Re: [PoC] pg_upgrade: allow to upgrade publisher node

Hi,

On Thu, Aug 10, 2023 at 04:30:40PM +0900, Masahiko Sawada wrote:
> On Thu, Aug 10, 2023 at 2:27 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > Sawada-San, Julien, and others, do you have any thoughts on the above point?
>
> IIUC during the old cluster running in the middle of pg_upgrade it
> doesn't accept TCP connections. I'm not sure we need to worry about
> the case where someone in the same server attempts to create
> replication slots during the upgrade.

AFAICS this is only true for non-Windows platform, so we would still need some
extra safeguards on Windows. Having those on all platforms will probably be
simpler and won't hurt otherwise.

> The same is true for other objects, as Amit mentioned.

I disagree. As I mentioned before any module registered in
shared_preload_libraries can spawn background workers which can perform any
activity. There were previous reports of corruption because of multi-xact
being generated by such bgworkers during pg_upgrade, I'm pretty sure that there
are some modules that create objects (automatic partitioning tools for
instance). It's also unclear to me what would happen if some writes are
performed by such module at various points of the pg_upgrade process. Couldn't
that lead to either data loss or broken slot (as it couldn't stream changes
from older major version)?