Merge pull request #504 from lf-lang/keepalive

Do not exit if there is no timeout, fast is true, and keepalive is true

core/threaded/reactor_threaded.c

@@ -195,7 +195,7 @@ void lf_set_present(lf_port_base_t* port) {
 }
 
 bool wait_until(instant_t wait_until_time, lf_cond_t* condition) {
-  if (!fast) {
+  if (!fast || (wait_until_time == FOREVER && keepalive_specified)) {
     LF_PRINT_DEBUG("-------- Waiting until physical time " PRINTF_TIME, wait_until_time - start_time);
     // Check whether we actually need to wait, or if we have already passed the timepoint.
     interval_t wait_duration = wait_until_time - lf_time_physical();