Hi Magnus,
It looks to me like the path to do_pg_start_backup() outside of a
transaction context comes from your initial commit of the base backup
facility.
The problem is that you're not allowed to do anything leading to a
syscache/catcache lookup in those contexts.
Greetings,
Andres Freund
--
Andres Freund http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services