This is only relevant with JACK, where different implementations
use threads for the latency callback.
With jack 2, jack_port_register() blocks and the jack_latency_callback
arrives in a different thread: https://pastebin.com/mitGBwpq
with jack 1 the callback arrives in sync
In either case this usually happens while
_adding_routes_in_progress == true and Ardour holds the process-lock,
because jack2 can process in parallel with reconfiguring latency
See also 1983f56592
b5e479dfc3