bugfix: Fix a bug which caused failures to join rooms over federation (#917)

* bugfix: Fix a bug which caused failures to join rooms over federation

The cause of this was the semantics of `/send_join`'s `auth_chain` response.

Previously, we would only send back the auth chain *for the join event* and
not the entire room state. However, we would then try to check that the
room state is valid, and then be missing auth events.

Now, we send back the entire auth chain for all room state in `/send_join`.
The spec needs to be clarified that this is what the chain should be.

* refactor: split out grabbing state to reduce cyclo complexity

roomserver/query/query.go

@@ -637,12 +637,6 @@ func (r *RoomserverQueryAPI) QueryStateAndAuthChain(
 	request *api.QueryStateAndAuthChainRequest,
 	response *api.QueryStateAndAuthChainResponse,
 ) error {
-	// TODO: get the correct room version
-	roomState, err := state.GetStateResolutionAlgorithm(state.StateResolutionAlgorithmV1, r.DB)
-	if err != nil {
-		return err
-	}
-
 	response.QueryStateAndAuthChainRequest = *request
 	roomNID, err := r.DB.RoomNID(ctx, request.RoomID)
 	if err != nil {
@@ -653,31 +647,21 @@ func (r *RoomserverQueryAPI) QueryStateAndAuthChain(
 	}
 	response.RoomExists = true
 
-	prevStates, err := r.DB.StateAtEventIDs(ctx, request.PrevEventIDs)
+	stateEvents, err := r.loadStateAtEventIDs(ctx, request.PrevEventIDs)
 	if err != nil {
-		switch err.(type) {
-		case types.MissingEventError:
-			return nil
-		default:
 		return err
 	}
-	}
 	response.PrevEventsExist = true
 
-	// Look up the currrent state for the requested tuples.
+	// add the auth event IDs for the current state events too
-	stateEntries, err := roomState.LoadCombinedStateAfterEvents(
+	var authEventIDs []string
-		ctx, prevStates,
+	authEventIDs = append(authEventIDs, request.AuthEventIDs...)
-	)
+	for _, se := range stateEvents {
-	if err != nil {
+		authEventIDs = append(authEventIDs, se.AuthEventIDs()...)
-		return err
 	}
+	authEventIDs = util.UniqueStrings(authEventIDs) // de-dupe
 
-	stateEvents, err := r.loadStateEvents(ctx, stateEntries)
+	authEvents, err := getAuthChain(ctx, r.DB, authEventIDs)
-	if err != nil {
-		return err
-	}
-
-	authEvents, err := getAuthChain(ctx, r.DB, request.AuthEventIDs)
 	if err != nil {
 		return err
 	}
@@ -699,6 +683,34 @@ func (r *RoomserverQueryAPI) QueryStateAndAuthChain(
 	return err
 }
 
+func (r *RoomserverQueryAPI) loadStateAtEventIDs(ctx context.Context, eventIDs []string) ([]gomatrixserverlib.Event, error) {
+	// TODO: get the correct room version
+	roomState, err := state.GetStateResolutionAlgorithm(state.StateResolutionAlgorithmV1, r.DB)
+	if err != nil {
+		return nil, err
+	}
+
+	prevStates, err := r.DB.StateAtEventIDs(ctx, eventIDs)
+	if err != nil {
+		switch err.(type) {
+		case types.MissingEventError:
+			return nil, nil
+		default:
+			return nil, err
+		}
+	}
+
+	// Look up the currrent state for the requested tuples.
+	stateEntries, err := roomState.LoadCombinedStateAfterEvents(
+		ctx, prevStates,
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	return r.loadStateEvents(ctx, stateEntries)
+}
+
 // getAuthChain fetches the auth chain for the given auth events. An auth chain
 // is the list of all events that are referenced in the auth_events section, and
 // all their auth_events, recursively. The returned set of events contain the