Record previous TLI in end-of-recovery record (was Re: [COMMITTERS] pgsql: Fast promote mode skips checkpoint at end of recovery.)

(this is unrelated to the other discussion about this patch)

On 29.01.2013 02:07, Simon Riggs wrote:
> Fast promote mode skips checkpoint at end of recovery.
> pg_ctl promote -m fast will skip the checkpoint at end of recovery so that we
> can achieve very fast failover when the apply delay is low. Write new WAL record
> XLOG_END_OF_RECOVERY to allow us to switch timeline correctly for downstream log
> readers. If we skip synchronous end of recovery checkpoint we request a normal
> spread checkpoint so that the window of re-recovery is low.

It just occurred to me that it would be really nice if the
end-of-recovery record, and the timeline-switching shutdown checkpoint
record too for that matter, would include the previous timeline's ID
that we forked from, in addition to the new TLI. Although it's not
required for anything at the moment, it would be useful debugging
information. It would allow reconstructing timeline history files from
the WAL; that might come handy.

Barring objections, I'll add that.

- Heikki