Optimise checking other servers allowed to see events (#2596)

* Try optimising checking if server is allowed to see event

* Fix error

* Handle case where snapshot NID is 0

* Fix query

* Update SQL

* Clean up `CheckServerAllowedToSeeEvent`

* Not supported on SQLite

* Maybe placate the unit tests

* Review comments

1 files changed, 23 insertions, 0 deletions

diff --git a/roomserver/state/state.go b/roomserver/state/state.go
index d1d24b09..ca0c69f2 100644
--- a/roomserver/state/state.go
+++ b/roomserver/state/state.go
@@ -124,6 +124,29 @@ func (v *StateResolution) LoadStateAtEvent(
 	return stateEntries, nil
 }
 
+// LoadStateAtEvent loads the full state of a room before a particular event.
+func (v *StateResolution) LoadStateAtEventForHistoryVisibility(
+	ctx context.Context, eventID string,
+) ([]types.StateEntry, error) {
+	span, ctx := opentracing.StartSpanFromContext(ctx, "StateResolution.LoadStateAtEvent")
+	defer span.Finish()
+
+	snapshotNID, err := v.db.SnapshotNIDFromEventID(ctx, eventID)
+	if err != nil {
+		return nil, fmt.Errorf("LoadStateAtEvent.SnapshotNIDFromEventID failed for event %s : %w", eventID, err)
+	}
+	if snapshotNID == 0 {
+		return nil, fmt.Errorf("LoadStateAtEvent.SnapshotNIDFromEventID(%s) returned 0 NID, was this event stored?", eventID)
+	}
+
+	stateEntries, err := v.LoadStateAtSnapshot(ctx, snapshotNID)
+	if err != nil {
+		return nil, err
+	}
+
+	return stateEntries, nil
+}
+
 // LoadCombinedStateAfterEvents loads a snapshot of the state after each of the events
 // and combines those snapshots together into a single list. At this point it is
 // possible to run into duplicate (type, state key) tuples.