mxqd: Enforce minimum job runtime of 30 seconds

Throttle througput for junk jobs. We don't want to busy
spin in the daemon and hammer the database if user
submits several jobs which immediately die or finish.

mxqd.c

@@ -1081,6 +1081,13 @@ int reaper_process(struct mxq_server *server,struct mxq_group_list *glist, struc
     }
     gettimeofday(&now, NULL);
     timersub(&now, &job->stats_starttime, &realtime);
+
+    if (realtime.tv_sec<30) {
+        int wait=30-realtime.tv_sec;
+        mx_log_warning("user process finished to fast (%ld seconds) : delaying termination for %d seconds",realtime.tv_sec,wait);
+        sleep(wait);
+    }
+
     res = getrusage(RUSAGE_CHILDREN, &rusage);
     if (res < 0) {
         mx_log_err("reaper: getrusage: %m");