当`this`变成nullptr时，如何在类成员线程中处理quick_exit？

Question

我有这个小sn-p代码：

// (...)
class Time 
{
  std::atomic<bool> m_running;
  std::thread m_worker;
  // ...
};


Time::Time()
{
  // ...
  m_running = true;
  m_worker  = std::move(std::thread(std::bind(&Time::Worker, this)));
}


bool Time::HasTimedOut() const
{
  return (!m_disabled) &&
         (IsPending() && (GetRunTime() >= m_maximum_timeout) && (CloseHandlesDiff() >= m_minimum_close_time));
}

Time::~Time()
{
  if (m_running)
  {
    m_running = false;
    m_worker.join();
  }
}

void Time::Worker()
{
  while (m_running)
  {
    if (time_data->HasTimedOut())
    {
      time_data->RunTimedOutCallback();
    }

    if (m_count < 0)
    {
      m_count = 0;
    }

    if (m_running)
    {
      std::this_thread::sleep_for(std::chrono::milliseconds(20));
    }
  }
}

std::shared_ptr<Time> time_data(std::make_shared<Time>());

令我惊讶的是，我得到了一个核心转储，而来自 gdb 的回溯命令显示了这一点：

(gdb) bt
#0  0x09438408 in monitor::Time::HasTimedOut (this=0x0)
    at monitor.cxx: // return (!m_disabled) &&
#1  0x09438a84 in monitor::Time::Worker (this=0xbd96dd8)
    monitor.cxx: // if(time_data->HasTimedOut()
#2  0x0943cf81 in std::__invoke_impl<void, void (monitor::Time::*&)(), monitor::Time*&> (
    __f=@0xbd96e54: (void (monitor::Time::*)(monitor::Time * const)) 0x94389f0 <monitor::Time::Worker()>, __t=@0xbd96e5c: 0xbd96dd8)

一个 nullptr 似乎是问题所在（SEGFAULT）：

(this=0x0)

这意味着我的类被销毁了，而没有调用析构函数。

据我所知/怀疑，当我的应用程序的操作系统/看门狗强制退出/快速终止时，这可能是可能的。

有没有办法解决这个问题？也许一些 shared_ptr 原子包装，我可以检查 shared_ptr 是否为 nullptr，是否有一些原子 if-not-null-execute-this？话又说回来..这确实发生在执行中期。

我知道可以添加快速退出挂钩，但有时使用快速退出是出于很好的理由，减慢快速退出的速度将违反设计。处理这个成为 nullptr 的最佳方法是什么？

或者我是否应该让 SEGFALT 发生，因为无论如何应用程序正在快速退出？

这是完整的堆栈，但可能不会添加更多有用的信息：

(gdb) bt
(gdb) bt
#0  0x09438408 in monitor::Time::HasTimedOut (this=0x0)
    at /opt/procesleiding/vptlib/lib/oracle_monitor.cxx:111
#1  0x09438a84 in monitor::Time::Worker (this=0xbd96dd8)
    at /opt/procesleiding/vptlib/lib/oracle_monitor.cxx:142
#2  0x0943cf81 in std::__invoke_impl<void, void (monitor::Time::*&)(), monitor::Time*&> (
    __f=@0xbd96e54: (void (monitor::Time::*)(monitor::Time * const)) 0x94389f0 <monitor::Time::Worker()>, __t=@0xbd96e5c: 0xbd96dd8)
    at /usr/include/c++/7/bits/invoke.h:73
#3  0x0943c99f in std::__invoke<void (monitor::Time::*&)(), monitor::Time*&> (
    __fn=@0xbd96e54: (void (monitor::Time::*)(monitor::Time * const)) 0x94389f0 <monitor::Time::Worker()>, __args#0=@0xbd96e5c: 0xbd96dd8)
    at /usr/include/c++/7/bits/invoke.h:95
#4  0x0943c70c in std::_Bind<void (monitor::Time::*(monitor::Time*))()>::__call<void, , 0u>(std::tuple<>&&, std::_Index_tuple<0u>) (
    this=0xbd96e54, __args=...)
    at /usr/include/c++/7/functional:467
#5  0x0943c28c in std::_Bind<void (monitor::Time::*(monitor::Time*))()>::operator()<, void>() (this=0xbd96e54)
    at /usr/include/c++/7/functional:551
#6  0x0943bcaf in std::__invoke_impl<void, std::_Bind<void (monitor::Time::*(monitor::Time*))()>>(std::__invoke_other, std::_Bind<void (monitor::Time::*(monitor::Time*))()>&&) (__f=...) at /usr/include/c++/7/bits/invoke.h:60
#7  0x0943b022 in std::__invoke<std::_Bind<void (monitor::Time::*(monitor::Time*))()>>(std::_Bind<void (monitor::Time::*(monitor::Time*))()>&&) (__fn=...) at /usr/include/c++/7/bits/invoke.h:95
#8  0x0943e2a6 in std::thread::_Invoker<std::tuple<std::_Bind<void (monitor::Time::*(monitor::Time*))()> > >::_M_invoke<0u>(std::_Index_tuple<0u>) (this=0xbd96e54) at /usr/include/c++/7/thread:234
#9  0x0943e15c in std::thread::_Invoker<std::tuple<std::_Bind<void (monitor::Time::*(monitor::Time*))()> > >::operator()() (this=0xbd96e54) at /usr/include/c++/7/thread:243
#10 0x0943e067 in std::thread::_State_impl<std::thread::_Invoker<std::tuple<std::_Bind<void (monitor::Time::*(monitor::Time*))()> > > >::_M_run() (this=0xbd96e50) at /usr/include/c++/7/thread:186

Answer 1

您的代码存在竞争条件。

Time::Time()
{
  // ...
  m_running = true;
  m_worker  = std::move(std::thread(std::bind(&Time::Worker, this)));
}

从这里开始

std::shared_ptr<Time> time_data(std::make_shared<Time>());

已完成。

简单的线程在std::shared_ptr<Time> time_data(std::make_shared<Time>());完成之前到达monitor::Time::HasTimedOut。

生成线程不在构造函数中，而是在分配time_data 后调用的单独方法中。

无论如何，如果您的Timer 根本不使用time_data 全局变量会更好。