
      _M_gate1.wait(__lk, [=]{ return !_M_write_entered(); });
      _M_state |= _S_write_entered;
      // Then wait until there are no more readers.
      _M_gate2.wait(__lk, [=]{ return _M_readers() == 0; });
    }

    bool
    try_lock()
    {
      unique_lock<mutex> __lk(_M_mut, try_to_lock);
      if (__lk.owns_lock() && _M_state == 0)
        {
          _M_state = _S_write_entered;
          return true;
        }
      return false;
    }

    void
    unlock()
    {
      lock_guard<mutex> __lk(_M_mut);
      __glibcxx_assert( _M_write_entered() );
      _M_state = 0;
      // call notify_all() while mutex is held so that another thread can't
      // lock and unlock the mutex then destroy *this before we make the call.
      _M_gate1.notify_all();
    }

    // Shared ownership

    void
    lock_shared()
    {
      unique_lock<mutex> __lk(_M_mut);
      _M_gate1.wait(__lk, [=]{ return _M_state < _S_max_readers; });
      ++_M_state;
    }

    bool
    try_lock_shared()
    {
      unique_lock<mutex> __lk(_M_mut, try_to_lock);
      if (!__lk.owns_lock())
        return false;
      if (_M_state < _S_max_readers)
        {
          ++_M_state;
          return true;
        }
      return false;
    }

    void
    unlock_shared()
    {
      lock_guard<mutex> __lk(_M_mut);
      __glibcxx_assert( _M_readers() > 0 );
      auto __prev = _M_state--;
      if (_M_write_entered())
        {
          // Wake the queued writer if there are no more readers.
          if (_M_readers() == 0)
            _M_gate2.notify_one();
          // No need to notify gate1 because we give priority to the queued
          // writer, and that writer will eventually notify gate1 after it
          // clears the write-entered flag.
        }
      else
        {
          // Wake any thread that was blocked on reader overflow.
          if (__prev == _S_max_readers)
            _M_gate1.notify_one();
        }
    }
  };
#endif

#if __cplusplus > 201402L
  /// The standard shared mutex type.
  class shared_mutex
  {
  public:
    shared_mutex() = default;
    ~shared_mutex() = default;

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

    // Exclusive ownership

    void lock() { _M_impl.lock(); }
    bool try_lock() { return _M_impl.try_lock(); }
    void unlock() { _M_impl.unlock(); }

    // Shared ownership

    void lock_shared() { _M_impl.lock_shared(); }
    bool try_lock_shared() { return _M_impl.try_lock_shared(); }
    void unlock_shared() { _M_impl.unlock_shared(); }

#if _GLIBCXX_USE_PTHREAD_RWLOCK_T
    typedef void* native_handle_type;
    native_handle_type native_handle() { return _M_impl.native_handle(); }

  private:
    __shared_mutex_pthread _M_impl;
#else
  private:
    __shared_mutex_cv _M_impl;
#endif
  };
#endif // C++17

#if _GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK
  using __shared_timed_mutex_base = __shared_mutex_pthread;
#else
  using __shared_timed_mutex_base = __shared_mutex_cv;
#endif

  /// The standard shared timed mutex type.
  class shared_timed_mutex
  : private __shared_timed_mutex_base
  {
    using _Base = __shared_timed_mutex_base;

    // Must use the same clock as condition_variable for __shared_mutex_cv.
    typedef chrono::system_clock        __clock_t;

  public:
    shared_timed_mutex() = default;
    ~shared_timed_mutex() = default;

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

    // Exclusive ownership

    void lock() { _Base::lock(); }
    bool try_lock() { return _Base::try_lock(); }
    void unlock() { _Base::unlock(); }

    template<typename _Rep, typename _Period>
      bool
      try_lock_for(const chrono::duration<_Rep, _Period>& __rel_time)
      {
        return try_lock_until(__clock_t::now() + __rel_time);
      }

    // Shared ownership

    void lock_shared() { _Base::lock_shared(); }
    bool try_lock_shared() { return _Base::try_lock_shared(); }
    void unlock_shared() { _Base::unlock_shared(); }

    template<typename _Rep, typename _Period>
      bool
      try_lock_shared_for(const chrono::duration<_Rep, _Period>& __rel_time)
      {
        return try_lock_shared_until(__clock_t::now() + __rel_time);
      }

#if _GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK

    // Exclusive ownership

    template<typename _Duration>
      bool
      try_lock_until(const chrono::time_point<__clock_t, _Duration>& __atime)
      {
        auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
        auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);

        __gthread_time_t __ts =
          {
            static_cast<std::time_t>(__s.time_since_epoch().count()),
            static_cast<long>(__ns.count())
          };

        int __ret = pthread_rwlock_timedwrlock(&_M_rwlock, &__ts);
        // On self-deadlock, we just fail to acquire the lock.  Technically,
        // the program violated the precondition.
        if (__ret == ETIMEDOUT || __ret == EDEADLK)
          return false;
        // Errors not handled: EINVAL
        __glibcxx_assert(__ret == 0);
        return true;
      }

    template<typename _Clock, typename _Duration>
      bool
      try_lock_until(const chrono::time_point<_Clock, _Duration>& __abs_time)
      {
        // DR 887 - Sync unknown clock to known clock.
        const typename _Clock::time_point __c_entry = _Clock::now();
        const __clock_t::time_point __s_entry = __clock_t::now();
        const auto __delta = __abs_time - __c_entry;
        const auto __s_atime = __s_entry + __delta;
        return try_lock_until(__s_atime);
      }

    // Shared ownership

    template<typename _Duration>
      bool
      try_lock_shared_until(const chrono::time_point<__clock_t,
                            _Duration>& __atime)
      {
        auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
        auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);

        __gthread_time_t __ts =
          {
            static_cast<std::time_t>(__s.time_since_epoch().count()),
            static_cast<long>(__ns.count())
          };

        int __ret;
        // Unlike for lock(), we are not allowed to throw an exception so if
        // the maximum number of read locks has been exceeded, or we would
        // deadlock, we just try to acquire the lock again (and will time out
        // eventually).
        // In cases where we would exceed the maximum number of read locks
        // throughout the whole time until the timeout, we will fail to
        // acquire the lock even if it would be logically free; however, this
        // is allowed by the standard, and we made a "strong effort"
        // (see C++14 30.4.1.4p26).
        // For cases where the implementation detects a deadlock we
        // intentionally block and timeout so that an early return isn't
        // mistaken for a spurious failure, which might help users realise
        // there is a deadlock.
        do
          __ret = pthread_rwlock_timedrdlock(&_M_rwlock, &__ts);
        while (__ret == EAGAIN || __ret == EDEADLK);
        if (__ret == ETIMEDOUT)
          return false;
        // Errors not handled: EINVAL
        __glibcxx_assert(__ret == 0);
        return true;
      }

    template<typename _Clock, typename _Duration>
      bool
      try_lock_shared_until(const chrono::time_point<_Clock,
                                                     _Duration>& __abs_time)
      {
        // DR 887 - Sync unknown clock to known clock.
        const typename _Clock::time_point __c_entry = _Clock::now();
        const __clock_t::time_point __s_entry = __clock_t::now();
        const auto __delta = __abs_time - __c_entry;
        const auto __s_atime = __s_entry + __delta;
        return try_lock_shared_until(__s_atime);
      }

#else // ! (_GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK)

    // Exclusive ownership

    template<typename _Clock, typename _Duration>
      bool
      try_lock_until(const chrono::time_point<_Clock, _Duration>& __abs_time)
      {
        unique_lock<mutex> __lk(_M_mut);
        if (!_M_gate1.wait_until(__lk, __abs_time,
                                 [=]{ return !_M_write_entered(); }))
          {
            return false;
          }
        _M_state |= _S_write_entered;
        if (!_M_gate2.wait_until(__lk, __abs_time,
                                 [=]{ return _M_readers() == 0; }))
          {
            _M_state ^= _S_write_entered;
            // Wake all threads blocked while the write-entered flag was set.
            _M_gate1.notify_all();
            return false;
          }
        return true;
      }

    // Shared ownership

    template <typename _Clock, typename _Duration>
      bool
      try_lock_shared_until(const chrono::time_point<_Clock,
                                                     _Duration>& __abs_time)
      {
        unique_lock<mutex> __lk(_M_mut);
        if (!_M_gate1.wait_until(__lk, __abs_time,
                                 [=]{ return _M_state < _S_max_readers; }))
          {
            return false;
          }
        ++_M_state;
        return true;
      }

#endif // _GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK
  };
#endif // _GLIBCXX_HAS_GTHREADS

  /// shared_lock
  template<typename _Mutex>
    class shared_lock
    {
    public:
      typedef _Mutex mutex_type;

      // Shared locking

      shared_lock() noexcept : _M_pm(nullptr), _M_owns(false) { }

      explicit
      shared_lock(mutex_type& __m)
      : _M_pm(std::__addressof(__m)), _M_owns(true)
      { __m.lock_shared(); }

      shared_lock(mutex_type& __m, defer_lock_t) noexcept
      : _M_pm(std::__addressof(__m)), _M_owns(false) { }

      shared_lock(mutex_type& __m, try_to_lock_t)
      : _M_pm(std::__addressof(__m)), _M_owns(__m.try_lock_shared()) { }

      shared_lock(mutex_type& __m, adopt_lock_t)
      : _M_pm(std::__addressof(__m)), _M_owns(true) { }

      template<typename _Clock, typename _Duration>
        shared_lock(mutex_type& __m,
                    const chrono::time_point<_Clock, _Duration>& __abs_time)
      : _M_pm(std::__addressof(__m)),
        _M_owns(__m.try_lock_shared_until(__abs_time)) { }

      template<typename _Rep, typename _Period>
        shared_lock(mutex_type& __m,
                    const chrono::duration<_Rep, _Period>& __rel_time)
      : _M_pm(std::__addressof(__m)),
        _M_owns(__m.try_lock_shared_for(__rel_time)) { }

      ~shared_lock()
      {
        if (_M_owns)
          _M_pm->unlock_shared();
      }

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

      shared_lock(shared_lock&& __sl) noexcept : shared_lock()
      { swap(__sl); }

      shared_lock&
      operator=(shared_lock&& __sl) noexcept
      {
        shared_lock(std::move(__sl)).swap(*this);
        return *this;
      }

      void
      lock()
      {
        _M_lockable();
        _M_pm->lock_shared();
        _M_owns = true;
      }

      bool
      try_lock()
      {
        _M_lockable();
        return _M_owns = _M_pm->try_lock_shared();
      }

      template<typename _Rep, typename _Period>
        bool
        try_lock_for(const chrono::duration<_Rep, _Period>& __rel_time)
        {
          _M_lockable();
          return _M_owns = _M_pm->try_lock_shared_for(__rel_time);
        }

      template<typename _Clock, typename _Duration>
        bool
        try_lock_until(const chrono::time_point<_Clock, _Duration>& __abs_time)
        {
          _M_lockable();
          return _M_owns = _M_pm->try_lock_shared_until(__abs_time);
        }

      void
      unlock()
      {
        if (!_M_owns)
          __throw_system_error(int(errc::resource_deadlock_would_occur));
        _M_pm->unlock_shared();
        _M_owns = false;
      }

      // Setters

      void
      swap(shared_lock& __u) noexcept
      {
        std::swap(_M_pm, __u._M_pm);
        std::swap(_M_owns, __u._M_owns);
      }

      mutex_type*
      release() noexcept
      {
        _M_owns = false;
        return std::exchange(_M_pm, nullptr);
      }

      // Getters

      bool owns_lock() const noexcept { return _M_owns; }

      explicit operator bool() const noexcept { return _M_owns; }

      mutex_type* mutex() const noexcept { return _M_pm; }

    private:
      void
      _M_lockable() const
      {
        if (_M_pm == nullptr)
          __throw_system_error(int(errc::operation_not_permitted));
        if (_M_owns)
          __throw_system_error(int(errc::resource_deadlock_would_occur));
      }

      mutex_type*       _M_pm;
      bool              _M_owns;
    };

  /// Swap specialization for shared_lock
  template<typename _Mutex>
    void
    swap(shared_lock<_Mutex>& __x, shared_lock<_Mutex>& __y) noexcept
    { __x.swap(__y); }

#endif // _GLIBCXX_USE_C99_STDINT_TR1

  // @} group mutexes
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace

#endif // C++14

#endif // _GLIBCXX_SHARED_MUTEX