Mail Archives: cygwin/2021/08/29/17:11:06

Ken Brown via Cygwin <cygwin AT cygwin DOT com> · Sun, 29 Aug 2021 17:09:41 -0400

On 8/29/2021 3:37 PM, Takashi Yano wrote:
> On Sun, 29 Aug 2021 11:57:04 -0400
> Ken Brown wrote:
>> On 8/29/2021 5:07 AM, Takashi Yano via Cygwin wrote:
>>> On Sat, 28 Aug 2021 18:41:02 +0900
>>> Takashi Yano wrote:
>>>> On Sat, 28 Aug 2021 10:43:27 +0200
>>>> Corinna Vinschen wrote:
>>>>> On Aug 28 02:21, Takashi Yano via Cygwin wrote:
>>>>>> On Fri, 27 Aug 2021 12:00:50 -0400
>>>>>> Ken Brown wrote:
>>>>>>> Two years ago I thought I needed nt_create to avoid problems when calling
>>>>>>> set_pipe_non_blocking.  Are you saying that's not an issue?  Is
>>>>>>> set_pipe_non_blocking unnecessary?  Is that the point of your modification to
>>>>>>> raw_read?
>>>>>>
>>>>>> Yes. Instead of making windows read function itself non-blocking,
>>>>>> it is possible to check if the pipe can be read before read using
>>>>>> PeekNamedPipe(). If the pipe cannot be read right now, EAGAIN is
>>>>>> returned.
>>>>>
>>>>> The problem is this:
>>>>>
>>>>>     if (PeekNamedPipe())
>>>>>       ReadFile(blocking);
>>>>>
>>>>> is not atomic.  I. e., if PeekNamedPipe succeeds, nothing keeps another
>>>>> thread from draining the pipe between the PeekNamedPipe and the ReadFile
>>>>> call.  And as soon as ReadFile runs, it hangs indefinitely and we can't
>>>>> stop it via a signal.
>>>>
>>>> Hmm, you are right. Mutex guard seems to be necessary like pty code
>>>> if we go this way.
>>>
>>> I have found that set_pipe_non_blocking() succeeds for both read and
>>> write pipes if the write pipe is created by CreateNamedPipe() and the
>>> read pipe is created by CreateFile() contrary to the current create()
>>> code. Therefore, not only nt_create() but also PeekNamedPipe() become
>>> unnecessary.
>>>
>>> Please see the revised patch attached.
>>
>> That's a great idea.
>>
>> I've applied your two patches to the topic/pipe branch.  I also rebased it and
>> did a forced push in order to bring in Corinna's loader script fix.  So you'll
>> have to do 'git fetch' and 'git rebase --hard origin/topic/pipe'.
>>
>> Does this now fix all known problems with pipes?
> 
> Only the small thing remaining is pipe mode. If the pipe mode
> is changed to byte mode, the following issue will be solved.
> https://cygwin.com/pipermail/cygwin/2021-March/247987.html
> 
> How about the simple patch attached?
> 
> The comment in pipe code says:
>       Note that the write side of the pipe is opened as PIPE_TYPE_MESSAGE.
>       This *seems* to more closely mimic Linux pipe behavior and is
>       definitely required for pty handling since fhandler_pty_master
>       writes to the pipe in chunks, terminated by newline when CANON mode
>       is specified.
> 
> This mentions about pty behaiviour in canonical mode, however, the
> pty pipe is created as message mode even with this patch. Are there
> any other reasons that message mode is preferred for pipe?

No idea.  All I remember is that there was a lot of discussion around the time 
that it was decided to use PIPE_TYPE_MESSAGE by default.  Corinna probably 
remembers the reasons.

Ken

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Date:	Sun, 29 Aug 2021 17:09:41 -0400
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From:	Ken Brown via Cygwin <cygwin AT cygwin DOT com>
Reply-To:	Ken Brown <kbrown AT cornell DOT edu>
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