Create singletons for StateFilter.{all,none}() (#11836)

No point recreating these for each call, since they are frozen

Author: richvdh

changelog.d/11836.misc

@@ -0,0 +1 @@
+Minor performance improvement in room state lookup.

synapse/storage/state.py

@@ -74,21 +74,21 @@ def __attrs_post_init__(self):
 
     @staticmethod
     def all() -> "StateFilter":
-        """Creates a filter that fetches everything.
+        """Returns a filter that fetches everything.
 
         Returns:
-            The new state filter.
+            The state filter.
         """
-        return StateFilter(types=frozendict(), include_others=True)
+        return _ALL_STATE_FILTER
 
     @staticmethod
     def none() -> "StateFilter":
-        """Creates a filter that fetches nothing.
+        """Returns a filter that fetches nothing.
 
         Returns:
             The new state filter.
         """
-        return StateFilter(types=frozendict(), include_others=False)
+        return _NONE_STATE_FILTER
 
     @staticmethod
     def from_types(types: Iterable[Tuple[str, Optional[str]]]) -> "StateFilter":
@@ -527,6 +527,10 @@ def approx_difference(self, other: "StateFilter") -> "StateFilter":
         )
 
 
+_ALL_STATE_FILTER = StateFilter(types=frozendict(), include_others=True)
+_NONE_STATE_FILTER = StateFilter(types=frozendict(), include_others=False)
+
+
 class StateGroupStorage:
     """High level interface to fetching state for event."""