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

Dear Amit,

> I see your point related to WALAVAIL_REMOVED status of the slot but
> did you test the scenario I have explained in my comment? Basically, I
> want to know whether it can impact the user in some way. So, please
> check whether the corresponding subscriptions will be allowed to drop.
> You can test it both before and after the upgrade.

Yeah, this is a real issue. I have tested and confirmed the expected things.
Even if the status of the slot is 'lost', it may be needed for dropping
subscriptions properly.

* before upgrading, the subscription which refers the lost slot could be dropped
* after upgrading, the subscription could not be dropped as-is.
  users must ALTER SUBSCRIPTION sub SET (slot_name = NONE);

Followings are the stepped what I did:

## Setup

1. constructed a logical replication system
2. disabled the subscriber once
3. consumed many WALs so that the status of slot became 'lost'

```
publisher=# SELECT slot_name, wal_status FROM pg_replication_slots ;
slot_name | wal_status 
-----------+------------
sub       | lost
(1 row)
```

# testcase a - try to drop sub. before upgrading

a-1. enabled the subscriber again.
  At that time following messages are shown on subscriber log:
```
ERROR:  could not start WAL streaming: ERROR:  can no longer get changes from replication slot "sub"
DETAIL:  This slot has been invalidated because it exceeded the maximum reserved size.
```

a-2. did DROP SUBSCRIPTION ...
a-3. succeeded.

```
subscriber=# DROP SUBSCRIPTION sub;
NOTICE:  dropped replication slot "sub" on publisher
DROP SUBSCRIPTION
```

# testcase b - try to drop sub. after upgrading

b-1. did pg_upgrade command
b-2. enabled the subscriber. From that point an apply worker connected to new node...
b-3. did DROP SUBSCRIPTION ...
b-4. failed with the message:

```
subscriber=# DROP SUBSCRIPTION sub;
ERROR:  could not drop replication slot "sub" on publisher: ERROR:  replication slot "sub" does not exist
```

The workaround was to disassociate the slot, which was written in the document. 

```
subscriber =# ALTER SUBSCRIPTION sub DISABLE;
ALTER SUBSCRIPTION
subscriber =# ALTER SUBSCRIPTION sub SET (slot_name = NONE);
ALTER SUBSCRIPTION
subscriber =# DROP SUBSCRIPTION sub;
DROP SUBSCRIPTION
```

PSA the script for emulating above tests.

Best Regards,
Hayato Kuroda
FUJITSU LIMITED