Fix booting slots stuck due when cooldown blocks termination

agent/nix_builder_autoscaler/reconciler.py

@@ -328,10 +328,28 @@ class Reconciler:
         return (now - last_change).total_seconds()
 
     def _begin_termination(self, slot: dict, reason: str, extra: dict | None = None) -> None:
-        if not self._can_start_termination():
-            return
         slot_id = slot["slot_id"]
         instance_id = slot.get("instance_id")
+        ec2_state = None
+        if extra is not None:
+            ec2_state = extra.get("ec2_state")
+
+        if not self._can_start_termination():
+            # If cooldown is active but EC2 is already terminal/stopped, do not
+            # keep the slot stuck in its current state.
+            if instance_id is None or ec2_state in _TERMINAL_OR_STOPPED_STATES:
+                self._db.update_slot_state(
+                    slot_id, SlotState.EMPTY, instance_id=None, instance_ip=None, lease_count=0
+                )
+                payload = {"slot_id": slot_id}
+                if instance_id:
+                    payload["instance_id"] = instance_id
+                if extra:
+                    payload.update(extra)
+                log.warning(reason, extra=payload)
+                log.info("slot_emptied", extra={"slot_id": slot_id})
+            return
+
         started_terminating = False
         if instance_id:
             self._terminate_instance_best_effort(slot_id, instance_id)

agent/nix_builder_autoscaler/tests/test_reconciler.py

@@ -228,3 +228,26 @@ def test_termination_cooldown_spaces_terminations() -> None:
     assert slot is not None
     assert slot["state"] == SlotState.TERMINATING.value
     assert runtime.terminate_calls == ["i-6", "i-7"]
+
+
+def test_booting_terminal_state_bypasses_termination_cooldown() -> None:
+    db, runtime, reconciler, clock = _make_env(termination_cooldown=30)
+
+    # Start cooldown by triggering one termination.
+    runtime.instances["i-8"] = _Instance(state="running", slot_id="slot001")
+    db.update_slot_state("slot001", SlotState.DRAINING, instance_id="i-8", lease_count=0)
+    reconciler.tick()
+    assert runtime.terminate_calls == ["i-8"]
+
+    # During cooldown, a booting slot whose instance is already terminated
+    # should be emptied immediately without waiting for cooldown expiry.
+    runtime.instances["i-9"] = _Instance(state="terminated", slot_id="slot001")
+    db.update_slot_state("slot001", SlotState.BOOTING, instance_id="i-9")
+    clock.advance(1)
+    reconciler.tick()
+
+    slot = db.get_slot("slot001")
+    assert slot is not None
+    assert slot["state"] == SlotState.EMPTY.value
+    assert slot["instance_id"] is None
+    assert runtime.terminate_calls == ["i-8"]