Keep lower-numbered lobbies during autoscaling

src/lib/server/autoscaling/planner.ts

@@ -57,8 +57,14 @@ export function planScaling(policy: ScalingPolicy, snapshot: ScalingSnapshot, no
         return available ? result('create', 'Replenish empty lobby capacity.', available) : result('wait', 'No unused lobby identity is available.');
     }
     if (occupied + empty > desired && empty > policy.spares && occupied + empty > policy.minimum) {
-        const candidates = members.filter(m => m.state === 'empty' && m.emptySince !== null && m.observedAt! - m.emptySince >= policy.emptyMinutes * 60_000)
-            .sort((a, b) => a.emptySince! - b.emptySince! || a.name.localeCompare(b.name, 'en', { numeric: true }));
+        // Protect the lowest-numbered spares before checking removal timers.
+        // Lobby 1 must survive even when a higher lobby has only just emptied.
+        const orderedEmpty = members.filter(m => m.state === 'empty').sort((a, b) =>
+            (parseServerName(a.name)?.lobby ?? Number.MAX_SAFE_INTEGER) - (parseServerName(b.name)?.lobby ?? Number.MAX_SAFE_INTEGER)
+            || a.name.localeCompare(b.name, 'en', { numeric: true }));
+        const requiredEmpty = Math.max(policy.spares, policy.minimum - occupied);
+        const candidates = orderedEmpty.slice(requiredEmpty).reverse()
+            .filter(m => m.emptySince !== null && m.observedAt! - m.emptySince >= policy.emptyMinutes * 60_000);
         if (candidates.length) return result('destroy', 'Surplus lobby has remained empty.', candidates[0].name);
         return result('wait', 'Waiting for a continuous empty interval.');
     }