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;
