Setting termios VMIN > 0 and VTIME > 0 on non blocking file

[Åke Rehnman]

On 2020-03-14 11:23, Åke Rehnman wrote:
>
> On 2020-03-13 11:12, Corinna Vinschen wrote:
>> On Mar 12 18:04, Åke Rehnman via Cygwin wrote:
>>> On 2020-03-12 16:08, Corinna Vinschen wrote:
>>>> On Mar 12 15:44, Corinna Vinschen wrote:
>>>>> On Mar 12 15:20, Åke Rehnman via Cygwin wrote:
>>>>>> I think the problem is if the number of bytes requested are more 
>>>>>> than what
>>>>> To clarify: number of bytes == VMIN?
>>> no number of bytes requested from ReadFile(). As far as I know Win32 
>>> has no
>>> concept of VMIN.
>> Right, just this weird TimeoutMultiplier, but nevertheless I wasn't sure
>> what you meant.
>>
>>>>>> is in the buffer it is going to overlap the read function 
>>>>>> (because of VTIME)
>>>>>> and immediately after that CancelIO is called. Contrary to what 
>>>>>> is mentioned
>>>>>> in the source code I think CancelIO is actually discarding data...
>>>>> So far we didn't have that experience.  CancelIO is usually safe
>>>>> in this regard.
>>> The data is MIA somehow...
>> Yes, but we're calling CancelIo in other circumstances in Cygwin and
>> there were no reports of missing data.  CancelIo is just supposed to
>> terminate the currently running overlapped IO, not to discard any
>> in-flight data.  If that's different for serial IO, there would be no
>> way to terminate serial overlapped IO gracefully.  Well, yeah, it's
>> Windows, but still...
> I have scrutinized the microsoft serial driver reference 
> implementation 
> (https://github.com/microsoft/Windows-driver-samples/blob/master/serial/serial/read.c) 
> and it looks to me as CancelIo does not purge any data. However since 
> the driver is vendor dependent in my case FTDI it is impossible to 
> know exactly what is going on since it is closed source.
>>>>>> -    if ((vmin_ > 0) && (vtime_ == 0))
>>>>>> +       if (is_nonblocking())
>>>>>> +       {
>>>>>> +               to.ReadIntervalTimeout = MAXDWORD;
>>>>>> +       }
>>>>>> +    else if ((vmin_ > 0) && (vtime_ == 0))
>>>>> What if you switch to !O_NONBLOCK after calling tcsetattr?  The
>>>>> setting of ReadIntervalTimeout would be lost then.
>>>>>
>>>>> Either we have to repeat calling SetCommTimeouts every time
>>>>> we switch mode, or we have to do the above setting temporary
>>>>> every time we call ReadFile in non blocking mode.
>>> True.
>>>> What about this:
>>>>
>>>> diff --git a/winsup/cygwin/fhandler_serial.cc 
>>>> b/winsup/cygwin/fhandler_serial.cc
>>>> --- a/winsup/cygwin/fhandler_serial.cc
>>>> +++ b/winsup/cygwin/fhandler_serial.cc
>>>> @@ -68,6 +68,16 @@ fhandler_serial::raw_read (void *ptr, size_t& ulen)
>>>>        goto err;
>>>>          else if (ev)
>>>>        termios_printf ("error detected %x", ev);
>>>> +      else if (is_nonblocking ())
>>>> +    {
>>>> +      if (!st.cbInQue)
>>>> +        {
>>>> +          tot = -1;
>>>> +          set_errno (EAGAIN);
>>>> +          goto out;
>>>> +        }
>>>> +      inq = st.cbInQue;
>>>> +    }
>>>>          else if (st.cbInQue && !vtime_)
>>>>        inq = st.cbInQue;
>>>>          else if (!is_nonblocking () && !overlapped_armed)
>>> Looks promising. I will try it.
> Your patch works (for my test case and screen). Question is if we have 
> to consider the case where ulen==0 ...
>>>
>>> BTW there is a gremlin in the "else if (ev)" line....
>> A gremlin?  Would you mind to explain?  Btw., if you find a bug
>> in the code, we do take patches :) https://cygwin.com/contrib.html
> If we have an error event in ev it will make a blocking read even if 
> VTIME==0.
>
I forgot, also any CancelIo should be terminated with a blocking 
GetOverlappedResult() see this excellent blog post 
https://devblogs.microsoft.com/oldnewthing/20110202-00/?p=11613

    // took longer than 1 second - cancel it and give up
    CancelIo(h);
    WaitForSingleObject(o.hEvent, INFINITE); // added // Alternatively: 
GetOverlappedResult(h, &o, TRUE);
    return WAIT_TIMEOUT;

/Ake