Fix OpenTTD#12509: Maintain timer sort invariants when changing period

src/timer/timer.h

@@ -98,7 +98,7 @@ class IntervalTimer : public BaseTimer<TTimerType> {
 	 */
 	void SetInterval(const TPeriod interval, bool reset = true)
 	{
-		this->period = interval;
+		TimerManager<TTimerType>::ChangeRegisteredTimerPeriod(*this, interval);
 		if (reset) this->storage = {};
 	}
 
@@ -150,7 +150,7 @@ class TimeoutTimer : public BaseTimer<TTimerType> {
 	 */
 	void Reset(const TPeriod timeout)
 	{
-		this->period = timeout;
+		TimerManager<TTimerType>::ChangeRegisteredTimerPeriod(*this, timeout);
 		this->fired = false;
 		this->storage = {};
 	}

src/timer/timer_manager.h

@@ -56,6 +56,20 @@ class TimerManager {
 		GetTimers().erase(&timer);
 	}
 
+	/**
+	 * Change the period of a registered timer.
+	 *
+	 * @param timer The timer to change the period of.
+	 * @param new_period The new period value.
+	 */
+	static void ChangeRegisteredTimerPeriod(BaseTimer<TTimerType> &timer, TPeriod new_period)
+	{
+		/* Unregistration and re-registration is necessary because the period is used as the sort key in base_timer_sorter */
+		UnregisterTimer(timer);
+		timer.period = new_period;
+		RegisterTimer(timer);
+	}
+
 #ifdef WITH_ASSERT
 	/**
 	 * Validate that a new period is actually valid.