Re: cygwin-2 process handling

[Corinna Vinschen <corinna-cygwin at cygwin dot com>]

Hi Takashi,

On May 21 20:53, Takashi Yano wrote:
> [...]
> I was looking into this problem, and found the PeekNamedPipe()
> call is blocked in fhandler_pty_master::close() when the problem
> occurs.
> 
> I had not noticed that, 
> 
> https://msdn.microsoft.com/en-us/library/windows/desktop/aa365779(v=vs.85).aspx
> 
> says:
> > The PeekNamedPipe function can block thread execution the same
> > way any I/O function can when called on a synchronous handle in
> > a multi-threaded application.
> 
> I have made a patch attached to resolve this problem. Please have
> a look.
> 
> With this patch, NtQueryObject() is used instead of PeekNamedPipe()
> to detect closure of the last master handle.
> 
> Please refer to the following discussion for more details.
> http://cygwin.com/ml/cygwin/2015-03/msg00221.html
> 
> 
> ChageLog is as follows.
> 
> 2015-05-21  Takashi Yano  <...>
> 
> 	* fhandler_tty.cc (fhandler_pty_master::close): Use NtQueryObject()
> 	instead of PeekNamedPipe() to detect closing the last master handle.
> [...]
> +  if (output_handle_local)
> +    {
> +      OBJECT_BASIC_INFORMATION obi;
> +      NTSTATUS status;
> +      ULONG hdl_cnt = 0;
> +
> +      status = NtQueryObject (output_handle_local, ObjectBasicInformation,
> +	  &obi, sizeof obi, NULL);
> +      if (!NT_SUCCESS (status))
> +	debug_printf ("NtQueryObject: %y", status);
> +      else
> +	hdl_cnt = obi.HandleCount;
> +      termios_printf("HandleCount: %d", hdl_cnt);
> +      if (hdl_cnt == 1)
> +	SetEvent (input_available_event);
> +      CloseHandle (output_handle_local);
> +    }
 
Isn't that racy?  Consider two processes doing that at the same time.
Both calls to NtQueryObject could come up with hdl_cnt == 2 and the
problem persists.

Wouldn't it be safer to call SetEvent(input_available_event) all the
time from here?


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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