Re: pg_stop_backup(wait_for_archive := true) on standby server

On Wed, Jul 5, 2017 at 4:57 PM, Michael Paquier
<michael(dot)paquier(at)gmail(dot)com> wrote:
> Why not refactoring a bit do_pg_stop_backup() so as the wait phase
> happens even if a backup is started in recovery? Now wait_for_archive
> is ignored because no wait is happening and the stop point is directly
> returned back to the caller. For the wait actually happening, I don't
> have a better idea than documenting the fact that enforcing manually a
> segment switch on the primary needs to happen. That's better than
> having users including WAL in their base backups but not actually
> having everything they need. And I think that documenting that
> properly is better than restricting things that should work.

While looking at that in more details, I got surprised by two things:
1) No backup history file is generated on a standby during a base backup.
2) Because of 1), those files are not archived even if archive_mode = always.

This sounds to me like a missing optimization of archive_mode =
always, and the following comment block in xlog.c is at least
incorrect as an archiver can be invoked in recovery:
* XXX currently a backup history file is for informational and debug
* purposes only. It's not essential for an online backup. Furthermore,
* even if it's created, it will not be archived during recovery because
* an archiver is not invoked. So it doesn't seem worthwhile to write a
* backup history file during recovery.

So I would suggest the following things to address this issue:
1) Generate a backup history file for backups taken at recovery as well.
2) Archive it if archive_mode = always.
3) Wait for both the segment of the stop point and the backup history
files to be archived before returning back to the caller of
pg_stop_backup.

It would be nice to get all that addressed in 10. Thoughts?
--
Michael