Re: O(n) tasks cause lengthy startups and checkpoints

Hi,

On 2022-02-16 20:14:04 -0800, Nathan Bossart wrote:
> >> - while ((spc_de = ReadDirExtended(spc_dir, "pg_tblspc", LOG)) != NULL)
> >> + while (!ShutdownRequestPending &&
> >> + (spc_de = ReadDirExtended(spc_dir, "pg_tblspc", LOG)) != NULL)
> >
> > Uh, huh? It strikes me as a supremely bad idea to have functions *silently*
> > not do their jobs when ShutdownRequestPending is set, particularly without a
> > huge fat comment.
>
> The idea was to avoid delaying shutdown because we're waiting for the
> custodian to finish relatively nonessential tasks. Another option might be
> to just exit immediately when the custodian receives a shutdown request.

I think we should just not do either of these and let the functions
finish. For the cases where shutdown really needs to be immediate
there's, uhm, immediate mode shutdowns.

> > Why does this not open us up to new xid wraparound issues? Before there was a
> > hard bound on how long these files could linger around. Now there's not
> > anymore.
>
> Sorry, I'm probably missing something obvious, but I'm not sure how this
> adds transaction ID wraparound risk. These files are tied to LSNs, and
> AFAIK they won't impact slots' xmins.

They're accessed by xid. The LSN is just for cleanup. Accessing files
left over from a previous transaction with the same xid wouldn't be
good - we'd read wrong catalog state for decoding...

Andres