Factor out a separate `EventContext.for_outlier` (#10883)

Constructing an EventContext for an outlier is actually really simple, and
there's no sense in going via an `async` method in the `StateHandler`.

This also means that we can resolve a bunch of FIXMEs.

1 files changed, 10 insertions, 4 deletions

diff --git a/synapse/events/snapshot.py b/synapse/events/snapshot.py
index f8d898c3b1..5ba01eeef9 100644
--- a/synapse/events/snapshot.py
+++ b/synapse/events/snapshot.py
@@ -80,9 +80,7 @@ class EventContext:
 
             (type, state_key) -> event_id
 
-            FIXME: what is this for an outlier? it seems ill-defined. It seems like
-            it could be either {}, or the state we were given by the remote
-            server, depending on $THINGS
+            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
@@ -96,7 +94,7 @@ class EventContext:
 
             (type, state_key) -> event_id
 
-            FIXME: again, what is this for an outlier?
+            For an outlier, this is {}
 
             As with _current_state_ids, this is a private attribute. It should be
             accessed via get_prev_state_ids.
@@ -130,6 +128,14 @@ class EventContext:
             delta_ids=delta_ids,
         )
 
+    @staticmethod
+    def for_outlier():
+        """Return an EventContext instance suitable for persisting an outlier event"""
+        return EventContext(
+            current_state_ids={},
+            prev_state_ids={},
+        )
+
     async def serialize(self, event: EventBase, store: "DataStore") -> dict:
         """Converts self to a type that can be serialized as JSON, and then
         deserialized by `deserialize`