Use homeserver URL field also as a filter field

src/backend/backend.py

@@ -84,6 +84,8 @@ class Backend:
             - `("<user_id>", "<room_id>", "filtered_members")`:
               See `models.special_models.FilteredMembers` docstring
 
+            - `("filtered_homeservers")`:
+              See `models.special_models.FilteredHomeservers` docstring
 
         clients: A `{user_id: MatrixClient}` dict for the logged-in clients
             we managed. Every client is logged to one matrix account.

src/backend/models/model_store.py

@@ -6,7 +6,9 @@ from typing import Dict
 
 from . import SyncId
 from .model import Model
-from .special_models import AllRooms, FilteredMembers, MatchingAccounts
+from .special_models import (
+    AllRooms, FilteredMembers, FilteredHomeservers, MatchingAccounts,
+)
 
 
 @dataclass(frozen=True)
@@ -36,6 +38,8 @@ class ModelStore(UserDict):
             model = AllRooms(self["accounts"])
         elif key == "matching_accounts":
             model = MatchingAccounts(self["all_rooms"])
+        elif key == "filtered_homeservers":
+            model = FilteredHomeservers()
         elif is_tuple and len(key) == 3 and key[2] == "filtered_members":
             model = FilteredMembers(user_id=key[0], room_id=key[1])
         else:

src/backend/models/special_models.py

@@ -115,3 +115,14 @@ class FilteredMembers(FieldSubstringFilter):
 
     def accept_source(self, source: Model) -> bool:
         return source.sync_id == (self.user_id, self.room_id, "members")
+
+
+class FilteredHomeservers(FieldSubstringFilter):
+    """Filtered list of public Matrix homeservers."""
+
+    def __init__(self) -> None:
+        super().__init__(sync_id="filtered_homeservers", fields=("id",))
+
+
+    def accept_source(self, source: Model) -> bool:
+        return source.sync_id == "homeservers"