Activate fd events in a pseudorandom order on older backends.

New backends like poll and kqueue and so on add fds to the queue in
the order that they are triggered.  But the select backend currently
activates low-numbered fds first, whereas the poll and win32 backends
currently favor whatever fds have been on for the longest.  This is no
good for fairness.

svn:r1318

ChangeLog

@@ -33,6 +33,7 @@ Changes in 2.0.2-alpha:
  o Fix a deadlock when suspending reads in a bufferevent due to a full buffer. (Spotted by Joachim Bauch.)
  o Fix a memory error when freeing a thread-enabled event base with registered events. (Spotted by Joachim Bauch.)
  o Try to contain degree of failure when running on a win32 version so heavily firewalled that we can't fake a socketpair.
+ o Activate fd events in a pseudorandom order with O(N) backends, so that we don't systematically favor low fds (select) or earlier-added fds (poll, win32).
 
 
 Changes in 2.0.1-alpha:

WIN32-Code/win32.c

@@ -282,8 +282,9 @@ win32_dispatch(struct event_base *base, struct timeval *tv)
 {
 	struct win32op *win32op = base->evbase;
 	int res = 0;
-	unsigned i;
+	unsigned j, i;
 	int fd_count;
+	SOCKET s;
 
 	fd_set_copy(win32op->readset_out, win32op->readset_in);
 	fd_set_copy(win32op->exset_out, win32op->readset_in);
@@ -314,19 +315,33 @@ win32_dispatch(struct event_base *base, struct timeval *tv)
 		evsig_process(base);
 	}
 
-	for (i=0; i<win32op->readset_out->fd_count; ++i) {
+	if (win32op->readset_out->fd_count) {
-		SOCKET s = win32op->readset_out->fd_array[i];
+		i = rand() % win32op->readset_out->fd_count;
+		for (j=0; j<win32op->readset_out->fd_count; ++j) {
+			if (++i >= win32op->readset_out->fd_count)
+				i = 0;
+			s = win32op->readset_out->fd_array[i];
 			evmap_io_active(base, s, EV_READ);
 		}
-	for (i=0; i<win32op->exset_out->fd_count; ++i) {
+	}
-		SOCKET s = win32op->exset_out->fd_array[i];
+	if (win32op->exset_out->fd_count) {
+		i = rand() % win32op->exset_out->fd_count;
+		for (j=0; j<win32op->exset_out->fd_count; ++j) {
+			if (++i >= win32op-exset_out->fd_count)
+				i = 0;
+			s = win32op->exset_out->fd_array[i];
 			evmap_io_active(base, s, EV_READ);
 		}
-	for (i=0; i<win32op->writeset_out->fd_count; ++i) {
+	}
+	if (win32op->writeset_out->fd_count) {
+		i = rand() % win32op->writeset_out->fd_count;
+		for (j=0; j<win32op->writeset_out->fd_count; ++j) {
+			if (++i >= win32op-exset_out->fd_count)
+				i = 0;
 			SOCKET s = win32op->writeset_out->fd_array[i];
 			evmap_io_active(base, s, EV_WRITE);
 		}
-
+	}
 	return (0);
 }
 

poll.c

@@ -117,7 +117,7 @@ poll_check_ok(struct pollop *pop)
 static int
 poll_dispatch(struct event_base *base, struct timeval *tv)
 {
-	int res, i, msec = -1, nfds;
+	int res, i, j, msec = -1, nfds;
 	struct pollop *pop = base->evbase;
 
 	poll_check_ok(pop);
@@ -142,11 +142,15 @@ poll_dispatch(struct event_base *base, struct timeval *tv)
 
 	event_debug(("%s: poll reports %d", __func__, res));
 
-	if (res == 0)
+	if (res == 0 || nfds == 0)
 		return (0);
 
-	for (i = 0; i < nfds; i++) {
+	i = random() % nfds;
-		int what = pop->event_set[i].revents;
+	for (j = 0; j < nfds; j++) {
+		int what;
+		if (++i == nfds)
+			i = 0;
+		what = pop->event_set[i].revents;
 		if (!what)
 			continue;
 

select.c

@@ -114,7 +114,7 @@ check_selectop(struct selectop *sop)
 static int
 select_dispatch(struct event_base *base, struct timeval *tv)
 {
-	int res, i;
+	int res, i, j;
 	struct selectop *sop = base->evbase;
 
 	check_selectop(sop);
@@ -144,7 +144,10 @@ select_dispatch(struct event_base *base, struct timeval *tv)
 	event_debug(("%s: select reports %d", __func__, res));
 
 	check_selectop(sop);
-	for (i = 0; i <= sop->event_fds; ++i) {
+	i = random() % (sop->event_fds+1);
+	for (j = 0; j <= sop->event_fds; ++j) {
+		if (++i >= sop->event_fds+1)
+			i = 0;
 		res = 0;
 		if (FD_ISSET(i, sop->event_readset_out))
 			res |= EV_READ;