Allow ThirdPartyEventRules modules to manipulate public room state (#8292)

This PR allows `ThirdPartyEventRules` modules to view, manipulate and block changes to the state of whether a room is published in the public rooms directory.

While the idea of whether a room is in the public rooms list is not kept within an event in the room, `ThirdPartyEventRules` generally deal with controlling which modifications can happen to a room. Public rooms fits within that idea, even if its toggle state isn't controlled through a state event. 

4 files changed, 131 insertions, 6 deletions

diff --git a/synapse/events/third_party_rules.py b/synapse/events/third_party_rules.py
index 9d5310851c..fed459198a 100644
--- a/synapse/events/third_party_rules.py
+++ b/synapse/events/third_party_rules.py
@@ -12,10 +12,12 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+from typing import Callable
 
 from synapse.events import EventBase
 from synapse.events.snapshot import EventContext
-from synapse.types import Requester
+from synapse.module_api import ModuleApi
+from synapse.types import Requester, StateMap
 
 
 class ThirdPartyEventRules:
@@ -38,7 +40,7 @@ class ThirdPartyEventRules:
 
         if module is not None:
             self.third_party_rules = module(
-                config=config, http_client=hs.get_simple_http_client()
+                config=config, module_api=ModuleApi(hs, hs.get_auth_handler()),
             )
 
     async def check_event_allowed(
@@ -106,6 +108,46 @@ class ThirdPartyEventRules:
         if self.third_party_rules is None:
             return True
 
+        state_events = await self._get_state_map_for_room(room_id)
+
+        ret = await self.third_party_rules.check_threepid_can_be_invited(
+            medium, address, state_events
+        )
+        return ret
+
+    async def check_visibility_can_be_modified(
+        self, room_id: str, new_visibility: str
+    ) -> bool:
+        """Check if a room is allowed to be published to, or removed from, the public room
+        list.
+
+        Args:
+            room_id: The ID of the room.
+            new_visibility: The new visibility state. Either "public" or "private".
+
+        Returns:
+            True if the room's visibility can be modified, False if not.
+        """
+        if self.third_party_rules is None:
+            return True
+
+        check_func = getattr(self.third_party_rules, "check_visibility_can_be_modified")
+        if not check_func or not isinstance(check_func, Callable):
+            return True
+
+        state_events = await self._get_state_map_for_room(room_id)
+
+        return await check_func(room_id, state_events, new_visibility)
+
+    async def _get_state_map_for_room(self, room_id: str) -> StateMap[EventBase]:
+        """Given a room ID, return the state events of that room.
+
+        Args:
+            room_id: The ID of the room.
+
+        Returns:
+            A dict mapping (event type, state key) to state event.
+        """
         state_ids = await self.store.get_filtered_current_state_ids(room_id)
         room_state_events = await self.store.get_events(state_ids.values())
 
@@ -113,7 +155,4 @@ class ThirdPartyEventRules:
         for key, event_id in state_ids.items():
             state_events[key] = room_state_events[event_id]
 
-        ret = await self.third_party_rules.check_threepid_can_be_invited(
-            medium, address, state_events
-        )
-        return ret
+        return state_events
diff --git a/synapse/handlers/directory.py b/synapse/handlers/directory.py
index 6f15c68240..ad5683d251 100644
--- a/synapse/handlers/directory.py
+++ b/synapse/handlers/directory.py
@@ -46,6 +46,7 @@ class DirectoryHandler(BaseHandler):
         self.config = hs.config
         self.enable_room_list_search = hs.config.enable_room_list_search
         self.require_membership = hs.config.require_membership_for_aliases
+        self.third_party_event_rules = hs.get_third_party_event_rules()
 
         self.federation = hs.get_federation_client()
         hs.get_federation_registry().register_query_handler(
@@ -454,6 +455,15 @@ class DirectoryHandler(BaseHandler):
                 # per alias creation rule?
                 raise SynapseError(403, "Not allowed to publish room")
 
+            # Check if publishing is blocked by a third party module
+            allowed_by_third_party_rules = await (
+                self.third_party_event_rules.check_visibility_can_be_modified(
+                    room_id, visibility
+                )
+            )
+            if not allowed_by_third_party_rules:
+                raise SynapseError(403, "Not allowed to publish room")
+
         await self.store.set_room_is_public(room_id, making_public)
 
     async def edit_published_appservice_room_list(
diff --git a/synapse/handlers/room.py b/synapse/handlers/room.py
index f1a6699cd4..f14f791586 100644
--- a/synapse/handlers/room.py
+++ b/synapse/handlers/room.py
@@ -681,6 +681,15 @@ class RoomCreationHandler(BaseHandler):
             creator_id=user_id, is_public=is_public, room_version=room_version,
         )
 
+        # Check whether this visibility value is blocked by a third party module
+        allowed_by_third_party_rules = await (
+            self.third_party_event_rules.check_visibility_can_be_modified(
+                room_id, visibility
+            )
+        )
+        if not allowed_by_third_party_rules:
+            raise SynapseError(403, "Room visibility value not allowed.")
+
         directory_handler = self.hs.get_handlers().directory_handler
         if room_alias:
             await directory_handler.create_association(
diff --git a/synapse/module_api/__init__.py b/synapse/module_api/__init__.py
index fcbd5378c4..646f09d2bc 100644
--- a/synapse/module_api/__init__.py
+++ b/synapse/module_api/__init__.py
@@ -14,13 +14,18 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import logging
+from typing import TYPE_CHECKING
 
 from twisted.internet import defer
 
+from synapse.http.client import SimpleHttpClient
 from synapse.http.site import SynapseRequest
 from synapse.logging.context import make_deferred_yieldable, run_in_background
 from synapse.types import UserID
 
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
 """
 This package defines the 'stable' API which can be used by extension modules which
 are loaded into Synapse.
@@ -43,6 +48,27 @@ class ModuleApi:
         self._auth = hs.get_auth()
         self._auth_handler = auth_handler
 
+        # We expose these as properties below in order to attach a helpful docstring.
+        self._http_client = hs.get_simple_http_client()  # type: SimpleHttpClient
+        self._public_room_list_manager = PublicRoomListManager(hs)
+
+    @property
+    def http_client(self):
+        """Allows making outbound HTTP requests to remote resources.
+
+        An instance of synapse.http.client.SimpleHttpClient
+        """
+        return self._http_client
+
+    @property
+    def public_room_list_manager(self):
+        """Allows adding to, removing from and checking the status of rooms in the
+        public room list.
+
+        An instance of synapse.module_api.PublicRoomListManager
+        """
+        return self._public_room_list_manager
+
     def get_user_by_req(self, req, allow_guest=False):
         """Check the access_token provided for a request
 
@@ -266,3 +292,44 @@ class ModuleApi:
         await self._auth_handler.complete_sso_login(
             registered_user_id, request, client_redirect_url,
         )
+
+
+class PublicRoomListManager:
+    """Contains methods for adding to, removing from and querying whether a room
+    is in the public room list.
+    """
+
+    def __init__(self, hs: "HomeServer"):
+        self._store = hs.get_datastore()
+
+    async def room_is_in_public_room_list(self, room_id: str) -> bool:
+        """Checks whether a room is in the public room list.
+
+        Args:
+            room_id: The ID of the room.
+
+        Returns:
+            Whether the room is in the public room list. Returns False if the room does
+            not exist.
+        """
+        room = await self._store.get_room(room_id)
+        if not room:
+            return False
+
+        return room.get("is_public", False)
+
+    async def add_room_to_public_room_list(self, room_id: str) -> None:
+        """Publishes a room to the public room list.
+
+        Args:
+            room_id: The ID of the room.
+        """
+        await self._store.set_room_is_public(room_id, True)
+
+    async def remove_room_from_public_room_list(self, room_id: str) -> None:
+        """Removes a room from the public room list.
+
+        Args:
+            room_id: The ID of the room.
+        """
+        await self._store.set_room_is_public(room_id, False)