Add test for nil senderID check

eventauth_test.go

@@ -20,6 +20,7 @@ import (
 	"testing"
 
 	"github.com/matrix-org/gomatrixserverlib/spec"
+	"github.com/stretchr/testify/assert"
 )
 
 func stateNeededEquals(a, b StateNeeded) bool {
@@ -1116,6 +1117,39 @@ func TestDemoteUserDefaultPowerLevelBelowOwn(t *testing.T) {
 	}
 }
 
+func NilUserIDForBadSenderTest(roomID spec.RoomID, senderID spec.SenderID) (*spec.UserID, error) {
+	if senderID == "@baduser" {
+		return nil, nil
+	}
+
+	return spec.NewUserID(string(senderID), true)
+}
+
+func TestPowerLevelCheckShouldNotPanic(t *testing.T) {
+	powerChangeShouldSucceed, err := MustGetRoomVersion(RoomVersionV1).NewEventFromTrustedJSON(spec.RawJSON(`{
+		"type": "m.room.power_levels",
+		"state_key": "",
+		"sender": "@u1:a",
+		"room_id": "!r1:a",
+		"event_id": "$e5:a",
+		"content": {
+			"users_default": 50,
+			"users": {
+				"@baduser": 0
+			},
+			"redact": 100
+		}
+	}`), false)
+	if err != nil {
+		t.Fatal(err)
+	}
+	assert.NotPanics(t, func() {
+		if err := Allowed(powerChangeShouldSucceed, powerLevelTestRoom, NilUserIDForBadSenderTest); err == nil {
+			panic("Event should not be allowed")
+		}
+	}, "")
+}
+
 func TestPromoteUserDefaultLevelAboveOwn(t *testing.T) {
 	// User shouldn't be able to promote the user default
 	// level above their own effective level.

eventcontent.go

@@ -76,6 +76,10 @@ func NewCreateContentFromAuthEvents(authEvents AuthEventProvider, userIDForSende
 		err = errorf("invalid sender userID: %s", err.Error())
 		return
 	}
+	if sender == nil {
+		err = errorf("userID not found for sender: %s in room %s", createEvent.SenderID(), createEvent.RoomID().String())
+		return
+	}
 	c.senderDomain = string(sender.Domain())
 	return
 }