RE: [PoC] pg_upgrade: allow to upgrade publisher node

Dear Amit,

Thanks for your suggestion! PSA new version.

> The other problem is that pg_resetwal removes all pre-existing WAL
> files which in this case could lead to the removal of the WAL file
> corresponding to restart_lsn. This is because at least the shutdown
> checkpoint record will be written after the creation of slots which
> could be in the new file used for restart_lsn. Then when we invoke
> pg_resetwal, it can remove that file.
> 
> One idea to deal with this could be to do the reset WAL stuff
> (FindEndOfXLOG(), KillExistingXLOG(), KillExistingArchiveStatus(),
> WriteEmptyXLOG()) in a separate function (say in pg_upgrade) and then
> create slots. If we do this, then we additionally need an option in
> pg_resetwal which skips resetting the WAL as that would have been done
> before creating the slots.

Based on above idea, I made new version patch which some functionalities were
exported from pg_resetwal. In this approach, pg_upgrade itself removed WALs and
then create logical slots, then pg_resetwal would be called with new option
--no-switch, which avoid to switch a WAL segment file. The option is only used
for the upgrading purpose so it is not written in doc and usage(). This option
is not required if pg_resetwal -o does not discard WAL records. Please see the
fork thread [1].

We do not have to reserve future restart_lsn while creating a slot, so the binary
function binary_upgrade_create_logical_replication_slot() was removed.

Another advantage of this approach is to avoid calling pg_log_standby_snapshot()
after the pg_resetwal. This was needed because of two reasons, but they were
resolved automatically.
  1) pg_resetwal removes all WAL files.
  2) Logical slots requires a RUNNING_XACTS record for building a snapshot.
 
[1]: https://www.postgresql.org/message-id/CAA4eK1KRyPMiY4fW98qFofsYrPd87Oc83zDNxSeHfTYh_asdBg%40mail.gmail.com

Best Regards,
Hayato Kuroda
FUJITSU LIMITED