Fix #2084 - incorrect /event_auth response (#2085)

* Fix #2084

* Return early

* Linting

2 files changed, 19 insertions, 0 deletions

diff --git a/roomserver/api/query.go b/roomserver/api/query.go
index 599156bb..28321715 100644
--- a/roomserver/api/query.go
+++ b/roomserver/api/query.go
@@ -226,6 +226,10 @@ type QueryStateAndAuthChainRequest struct {
 	PrevEventIDs []string `json:"prev_event_ids"`
 	// The list of auth events for the event. Used to calculate the auth chain
 	AuthEventIDs []string `json:"auth_event_ids"`
+	// If true, the auth chain events for the auth event IDs given will be fetched only. Prev event IDs are ignored.
+	// If false, state and auth chain events for the prev event IDs and entire current state will be included.
+	// TODO: not a great API shape. It serves 2 main uses: false=>response for send_join, true=>response for /event_auth
+	OnlyFetchAuthChain bool `json:"only_fetch_auth_chain"`
 	// Should state resolution be ran on the result events?
 	// TODO: check call sites and remove if we always want to do state res
 	ResolveState bool `json:"resolve_state"`
diff --git a/roomserver/internal/query/query.go b/roomserver/internal/query/query.go
index b80f08ab..28b140c7 100644
--- a/roomserver/internal/query/query.go
+++ b/roomserver/internal/query/query.go
@@ -457,6 +457,21 @@ func (r *Queryer) QueryStateAndAuthChain(
 	response.RoomExists = true
 	response.RoomVersion = info.RoomVersion
 
+	// handle this entirely separately to the other case so we don't have to pull out
+	// the entire current state of the room
+	// TODO: this probably means it should be a different query operation...
+	if request.OnlyFetchAuthChain {
+		var authEvents []*gomatrixserverlib.Event
+		authEvents, err = GetAuthChain(ctx, r.DB.EventsFromIDs, request.AuthEventIDs)
+		if err != nil {
+			return err
+		}
+		for _, event := range authEvents {
+			response.AuthChainEvents = append(response.AuthChainEvents, event.Headered(info.RoomVersion))
+		}
+		return nil
+	}
+
 	var stateEvents []*gomatrixserverlib.Event
 	stateEvents, err = r.loadStateAtEventIDs(ctx, *info, request.PrevEventIDs)
 	if err != nil {