Re: pgsql: rm_cleanup functions need to be allowed to write WAL entries.

On Sat, 2009-08-08 at 12:46 -0400, Tom Lane wrote:
> Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> > Resetting it back seems fragile, since in crash recovery we call it
> > again almost immediately during CreateCheckPoint(). That only works if
> > LocalSetXLogInsertAllowed() has no side effects. I understand Heikki's
> > wish to have safeguards in place, so we should document that
> > LocalSetXLogInsertAllowed() can be executed twice without problem.
>
> Done.
>
> My initial thought had actually been to get rid of the use of
> LocalSetXLogInsertAllowed inside CreateCheckPoint, since with this
> patch we are calling it from the same bit of code that is about
> to call CreateCheckPoint --- leaving the flag set throughout that
> bit would be fine. However that would only work as intended if
> the checkpoint were done locally; if somehow we'd launched the
> bgwriter and the checkpoint request got sent over there, it'd fail.
> I don't believe this is currently possible during a crash recovery
> scenario, but on the whole it seemed more fragile to do it that way
> than in the code as-committed.

OK

> In principle, at least, it seems
> possible that the rm_cleanup and checkpoint actions could be done
> in different processes, and this setup preserves the freedom to
> let that happen.

Good.

I want to move in the direction of having two cleanup routines, one
executed before recovery ends and one done afterwards, so it can write
WAL. Perhaps these would be called rm_makesafe() and rm_repair(). Rough
thinking at this stage.

The rm_repair() would execute in a separate process once we're up.

--
Simon Riggs www.2ndQuadrant.com