fix error source implementing stringer interface with fallback to def…

…ault if not set

backend/error_source.go

@@ -21,10 +21,20 @@ const (
 	DefaultErrorSource ErrorSource = ErrorSourcePlugin
 )
 
+// IsValid return true if es is ErrorSourceDownstream or ErrorSourcePlugin.
 func (es ErrorSource) IsValid() bool {
 	return es == ErrorSourceDownstream || es == ErrorSourcePlugin
 }
 
+// String returns the string representation of es. If es is not valid, DefaultErrorSource is returned.
+func (es ErrorSource) String() string {
+	if !es.IsValid() {
+		return string(DefaultErrorSource)
+	}
+
+	return string(es)
+}
+
 // ErrorSourceFromStatus returns an [ErrorSource] based on provided HTTP status code.
 func ErrorSourceFromHTTPStatus(statusCode int) ErrorSource {
 	switch statusCode {

backend/error_source_test.go

@@ -17,8 +17,11 @@ import (
 func TestErrorSource(t *testing.T) {
 	var es ErrorSource
 	require.False(t, es.IsValid())
+	require.Equal(t, "plugin", es.String())
 	require.True(t, ErrorSourceDownstream.IsValid())
+	require.Equal(t, "downstream", ErrorSourceDownstream.String())
 	require.True(t, ErrorSourcePlugin.IsValid())
+	require.Equal(t, "plugin", ErrorSourcePlugin.String())
 }
 
 func TestIsDownstreamError(t *testing.T) {