SET SESSION AUTHORIZATION command doesn't update status of backend

Hi,

Nikita Degtyarev (n.degtyarev@postgrespro.ru) reported a suspicious
behaviour of a SET SESSION AUTHORIZATION command. If you switch user in
a session:

CREATE USER abc;
SET SESSION AUTHORIZATION abc;

command "SELECT SESSION_USER, CURRENT_USER" shows us:

session_user | current_user
--------------+--------------
abc | abc

But if you launch a job:
SELECT pg_sleep(60);

and will see into the pg_stat_activity:

SELECT usename,query FROM pg_stat_activity
WHERE backend_type = 'client backend';

it tell you that this job is executing with the original user:

usename | query
---------+--------------------------------------------
andrey | SELECT usename,query FROM pg_stat_activity+
| WHERE backend_type = 'client backend';
abc | SELECT pg_sleep(60);

The deal is in missed update of PgBackendStatus after updating of
session user.
The attached patch fixes this bug.

--
regards,
Andrey Lepikhov
Postgres Professional