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Date: Mon, 18 May 2026 10:13:26 +0900
To: cygwin AT cygwin DOT com
Subject: Re: On exit, winsup/cygwin/fhandler/console.cc waits on closed
 handle, deadlock possible
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From: Takashi Yano via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>
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On Mon, 18 May 2026 10:04:04 +0900
Takashi Yano wrote:
> On Thu, 14 May 2026 23:11:48 +0300
> Ivan Krylov wrote:
> > On Wed, 13 May 2026 12:59:59 +0300
> > Ivan Krylov <ikrylov AT disroot DOT org> wrote:
> > 
> > > I think it doesn't close the race window completely.
> > 
> > Here's a patch that I have more confidence in:
> > 
> > diff --git a/winsup/cygwin/fhandler/console.cc b/winsup/cygwin/fhandler/console.cc
> > index c76347f6f..a97df4cef 100644
> > --- a/winsup/cygwin/fhandler/console.cc
> > +++ b/winsup/cygwin/fhandler/console.cc
> > @@ -62,6 +62,7 @@ fhandler_console::console_state NO_COPY
> >  
> >  static bool NO_COPY inside_pcon_checked = false;
> >  static bool NO_COPY inside_pcon = false;
> > +static LONG NO_COPY inside_pcon_cleanup = 0;
> >  static int NO_COPY parent_pty;
> >  static HANDLE NO_COPY parent_pty_input_mutex = NULL;
> >  
> > @@ -530,8 +531,13 @@ fhandler_console::cons_master_thread (handle_set_t *p, tty *ttyp)
> >  	default: /* Error */
> >  	  free (input_rec);
> >  	  free (input_tmp);
> > -	  if (inside_pcon && parent_pty_input_mutex)
> > +	  if (inside_pcon && parent_pty_input_mutex) {
> >  	    ReleaseMutex (parent_pty_input_mutex);
> > +	    if (InterlockedCompareExchange(&inside_pcon_cleanup, 0, 0)) {
> > +	      ForceCloseHandle (parent_pty_input_mutex);
> > +	      inside_pcon = false;
> > +	    }
> > +	  }
> >  	  ReleaseMutex (p->input_mutex);
> >  	  return;
> >  	}
> > @@ -713,8 +719,13 @@ remove_record:
> >  	  while (true);
> >  	}
> >  skip_writeback:
> > -      if (inside_pcon && parent_pty_input_mutex)
> > +      if (inside_pcon && parent_pty_input_mutex) {
> >  	ReleaseMutex (parent_pty_input_mutex);
> > +	if (InterlockedCompareExchange(&inside_pcon_cleanup, 0, 0)) {
> > +	  ForceCloseHandle (parent_pty_input_mutex);
> > +	  inside_pcon = false;
> > +	}
> > +      }
> >        ReleaseMutex (p->input_mutex);
> >        cygwait (40);
> >      }
> > @@ -2048,7 +2059,7 @@ fhandler_console::pcon_hand_over_proc (void)
> >    else
> >      system_printf("Acquiring pcon_ho_mutex failed.");
> >    ReleaseMutex (mtx);
> > -  ForceCloseHandle (parent_pty_input_mutex);
> > +  InterlockedExchange(&inside_pcon_cleanup, 1);
> >  }
> >  
> >  bool
> > 
> > Instead of manipulating the mutex handle shared between threads and
> > leaving a race window between if(parent_pty_input_mutex) and
> > WaitForSingleObject(parent_pty_input_mutex, ...), tell the
> > cons_master_thread to close the handle and stop using it.
> 
> Thanks for the report and the patch.
> 
> The root cause of the problem is closing parent_pty_input_mutex before
> stopping cons_master_thread.
> 
> What abount just moving
> > -  ForceCloseHandle (parent_pty_input_mutex);
> into fhandler_console::close() like:
> 
> diff --git a/winsup/cygwin/fhandler/console.cc b/winsup/cygwin/fhandler/console.cc
> index c76347f6f..c4950885a 100644
> --- a/winsup/cygwin/fhandler/console.cc
> +++ b/winsup/cygwin/fhandler/console.cc
> @@ -2048,7 +2048,6 @@ fhandler_console::pcon_hand_over_proc (void)
>    else
>      system_printf("Acquiring pcon_ho_mutex failed.");
>    ReleaseMutex (mtx);
> -  ForceCloseHandle (parent_pty_input_mutex);
>  }
>  
>  bool
> @@ -2157,6 +2156,9 @@ fhandler_console::close (int flag)
>    CloseHandle (output_mutex);
>    output_mutex = NULL;
>  
> +  ForceCloseHandle (parent_pty_input_mutex);

Hmm, this should be:
+  if (parent_pty_input_mutex)
+    CloseHandle (parent_pty_input_mutex);

> +  parent_pty_input_mutex = NULL;
> +
>    WaitForSingleObject (shared_info_mutex, INFINITE);
>    if (--shared_info_state[unit] == 0 && shared_console_info[unit])
>      {


-- 
Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>

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