ec2: Fix --allow-recreate not recreating terminated spot instances.

Until now, `nixops deploy --check --allow-recreate` would fail to
recreate terminated spot instances, instead emitting the error:

    Exception: spot instance request got fulfilled unexpectedly as instance ‘i-0543dd70c68337dea’

This was because `_reset_state()` did not reset the
`self._cached_instance`, and so the `_get_instance()` call in
`create_instance()` would continue to refer to the ID of the disappeared
instance, eventually producing the error.

Author: nh2

nixops_aws/backends/ec2.py

@@ -201,6 +201,11 @@ def _reset_state(self):
             self.spot_instance_request_id = None
             self.spot_instance_price = None
 
+        # The `_cached_instance` also contains an instance `.id`;
+        # reset it just as we reset `vm_id`, otherwise `_get_instance()`
+        # will incorrectly refer to the old one after resetting.
+        self._cached_instance = None
+
     def get_ssh_name(self):
         retVal = None
         if self.use_private_ip_address:
@@ -907,6 +912,8 @@ def _wait_for_spot_request_fulfillment(self, request_id):
             time.sleep(3)
         self.log_end("")
 
+        assert self._cached_instance is None
+
         instance = self._retry(
             lambda: self._get_instance(instance_id=request.instance_id)
         )
@@ -1031,8 +1038,9 @@ def _cancel_spot_request(self):
             if request.instance_id is not None and request.instance_id != self.vm_id:
                 if self.vm_id is not None:
                     raise Exception(
-                        "spot instance request got fulfilled unexpectedly as instance ‘{0}’".format(
-                            request.instance_id
+                        "spot instance request got fulfilled unexpectedly as instance ‘{0}’; expected ‘{1}’".format(
+                            request.instance_id,
+                            self.vm_id,
                         )
                     )
                 self.vm_id = request.instance_id