Don't re-run state resolution on a single trusted state snapshot (#1526)

* Don't re-run state resolution on a single trusted state snapshot

* Lint

* Check if backward extremity is create event before checking missing state

federationapi/routing/send.go

@@ -476,6 +476,7 @@ func checkAllowedByState(e gomatrixserverlib.Event, stateEvents []gomatrixserver
 	return gomatrixserverlib.Allowed(e, &authUsingState)
 }
 
+// nolint:gocyclo
 func (t *txnReq) processEventWithMissingState(ctx context.Context, e gomatrixserverlib.Event, roomVersion gomatrixserverlib.RoomVersion) error {
 	// Do this with a fresh context, so that we keep working even if the
 	// original request times out. With any luck, by the time the remote
@@ -513,36 +514,71 @@ func (t *txnReq) processEventWithMissingState(ctx context.Context, e gomatrixser
 	backwardsExtremity := &newEvents[0]
 	newEvents = newEvents[1:]
 
+	type respState struct {
+		// A snapshot is considered trustworthy if it came from our own roomserver.
+		// That's because the state will have been through state resolution once
+		// already in QueryStateAfterEvent.
+		trustworthy bool
+		*gomatrixserverlib.RespState
+	}
+
 	// at this point we know we're going to have a gap: we need to work out the room state at the new backwards extremity.
 	// Therefore, we cannot just query /state_ids with this event to get the state before. Instead, we need to query
 	// the state AFTER all the prev_events for this event, then apply state resolution to that to get the state before the event.
-	var states []*gomatrixserverlib.RespState
+	var states []*respState
 	for _, prevEventID := range backwardsExtremity.PrevEventIDs() {
 		// Look up what the state is after the backward extremity. This will either
 		// come from the roomserver, if we know all the required events, or it will
 		// come from a remote server via /state_ids if not.
-		var prevState *gomatrixserverlib.RespState
-		prevState, err = t.lookupStateAfterEvent(gmectx, roomVersion, backwardsExtremity.RoomID(), prevEventID)
-		if err != nil {
-			util.GetLogger(ctx).WithError(err).Errorf("Failed to lookup state after prev_event: %s", prevEventID)
-			return err
+		prevState, trustworthy, lerr := t.lookupStateAfterEvent(gmectx, roomVersion, backwardsExtremity.RoomID(), prevEventID)
+		if lerr != nil {
+			util.GetLogger(ctx).WithError(lerr).Errorf("Failed to lookup state after prev_event: %s", prevEventID)
+			return lerr
 		}
 		// Append the state onto the collected state. We'll run this through the
 		// state resolution next.
-		states = append(states, prevState)
+		states = append(states, &respState{trustworthy, prevState})
 	}
 
 	// Now that we have collected all of the state from the prev_events, we'll
 	// run the state through the appropriate state resolution algorithm for the
-	// room. This does a couple of things:
+	// room if needed. This does a couple of things:
 	// 1. Ensures that the state is deduplicated fully for each state-key tuple
 	// 2. Ensures that we pick the latest events from both sets, in the case that
 	//    one of the prev_events is quite a bit older than the others
-	resolvedState, err := t.resolveStatesAndCheck(gmectx, roomVersion, states, backwardsExtremity)
+	resolvedState := &gomatrixserverlib.RespState{}
+	switch len(states) {
+	case 0:
+		extremityIsCreate := backwardsExtremity.Type() == gomatrixserverlib.MRoomCreate && backwardsExtremity.StateKeyEquals("")
+		if !extremityIsCreate {
+			// There are no previous states and this isn't the beginning of the
+			// room - this is an error condition!
+			util.GetLogger(ctx).Errorf("Failed to lookup any state after prev_events")
+			return fmt.Errorf("expected %d states but got %d", len(backwardsExtremity.PrevEventIDs()), len(states))
+		}
+	case 1:
+		// There's only one previous state - if it's trustworthy (came from a
+		// local state snapshot which will already have been through state res),
+		// use it as-is. There's no point in resolving it again.
+		if states[0].trustworthy {
+			resolvedState = states[0].RespState
+			break
+		}
+		// Otherwise, if it isn't trustworthy (came from federation), run it through
+		// state resolution anyway for safety, in case there are duplicates.
+		fallthrough
+	default:
+		respStates := make([]*gomatrixserverlib.RespState, len(states))
+		for i := range states {
+			respStates[i] = states[i].RespState
+		}
+		// There's more than one previous state - run them all through state res
+		resolvedState, err = t.resolveStatesAndCheck(gmectx, roomVersion, respStates, backwardsExtremity)
 		if err != nil {
 			util.GetLogger(ctx).WithError(err).Errorf("Failed to resolve state conflicts for event %s", backwardsExtremity.EventID())
 			return err
 		}
+	}
 
 	// First of all, send the backward extremity into the roomserver with the
 	// newly resolved state. This marks the "oldest" point in the backfill and
@@ -581,16 +617,16 @@ func (t *txnReq) processEventWithMissingState(ctx context.Context, e gomatrixser
 
 // lookupStateAfterEvent returns the room state after `eventID`, which is the state before eventID with the state of `eventID` (if it's a state event)
 // added into the mix.
-func (t *txnReq) lookupStateAfterEvent(ctx context.Context, roomVersion gomatrixserverlib.RoomVersion, roomID, eventID string) (*gomatrixserverlib.RespState, error) {
+func (t *txnReq) lookupStateAfterEvent(ctx context.Context, roomVersion gomatrixserverlib.RoomVersion, roomID, eventID string) (*gomatrixserverlib.RespState, bool, error) {
 	// try doing all this locally before we resort to querying federation
 	respState := t.lookupStateAfterEventLocally(ctx, roomID, eventID)
 	if respState != nil {
-		return respState, nil
+		return respState, true, nil
 	}
 
 	respState, err := t.lookupStateBeforeEvent(ctx, roomVersion, roomID, eventID)
 	if err != nil {
-		return nil, fmt.Errorf("t.lookupStateBeforeEvent: %w", err)
+		return nil, false, fmt.Errorf("t.lookupStateBeforeEvent: %w", err)
 	}
 
 	servers := t.getServers(ctx, roomID)
@@ -602,11 +638,11 @@ func (t *txnReq) lookupStateAfterEvent(ctx context.Context, roomVersion gomatrix
 	h, err := t.lookupEvent(ctx, roomVersion, eventID, false, servers)
 	switch err.(type) {
 	case verifySigError:
-		return respState, nil
+		return respState, false, nil
 	case nil:
 		// do nothing
 	default:
-		return nil, fmt.Errorf("t.lookupEvent: %w", err)
+		return nil, false, fmt.Errorf("t.lookupEvent: %w", err)
 	}
 	t.haveEvents[h.EventID()] = h
 	if h.StateKey() != nil {
@@ -624,7 +660,7 @@ func (t *txnReq) lookupStateAfterEvent(ctx context.Context, roomVersion gomatrix
 		}
 	}
 
-	return respState, nil
+	return respState, false, nil
 }
 
 func (t *txnReq) lookupStateAfterEventLocally(ctx context.Context, roomID, eventID string) *gomatrixserverlib.RespState {