Make check_event_for_spam async

Method `check_event_for_spam` in both `class SpamCheck` and in plug-in spam checkers is currently
sync. This makes it impossible to write spam checkers that need to e.g. check with a database
before responding.

Signed-off-by: Your Name <davidt@element.io>

Author: Yoric

changelog.d/8890.feature

@@ -0,0 +1 @@
+Spam-checkers may now define their method `check_event_for_spam` as `async`. This will simplify the work of spam-checkers that need to e.g. request data from a database without blocking the server.

docs/spam_checker.md

@@ -37,7 +37,7 @@ class ExampleSpamChecker:
         self.config = config
         self.api = api
 
-    def check_event_for_spam(self, foo):
+    async def check_event_for_spam(self, foo):
         return False  # allow all events
 
     def user_may_invite(self, inviter_userid, invitee_userid, room_id):

synapse/events/spamcheck.py

@@ -19,6 +19,7 @@
 
 from synapse.spam_checker_api import RegistrationBehaviour
 from synapse.types import Collection
+from synapse.util.async_helpers import maybe_awaitable
 
 if TYPE_CHECKING:
     import synapse.events
@@ -39,7 +40,7 @@ def __init__(self, hs: "synapse.server.HomeServer"):
             else:
                 self.spam_checkers.append(module(config=config))
 
-    def check_event_for_spam(self, event: "synapse.events.EventBase") -> bool:
+    async def check_event_for_spam(self, event: "synapse.events.EventBase") -> bool:
         """Checks if a given event is considered "spammy" by this server.
 
         If the server considers an event spammy, then it will be rejected if
@@ -53,7 +54,7 @@ def check_event_for_spam(self, event: "synapse.events.EventBase") -> bool:
             True if the event is spammy.
         """
         for spam_checker in self.spam_checkers:
-            if spam_checker.check_event_for_spam(event):
+            if await maybe_awaitable(spam_checker.check_event_for_spam(event)):
                 return True
 
         return False

synapse/federation/federation_base.py

@@ -18,7 +18,7 @@
 from typing import Iterable, List
 
 from twisted.internet import defer
-from twisted.internet.defer import Deferred, DeferredList
+from twisted.internet.defer import Deferred, DeferredList, ensureDeferred
 from twisted.python.failure import Failure
 
 from synapse.api.constants import MAX_DEPTH, EventTypes, Membership
@@ -35,6 +35,7 @@
     make_deferred_yieldable,
 )
 from synapse.types import JsonDict, get_domain_from_id
+from synapse.util.async_helpers import maybe_awaitable
 
 logger = logging.getLogger(__name__)
 
@@ -74,11 +75,11 @@ def _check_sigs_and_hashes(
               * throws a SynapseError if the signature check failed.
             The deferreds run their callbacks in the sentinel
         """
-        deferreds = _check_sigs_on_pdus(self.keyring, room_version, pdus)
+        initial_deferreds = _check_sigs_on_pdus(self.keyring, room_version, pdus)
 
         ctx = current_context()
 
-        def callback(_, pdu: EventBase):
+        async def callback(_, pdu: EventBase):
             with PreserveLoggingContext(ctx):
                 if not check_event_content_hash(pdu):
                     # let's try to distinguish between failures because the event was
@@ -105,7 +106,7 @@ def callback(_, pdu: EventBase):
                         )
                     return redacted_event
 
-                if self.spam_checker.check_event_for_spam(pdu):
+                if await maybe_awaitable(self.spam_checker.check_event_for_spam(pdu)):
                     logger.warning(
                         "Event contains spam, redacting %s: %s",
                         pdu.event_id,
@@ -125,12 +126,23 @@ def errback(failure: Failure, pdu: EventBase):
                 )
             return failure
 
-        for deferred, pdu in zip(deferreds, pdus):
-            deferred.addCallbacks(
-                callback, errback, callbackArgs=[pdu], errbackArgs=[pdu]
-            )
+        # Here, we require a little gymnastics to return
+        # deferreds that accept awaitables as callbacks
+        deferred_with_callbacks = []
+        for deferred, pdu in zip(initial_deferreds, pdus):
+
+            async def awaitable():
+                result = None
+                try:
+                    result = await deferred
+                except Exception as e:
+                    return errback(e, pdu)
+                else:
+                    return await callback(result, pdu)
+
+            deferred_with_callbacks.append(ensureDeferred(awaitable))
 
-        return deferreds
+        return deferred_with_callbacks
 
 
 class PduToCheckSig(

synapse/handlers/message.py

@@ -51,7 +51,7 @@
 from synapse.storage.state import StateFilter
 from synapse.types import Requester, RoomAlias, StreamToken, UserID, create_requester
 from synapse.util import json_decoder, json_encoder
-from synapse.util.async_helpers import Linearizer
+from synapse.util.async_helpers import Linearizer, maybe_awaitable
 from synapse.util.metrics import measure_func
 from synapse.visibility import filter_events_for_client
 
@@ -744,7 +744,9 @@ async def create_and_send_nonmember_event(
                 event.sender,
             )
 
-            spam_error = self.spam_checker.check_event_for_spam(event)
+            spam_error = await maybe_awaitable(
+                self.spam_checker.check_event_for_spam(event)
+            )
             if spam_error:
                 if not isinstance(spam_error, str):
                     spam_error = "Spam is not permitted here"