FO: Use tevent_req_defer_callback() when notifying callers
If a fo_resolve_service callback would modify the server->common member
in any way, for example by dereferencing the server and lowering the
refcount to 0, which would free the common structure, then the next
iteration of fo_resolve_service_done would access memory that was
for more details.
Reviewed-by: Pavel Březina <firstname.lastname@example.org>
(cherry picked from commit a92f68763a57b211a1bf6b80b6dd80c4a1aa2738)