Re: Regression (last snapshot)

[Ken Brown <kbrown at cornell dot edu>]

On 7/29/2019 11:40 AM, Corinna Vinschen wrote:
> On Jul 29 17:23, Corinna Vinschen wrote:
>> On Jul 29 14:26, Ken Brown wrote:
>>> On 7/29/2019 9:47 AM, Corinna Vinschen wrote:
>>>> On Jul 29 13:18, Ken Brown wrote:
>>>>> $ strace -o trace.out ls -lL <(grep bash .bashrc)
>>>>> ls: cannot access '/dev/fd/63': No such file or directory
>>>>
>>>> No, please run bash:
>>>>
>>>>     strace -o trace.out bash -c 'ls -lL <(grep bash .bashrc)'
>>>>
>>>> Otherwise there's no process actually creating the pipe, given the <()
>>>> expression is a bash expression.
>>>
>>> Yes, of course.  I should have realized this since it's exactly what I
>>> did under gdb.  Anyway, the result is the same as it was under gdb: If
>>> I run the command under strace, I don't see the broken pipe error.
>>>
>>> Is it possible that debugging causes an fd to the read end of the pipe
>>> to stay open longer, thereby preventing the error?
>>
>> The fact that you observe it sporadically points to a race condition.
>> Debugging serializes stuff usually running in parallel, potentially
>> eliminating the race.
> 
> Is there any chance this is a BLODA problem?

I doubt it.  I'm seeing this on two different computers, and I haven't seen any 
other symptoms suggesting BLODA.

>  If /dev/fd/63 doesn't
> exist in ls, it would mean ls didn't inherit the FIFO, which sounds
> very unlikely.

Actually I never saw an ls error saying /dev/fd/63 doesn't exist, except in my 
incorrect run of strace.

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.

Ken

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