Re-read subscription state after lock in AlterSubscription

Hi hackers,

while playing with the new ALTER SUBSCRIPTION parameter added in a5918fddf10,
I realized that the subscription is not re-read once we acquire the lock in
AlterSubscription().

This pre-existing issue is now more visible after a5918fddf10:

1/ two concurrent ALTER SUBSCRIPTION SET (conflict_log_destination = 'table')
could result in the second session attempting to create an already-existing
conflict log table, producing a confusing "relation already exists" error:

ERROR:  relation "pg_conflict_log_24614" already exists

It's confusing because ALTER SUBSCRIPTION SET (conflict_log_destination = 'table')
would not report an error if the conflict table already exists (and no concurrent
ALTER is running).

2/ a concurrent DROP followed by the ALTER would emit a NOTICE about creating the
conflict log table before failing with "referenced subscription was concurrently
dropped". That sounds like a weird messaging:

NOTICE:  created conflict log table "pg_conflict.pg_conflict_log_24620" for subscription "mysub"
ERROR:  referenced subscription was concurrently dropped

The attached fixes it by:

- Re-reading the subscription tuple after LockSharedObject() and refreshing the
  Subscription struct.
- Moving the local variable assignments to after the re-read.
- Re-checking the password_required privilege restriction after the re-read.

Remarks:

1/ not re-checking password_required after the re-read would still produce a
"tuple concurrently updated" error, but re-checking it allows us to display a
better error message.

2/ the ownership check is intentionally not re-done after the lock because
AlterSubscriptionOwner() does not take AccessExclusiveLock on the subscription
object: it only takes RowExclusiveLock on the pg_subscription catalog table.
This means ownership can change regardless of our lock, making a re-check after
lock acquisition pointless. The existing "tuple concurrently updated" error from
CatalogTupleUpdate() already provides a protection if ownership changes
concurrently.

3/ the "privileges" checks are still also done before the lock acquisition because
we don't want to lock an object we don't have privileges on.

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com