Re: Outdated replication protocol error?

On Wed, 2021-06-16 at 16:17 -0700, Andres Freund wrote:
> I think we should explicitly compute the current timeline before
> using
> ThisTimelineID. E.g. in StartReplication() call a new version of
> GetFlushRecPtr() that also returns the current timeline id.

I think all we need to do is follow the pattern in IdentifySystem() by
calling:

    am_cascading_walsender = RecoveryInProgress();

first. There are three cases:

1. If the server was a primary the last time RecoveryInProgress() was
called, and it's still a primary, then it returns false immediately.
ThisTimeLineID should be set properly already.

2. If the server was a secondary the last time RecoveryInProgress() was
called, and now it's a primary, then it updates ThisTimeLineID in
InitXLOGAccess() and returns false.

3. If the server was a secondary the last time, and is still a
secondary, then it returns true. Then, StartReplication() will call
GetStandbyFlushRecPtr(), which will update ThisTimeLineID.

It was confusing to me for a while because I was trying to sort out
whether am_cascading_walsender and/or ThisTimeLineID could be out of
date, and how that would result in not updating ThisTimeLineID; and
also why there was a difference between IdentifySystem() and
StartReplication().

Simple patch attached. I didn't test it yet, but wanted to post my
analysis.

Regards,
	Jeff Davis