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

Dear Amit,

Thanks for reviewing! New patch is available at [1].

>
> Some more comments:
> 1. Let's restruture binary_upgrade_validate_wal_logical_end() a bit.
> First, let's change its name to binary_upgrade_slot_has_pending_wal()
> or something like that. Then move the context creation and free
> related code into DecodingContextHasDecodedItems(). We can rename
> DecodingContextHasDecodedItems() as
> pg_logical_replication_slot_has_pending_wal() and place it in
> slotfuncs.c. This will make the code structure similar to other slot
> functions like pg_replication_slot_advance().

Seems clearer than mine. Fixed.

> 2. + * Returns true if there are no changes after the confirmed_flush_lsn.
>
> How about something like: "Returns true if there are no decodable WAL
> records after the confirmed_flush_lsn."?

Fixed.

> 3. Shouldn't we need to call CheckSlotPermissions() in
> binary_upgrade_validate_wal_logical_end?

Added, but actually it is not needed. This is because only superusers can connect
to the server while upgrading. Please see below codes in InitPostgres().

```
if (IsBinaryUpgrade && !am_superuser)
{
ereport(FATAL,
(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
errmsg("must be superuser to connect in binary upgrade mode")));
}
```

> 4.
> + /*
> + * Also, set processing_required flag if the message is not
> + * transactional. It is needed to notify the message's existence to
> + * the caller side. Usually, the flag is set when either the COMMIT or
> + * ABORT records are decoded, but this must be turned on here because
> + * the non-transactional logical message is decoded without waiting
> + * for these records.
> + */
>
> The first sentence of the comments doesn't seem to be required as that
> just says what the code does. So, let's slightly change it to: "We
> need to set processing_required flag to notify the message's existence
> to the caller side. Usually, the flag is set when either the COMMIT or
> ABORT records are decoded, but this must be turned on here because the
> non-transactional logical message is decoded without waiting for these
> records."

Fixed.

[1]: https://www.postgresql.org/message-id/TYAPR01MB5866B0614F80CE9F5EF051BDF5D3A%40TYAPR01MB5866.jpnprd01.prod.outlook.com

Best Regards,
Hayato Kuroda
FUJITSU LIMITED