From 7bd028861b6ed5fae2ed4283df8a8ead17779d85 Mon Sep 17 00:00:00 2001
From: Maxim Sobolev <sobomax@FreeBSD.org>
Date: Wed, 17 Oct 2012 00:44:34 +0000
Subject: [PATCH] o Use nanosleep(2) to sleep exact amount of time till the
 next second, not multiple of 1 second, which results in actual time to drift
 back and forth every run within 1 second of the actual action has been set
 for.

Suggested by:   Ian Lepore

o Schedule the first run in 1 second after starting up, not on the
boundary of the next minute, which results in the every_second jobs
not being run.
---
 usr.sbin/cron/cron/cron.c | 54 ++++++++++++++++++++++++++++++++++-----
 1 file changed, 48 insertions(+), 6 deletions(-)

diff --git a/usr.sbin/cron/cron/cron.c b/usr.sbin/cron/cron/cron.c
index 011458a537f1..5f3e7fd5c501 100644
--- a/usr.sbin/cron/cron/cron.c
+++ b/usr.sbin/cron/cron/cron.c
@@ -341,37 +341,73 @@ cron_tick(db)
  */
 static void
 cron_sync() {
+#if 0
  	register struct tm	*tm;
+#endif
 
-	TargetTime = time((time_t*)0);
+	TargetTime = time((time_t*)0) + 1;
+#if 0
 	tm = localtime(&TargetTime);
 	TargetTime += (60 - tm->tm_sec);
+#endif
 }
 
+static int
+timeval_subtract(struct timespec *result, struct timeval *x, struct timeval *y)
+{
+	int nsec;
+
+	/* Perform the carry for the later subtraction by updating y. */
+	if (x->tv_usec < y->tv_usec) {
+		nsec = (y->tv_usec - x->tv_usec) / 1000000 + 1;
+		y->tv_usec -= 1000000 * nsec;
+		y->tv_sec += nsec;
+	}
+	if (x->tv_usec - y->tv_usec > 1000000) {
+		nsec = (x->tv_usec - y->tv_usec) / 1000000;
+		y->tv_usec += 1000000 * nsec;
+		y->tv_sec -= nsec;
+	}
+     
+	/* tv_nsec is certainly positive. */
+	result->tv_sec = x->tv_sec - y->tv_sec;
+	result->tv_nsec = (x->tv_usec - y->tv_usec) * 1000;
+     
+	/* Return difference in seconds */
+	return (x->tv_sec - y->tv_sec);
+}
 
 static void
 cron_sleep(db)
 	cron_db	*db;
 {
-	int	seconds_to_wait = 0;
+	int seconds_to_wait;
+	int rval;
+	struct timeval ctime, ttime;
+	struct timespec stime, remtime;
 
 	/*
 	 * Loop until we reach the top of the next minute, sleep when possible.
 	 */
 
 	for (;;) {
-		seconds_to_wait = (int) (TargetTime - time((time_t*)0));
+		gettimeofday(&ctime, NULL);
+		ttime.tv_sec = TargetTime;
+		ttime.tv_usec = 0;
+		timeval_subtract(&stime, &ttime, &ctime);
 
 		/*
 		 * If the seconds_to_wait value is insane, jump the cron
 		 */
 
-		if (seconds_to_wait < -600 || seconds_to_wait > 600) {
+		if (stime.tv_sec < -600 || stime.tv_sec > 600) {
 			cron_clean(db);
 			cron_sync();
 			continue;
 		}
 
+		seconds_to_wait = (stime.tv_nsec > 0) ? stime.tv_sec + 1 : stime.tv_sec;
+
 		Debug(DSCH, ("[%d] TargetTime=%ld, sec-to-wait=%d\n",
 			getpid(), (long)TargetTime, seconds_to_wait))
 
@@ -380,13 +416,19 @@ cron_sleep(db)
 		 * to run, break
 		 */
 
-		if (seconds_to_wait <= 0)
+		if (stime.tv_sec < 0)
 			break;
 		if (job_runqueue() == 0) {
 			Debug(DSCH, ("[%d] sleeping for %d seconds\n",
 				getpid(), seconds_to_wait))
 
-			sleep(seconds_to_wait);
+			for (;;) {
+				rval = nanosleep(&stime, &remtime);
+				if (rval == 0 || errno != EINTR)
+					break;
+				stime.tv_sec = remtime.tv_sec;
+				stime.tv_nsec = remtime.tv_nsec;
+			}
 		}
 	}
 }