Allow denying or shadow banning registrations via the spam checker (#8034)

synapse/spam_checker_api/__init__.py

@@ -13,6 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import logging
+from enum import Enum
 
 from twisted.internet import defer
 
@@ -25,6 +26,16 @@ if MYPY:
 logger = logging.getLogger(__name__)
 
 
+class RegistrationBehaviour(Enum):
+    """
+    Enum to define whether a registration request should allowed, denied, or shadow-banned.
+    """
+
+    ALLOW = "allow"
+    SHADOW_BAN = "shadow_ban"
+    DENY = "deny"
+
+
 class SpamCheckerApi(object):
     """A proxy object that gets passed to spam checkers so they can get
     access to rooms and other relevant information.