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Re: [bug-bash] Named fifo's causing hanging bash scripts


From: Chet Ramey
Subject: Re: [bug-bash] Named fifo's causing hanging bash scripts
Date: Fri, 16 Jan 2015 10:46:02 -0500
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On 1/16/15 10:32 AM, Dr. Werner Fink wrote:
> On Fri, Jan 16, 2015 at 09:22:36AM -0500, Chet Ramey wrote:
>> On 1/13/15 4:29 AM, Dr. Werner Fink wrote:
>>
>>>>> Bash Version: 4.3
>>>>> Patch Level: 33
>>>>> Release Status: release
>>>>>
>>>>> Description:
>>>>>         Named fifo's causing hanging bash scripts like
>>>>>
>>>>>         while IFS="|" read a b c ; do
>>>>>           [shell code]
>>>>>         done < <(shell code)
>>>>>
>>>>>         can cause random hangs of the bash.    An strace shows that the 
>>>>> bash
>>>>>         stays in wait4()
>>>>
>>>> And when you attach to one of the hanging bash processes using gdb, what
>>>> does the stack traceback look like?
>>>
>>> Yes (and sorry for the wrong email address as this was done on a clean 
>>> virtual sysstem)
>>>
>>> there are two hanging bash processes together with the find command:
>>>
>>> werner   19062  0.8  0.0  11864  2868 ttyS0    S+   10:21   0:00 bash -x 
>>> /tmp/brp-25-symlink
>>> werner   19063  0.0  0.0  11860  1920 ttyS0    S+   10:21   0:00 bash -x 
>>> /tmp/brp-25-symlink
>>> werner   19064  0.2  0.0  16684  2516 ttyS0    S+   10:21   0:00 find . 
>>> -type l -printf %p|%h|%l n
>>>
>>> the gdb -p 19062 and gdb -p 19063 show
>>>
>>> (gdb) bt
>>> #0  0x00007f530818a65c in waitpid () from /lib64/libc.so.6
>>> #1  0x000000000042b233 in waitchld (address@hidden, wpid=19175) at 
>>> jobs.c:3235
>>> #2  0x000000000042c6da in wait_for (address@hidden) at jobs.c:2496
>>
>> What do ps and gdb tell you about pid 19175 (and the corresponding pid in
>> the call to waitchld in the other traceback)?  Running, terminated, reaped,
>> other?
> 
>   d136:~ # ps 10942
>     PID TTY      STAT   TIME COMMAND
>   d136:~ #
> 
> ... the process does not exists anymore. I guess that this could belong to
> the sed commands of the script.  

This is why I need to be able to reproduce it.  If the process got reaped,
when would it have happened and why would the call to wait_for() have
found a valid CHILD struct for it?  The whole loop runs with SIGCHLD
blocked, so it's not as if the signal handler could have reaped the
child out from under it.  I have questions but no way to find answers.


- -- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, ITS, CWRU    address@hidden    http://cnswww.cns.cwru.edu/~chet/
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