core/timer: Always use inactive_exit_timestamp if it is set

If we're doing a daemon-reload, we'll be going from TIMER_DEAD => TIMER_WAITING,
so we won't use inactive_exit_timestamp because TIMER_DEAD != UNIT_ACTIVE, even
though inactive_exit_timestamp is serialized/deserialized and will be valid after
the daemon-reload.

This issue can lead to timers never firing as we'll always calculate the next
elapse based on the current realtime on daemon-reload, so if daemon-reload happens
often enough, the elapse interval will be moved into the future every time, which
means the timer will never trigger.

To fix the issue, let's always use inactive_exit_timestamp if it is set, and only
fall back to the current realtime if it is not set.

(cherry picked from commit 6546045)
(cherry picked from commit aa48ecb)
(cherry picked from commit 48445d2)

Author: DaanDeMeyer

src/core/timer.c

@@ -401,12 +401,10 @@ static void timer_enter_waiting(Timer *t, bool time_change) {
 
                         if (t->last_trigger.realtime > 0)
                                 b = t->last_trigger.realtime;
-                        else {
-                                if (state_translation_table[t->state] == UNIT_ACTIVE)
-                                        b = UNIT(t)->inactive_exit_timestamp.realtime;
-                                else
-                                        b = ts.realtime;
-                        }
+                        else if (dual_timestamp_is_set(&UNIT(t)->inactive_exit_timestamp))
+                                b = UNIT(t)->inactive_exit_timestamp.realtime;
+                        else
+                                b = ts.realtime;
 
                         r = calendar_spec_next_usec(v->calendar_spec, b, &v->next_elapse);
                         if (r < 0)