Keep restricted engines reserved through asynchronous retirement

restricted-execution/service.mjs

@@ -239,15 +239,17 @@ export async function serveRestrictedRegistry(config, io = process) {
       }
       const engine = call.engine;
       if (engine) {
-        // Revoke the frame token before returning an engine to the idle pool.
-        // A delayed child request can never acquire the next call's authority.
-        if (engine.current === call) engine.current = null;
         if (!stopped && reason.code === "INVOCATION_ENDED" && !usageFailure && !engine.destroying) {
           const rss = (await engineUsage(engine)).memoryBytes;
           metrics.peakRSSBytes = Math.max(metrics.peakRSSBytes, rss);
           // Bound retained engine heaps as well as lifetime. Hard aggregate
           // physical memory remains the supervising process/cgroup's contract.
-          if (rss <= 128 * 1024 * 1024) {
+          // Keep the lease busy through every await. Another artifact can evict
+          // an idle lease immediately; never publish an engine still retiring
+          // or one whose process died while its final usage read was pending.
+          if (rss <= 128 * 1024 * 1024 && !stopped && !engine.destroying && engine.current === call) {
+            // Revoke the frame token atomically with publishing idle capacity.
+            engine.current = null;
             idleEngines.push(engine);
             engine.idleUntil = Date.now() + limits.engineIdleMs;
             engine.idleTimer = setTimeout(() => void destroyEngine(engine).catch(fail), limits.engineIdleMs);