EPOLLPRI causes the waiter to report activity on a socket when there
is OOB data available. Since HTTP does not make use of OOB the data is
never read and the socket thus is always ready for read according to
the waiter. This causes the waiter to continously assign the session
to a worker only for the worker to find out there is no data sending
it back to the waiter. This continues until the request timeout_idle
has elapsed.

This problem does no constitute a DOS attack vector as the linger
timeout in the session SES_RxReq is still invoked, and this one does
not trigger on PRI. So the worker sleeps on that poll for
timeout_linger before sending the session back to the waiter, and
eventually is dropped on timeout_idle.