Re: pgsql: Remove the restriction that the relmap must be 512 bytes.

On Wed, Jul 27, 2022 at 3:17 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> writes:
> >> Hmm, but that doesn't actually produce nice behavior. It just goes
> >> into an infinite loop, like this:
> >> So now I'm back to being unsure what to do here.
>
> > I vote to go for the catversion bump for now.
>
> What this is showing us is that any form of corruption in the relmapper
> file causes very unpleasant behavior. We probably had better do something
> about that, independently of this issue.

I'm not sure how important that is, but it certainly wouldn't hurt.

> In the meantime, I still think bumping the file magic number is a better
> answer. It won't make the behavior any worse for un-updated code than
> it is already.

But it also won't make it any better, so why even bother? The goal of
catversion bumps is to replace crashes or unpredictable misbehavior
with a nice error message that tells you exactly what the problem is.
Here we'd just be replacing an infinite series of crashes with an
infinite series of crashes with a slightly different error message.
It's probably worth comparing those error messages:

FATAL: could not read file "global/pg_filenode.map": read 512 of 524
FATAL: relation mapping file "global/pg_filenode.map" contains invalid data

The first message is what you get now. The second message is what you
get with the proposed change to the magic number. I would argue that
the second message is actually worse than the first one, because the
first one actually gives you some hint what the problem is, whereas
the second one really just says that an unspecified bad thing
happened.

In short, I think Alvaro's idea is unprincipled but solves the problem
of making it clear that a new initdb is required. Your idea is
principled but does not solve any problem.

--
Robert Haas
EDB: http://www.enterprisedb.com