Clarify authenticator interface

letsencrypt/client/auth_handler.py

@@ -151,8 +151,10 @@ class AuthHandler(object):  # pylint: disable=too-many-instance-attributes
             flat_auth.extend(ichall.chall for ichall in self.dv_c[dom])
 
         try:
-            client_resp = self.client_auth.perform(flat_client)
-            dv_resp = self.dv_auth.perform(flat_auth)
+            if flat_client:
+                client_resp = self.client_auth.perform(flat_client)
+            if flat_auth:
+                dv_resp = self.dv_auth.perform(flat_auth)
         # This will catch both specific types of errors.
         except errors.LetsEncryptAuthHandlerError as err:
             logging.critical("Failure in setting up challenges:")
@@ -212,9 +214,13 @@ class AuthHandler(object):  # pylint: disable=too-many-instance-attributes
         # These are indexed challenges... give just the challenges to the auth
         # Chose to make these lists instead of a generator to make it easier to
         # work with...
-        self.dv_auth.cleanup([ichall.chall for ichall in self.dv_c[domain]])
-        self.client_auth.cleanup(
-            [ichall.chall for ichall in self.client_c[domain]])
+        dv_list = [ichall.chall for ichall in self.dv_c[domain]]
+        client_list = [ichall.chall for ichall in self.client_c[domain]]
+        if dv_list:
+            self.dv_auth.cleanup(dv_list)
+        if client_list:
+            self.client_auth.cleanup(client_list)
+
 
     def _cleanup_state(self, delete_list):
         """Cleanup state after an authorization is received.

letsencrypt/client/interfaces.py

@@ -30,6 +30,9 @@ class IAuthenticator(zope.interface.Interface):
 
         :param list chall_list: List of namedtuple types defined in
             :mod:`letsencrypt.client.challenge_util` (``DvsniChall``, etc.).
+            - chall_list will never be empty
+            - chall_list will only contain types found within
+                :func:`get_chall_pref`
 
         :returns: ACME Challenge responses or if it cannot be completed then:
 
@@ -43,20 +46,16 @@ class IAuthenticator(zope.interface.Interface):
         """
 
     def cleanup(chall_list):
-        """Revert changes and shutdown after challenges complete."""
+        """Revert changes and shutdown after challenges complete.
 
+        :param list chall_list: List of namedtuple types defined in
+            :mod:`letsencrypt.client.challenge_util` (``DvsniChall``, etc.)
 
-class IChallenge(zope.interface.Interface):
-    """Let's Encrypt challenge."""
+            - Only challenges given previously in the perform function will be
+            found in chall_list.
+            - chall_list will never be empty
 
-    def perform():
-        """Perform the challenge."""
-
-    def generate_response():
-        """Generate response."""
-
-    def cleanup():
-        """Cleanup."""
+        """
 
 
 class IConfig(zope.interface.Interface):