Re: Re: backend hangs at immediate shutdown (Re: Back-branch update releases coming in a couple weeks)

On 2013-06-21 23:19:27 -0400, Tom Lane wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> > On Fri, Jun 21, 2013 at 5:44 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >> The traditional theory has been that that would be less robust, not
> >> more so. Child backends are (mostly) able to carry out queries whether
> >> or not the postmaster is around.
>
> > I think that's the Tom Lane theory. The Robert Haas theory is that if
> > the postmaster has died, there's no reason to suppose that it hasn't
> > corrupted shared memory on the way down, or that the system isn't
> > otherwise heavily fuxxored in some way.
>
> Eh? The postmaster does its level best never to touch shared memory
> (after initialization anyway).

I am not sure that will never happen - but I think the chain of argument
misses the main point. Normally we rely on the postmaster to kill off
all other backends if a backend PANICs or segfaults for all the known
reasons. As soon as there's no postmaster anymore we loose that
capability.
And *that* is scary imo. Especially as I would say the chance of getting
PANICs or segfaults increases if there's no postmaster anymore since we
might reach code branches we otherwise won't.

> >> True, you can't make new connections,
> >> but how does killing the existing children make that better?
>
> > It allows you to start a new postmaster in a timely fashion, instead
> > of waiting for an idle connection that may not ever terminate without
> > operator intervention.

And it's no always easy to figure out which cluster those backends
belong to if there are multiple postgres instances running as the same user.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services