Detect the case where time goes backward and force an event cache update.

If someone resets the clock after bootinfo starts running, then the
time stamp in the cache keeping track of the last time an event was sent
from a node will suddenly be in the (potentially far distant) future
and we will stop sending events for a (potentially long) time.

pxe/bootinfo.c

@@ -246,7 +246,15 @@ bicache_needevent(struct in_addr ipaddr)
 			info("Timestamps: old:%ld new:%ld\n", oldtt, tt);
 		}
 
-		if (tt - oldtt <= MINEVENTTIME) {
+		/*
+		 * XXX sanity check, in case time goes backward while we
+		 * are running.
+		 */
+		if (tt < oldtt) {
+			info("%s: Whoa! time went backwards (%ld -> %ld),"
+			     "fixing...\n",
+			     inet_ntoa(ipaddr), oldtt, tt);
+		} else if (tt - oldtt <= MINEVENTTIME) {
 			rval = 0;
 			info("%s: no event will be sent: last:%ld cur:%ld\n",
 			     inet_ntoa(ipaddr), oldtt, tt);