Concurrent CREATE USER MAPPING and DROP SERVER

Hello.

I think I may have found a bug in the CREATE USER MAPPING / DROP SERVER
handling.

It is possible to create a user mapping on a server and drop said server in
two concurrent transactions, and have both succeed.

This results in an orphan row in the pg_user_mapping table.

I would expect one of those transactions to fail.

This is more easily explained with the following example:

CREATE extension postgres_fdw;

CREATE server pg_server FOREIGN DATA WRAPPER postgres_fdw

-- SESSION 1
BEGIN;
CREATE USER MAPPING FOR postgres SERVER pg_server OPTIONS (user 'user');

-- SESSION 2
BEGIN;
DROP SERVER pg_server;

-- SESSION 1
COMMIT;

-- SESSION 2
COMMIT;

select umserver, s.oid from pg_user_mapping left join pg_foreign_server s on
s.oid = pg_user_mapping.umserver;

┌──────────┬─────┐
│ umserver │ oid │
├──────────┼─────┤
│62172 │ ¤ │
└──────────┴─────┘