BUG #15036: Un-killable queries Hanging in BgWorkerShutdown

The following bug has been logged on the website:

Bug reference:      15036
Logged by:          David Kohn
Email address:      djk447@gmail.com
PostgreSQL version: 10.1
Operating system:   Ubuntu 16.04
Description:

I have been experiencing a consistent problem with queries that I cannot
kill with pg_cancel_backend or pg_terminate_backend. In many cases they have
been running for days and are in a transaction so it eventually causes
rather large bloat etc problems. All the backends are in the IPC wait_event.
The backends appear to either be a main client_backend, in which case
wait_event_type fields in pg_stat_activity say BgWorkerShutdown and for the
background workers I see two (though I'm not sure that that this is all of
them): BtreePage and MessageQueuePutMessage. I'm quite sure the clients for
these are dead, they had statement timeouts set to an hour at most, they
might have died sooner than that of other causes. I assume this is a bug and
I should be reporting it here, but if I'm putting it on the wrong list let
me know and I'll move it! 
Best,
David