You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: cert_authenticator_test.go
    path of file: ./repos/istio/security/pkg/server/ca/authenticate
    
    the code of the file until where you have to start completion: // Copyright Istio 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 authenticate

import (
	"context"
	"crypto/tls"
	"crypto/x509"
	"crypto/x509/pkix"
	"reflect"
	"testing"

	"google.golang.org/grpc/cr
    
func TestAuthenticate_clientCertAuthenticator(t *testing.T) {
	callerID := "test.identity"
	ids := []util.Identity{
		{Type: util.TypeURI, Value: []byte(callerID)},
	}
	sanExt, err := util.BuildSANExtension(ids)
	if err != nil {
		t.Error(err)
	}

	testCases := map[string]struct {
		certChain          [][]*x509.Certificate
		caller             *security.Caller
		authenticateErrMsg string
		fakeAuthInfo       *mockAuthInfo
	}{
		"No client certificate": {
			certChain:          nil,
			caller:             nil,
			authenticateErrMsg: "no client certificate is presented",
		},
		"Unsupported auth type": {
			certChain:          nil,
			caller:             nil,
			authenticateErrMsg: "unsupported auth type: \"not-tls\"",
			fakeAuthInfo:       &mockAuthInfo{"not-tls"},
		},
		"Empty cert chain": {
			certChain:          [][]*x509.Certificate{},
			caller:             nil,
			authenticateErrMsg: "no verified chain is found",
		},
		"Certificate has no SAN": {
			certChain: [][]*x509.Certificate{
				{
					{
						Version: 1,
					},
				},
			},
			authenticateErrMsg: "the SAN extension does not exist",
		},
		"With client certificate": {
			certChain: [][]*x509.Certificate{
				{
					{
						Extensions: []pkix.Extension{*sanExt},
					},
				},
			},
			caller: &security.Caller{Identities: []string{callerID}},
		},
	}

	auth := &ClientCertAuthenticator{}

	for id, tc := range testCases {
		ctx := context.Background()
		if tc.certChain != nil {
			tlsInfo := credentials.TLSInfo{
				State: tls.ConnectionState{VerifiedChains: tc.certChain},
			}
			p := &peer.Peer{AuthInfo: tlsInfo}
			ctx = peer.NewContext(ctx, p)
		}
		if tc.fakeAuthInfo != nil {
			ctx = peer.NewContext(ctx, &peer.Peer{AuthInfo: tc.fakeAuthInfo})
		}

		result, err := auth.Authenticate(security.AuthContext{GrpcContext: ctx})
		if len(tc.authenticateErrMsg) > 0 {
			if err == nil {
				t.Errorf("Case %s: Succeeded. Error expected: %v", id, err)
			} else if err.Error() != tc.authenticateErrMsg {
				t.Errorf("Case %s: Incorrect error message: want %s but got %s",
					id, tc.authenticateErrMsg, err.Error())
			}
			continue
		} else if err != nil {
			t.Fatalf("Case %s: Unexpected Error: %v", id, err)
		}

		if !reflect.DeepEqual(tc.caller, result) {
			t.Errorf("Case %q: Unexpected authentication result: want %v but got %v",
				id, tc.caller, result)
		}
	}
}