RE: Bug in ALTER SUBSCRIPTION ... SERVER / ... CONNECTION with broken old server

Dear Chao,

> I tested the v19 new feature CREATE SUBSCRIPTION ... SERVER yesterday, and
> found an issue: once the old server becomes broken, the subscription cannot be
> recovered by switching it to a good server.

Thanks for testing. I could reproduce the same issue. In addition to yours, I found
DROP SUBSCRIPTION cannot be done anymore. To switch the connection or drop it,
I had to create the same user mapping must be created again.

```
postgres=# DROP SUBSCRIPTION sub_bug ;
ERROR: user mapping not found for user "postgres", server "old_srv"
postgres=# CREATE USER MAPPING FOR CURRENT_USER SERVER old_srv
OPTIONS (user 'dummy', password 'dummy');
CREATE USER MAPPING
postgres=# DROP SUBSCRIPTION sub_bug ;
DROP SUBSCRIPTION
```

Before deep dive to your fix, I'm unclear why dropping the active USER MAPPING is
allowed. Personally, it should be avoided anyway. Do you know why it's not restricted?

Best regards,
Hayato Kuroda
FUJITSU LIMITED