YP can keep source tree read lock for too long
yp_touch keeps rlock on source tree when updating YP directory servers, if one
of the stated YP servers becomes inaccessible then curl will block for upto 30
seconds per server per source. Any new source connecting or disconnecting will
block on the wlock until the yp thread finishes all existing YP source updates.
Test with a listening port that does not send anything back.