shared_mutex

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

// Copyright (C) 2013-2015 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 <= 201103L
# include <bits/c++14_warning.h>
#else

#include <bits/c++config.h>
#include <mutex>
#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

#define __cpp_lib_shared_timed_mutex 201402

  /// shared_timed_mutex
  class shared_timed_mutex
  {
#if _GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK
    typedef chrono::system_clock   __clock_t;

#ifdef PTHREAD_RWLOCK_INITIALIZER
    pthread_rwlock_t       _M_rwlock = PTHREAD_RWLOCK_INITIALIZER;

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

  public:
    shared_timed_mutex()
    {
      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_DEBUG_ASSERT(__ret == 0);
    }

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

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

    // Exclusive ownership

    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_DEBUG_ASSERT(__ret == 0);
    }

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

    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);
      }

    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_DEBUG_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);
      }

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

    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);
      }

    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_DEBUG_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);
      }

    void
    unlock_shared()
    {
      unlock();
    }

#else // ! (_GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK)

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

    // 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_timed_mutex() : _M_state(0) {}

    ~shared_timed_mutex()
    {
      _GLIBCXX_DEBUG_ASSERT( _M_state == 0 );
    }

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

    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);
      }

    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;
      }

    void
    unlock()
    {
      lock_guard<mutex> __lk(_M_mut);
      _GLIBCXX_DEBUG_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;
    }

    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);
      }

    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;
      }

    void
    unlock_shared()
    {
      lock_guard<mutex> __lk(_M_mut);
      _GLIBCXX_DEBUG_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 // _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(&__m), _M_owns(true)
      { __m.lock_shared(); }

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

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

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

      template<typename _Clock, typename _Duration>
   shared_lock(mutex_type& __m,
               const chrono::time_point<_Clock, _Duration>& __abs_time)
      : _M_pm(&__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(&__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