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, 4 insertions, 30 deletions

diff --git a/synapse/state/__init__.py b/synapse/state/__init__.py

index 463ce58dae..c981df3f18 100644
--- a/synapse/state/__init__.py
+++ b/synapse/state/__init__.py
@@ -263,7 +263,9 @@ class StateHandler:
     async def compute_event_context(
         self, event: EventBase, old_state: Optional[Iterable[EventBase]] = None
     ) -> EventContext:
-        """Build an EventContext structure for the event.
+        """Build an EventContext structure for a non-outlier event.
+
+        (for an outlier, call EventContext.for_outlier directly)
 
         This works out what the current state should be for the event, and
         generates a new state group if necessary.
@@ -278,35 +280,7 @@ class StateHandler:
             The event context.
         """
 
-        if event.internal_metadata.is_outlier():
-            # If this is an outlier, then we know it shouldn't have any current
-            # state. Certainly store.get_current_state won't return any, and
-            # persisting the event won't store the state group.
-
-            # FIXME: why do we populate current_state_ids? I thought the point was
-            # that we weren't supposed to have any state for outliers?
-            if old_state:
-                prev_state_ids = {(s.type, s.state_key): s.event_id for s in old_state}
-                if event.is_state():
-                    current_state_ids = dict(prev_state_ids)
-                    key = (event.type, event.state_key)
-                    current_state_ids[key] = event.event_id
-                else:
-                    current_state_ids = prev_state_ids
-            else:
-                current_state_ids = {}
-                prev_state_ids = {}
-
-            # We don't store state for outliers, so we don't generate a state
-            # group for it.
-            context = EventContext.with_state(
-                state_group=None,
-                state_group_before_event=None,
-                current_state_ids=current_state_ids,
-                prev_state_ids=prev_state_ids,
-            )
-
-            return context
+        assert not event.internal_metadata.is_outlier()
 
         #
         # first of all, figure out the state before the event