forward requester id to check username for spam callbacks

changelog.d/17916.feature

@@ -0,0 +1,3 @@
+Module developers will have access to user id of requester when adding `check_username_for_spam` callbacks to `spam_checker_module_callbacks`.
+
+Contributed by [email protected]

docs/modules/spam_checker_callbacks.md

@@ -245,7 +245,7 @@ this callback.
 _First introduced in Synapse v1.37.0_
 
 ```python
-async def check_username_for_spam(user_profile: synapse.module_api.UserProfile) -> bool
+async def check_username_for_spam(user_profile: synapse.module_api.UserProfile, requester_id: str) -> bool
 ```
 
 Called when computing search results in the user directory. The module must return a

docs/spam_checker.md

@@ -72,8 +72,8 @@ class ExampleSpamChecker:
     async def user_may_publish_room(self, userid, room_id):
         return True  # allow publishing of all rooms
 
-    async def check_username_for_spam(self, user_profile):
-        return False  # allow all usernames
+    async def check_username_for_spam(self, user_profile, requester_id):
+        return False  # allow all usernames regardless of requester
 
     async def check_registration_for_spam(
         self,

synapse/handlers/user_directory.py

@@ -161,7 +161,7 @@ async def search_users(
         non_spammy_users = []
         for user in results["results"]:
             if not await self._spam_checker_module_callbacks.check_username_for_spam(
-                user
+                user, user_id
             ):
                 non_spammy_users.append(user)
         results["results"] = non_spammy_users

synapse/module_api/callbacks/spamchecker_callbacks.py

@@ -31,6 +31,7 @@
     Optional,
     Tuple,
     Union,
+    cast,
 )
 
 # `Literal` appears with Python 3.8.
@@ -168,7 +169,10 @@
         ]
     ],
 ]
-CHECK_USERNAME_FOR_SPAM_CALLBACK = Callable[[UserProfile], Awaitable[bool]]
+CHECK_USERNAME_FOR_SPAM_CALLBACK = Union[
+    Callable[[UserProfile], Awaitable[bool]],
+    Callable[[UserProfile, str], Awaitable[bool]],
+]
 LEGACY_CHECK_REGISTRATION_FOR_SPAM_CALLBACK = Callable[
     [
         Optional[dict],
@@ -716,7 +720,9 @@ async def user_may_publish_room(
 
         return self.NOT_SPAM
 
-    async def check_username_for_spam(self, user_profile: UserProfile) -> bool:
+    async def check_username_for_spam(
+        self, user_profile: UserProfile, requester_id: str
+    ) -> bool:
         """Checks if a user ID or display name are considered "spammy" by this server.
 
         If the server considers a username spammy, then it will not be included in
@@ -727,15 +733,33 @@ async def check_username_for_spam(self, user_profile: UserProfile) -> bool:
                 * user_id
                 * display_name
                 * avatar_url
+            requester_id: The user ID of the user making the user directory search request.
 
         Returns:
             True if the user is spammy.
         """
         for callback in self._check_username_for_spam_callbacks:
             with Measure(self.clock, f"{callback.__module__}.{callback.__qualname__}"):
+                checker_args = inspect.signature(callback)
                 # Make a copy of the user profile object to ensure the spam checker cannot
                 # modify it.
-                res = await delay_cancellation(callback(user_profile.copy()))
+                # Also ensure backwards compatibility with spam checker callbacks
+                # that don't expect the requester_id argument.
+                if len(checker_args.parameters) == 2:
+                    callback_with_requester_id = cast(
+                        Callable[[UserProfile, str], Awaitable[bool]], callback
+                    )
+                    res = await delay_cancellation(
+                        callback_with_requester_id(user_profile.copy(), requester_id)
+                    )
+                else:
+                    callback_without_requester_id = cast(
+                        Callable[[UserProfile], Awaitable[bool]], callback
+                    )
+                    res = await delay_cancellation(
+                        callback_without_requester_id(user_profile.copy())
+                    )
+
             if res:
                 return True
 

tests/handlers/test_user_directory.py

@@ -820,6 +820,38 @@ async def block_all(user_profile: UserProfile) -> bool:
         s = self.get_success(self.handler.search_users(u1, "user2", 10))
         self.assertEqual(len(s["results"]), 0)
 
+        async def allow_all_expects_requester_id(
+            user_profile: UserProfile, requester_id: str
+        ) -> bool:
+            # Allow all users.
+            return False
+
+        # Configure a spam checker that does not filter any users.
+        spam_checker = self.hs.get_module_api_callbacks().spam_checker
+        spam_checker._check_username_for_spam_callbacks = [
+            allow_all_expects_requester_id
+        ]
+
+        # The results do not change:
+        # We get one search result when searching for user2 by user1.
+        s = self.get_success(self.handler.search_users(u1, "user2", 10))
+        self.assertEqual(len(s["results"]), 1)
+
+        # Configure a spam checker that filters all users.
+        async def block_all_expects_requester_id(
+            user_profile: UserProfile, requester_id: str
+        ) -> bool:
+            # All users are spammy.
+            return True
+
+        spam_checker._check_username_for_spam_callbacks = [
+            block_all_expects_requester_id
+        ]
+
+        # User1 now gets no search results for any of the other users.
+        s = self.get_success(self.handler.search_users(u1, "user2", 10))
+        self.assertEqual(len(s["results"]), 0)
+
     @override_config(
         {
             "spam_checker": {