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From:	Ken Brown <kbrown AT cornell DOT edu>
To:	Jon Turney <jon DOT turney AT dronecode DOT org DOT uk>,
	The Cygwin Mailing List <cygwin AT cygwin DOT com>
Subject:	Re: Regression (last snapshot)
Date:	Thu, 1 Aug 2019 15:30:09 +0000
Message-ID:	<12269538-c387-1f1e-ab5a-b36b60d09937@cornell.edu>
References:	<20190722164509 DOT GG21169 AT calimero DOT vinschen DOT de> <eb8007691b20df0eb8032ca3d2fa445f AT smtp-cloud8 DOT xs4all DOT net> <b3b04db5-7f03-8281-f376-6cd10207ec01 AT cornell DOT edu> <4b59209a91e8384ec000e2724696791c AT smtp-cloud7 DOT xs4all DOT net> <935d8ce5-fd5c-3010-4664-bb2dc9b7ca2f AT cornell DOT edu> <20190729084552 DOT GL11632 AT calimero DOT vinschen DOT de> <eea273ca-f190-c91a-7892-9407bfcf4326 AT cornell DOT edu> <20190729134700 DOT GO11632 AT calimero DOT vinschen DOT de> <ced2750a-0a52-1396-b6c1-f4228116bde7 AT cornell DOT edu> <20190729152327 DOT GQ11632 AT calimero DOT vinschen DOT de> <20190729154049 DOT GR11632 AT calimero DOT vinschen DOT de> <c5d01965-80b9-cece-54d2-f7462f84c0a2 AT cornell DOT edu> <8e57d4da-722d-1b9a-bd28-5f96ed182f6a AT cornell DOT edu> <0a09679b-1a0f-613b-04c8-1a63da8a00c2 AT cornell DOT edu> <33aead20-540c-ee3b-0d38-ff053fbac040 AT dronecode DOT org DOT uk>
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On 8/1/2019 10:27 AM, Jon Turney wrote:
> On 31/07/2019 19:00, Ken Brown wrote:
>> On 7/31/2019 11:53 AM, Ken Brown wrote:
>>> On 7/29/2019 2:55 PM, Ken Brown wrote:
>>>> Here's the error that I can reproduce easily in xterm:
>>>>
>>>> $ ls <(grep bash .bashrc)
>>>> /dev/fd/63
>>>> grep: write error: Broken pipe
>>>>
>>>> This happens 98% of the time.  Notice that I used plain 'ls' rather 
>>>> than the
>>>> original 'ls -lL'.  With the latter, I get the broken pipe error 60% 
>>>> of the time
>>>> rather than 98%:
>>>>
>>>> $ ls -lL <(grep bash .bashrc)
>>>> pr-------- 1 kbrown None 0 2019-07-29 14:46 /dev/fd/63
>>>> grep: write error: Broken pipe
>>>>
>>>> What about the explanation I tried earlier, but perhaps not clearly: 
>>>> ls prints
>>>> /dev/fd/63 and then exits, thereby closing the read end of the pipe, 
>>>> while grep
>>>> (running asynchronously) hasn't finished writing to the write end of 
>>>> the pipe.
>>>>
>>>> The fact that I get the broken pipe error more often with plain 'ls' 
>>>> than with
>>>> 'ls -lL' is consistent with that.  And the fact that I get no errors 
>>>> with 'cat
>>>> <(grep bash .bashrc)' is also consistent with it, since cat doesn't 
>>>> exit until
>>>> grep has finished writing.
>>>>
>>>> On the other hand, this doesn't explain why I see the error only 
>>>> under xterm,
>>>> nor does it explain why you can't reproduce it at all.
>>>
>>> I've made some progress.  It turns out that the problem only occurs 
>>> in terminals
>>> launched from the xwin-xdg-menu tray icon.  I can even launch a 
>>> mintty window
>>> from that icon (System Tools -> Cygwin Terminal) and I'll see the 
>>> problem.  On
>>> the other hand, I can launch an xterm without using that icon (e.g., 
>>> 'DISPLAY=:0
>>> xterm -l&' from a mintty window) and I won't see the problem.
>>>
>>> So the issue has something to do with how xwin-xdg-menu launches 
>>> applications,
>>> and how that interacts with bash's process substitution.  I've just 
>>> downloaded
>>> the xwin-xdg-menu source and will see if I can figure out what's 
>>> going on.  I've
>>> also added Jon to the CC in case he has a chance to take a look.
>>
>> OK, when xwin-xdg-menu launches an application, it creates two pipes 
>> and sets
>> the application's stdout and stderr to the write ends of those pipes.  
>> For
>> example, here's what I see when I launch mintty from xwin-xdg-menu:
>>
>> $ pgrep mintty
>> 5375
>>
>> $ ls -l /proc/5375/fd
>> total 0
>> lrwxrwxrwx 1 kbrown None 0 2019-07-31 13:37 0 -> /dev/null
>> lrwxrwxrwx 1 kbrown None 0 2019-07-31 13:37 1 -> pipe:[38654736160]
>> lrwxrwxrwx 1 kbrown None 0 2019-07-31 13:37 2 -> pipe:[42949703456]
>> lrwxrwxrwx 1 kbrown None 0 2019-07-31 13:37 3 -> /dev/ptmx
>> lrwxrwxrwx 1 kbrown None 0 2019-07-31 13:37 4 -> /dev/windows
>>
>> These pipes are apparently used for logging.
> 
> That's correct.  Writes to those pipes are drained into the session 
> logfile (~/.xsession-errors), which you can view with the 'View logfile' 
> menu item.
> 
>> I don't see how this would explain my observations (grep reporting a 
>> broken
>> pipe), but it is at least a difference between a terminal started from
>> xwin-xdg-menu and an "ordinary" terminal.
>>
>> Anyway, I don't see any evidence here of a Cygwin bug.
> 
> I'm not so sure: these pipes for stdout/stderr of the mintty process 
> shouldn't have any influence at all on the child bash process (since 
> it's stdin/stdout/stderr is the slave end of a pty), but apparently they 
> somehow do?

Well, I can't be sure that the pipes are responsible.  It's just that 
the existence of the pipes is the only difference I could spot between 
an ordinary terminal and a terminal started from xwin-xdg-menu.

Is it possible that the logging somehow slows things down or changes the 
buffering, so that the grep process takes longer to complete?  This 
would be consistent with my theory that the broken pipe error doesn't 
really represent a bug, but rather it reflects the fact that ls exits 
before grep has finished writing.

Ken

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