[pseudoIDs] More pseudo ID fixes - Part 2 (#3181)

Fixes include:
- Translating state keys that contain user IDs to their respective room
keys for both querying and sending state events
- **NOTE**: there may be design discussion needed on what should happen
when sender keys cannot be found for users
- A simple fix for kicking guests from rooms properly
- Logic for boundary history visibilities was slightly off (I'm
surprised this only manifested in pseudo ID room versions)

Signed-off-by: `Sam Wedgwood <sam@wedgwood.dev>`

syncapi/synctypes/clientevent.go

@@ -16,6 +16,8 @@
 package synctypes
 
 import (
+	"fmt"
+
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/matrix-org/gomatrixserverlib/spec"
 )
@@ -118,3 +120,31 @@ func ToClientEventDefault(userIDQuery spec.UserIDForSender, event gomatrixserver
 	}
 	return ToClientEvent(event, FormatAll, sender, sk)
 }
+
+// If provided state key is a user ID (state keys beginning with @ are reserved for this purpose)
+// fetch it's associated sender ID and use that instead. Otherwise returns the same state key back.
+//
+// # This function either returns the state key that should be used, or an error
+//
+// TODO: handle failure cases better (e.g. no sender ID)
+func FromClientStateKey(roomID spec.RoomID, stateKey string, senderIDQuery spec.SenderIDForUser) (*string, error) {
+	if len(stateKey) >= 1 && stateKey[0] == '@' {
+		parsedStateKey, err := spec.NewUserID(stateKey, true)
+		if err != nil {
+			// If invalid user ID, then there is no associated state event.
+			return nil, fmt.Errorf("Provided state key begins with @ but is not a valid user ID: %s", err.Error())
+		}
+		senderID, err := senderIDQuery(roomID, *parsedStateKey)
+		if err != nil {
+			return nil, fmt.Errorf("Failed to query sender ID: %s", err.Error())
+		}
+		if senderID == nil {
+			// If no sender ID, then there is no associated state event.
+			return nil, fmt.Errorf("No associated sender ID found.")
+		}
+		newStateKey := string(*senderID)
+		return &newStateKey, nil
+	} else {
+		return &stateKey, nil
+	}
+}