DROP ROLE: how to detect active sessions?

I need to track db activity by role. All web-related connections to the db use the same user name.
When a user logs in via the web, the application code runs:
SET ROLE "n"
where n is the ID value for the user in the "users" table.
(These numeric roles are created dynamically as needed).
We can then use the value of "current_user" to track activity.

So far so good.

The problem happens when we periodically drop roles for inactive users.
If I run DROP ROLE "n", and there happens to be an active session for that role,
we have a problem: in that session, "select current_user" fails with "invalid role OID: nnnn".

Is there any way to determine if a given role has any active sessions?

Thanks a bunch,

Ken Lalonde
Gap Adventures, Toronto