Limit filter to `limit/2` for before/after events on `/context` (#3332)

Part of https://github.com/matrix-org/dendrite/issues/3224

syncapi/routing/context.go

@@ -110,6 +110,7 @@ func Context(
 	}
 
 	stateFilter := synctypes.StateFilter{
+		Limit:                   filter.Limit,
 		NotSenders:              filter.NotSenders,
 		NotTypes:                filter.NotTypes,
 		Senders:                 filter.Senders,
@@ -157,6 +158,11 @@ func Context(
 		}
 	}
 
+	// Limit is split up for before/after events
+	if filter.Limit > 1 {
+		filter.Limit = filter.Limit / 2
+	}
+
 	eventsBefore, err := snapshot.SelectContextBeforeEvent(ctx, id, roomID, filter)
 	if err != nil && err != sql.ErrNoRows {
 		logrus.WithError(err).Error("unable to fetch before events")

syncapi/syncapi_test.go

@@ -1136,7 +1136,7 @@ func testContext(t *testing.T, dbType test.DBType) {
 		},
 		{
 			name:             "events are not limited",
-			wantBeforeLength: 7,
+			wantBeforeLength: 5,
 		},
 		{
 			name: "all events are limited",