Do not allow send_nonmember_event to be called with shadow-banned users.

changelog.d/8158.feature

@@ -0,0 +1 @@
+ Add support for shadow-banning users (ignoring any message send requests).

synapse/handlers/message.py

@@ -647,24 +647,35 @@ async def send_nonmember_event(
         event: EventBase,
         context: EventContext,
         ratelimit: bool = True,
+        ignore_shadow_ban: bool = False,
     ) -> int:
         """
         Persists and notifies local clients and federation of an event.
 
         Args:
-            requester
-            event the event to send.
-            context: the context of the event.
+            requester: The requester sending the event.
+            event: The event to send.
+            context: The context of the event.
             ratelimit: Whether to rate limit this send.
+            ignore_shadow_ban: True if shadow-banned users should be allowed to
+                send this event.
 
         Return:
             The stream_id of the persisted event.
+
+        Raises:
+            ShadowBanError if the requester has been shadow-banned.
         """
         if event.type == EventTypes.Member:
             raise SynapseError(
                 500, "Tried to send member event through non-member codepath"
             )
 
+        if not ignore_shadow_ban and requester.shadow_banned:
+            # We randomly sleep a bit just to annoy the requester.
+            await self.clock.sleep(random.randint(1, 10))
+            raise ShadowBanError()
+
         user = UserID.from_string(event.sender)
 
         assert self.hs.is_mine(user), "User must be our own: %s" % (user,)
@@ -725,6 +736,14 @@ async def create_and_send_nonmember_event(
 
         See self.create_event and self.send_nonmember_event.
 
+        Args:
+            requester: The requester sending the event.
+            event_dict: An entire event.
+            ratelimit: Whether to rate limit this send.
+            txn_id: The transaction ID.
+            ignore_shadow_ban: True if shadow-banned users should be allowed to
+                send this event.
+
         Raises:
             ShadowBanError if the requester has been shadow-banned.
         """
@@ -750,7 +769,11 @@ async def create_and_send_nonmember_event(
                 raise SynapseError(403, spam_error, Codes.FORBIDDEN)
 
             stream_id = await self.send_nonmember_event(
-                requester, event, context, ratelimit=ratelimit
+                requester,
+                event,
+                context,
+                ratelimit=ratelimit,
+                ignore_shadow_ban=ignore_shadow_ban,
             )
         return event, stream_id
 
@@ -1190,8 +1213,14 @@ async def _send_dummy_events_to_fill_extremities(self):
 
                     event.internal_metadata.proactively_send = False
 
+                    # Since this is a dummy-event it is OK if it is sent by a
+                    # shadow-banned user.
                     await self.send_nonmember_event(
-                        requester, event, context, ratelimit=False
+                        requester,
+                        event,
+                        context,
+                        ratelimit=False,
+                        ignore_shadow_ban=True,
                     )
                     dummy_event_sent = True
                     break