Commit 5a2fed911a broke parallel query

This is a script to reproduce the problem:

  CREATE ROLE fluff;
  CREATE ROLE duff LOGIN IN ROLE fluff;
  CREATE DATABASE scratch OWNER duff;
  REVOKE CONNECT, TEMP ON DATABASE scratch FROM PUBLIC;

  \c scratch duff
  CREATE TABLE large AS SELECT id FROM generate_series(1, 500000) AS id;
  GRANT SELECT ON large TO fluff;

  SET ROLE fluff;
  SELECT count(*) FROM large;

Since commit 5a2fed911a, this results in:

  ERROR:  permission denied for database "scratch"
  DETAIL:  User does not have CONNECT privilege.
  CONTEXT:  parallel worker

The following patch makes the problem disappear, but I am far
from certain that using the session user is always correct there:

diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index a024b1151d0..150ec3f52f8 100644
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -360,7 +360,7 @@ CheckMyDatabase(const char *name, bool am_superuser, bool override_allow_connect
          * and save a few cycles.)
          */
         if (!am_superuser &&
-            object_aclcheck(DatabaseRelationId, MyDatabaseId, GetUserId(),
+            object_aclcheck(DatabaseRelationId, MyDatabaseId, GetSessionUserId(),
                             ACL_CONNECT) != ACLCHECK_OK)
             ereport(FATAL,
                     (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),

Yours,
Laurenz Albe