Re: 4.3.42 : shopt -s lastpipe & "no record of process" warnings

[Jason Vas Dias]

OK, I found one fix - you probably
won't like it, but it fixes the oroblem,
and all the test cases run in
'make test' still work - especially
'run_jobs' and  'run_lastpipe' -
the patch simply removes the error
from the FIND_CHILD macro in jobs.c,
so it noe reads ;
jobs.c @ line 2377:
#define FIND_CHILD(pid,child) \
  do \
      {\
        child = find_pipeline (pid, 0, (int*)NULL); \
        if ( child == 0 ) \
           { \
              give_terminal_to (shell_pgrp, 0); \
              UNLOCK_CHILD (oset); \
              restore_sigint_handler (); \
              return (termination_state = 0): \
           } \
     } \
  while (0)

So  I've removed the manufactured
"No record of process X" error altogether,
and yet all test cases still pass - it seems
it was unecessary.

Will a fix for this issue appear in a 'bash43-43+' patch
file or in the forthcoming bash-4.4 release ?
I think it should, as it is rather unfair of bash to make its
internal bookkeeping errors appear as if they could be
user programming errors.

Regards,
Jason


On 05/06/2016, Jason Vas Dias <jason.vas.dias@gmail.com> wrote:
> The strace log shows that process 8277 is the
> bash subshell that runs f() , forks to create 8278,
> which forks to execve "some_executable" ;
> process 8277 then eads the 8278  output on a pipe,
> so it knows when the pipe is closed, amd the strace
> log shows 8277 first does a wait4, which returns 8278,
> but does NOT get a SIGCHLD event for 8278.
> Maybe this is the problem? Since the wait has
> already succeeded, no SIGCHLD will be generated.
> 8277 evidently is not aware that its 8278 child has exited,
> and goes on to issue a further wait4 for it which
> returns -1 with errno==ECHLD and then emits the
> message:
> "xxx.sh: line 46: No record of process 8278" .
> Line 46 in my script consist of
>   function f()
> .
> This seems buggy to me - I'll try developing a
> patch to fix it and will post back here.
>
> Regards, Jason
> On 05/06/2016, Jason Vas Dias <jason.vas.dias@gmail.com> wrote:
>> With a build of bash 4.3 patchlevel 42
>> on a Linux x86_64 system I am getting
>> warning messages on stderr when
>> running a certain script like:
>>    "wait_for: no record of process 8278" .
>> Running bash with the script as
>> input under strace shows that process 8277
>> does a successful wait4(-1,...) which DOES
>> return pid 8278 . So why is bash complaining
>> it has no record of it ?
>> Is bash getting its book-keeping wrong here?
>> The script is not using any background
>> jobs with '&' or using the 'wait' built-in.
>> It is simply doing something like:
>> <quote><pre>
>> shopt -s lastpipe;
>> set -o pipefail;
>> function f()
>> { some_executable "$@" | {
>>      while read line; do { ... ; } done;
>>    }
>>    return 0;
>> }
>> ...
>> f  $args | { while read result; do
>> ...; done ; }
>> </pre></quote>
>>
>> So I'd expect the initial bash process to run
>> a subshell bash to invoke the f() function,
>> which runs a command child that execve-s
>> "some_executable', parsing its output and writing
>> to the subshell bash on a pipe, which writes to the
>> parent bash on a pipe, which parses it & does whatever.
>> Without the lastpipe option, this would be the
>> other way round - the parent would run f, and
>> its output would be parsed in the subshell
>> running the f output parsing loop.
>> All this seems to work OK, but why the warning
>> message about "no record of process X"?
>> Or is this message indicating something has
>> gone seriously wrong ?
>> Thanks in advance for any replies,
>> Regards,
>> Jason
>>
>