Add a validation helper function.

I will use this in a subsequent PR as part of #12298

pkg/api/validation/validation.go

@@ -84,6 +84,20 @@ func ValidateLabels(labels map[string]string, fldPath *field.Path) field.ErrorLi
 	return allErrs
 }
 
+// ValidateHasLabel requires that api.ObjectMeta has a Label with key and expectedValue
+func ValidateHasLabel(meta api.ObjectMeta, fldPath *field.Path, key, expectedValue string) field.ErrorList {
+	allErrs := field.ErrorList{}
+	actualValue, found := meta.Labels[key]
+	if !found {
+		allErrs = append(allErrs, field.Required(fldPath.Child("labels"), key+"="+expectedValue))
+		return allErrs
+	}
+	if actualValue != expectedValue {
+		allErrs = append(allErrs, field.Invalid(fldPath.Child("labels"), meta.Labels, "expected "+key+"="+expectedValue))
+	}
+	return allErrs
+}
+
 // ValidateAnnotations validates that a set of annotations are correctly defined.
 func ValidateAnnotations(annotations map[string]string, fldPath *field.Path) field.ErrorList {
 	allErrs := field.ErrorList{}

pkg/api/validation/validation_test.go

@@ -5009,3 +5009,40 @@ func TestValidateConfigMapUpdate(t *testing.T) {
 		}
 	}
 }
+
+func TestValidateHasLabel(t *testing.T) {
+	successCase := api.ObjectMeta{
+		Name:      "123",
+		Namespace: "ns",
+		Labels: map[string]string{
+			"other": "blah",
+			"foo":   "bar",
+		},
+	}
+	if errs := ValidateHasLabel(successCase, field.NewPath("field"), "foo", "bar"); len(errs) != 0 {
+		t.Errorf("expected success: %v", errs)
+	}
+
+	missingCase := api.ObjectMeta{
+		Name:      "123",
+		Namespace: "ns",
+		Labels: map[string]string{
+			"other": "blah",
+		},
+	}
+	if errs := ValidateHasLabel(missingCase, field.NewPath("field"), "foo", "bar"); len(errs) == 0 {
+		t.Errorf("expected failure")
+	}
+
+	wrongValueCase := api.ObjectMeta{
+		Name:      "123",
+		Namespace: "ns",
+		Labels: map[string]string{
+			"other": "blah",
+			"foo":   "notbar",
+		},
+	}
+	if errs := ValidateHasLabel(wrongValueCase, field.NewPath("field"), "foo", "bar"); len(errs) == 0 {
+		t.Errorf("expected failure")
+	}
+}