Re: needless complexity in StartupXLOG

On Mon, Jul 26, 2021 at 03:53:20PM -0400, Robert Haas wrote:
> Yeah, and there again, it's a lot easier to test if we don't have as
> many cases. Now no single change is going to fix that, but the number
> of flag variables in xlog.c is simply bonkers. This particular stretch
> of code uses 3 of them to even decide whether to attempt the test in
> question, and all of those are set in complex ways depending on the
> values of still more flag variables. The comments where
> bgwriterLaunched is set claim that we only do this during archive
> recovery, not crash recovery, but the code depends on the value of
> ArchiveRecoveryRequested, not InArchiveRecovery. So I wonder if we
> can't get the bgwriter to run even during crash recovery in the
> scenario described by:

I'm not following along closely and maybe you're already aware of this one?
https://commitfest.postgresql.org/33/2706/
Background writer and checkpointer in crash recovery

@Thomas:
https://www.postgresql.org/message-id/CA%2BTgmoYmw%3D%3DTOJ6EzYb_vcjyS09NkzrVKSyBKUUyo1zBEaJASA%40mail.gmail.com
>          * It's possible that archive recovery was requested, but we don't
>          * know how far we need to replay the WAL before we reach consistency.
>          * This can happen for example if a base backup is taken from a
>          * running server using an atomic filesystem snapshot, without calling
>          * pg_start/stop_backup. Or if you just kill a running primary server
>          * and put it into archive recovery by creating a recovery signal
>          * file.
> 
> If we ran the bgwriter all the time during crash recovery, we'd know
> for sure whether that causes any problems. If we only do it like this
> in certain corner cases, it's much more likely that we have bugs.
> Grumble, grumble.

-- 
Justin