crypto/tls: report TLS version in ConnectionState.

Fixes #7231.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/68250043

src/pkg/crypto/tls/common.go

@@ -155,6 +155,7 @@ var supportedClientCertSignatureAlgorithms = []signatureAndHash{
 
 // ConnectionState records basic TLS details about the connection.
 type ConnectionState struct {
+	Version                    uint16                // TLS version used by the connection (e.g. VersionTLS12)
 	HandshakeComplete          bool                  // TLS handshake is complete
 	DidResume                  bool                  // connection resumes a previous TLS connection
 	CipherSuite                uint16                // cipher suite in use (TLS_RSA_WITH_RC4_128_SHA, ...)

src/pkg/crypto/tls/conn.go

@@ -969,6 +969,7 @@ func (c *Conn) ConnectionState() ConnectionState {
 	var state ConnectionState
 	state.HandshakeComplete = c.handshakeComplete
 	if c.handshakeComplete {
+		state.Version = c.vers
 		state.NegotiatedProtocol = c.clientProtocol
 		state.DidResume = c.didResume
 		state.NegotiatedProtocolIsMutual = !c.clientProtocolFallback

src/pkg/crypto/tls/handshake_server_test.go

@@ -195,6 +195,23 @@ func testHandshake(clientConfig, serverConfig *Config) (state ConnectionState, e
 	return
 }
 
+func TestVersion(t *testing.T) {
+	serverConfig := &Config{
+		Certificates: testConfig.Certificates,
+		MaxVersion:   VersionTLS11,
+	}
+	clientConfig := &Config{
+		InsecureSkipVerify: true,
+	}
+	state, err := testHandshake(clientConfig, serverConfig)
+	if err != nil {
+		t.Fatalf("handshake failed: %s", err)
+	}
+	if state.Version != VersionTLS11 {
+		t.Fatalf("Incorrect version %x, should be %x", state.Version, VersionTLS11)
+	}
+}
+
 func TestCipherSuitePreference(t *testing.T) {
 	serverConfig := &Config{
 		CipherSuites: []uint16{TLS_RSA_WITH_RC4_128_SHA, TLS_RSA_WITH_AES_128_CBC_SHA, TLS_ECDHE_RSA_WITH_RC4_128_SHA},