Uniformize spam-checker API, part 2: check_event_for_spam (#12808)

Signed-off-by: David Teller <davidt@element.io>

synapse/events/spamcheck.py

@@ -27,9 +27,10 @@ from typing import (
     Union,
 )
 
+from synapse.api.errors import Codes
 from synapse.rest.media.v1._base import FileInfo
 from synapse.rest.media.v1.media_storage import ReadableFileWrapper
-from synapse.spam_checker_api import RegistrationBehaviour
+from synapse.spam_checker_api import Allow, Decision, RegistrationBehaviour
 from synapse.types import RoomAlias, UserProfile
 from synapse.util.async_helpers import delay_cancellation, maybe_awaitable
 from synapse.util.metrics import Measure
@@ -40,9 +41,19 @@ if TYPE_CHECKING:
 
 logger = logging.getLogger(__name__)
 
+
 CHECK_EVENT_FOR_SPAM_CALLBACK = Callable[
     ["synapse.events.EventBase"],
-    Awaitable[Union[bool, str]],
+    Awaitable[
+        Union[
+            Allow,
+            Codes,
+            # Deprecated
+            bool,
+            # Deprecated
+            str,
+        ]
+    ],
 ]
 SHOULD_DROP_FEDERATED_EVENT_CALLBACK = Callable[
     ["synapse.events.EventBase"],
@@ -259,7 +270,7 @@ class SpamChecker:
 
     async def check_event_for_spam(
         self, event: "synapse.events.EventBase"
-    ) -> Union[bool, str]:
+    ) -> Union[Decision, str]:
         """Checks if a given event is considered "spammy" by this server.
 
         If the server considers an event spammy, then it will be rejected if
@@ -270,18 +281,36 @@ class SpamChecker:
             event: the event to be checked
 
         Returns:
-            True or a string if the event is spammy. If a string is returned it
-            will be used as the error message returned to the user.
+            - on `ALLOW`, the event is considered good (non-spammy) and should
+                be let through. Other spamcheck filters may still reject it.
+            - on `Code`, the event is considered spammy and is rejected with a specific
+                error message/code.
+            - on `str`, the event is considered spammy and the string is used as error
+                message. This usage is generally discouraged as it doesn't support
+                internationalization.
         """
         for callback in self._check_event_for_spam_callbacks:
             with Measure(
                 self.clock, "{}.{}".format(callback.__module__, callback.__qualname__)
             ):
-                res: Union[bool, str] = await delay_cancellation(callback(event))
-            if res:
-                return res
+                res: Union[Decision, str, bool] = await delay_cancellation(
+                    callback(event)
+                )
+                if res is False or res is Allow.ALLOW:
+                    # This spam-checker accepts the event.
+                    # Other spam-checkers may reject it, though.
+                    continue
+                elif res is True:
+                    # This spam-checker rejects the event with deprecated
+                    # return value `True`
+                    return Codes.FORBIDDEN
+                else:
+                    # This spam-checker rejects the event either with a `str`
+                    # or with a `Codes`. In either case, we stop here.
+                    return res
 
-        return False
+        # No spam-checker has rejected the event, let it pass.
+        return Allow.ALLOW
 
     async def should_drop_federated_event(
         self, event: "synapse.events.EventBase"