This fixes a recent regression in kqueue_scan() where the function can mistakenly return EWOULDBLOCK.
Currently, kqueue_scan() does one more scan attempt after a timeout. Usually, this gives no new events and the function bails out through the following code. Note that it clears `error'. if (kq->kq_count == 0) { /* * Successive loops are only necessary if there are more * ready events to gather, so they don't need to block. */ if ((tsp != NULL && !timespecisset(tsp)) || scan->kqs_nevent != 0) { splx(s); error = 0; goto done; } However, there can be a last-minute event activation, in which case the function processes the event queue. Unfortunately, the error variable preserves its value EWOULDBLOCK/EAGAIN that gets returned to the caller. kevent(2), or select(2) or poll(2), is not supposed to return this error. The issue emerged in r1.146 of kern_event.c when the final copyout() was moved outside kqueue_scan(). The copyout()'s return value used to override the EWOULDBLOCK. The following patch fixes the regression by clearing `error' at the start of each scan round. The clearing could be done conditionally after kqueue_sleep(). However, that does not seem as robust. OK? Index: kern/kern_event.c =================================================================== RCS file: src/sys/kern/kern_event.c,v retrieving revision 1.153 diff -u -p -r1.153 kern_event.c --- kern/kern_event.c 20 Dec 2020 12:54:05 -0000 1.153 +++ kern/kern_event.c 23 Dec 2020 07:10:24 -0000 @@ -977,6 +977,8 @@ kqueue_scan(struct kqueue_scan_state *sc retry: KASSERT(nkev == 0); + error = 0; + if (kq->kq_state & KQ_DYING) { error = EBADF; goto done;