Add extra group constants and validation to pkg/bootstrap/api.

This adds constants and validation for a new `auth-extra-groups` key on `bootstrap.kubernetes.io/token` secrets. This key allows a bootstrap token to authenticate to extra groups in addition to the `system:bootstrappers` group.

Extra groups are always applied in addition to the `system:bootstrappers` group, must begin with a `system:bootstrappers:` prefix, are limited in length, and are limited to a restricted set of characters (alphanumeric, colons, and dashes without a trailing colon/dash).

pkg/bootstrap/api/helpers.go

@@ -0,0 +1,34 @@
+/*
+Copyright 2017 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package api
+
+import (
+	"fmt"
+	"regexp"
+)
+
+var bootstrapGroupRegexp = regexp.MustCompile(`\A` + BootstrapGroupPattern + `\z`)
+
+// ValidateBootstrapGroupName checks if the provided group name is a valid
+// bootstrap group name. Returns nil if valid or a validation error if invalid.
+// TODO(mattmoyer): this validation should migrate out to client-go (see https://github.com/kubernetes/client-go/issues/114)
+func ValidateBootstrapGroupName(name string) error {
+	if bootstrapGroupRegexp.Match([]byte(name)) {
+		return nil
+	}
+	return fmt.Errorf("bootstrap group %q is invalid (must match %s)", name, BootstrapGroupPattern)
+}