Add is_encrypted filtering to Sliding Sync /sync

Based on [MSC3575](https://github.com/matrix-org/matrix-spec-proposals/pull/3575): Sliding Sync

changelog.d/17249.feature

@@ -0,0 +1 @@
+Add `is_encrypted` filtering to experimental [MSC3575](https://github.com/matrix-org/matrix-spec-proposals/pull/3575) Sliding Sync `/sync` endpoint.

synapse/handlers/sliding_sync.py

@@ -31,7 +31,7 @@ if TYPE_CHECKING or HAS_PYDANTIC_V2:
 else:
     from pydantic import Extra
 
-from synapse.api.constants import AccountDataTypes, Membership
+from synapse.api.constants import AccountDataTypes, EventTypes, Membership
 from synapse.events import EventBase
 from synapse.rest.client.models import SlidingSyncBody
 from synapse.types import JsonMapping, Requester, RoomStreamToken, StreamToken, UserID
@@ -661,8 +661,23 @@ class SlidingSyncHandler:
         if filters.spaces:
             raise NotImplementedError()
 
-        if filters.is_encrypted:
-            raise NotImplementedError()
+        # Filter for encrypted rooms
+        if filters.is_encrypted is not None:
+            # Make a copy so we don't run into an error: `Set changed size during iteration`
+            for room_id in list(filtered_room_id_set):
+                # TODO: Is there a good method to look up all rooms at once? (N+1 query problem)
+                is_encrypted = (
+                    await self.storage_controllers.state.get_current_state_event(
+                        room_id, EventTypes.RoomEncryption, ""
+                    )
+                )
+
+                # If we're looking for encrypted rooms, filter out rooms that are not
+                # encrypted and vice versa
+                if (filters.is_encrypted and not is_encrypted) or (
+                    not filters.is_encrypted and is_encrypted
+                ):
+                    filtered_room_id_set.remove(room_id)
 
         if filters.is_invite:
             raise NotImplementedError()

tests/handlers/test_sliding_sync.py

@@ -1246,3 +1246,68 @@ class FilterRoomsTestCase(HomeserverTestCase):
         )
 
         self.assertEqual(falsy_filtered_room_ids, {room_id})
+
+    def test_filter_encrypted_rooms(self) -> None:
+        """
+        Test `filter.is_encrypted` for encrypted rooms
+        """
+        user1_id = self.register_user("user1", "pass")
+        user1_tok = self.login(user1_id, "pass")
+
+        # Create a normal room
+        room_id = self.helper.create_room_as(
+            user1_id,
+            is_public=False,
+            tok=user1_tok,
+        )
+
+        # Create an encrypted room
+        encrypted_room_id = self.helper.create_room_as(
+            user1_id,
+            is_public=False,
+            tok=user1_tok,
+        )
+        self.helper.send_state(
+            encrypted_room_id,
+            EventTypes.RoomEncryption,
+            {"algorithm": "m.megolm.v1.aes-sha2"},
+            tok=user1_tok,
+        )
+
+        # TODO: Better way to avoid the circular import? (see
+        # https://github.com/element-hq/synapse/pull/17187#discussion_r1619492779)
+        from synapse.handlers.sliding_sync import SlidingSyncConfig
+
+        # Try with `is_encrypted=True`
+        # -----------------------------
+        truthy_filters = SlidingSyncConfig.SlidingSyncList.Filters(
+            is_encrypted=True,
+        )
+
+        # Filter the rooms
+        truthy_filtered_room_ids = self.get_success(
+            self.sliding_sync_handler.filter_rooms(
+                UserID.from_string(user1_id),
+                {room_id, encrypted_room_id},
+                truthy_filters,
+            )
+        )
+
+        self.assertEqual(truthy_filtered_room_ids, {encrypted_room_id})
+
+        # Try with `is_encrypted=False`
+        # -----------------------------
+        falsy_filters = SlidingSyncConfig.SlidingSyncList.Filters(
+            is_encrypted=False,
+        )
+
+        # Filter the rooms
+        falsy_filtered_room_ids = self.get_success(
+            self.sliding_sync_handler.filter_rooms(
+                UserID.from_string(user1_id),
+                {room_id, encrypted_room_id},
+                falsy_filters,
+            )
+        )
+
+        self.assertEqual(falsy_filtered_room_ids, {room_id})