[PostgreSQL] Subnet entity with ServiceType grouped by both tables

The SQL clause "GROUP BY" in PostgreSQL requires the presence of all
fields provided in the "SELECT" clause; this is not happening in
MariaDB not MySQL.

Since [1] (and the related patch [2] that are part of the same bug
fix), when a resource with RBAC registers is selected, the "GROUP BY"
clause is added to reduce the number of returned registers. The
filed used is "id", present in all the RBAC controlled resources
('network', 'qospolicy', 'securitygroup', 'addressscope',
'subnetpool', 'addressgroup'). That is in opposition to what was
stated in the first paragraph "requires the presence of all
fields provided in the "SELECT" clause". However it is possible
to group only by a primary key [3].

In [4] a prior change was introduced that modifies the "Subnet" entity
to include a child table "SubnetServiceType". That introduces two new
fields ("SubnetServiceType" fields) that are not present in the
"GROUP BY" clause, causing the reported error in the LP bug.

This patch is adding these two fields (that form the composite primary
key of "SubnetServiceType" table) to the "GROUP BY" clause, when
needed; if the query is executed by an administrator, the RBAC query
won't be performed (an administrator has always permissions) and the
"GROUP BY" clause won't be issued.

[1]https://review.opendev.org/c/openstack/neutron-lib/+/884878
[2]https://review.opendev.org/c/openstack/neutron/+/884877
[3]https://learnsql.com/blog/must-appear-in-group-by-clause/
[4]https://review.opendev.org/c/openstack/neutron/+/744512

Closes-Bug: #2028003
Change-Id: I18e65d79e56fe5995076eb9166da23fc14c92fc5
(cherry picked from commit c831771)

Author: ralonsoh

neutron/objects/subnet.py

@@ -183,6 +183,14 @@ def _query_entity_service_subnets(cls, query, service_type):
             and_(Subnet.enable_dhcp.is_(True),
                  service_type == const.DEVICE_OWNER_DHCP)))
 
+        if query._group_by_clauses:
+            # If the "Subnet" query has a "GROUP BY" clause (that happens when
+            # a non-admin user has executed the query, that requires the join
+            # of the RBAC registers), it is needed to add the
+            # "SubnetServiceType" fields to this clause too.
+            query = query.group_by(ServiceType.subnet_id,
+                                   ServiceType.service_type)
+
         entity = orm.aliased(Subnet, query.subquery())
         return entity