Codechange: Add a priority field to TimerGameTick::TPeriod

Use this as the primary sort key for TimerGameTick::TPeriod,
to avoid container sort order changes on timer period saveload.
See: #12509

src/timer/timer_game_tick.cpp

@@ -21,13 +21,13 @@ TimerGameTick::TickCounter TimerGameTick::counter = 0;
 template<>
 void IntervalTimer<TimerGameTick>::Elapsed(TimerGameTick::TElapsed delta)
 {
-	if (this->period == 0) return;
+	if (this->period.value == 0) return;
 
 	this->storage.elapsed += delta;
 
 	uint count = 0;
-	while (this->storage.elapsed >= this->period) {
-		this->storage.elapsed -= this->period;
+	while (this->storage.elapsed >= this->period.value) {
+		this->storage.elapsed -= this->period.value;
 		count++;
 	}
 
@@ -40,11 +40,11 @@ template<>
 void TimeoutTimer<TimerGameTick>::Elapsed(TimerGameTick::TElapsed delta)
 {
 	if (this->fired) return;
-	if (this->period == 0) return;
+	if (this->period.value == 0) return;
 
 	this->storage.elapsed += delta;
 
-	if (this->storage.elapsed >= this->period) {
+	if (this->storage.elapsed >= this->period.value) {
 		this->callback();
 		this->fired = true;
 	}
@@ -64,7 +64,16 @@ bool TimerManager<TimerGameTick>::Elapsed(TimerGameTick::TElapsed delta)
 
 #ifdef WITH_ASSERT
 template<>
-void TimerManager<TimerGameTick>::Validate(TimerGameTick::TPeriod)
+void TimerManager<TimerGameTick>::Validate(TimerGameTick::TPeriod period)
 {
+	if (period.priority == TimerGameTick::Priority::NONE) return;
+
+	/* Validate we didn't make a developer error and scheduled more than one
+	 * entry on the same priority. There can only be one timer on
+	 * a specific priority, to ensure we are deterministic, and to avoid
+	 * container sort order invariant issues with timer period saveload. */
+	for (const auto &timer : TimerManager<TimerGameTick>::GetTimers()) {
+		assert(timer->period.priority != period.priority);
+	}
 }
 #endif /* WITH_ASSERT */