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

Dear Vignesh,

Thank you for making the PoC!

> Here is a patch which checks that there are no WAL records other than
> CHECKPOINT_SHUTDOWN WAL record to be consumed based on the discussion
> from [1].

Basically I agreed your approach. Thanks!

> Patch 0001 and 0002 is same as the patch posted by Kuroda-san, Patch
> 0003 exposes pg_get_wal_records_content to get the WAL records along
> with the WAL record type between start and end lsn. pg_walinspect
> contrib module already exposes a function for this requirement, I have
> moved this functionality to be exposed from the backend. Patch 0004
> has slight change in check function to check that there are no other
> records other than CHECKPOINT_SHUTDOWN to be consumed. The attached
> patch has the changes for the same.
> Thoughts?
>
> [1] -
> https://www.postgresql.org/message-id/CAA4eK1Kem-J5NM7GJCgyKP84pEN6
> RsG6JWo%3D6pSn1E%2BiexL1Fw%40mail.gmail.com

Few comments:

* Per comment from Amit [1], I used pg_get_wal_record_info() instead of pg_get_wal_records_info().
This function extract a next available WAL record, which can avoid huge scan if
the confirmed_flush is much behind.
* According to cfbot and my analysis, the 0001 cannot pass the test on macOS.
So I revived Julien's patch [2] as 0002 once. AFAIS the 0001 is not so welcomed.

Next patch will be available soon.

[1]: https://www.postgresql.org/message-id/CAA4eK1LWKkoyy-p-SAT0JTWa%3D6kXiMd%3Da6ZcArY9eU4a3g4TZg%40mail.gmail.com
[2]: https://www.postgresql.org/message-id/20230414061248.vdsxz2febjo3re6h%40jrouhaud

Best Regards,
Hayato Kuroda
FUJITSU LIMITED