Use proper algorithm for median computation; use fabs() for computing

an absolute value and fix poll loop to first generate messages and
then compute poll flags the write cases. This makes the timeout
workaround for constraints unneeded.  ok reyk@ tb@

src/usr.sbin/ntpd/constraint.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: constraint.c,v 1.43 2019/05/28 06:49:46 otto Exp $	*/
+/*	$OpenBSD: constraint.c,v 1.44 2019/05/30 13:42:19 otto Exp $	*/
 
 /*
  * Copyright (c) 2015 Reyk Floeter <reyk@openbsd.org>
@@ -41,6 +41,7 @@
 #include <ctype.h>
 #include <tls.h>
 #include <pwd.h>
+#include <math.h>
 
 #include "ntpd.h"
 
@@ -773,7 +774,7 @@ constraint_update(void)
 {
 	struct constraint *cstr;
 	int	 cnt, i;
-	time_t	*sum;
+	time_t	*values;
 	time_t	 now;
 
 	now = getmonotime();
@@ -784,29 +785,31 @@ constraint_update(void)
 			continue;
 		cnt++;
 	}
+	if (cnt == 0)
+		return;
 
-	if ((sum = calloc(cnt, sizeof(time_t))) == NULL)
+	if ((values = calloc(cnt, sizeof(time_t))) == NULL)
 		fatal("calloc");
 
 	i = 0;
 	TAILQ_FOREACH(cstr, &conf->constraints, entry) {
 		if (cstr->state != STATE_REPLY_RECEIVED)
 			continue;
-		sum[i++] = cstr->constraint + (now - cstr->last);
+		values[i++] = cstr->constraint + (now - cstr->last);
 	}
 
-	qsort(sum, cnt, sizeof(time_t), constraint_cmp);
+	qsort(values, cnt, sizeof(time_t), constraint_cmp);
 
 	/* calculate median */
 	i = cnt / 2;
 	if (cnt % 2 == 0)
-		if (sum[i - 1] < sum[i])
-			i -= 1;
+		conf->constraint_median = (values[i - 1] + values[i]) / 2;
+	else
+		conf->constraint_median = values[i];
 
 	conf->constraint_last = now;
-	conf->constraint_median = sum[i];
 
-	free(sum);
+	free(values);
 }
 
 void
@@ -826,7 +829,7 @@ int
 constraint_check(double val)
 {
 	struct timeval	tv;
-	double		constraint;
+	double		diff;
 	time_t		now;
 
 	if (conf->constraint_median == 0)
@@ -836,10 +839,9 @@ constraint_check(double val)
 	now = getmonotime();
 	tv.tv_sec = conf->constraint_median + (now - conf->constraint_last);
 	tv.tv_usec = 0;
-	constraint = gettime_from_timeval(&tv);
+	diff = fabs(val - gettime_from_timeval(&tv));
 
-	if (((val - constraint) > CONSTRAINT_MARGIN) ||
-	    ((constraint - val) > CONSTRAINT_MARGIN)) {
+	if (diff > CONSTRAINT_MARGIN) {
 		/* XXX get new constraint if too many errors happened */
 		if (conf->constraint_errors++ >
 		    (CONSTRAINT_ERROR_MARGIN * peer_cnt)) {

src/usr.sbin/ntpd/ntp.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: ntp.c,v 1.151 2019/05/29 18:48:33 otto Exp $ */
+/*	$OpenBSD: ntp.c,v 1.152 2019/05/30 13:42:19 otto Exp $ */
 
 /*
  * Copyright (c) 2003, 2004 Henning Brauer <henning@openbsd.org>
@@ -316,6 +316,11 @@ ntp_main(struct ntpd_conf *nconf, struct passwd *pw, int argc, char **argv)
 		    (peer_cnt == 0 && sensors_cnt == 0)))
 			priv_settime(0);	/* no good peers, don't wait */
 
+		TAILQ_FOREACH(cstr, &conf->constraints, entry) {
+			if (constraint_query(cstr) == -1)
+				continue;
+		}
+
 		if (ibuf_main->w.queued > 0)
 			pfd[PFD_PIPE_MAIN].events |= POLLOUT;
 		if (ibuf_dns->w.queued > 0)
@@ -330,15 +335,7 @@ ntp_main(struct ntpd_conf *nconf, struct passwd *pw, int argc, char **argv)
 		}
 		ctls = i;
 
-		TAILQ_FOREACH(cstr, &conf->constraints, entry) {
-			if (constraint_query(cstr) == -1)
-				continue;
-		}
-
 		now = getmonotime();
-		if (constraint_cnt)
-			nextaction = now + 1;
-
 		timeout = nextaction - now;
 		if (timeout < 0)
 			timeout = 0;