auth: a new auth token provider nop

This commit adds a new auth token provider named nop. The nop provider
refuses every Authenticate() request so CN based authentication can
only be allowed. If the tokenOpts parameter of auth.NewTokenProvider()
is empty, the provider will be used.

auth/nop.go

@@ -0,0 +1,35 @@
+// Copyright 2018 The etcd 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 auth
+
+import (
+	"context"
+)
+
+type tokenNop struct{}
+
+func (t *tokenNop) enable()                         {}
+func (t *tokenNop) disable()                        {}
+func (t *tokenNop) invalidateUser(string)           {}
+func (t *tokenNop) genTokenPrefix() (string, error) { return "", nil }
+func (t *tokenNop) info(ctx context.Context, token string, rev uint64) (*AuthInfo, bool) {
+	return nil, false
+}
+func (t *tokenNop) assign(ctx context.Context, username string, revision uint64) (string, error) {
+	return "", ErrAuthFailed
+}
+func newTokenProviderNop() (*tokenNop, error) {
+	return &tokenNop{}, nil
+}

auth/store.go

@@ -1055,6 +1055,8 @@ func NewTokenProvider(tokenOpts string, indexWaiter func(uint64) <-chan struct{}
 		return newTokenProviderSimple(indexWaiter), nil
 	case "jwt":
 		return newTokenProviderJWT(typeSpecificOpts)
+	case "":
+		return newTokenProviderNop()
 	default:
 		plog.Errorf("unknown token type: %s", tokenType)
 		return nil, ErrInvalidAuthOpts