Regression (last snapshot)

[Ken Brown]

On 7/29/2019 9:18 AM, Ken Brown wrote:
> On 7/29/2019 4:45 AM, Corinna Vinschen wrote:
>> On Jul 27 15:24, Ken Brown wrote:
>>> On 7/27/2019 6:21 AM, Houder wrote:
>>>> On Fri, 26 Jul 2019 22:12:43, Ken Brown  wrote:
>>>>
>>>>> On 7/22/2019 2:47 PM, Houder wrote:
>>>>
>>>>>> The specific regression as reported, has gone.
>>>>>>
>>>>>> 64-@@ uname -a
>>>>>> CYGWIN_NT-6.1 Seven 3.1.0s(0.339/5/3) 2019-07-22 16:43 x86_64 Cygwin
>>>>>> 64-@@ ls -lL <(grep bash .bashrc)
>>>>>> pr-------- 1 Henri None 0 Jul 22 20:36 /dev/fd/63
>>>>>
>>>> Over all the behavior has simularity w/ the error reported by David Karr:
>>>>
>>>>        https://cygwin.com/ml/cygwin/2019-07/msg00150.html
>>>>        ( Piping input from subprocess loses track of temp file )
>>>
>>> Thanks, I hadn't noticed that.
>>>
>>> The situation is more complicated than what I reported.  First, it happens even
>>> in cygwin-3.0.7, as David Karr's report suggests.  Second, it's true that I can
>>> only reproduce it under X11, but the pattern is not as regular as I thought.  I
>>> just ran the ls command 1000 times in an xterm window under cygwin-3.0.7, and I
>>> got the "Broken pipe" error 390 times, with a varying number of consecutive
>>> successful runs between the errors.
>>>
>>> Repeating this under the 20190722 or 20190725 snapshots gave slightly worse
>>> results (close to 500 errors).  Using my own unoptimized build of cygwin1.dll,
>>> the error count went up to about 650.
>>
>> I just tried this myself and I can't reproduce the problem.  1000 runs,
>> no error.
> 
> Interesting.  And you ran this under X11 in an xterm window?
> 
>>> I tried running under gdb, but I couldn't get grep to fail.  More precisely, I
>>> didn't see an error message from grep.  Every run looked like this:
>>>
>>> $ gdb bash
>>> GNU gdb (GDB) (Cygwin 8.2.1-1) 8.2.1
>>> [...]
>>> (gdb) r -c 'ls -lL <(grep bash .bashrc)'
>>> Starting program: /usr/bin/bash -c 'ls -lL <(grep bash .bashrc)'
>>> [...]
>>> pr-------- 1 kbrown None 0 2019-07-27 11:07 /dev/fd/63
>>> [...]
>>> [Inferior 1 (process 21712) exited normally]
>>>
>>> It would be better to be able to debug ls and/or grep, but I don't know how to
>>> get to subprocesses in gdb.  And I think I have to start with 'gdb bash' in
>>> order for the process substitution to happen.
>>
>> Yeah, subprocess debugging is a problem in GDB.  Given how this works,
>> you can at least take grep out of the picture.  Bash is doing all the
>> lifting, so it's just bash and ls.   Did you try to reproduce this under
>> strace?
> 
> Yes, but there I get an error (even under mintty) for a different reason:
> 
> $ strace -o trace.out ls -lL <(grep bash .bashrc)
> ls: cannot access '/dev/fd/63': No such file or directory
> 
> The strace output shows a call to fhandler_process::exists on /proc/45036/fd/63;
> here 45036 is the PID of 'ls'.  And then I see an EBADF error.  But I think
> what's happening here might be that bash is parsing '<(grep bash .bashrc)' too
> soon, so that '/dev/fd/63' isn't related to the 'ls' command.
> 
> By the way, I've just tried a different experiment, in which I simplify the ls
> command to 'ls <(grep bash .bashrc)'.  When I run this under xterm, I get the
> broken pipe error 98% of the time or more.  But it's fine under mintty.

I think I may have more-or-less figured out what's going on.  The "broken pipe" 
error simply means that ls has exited before grep has finished writing.  So grep 
is writing to a pipe that has no readers.  If I replace 'ls' by 'cat', I don't 
get any errors.

It remains a mystery to me why I was seeing the broken pipe only under X11, but 
I don't see any reason to think there's a problem.

Ken
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