Re: pg_upgrade and system() return value

Can someone comment on the attached patch? pg_upgrade was testing if
system() returned a non-zero value, while I am thinking I should be
adjusting system()'s return value with WEXITSTATUS().

Is there any possible bug in back branches just compariing system()'s
turn value to non-zero without calling WEXITSTATUS()? I never saw a bug
related to this. I am thinking of applying this just to git head.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +

Bruce Momjian <bruce(at)momjian(dot)us> writes:
> Can someone comment on the attached patch? pg_upgrade was testing if
> system() returned a non-zero value, while I am thinking I should be
> adjusting system()'s return value with WEXITSTATUS().

AFAIK it's not very good style to test the result as an integer, and
testing for -1 is not an improvement on that. Actually it's a
disimprovement, because the only case where the standard guarantees
anything about the integer representation is that zero corresponds
to "exited with status 0". See the Single Unix Spec, wherein system's
result code is defined in terms of wait's, and the wait man page saith

If and only if the status returned is from a terminated child process
that returned 0 from main() or passed 0 as the status argument to
_exit() or exit(), the value stored at the location pointed to by
stat_loc will be 0. Regardless of its value, this information may be
interpreted using the following macros ...

If you want to do something different, then you could test for
WIFEXITED && WEXITSTATUS == 0. (Testing the latter alone is flat
wrong.) But I'm not particularly convinced that that's an improvement
on what's there now. I note that your proposed patch would prevent
any possibility of printing debug information about failure cases,
since it loses the original result value.

In short: it's not broken now, but this patch would break it.

regards, tom lane

On Sun, Jan 20, 2013 at 11:14:47AM -0500, Tom Lane wrote:
> Bruce Momjian <bruce(at)momjian(dot)us> writes:
> > Can someone comment on the attached patch? pg_upgrade was testing if
> > system() returned a non-zero value, while I am thinking I should be
> > adjusting system()'s return value with WEXITSTATUS().
>
> AFAIK it's not very good style to test the result as an integer, and
> testing for -1 is not an improvement on that. Actually it's a
> disimprovement, because the only case where the standard guarantees
> anything about the integer representation is that zero corresponds
> to "exited with status 0". See the Single Unix Spec, wherein system's
> result code is defined in terms of wait's, and the wait man page saith
>
> If and only if the status returned is from a terminated child process
> that returned 0 from main() or passed 0 as the status argument to
> _exit() or exit(), the value stored at the location pointed to by
> stat_loc will be 0. Regardless of its value, this information may be
> interpreted using the following macros ...
>
> If you want to do something different, then you could test for
> WIFEXITED && WEXITSTATUS == 0. (Testing the latter alone is flat
> wrong.) But I'm not particularly convinced that that's an improvement
> on what's there now. I note that your proposed patch would prevent
> any possibility of printing debug information about failure cases,
> since it loses the original result value.
>
> In short: it's not broken now, but this patch would break it.

I thought checking for non-zero was sufficient too, but my Debian
Squeeze system() manual page says:

The value returned is -1 on error (e.g. fork(2) failed), and the
return status of the command otherwise.

I am good with the above sentence, but the next sentences have me
confused:

This latter return status is in the format specified in wait(2).
Thus, the exit code of the command will be WEXITSTATUS(status).
In case /bin/sh could not be executed, the exit status will be
that of a command that does exit(127).

I assume my pg_upgrade waitpid() code is OK:

ret = waitpid(-1, &work_status, wait_for_child ? 0 : WNOHANG);

/* no children or, for WNOHANG, no dead children */
if (ret <= 0 || !WIFEXITED(work_status))
return false;

if (WEXITSTATUS(work_status) != 0)
pg_log(PG_FATAL, "child worker exited abnormally: %s\n", strerror(errno));

Can that be simplified too?

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +