Refactor `EventContext` (#12689)

Refactor how the `EventContext` class works, with the intention of reducing the amount of state we fetch from the DB during event processing.

The idea here is to get rid of the cached `current_state_ids` and `prev_state_ids` that live in the `EventContext`, and instead defer straight to the database (and its caching). 

One change that may have a noticeable effect is that we now no longer prefill the `get_current_state_ids` cache on a state change. However, that query is relatively light, since its just a case of reading a table from the DB (unlike fetching state at an event which is more heavyweight). For deployments with workers this cache isn't even used.


Part of #12684

1 files changed, 32 insertions, 145 deletions

diff --git a/synapse/events/snapshot.py b/synapse/events/snapshot.py
index 8120c305df..9ccd24b298 100644
--- a/synapse/events/snapshot.py
+++ b/synapse/events/snapshot.py
@@ -17,11 +17,8 @@ import attr
 from frozendict import frozendict
 from typing_extensions import Literal
 
-from twisted.internet.defer import Deferred
-
 from synapse.appservice import ApplicationService
 from synapse.events import EventBase
-from synapse.logging.context import make_deferred_yieldable, run_in_background
 from synapse.types import JsonDict, StateMap
 
 if TYPE_CHECKING:
@@ -61,6 +58,9 @@ class EventContext:
             If ``state_group`` is None (ie, the event is an outlier),
             ``state_group_before_event`` will always also be ``None``.
 
+        state_delta_due_to_event: If `state_group` and `state_group_before_event` are not None
+            then this is the delta of the state between the two groups.
+
         prev_group: If it is known, ``state_group``'s prev_group. Note that this being
             None does not necessarily mean that ``state_group`` does not have
             a prev_group!
@@ -79,73 +79,47 @@ class EventContext:
         app_service: If this event is being sent by a (local) application service, that
             app service.
 
-        _current_state_ids: The room state map, including this event - ie, the state
-            in ``state_group``.
-
-            (type, state_key) -> event_id
-
-            For an outlier, this is {}
-
-            Note that this is a private attribute: it should be accessed via
-            ``get_current_state_ids``. _AsyncEventContext impl calculates this
-            on-demand: it will be None until that happens.
-
-        _prev_state_ids: The room state map, excluding this event - ie, the state
-            in ``state_group_before_event``. For a non-state
-            event, this will be the same as _current_state_events.
-
-            Note that it is a completely different thing to prev_group!
-
-            (type, state_key) -> event_id
-
-            For an outlier, this is {}
-
-            As with _current_state_ids, this is a private attribute. It should be
-            accessed via get_prev_state_ids.
-
         partial_state: if True, we may be storing this event with a temporary,
             incomplete state.
     """
 
+    _storage: "Storage"
     rejected: Union[Literal[False], str] = False
     _state_group: Optional[int] = None
     state_group_before_event: Optional[int] = None
+    _state_delta_due_to_event: Optional[StateMap[str]] = None
     prev_group: Optional[int] = None
     delta_ids: Optional[StateMap[str]] = None
     app_service: Optional[ApplicationService] = None
 
-    _current_state_ids: Optional[StateMap[str]] = None
-    _prev_state_ids: Optional[StateMap[str]] = None
-
     partial_state: bool = False
 
     @staticmethod
     def with_state(
+        storage: "Storage",
         state_group: Optional[int],
         state_group_before_event: Optional[int],
-        current_state_ids: Optional[StateMap[str]],
-        prev_state_ids: Optional[StateMap[str]],
+        state_delta_due_to_event: Optional[StateMap[str]],
         partial_state: bool,
         prev_group: Optional[int] = None,
         delta_ids: Optional[StateMap[str]] = None,
     ) -> "EventContext":
         return EventContext(
-            current_state_ids=current_state_ids,
-            prev_state_ids=prev_state_ids,
+            storage=storage,
             state_group=state_group,
             state_group_before_event=state_group_before_event,
+            state_delta_due_to_event=state_delta_due_to_event,
             prev_group=prev_group,
             delta_ids=delta_ids,
             partial_state=partial_state,
         )
 
     @staticmethod
-    def for_outlier() -> "EventContext":
+    def for_outlier(
+        storage: "Storage",
+    ) -> "EventContext":
         """Return an EventContext instance suitable for persisting an outlier event"""
-        return EventContext(
-            current_state_ids={},
-            prev_state_ids={},
-        )
+        return EventContext(storage=storage)
 
     async def serialize(self, event: EventBase, store: "DataStore") -> JsonDict:
         """Converts self to a type that can be serialized as JSON, and then
@@ -158,24 +132,14 @@ class EventContext:
             The serialized event.
         """
 
-        # We don't serialize the full state dicts, instead they get pulled out
-        # of the DB on the other side. However, the other side can't figure out
-        # the prev_state_ids, so if we're a state event we include the event
-        # id that we replaced in the state.
-        if event.is_state():
-            prev_state_ids = await self.get_prev_state_ids()
-            prev_state_id = prev_state_ids.get((event.type, event.state_key))
-        else:
-            prev_state_id = None
-
         return {
-            "prev_state_id": prev_state_id,
-            "event_type": event.type,
-            "event_state_key": event.get_state_key(),
             "state_group": self._state_group,
             "state_group_before_event": self.state_group_before_event,
             "rejected": self.rejected,
             "prev_group": self.prev_group,
+            "state_delta_due_to_event": _encode_state_dict(
+                self._state_delta_due_to_event
+            ),
             "delta_ids": _encode_state_dict(self.delta_ids),
             "app_service_id": self.app_service.id if self.app_service else None,
             "partial_state": self.partial_state,
@@ -193,16 +157,16 @@ class EventContext:
         Returns:
             The event context.
         """
-        context = _AsyncEventContextImpl(
+        context = EventContext(
             # We use the state_group and prev_state_id stuff to pull the
             # current_state_ids out of the DB and construct prev_state_ids.
             storage=storage,
-            prev_state_id=input["prev_state_id"],
-            event_type=input["event_type"],
-            event_state_key=input["event_state_key"],
             state_group=input["state_group"],
             state_group_before_event=input["state_group_before_event"],
             prev_group=input["prev_group"],
+            state_delta_due_to_event=_decode_state_dict(
+                input["state_delta_due_to_event"]
+            ),
             delta_ids=_decode_state_dict(input["delta_ids"]),
             rejected=input["rejected"],
             partial_state=input.get("partial_state", False),
@@ -250,8 +214,15 @@ class EventContext:
         if self.rejected:
             raise RuntimeError("Attempt to access state_ids of rejected event")
 
-        await self._ensure_fetched()
-        return self._current_state_ids
+        assert self._state_delta_due_to_event is not None
+
+        prev_state_ids = await self.get_prev_state_ids()
+
+        if self._state_delta_due_to_event:
+            prev_state_ids = dict(prev_state_ids)
+            prev_state_ids.update(self._state_delta_due_to_event)
+
+        return prev_state_ids
 
     async def get_prev_state_ids(self) -> StateMap[str]:
         """
@@ -266,94 +237,10 @@ class EventContext:
             Maps a (type, state_key) to the event ID of the state event matching
             this tuple.
         """
-        await self._ensure_fetched()
-        # There *should* be previous state IDs now.
-        assert self._prev_state_ids is not None
-        return self._prev_state_ids
-
-    def get_cached_current_state_ids(self) -> Optional[StateMap[str]]:
-        """Gets the current state IDs if we have them already cached.
-
-        It is an error to access this for a rejected event, since rejected state should
-        not make it into the room state. This method will raise an exception if
-        ``rejected`` is set.
-
-        Returns:
-            Returns None if we haven't cached the state or if state_group is None
-            (which happens when the associated event is an outlier).
-
-            Otherwise, returns the the current state IDs.
-        """
-        if self.rejected:
-            raise RuntimeError("Attempt to access state_ids of rejected event")
-
-        return self._current_state_ids
-
-    async def _ensure_fetched(self) -> None:
-        return None
-
-
-@attr.s(slots=True)
-class _AsyncEventContextImpl(EventContext):
-    """
-    An implementation of EventContext which fetches _current_state_ids and
-    _prev_state_ids from the database on demand.
-
-    Attributes:
-
-        _storage
-
-        _fetching_state_deferred: Resolves when *_state_ids have been calculated.
-            None if we haven't started calculating yet
-
-        _event_type: The type of the event the context is associated with.
-
-        _event_state_key: The state_key of the event the context is associated with.
-
-        _prev_state_id: If the event associated with the context is a state event,
-            then `_prev_state_id` is the event_id of the state that was replaced.
-    """
-
-    # This needs to have a default as we're inheriting
-    _storage: "Storage" = attr.ib(default=None)
-    _prev_state_id: Optional[str] = attr.ib(default=None)
-    _event_type: str = attr.ib(default=None)
-    _event_state_key: Optional[str] = attr.ib(default=None)
-    _fetching_state_deferred: Optional["Deferred[None]"] = attr.ib(default=None)
-
-    async def _ensure_fetched(self) -> None:
-        if not self._fetching_state_deferred:
-            self._fetching_state_deferred = run_in_background(self._fill_out_state)
-
-        await make_deferred_yieldable(self._fetching_state_deferred)
-
-    async def _fill_out_state(self) -> None:
-        """Called to populate the _current_state_ids and _prev_state_ids
-        attributes by loading from the database.
-        """
-        if self.state_group is None:
-            # No state group means the event is an outlier. Usually the state_ids dicts are also
-            # pre-set to empty dicts, but they get reset when the context is serialized, so set
-            # them to empty dicts again here.
-            self._current_state_ids = {}
-            self._prev_state_ids = {}
-            return
-
-        current_state_ids = await self._storage.state.get_state_ids_for_group(
-            self.state_group
+        assert self.state_group_before_event is not None
+        return await self._storage.state.get_state_ids_for_group(
+            self.state_group_before_event
         )
-        # Set this separately so mypy knows current_state_ids is not None.
-        self._current_state_ids = current_state_ids
-        if self._event_state_key is not None:
-            self._prev_state_ids = dict(current_state_ids)
-
-            key = (self._event_type, self._event_state_key)
-            if self._prev_state_id:
-                self._prev_state_ids[key] = self._prev_state_id
-            else:
-                self._prev_state_ids.pop(key, None)
-        else:
-            self._prev_state_ids = current_state_ids
 
 
 def _encode_state_dict(