1 files changed, 32 insertions, 13 deletions

diff --git a/synapse/storage/state.py b/synapse/storage/state.py
index e5400d681a..c76529cb57 100644
--- a/synapse/storage/state.py
+++ b/synapse/storage/state.py
@@ -25,12 +25,15 @@ from typing import (
 )
 
 import attr
+from frozendict import frozendict
 
 from synapse.api.constants import EventTypes
 from synapse.events import EventBase
 from synapse.types import MutableStateMap, StateMap
 
 if TYPE_CHECKING:
+    from typing import FrozenSet  # noqa: used within quoted type hint; flake8 sad
+
     from synapse.server import HomeServer
     from synapse.storage.databases import Databases
 
@@ -40,7 +43,7 @@ logger = logging.getLogger(__name__)
 T = TypeVar("T")
 
 
-@attr.s(slots=True)
+@attr.s(slots=True, frozen=True)
 class StateFilter:
     """A filter used when querying for state.
 
@@ -53,14 +56,19 @@ class StateFilter:
             appear in `types`.
     """
 
-    types = attr.ib(type=Dict[str, Optional[Set[str]]])
+    types = attr.ib(type="frozendict[str, Optional[FrozenSet[str]]]")
     include_others = attr.ib(default=False, type=bool)
 
     def __attrs_post_init__(self):
         # If `include_others` is set we canonicalise the filter by removing
         # wildcards from the types dictionary
         if self.include_others:
-            self.types = {k: v for k, v in self.types.items() if v is not None}
+            # this is needed to work around the fact that StateFilter is frozen
+            object.__setattr__(
+                self,
+                "types",
+                frozendict({k: v for k, v in self.types.items() if v is not None}),
+            )
 
     @staticmethod
     def all() -> "StateFilter":
@@ -69,7 +77,7 @@ class StateFilter:
         Returns:
             The new state filter.
         """
-        return StateFilter(types={}, include_others=True)
+        return StateFilter(types=frozendict(), include_others=True)
 
     @staticmethod
     def none() -> "StateFilter":
@@ -78,7 +86,7 @@ class StateFilter:
         Returns:
             The new state filter.
         """
-        return StateFilter(types={}, include_others=False)
+        return StateFilter(types=frozendict(), include_others=False)
 
     @staticmethod
     def from_types(types: Iterable[Tuple[str, Optional[str]]]) -> "StateFilter":
@@ -103,7 +111,12 @@ class StateFilter:
 
             type_dict.setdefault(typ, set()).add(s)  # type: ignore
 
-        return StateFilter(types=type_dict)
+        return StateFilter(
+            types=frozendict(
+                (k, frozenset(v) if v is not None else None)
+                for k, v in type_dict.items()
+            )
+        )
 
     @staticmethod
     def from_lazy_load_member_list(members: Iterable[str]) -> "StateFilter":
@@ -116,7 +129,10 @@ class StateFilter:
         Returns:
             The new state filter
         """
-        return StateFilter(types={EventTypes.Member: set(members)}, include_others=True)
+        return StateFilter(
+            types=frozendict({EventTypes.Member: frozenset(members)}),
+            include_others=True,
+        )
 
     def return_expanded(self) -> "StateFilter":
         """Creates a new StateFilter where type wild cards have been removed
@@ -173,7 +189,7 @@ class StateFilter:
             # We want to return all non-members, but only particular
             # memberships
             return StateFilter(
-                types={EventTypes.Member: self.types[EventTypes.Member]},
+                types=frozendict({EventTypes.Member: self.types[EventTypes.Member]}),
                 include_others=True,
             )
 
@@ -245,14 +261,15 @@ class StateFilter:
 
         return len(self.concrete_types())
 
-    def filter_state(self, state_dict: StateMap[T]) -> StateMap[T]:
-        """Returns the state filtered with by this StateFilter
+    def filter_state(self, state_dict: StateMap[T]) -> MutableStateMap[T]:
+        """Returns the state filtered with by this StateFilter.
 
         Args:
             state: The state map to filter
 
         Returns:
-            The filtered state map
+            The filtered state map.
+            This is a copy, so it's safe to mutate.
         """
         if self.is_full():
             return dict(state_dict)
@@ -324,14 +341,16 @@ class StateFilter:
             if state_keys is None:
                 member_filter = StateFilter.all()
             else:
-                member_filter = StateFilter({EventTypes.Member: state_keys})
+                member_filter = StateFilter(frozendict({EventTypes.Member: state_keys}))
         elif self.include_others:
             member_filter = StateFilter.all()
         else:
             member_filter = StateFilter.none()
 
         non_member_filter = StateFilter(
-            types={k: v for k, v in self.types.items() if k != EventTypes.Member},
+            types=frozendict(
+                {k: v for k, v in self.types.items() if k != EventTypes.Member}
+            ),
             include_others=self.include_others,
         )