Re: shared-memory based stats collector

On 2020-03-10 09:48:07 -0300, Alvaro Herrera wrote:
> On 2020-Mar-10, Kyotaro Horiguchi wrote:
>
> > At Mon, 9 Mar 2020 20:34:20 -0700, Andres Freund <andres(at)anarazel(dot)de> wrote in
> > > On 2020-03-10 12:27:25 +0900, Kyotaro Horiguchi wrote:
> > > > That's true, but I have the same concern with Tom. The archive bacame
> > > > too-tightly linked with other processes than actual relation.
> > >
> > > What's the problem here? We have a number of helper processes
> > > (checkpointer, bgwriter) that are attached to shared memory, and it's
> > > not a problem.
> >
> > That theoretically raises the chance of server-crash by a small amount
> > of probability. But, yes, it's absurd to prmise that archiver process
> > crashes.
>
> The case I'm worried about is a misconfigured archive_command that
> causes the archiver to misbehave (exit with a code other than 0); if
> that already doesn't happen, or we can make it not happen, then I'm okay
> with the changes to archiver.

Well, an exit(1) is also fine, afaict. No?

The archive command can just trigger either a FATAL or a LOG:

rc = system(xlogarchcmd);
if (rc != 0)
{
/*
* If either the shell itself, or a called command, died on a signal,
* abort the archiver. We do this because system() ignores SIGINT and
* SIGQUIT while waiting; so a signal is very likely something that
* should have interrupted us too. Also die if the shell got a hard
* "command not found" type of error. If we overreact it's no big
* deal, the postmaster will just start the archiver again.
*/
int lev = wait_result_is_any_signal(rc, true) ? FATAL : LOG;

if (WIFEXITED(rc))
{
ereport(lev,
(errmsg("archive command failed with exit code %d",
WEXITSTATUS(rc)),
errdetail("The failed archive command was: %s",
xlogarchcmd)));
}
else if (WIFSIGNALED(rc))
{
#if defined(WIN32)
ereport(lev,
(errmsg("archive command was terminated by exception 0x%X",
WTERMSIG(rc)),
errhint("See C include file \"ntstatus.h\" for a description of the hexadecimal value."),
errdetail("The failed archive command was: %s",
xlogarchcmd)));
#else
ereport(lev,
(errmsg("archive command was terminated by signal %d: %s",
WTERMSIG(rc), pg_strsignal(WTERMSIG(rc))),
errdetail("The failed archive command was: %s",
xlogarchcmd)));
#endif
}
else
{
ereport(lev,
(errmsg("archive command exited with unrecognized status %d",
rc),
errdetail("The failed archive command was: %s",
xlogarchcmd)));
}

snprintf(activitymsg, sizeof(activitymsg), "failed on %s", xlog);
set_ps_display(activitymsg, false);

return false;
}

I.e. there's only normal ways to shut down the archiver due to a failing
archvie command.

Greetings,

Andres Freund