unversioned: conversion from new to original label selector

pkg/api/unversioned/helpers.go

@@ -64,6 +64,35 @@ func LabelSelectorAsSelector(ps *LabelSelector) (labels.Selector, error) {
 	return selector, nil
 }
 
+// LabelSelectorAsMap converts the LabelSelector api type into a map of strings, ie. the
+// original structure of a label selector. Operators that cannot be converted into plain
+// labels (Exists, DoesNotExist, NotIn, and In with more than one value) will result in
+// an error.
+func LabelSelectorAsMap(ps *LabelSelector) (map[string]string, error) {
+	if ps == nil {
+		return nil, nil
+	}
+	selector := map[string]string{}
+	for k, v := range ps.MatchLabels {
+		selector[k] = v
+	}
+	for _, expr := range ps.MatchExpressions {
+		switch expr.Operator {
+		case LabelSelectorOpIn:
+			if len(expr.Values) != 1 {
+				return selector, fmt.Errorf("operator %q without a single value cannot be converted into the old label selector format", expr.Operator)
+			}
+			// Should we do anything in case this will override a previous key-value pair?
+			selector[expr.Key] = expr.Values[0]
+		case LabelSelectorOpNotIn, LabelSelectorOpExists, LabelSelectorOpDoesNotExist:
+			return selector, fmt.Errorf("operator %q cannot be converted into the old label selector format", expr.Operator)
+		default:
+			return selector, fmt.Errorf("%q is not a valid selector operator", expr.Operator)
+		}
+	}
+	return selector, nil
+}
+
 // ParseToLabelSelector parses a string representing a selector into a LabelSelector object.
 // Note: This function should be kept in sync with the parser in pkg/labels/selector.go
 func ParseToLabelSelector(selector string) (*LabelSelector, error) {