In dns_rpz_update_from_db we call setup_update which creates the db
iterator and calls dns_dbiterator_first. This unpauses the iterator and
might cause db->tree_lock to be acquired. We then do isc_task_send(...)
on an event to do quantum_update, which (correctly) after each iteration
calls dns_dbiterator_pause, and re-isc_task_sends itself.
That's an obvious bug, as we're holding a lock over an async task send -
if a task requesting write (e.g. prune_tree) is scheduled on the same
workers queue as update_quantum but before it, it will wait for the
write lock indefinitely, resulting in a deadlock.
To fix it we have to pause dbiterator in setup_update.
(cherry picked from commit 06021b3529)
6e63d7047d