Add ConnectionMetrics to expose session ticket/PSK usage

common.go

@@ -235,6 +235,13 @@ const (
 // include downgrade canaries even if it's using its highers supported version.
 var testingOnlyForceDowngradeCanary bool
 
+// ConnectionMetrics contains basic metrics about the connection.
+type ConnectionMetrics struct {
+	// ClientSentTicket is true if the client has sent a TLS 1.2 session ticket
+	// or a TLS 1.3 PSK in the ClientHello successfully.
+	ClientSentTicket bool
+}
+
 // ConnectionState records basic TLS details about the connection.
 type ConnectionState struct {
 	// Version is the TLS version used by the connection (e.g. VersionTLS12).

conn.go

@@ -46,6 +46,7 @@ type Conn struct {
 	// zero or one.
 	handshakes       int
 	extMasterSecret  bool
+	clientSentTicket bool // whether the client sent a session ticket or a PSK in the Client Hello
 	didResume        bool // whether this connection was a session resumption
 	didHRR           bool // whether a HelloRetryRequest was sent/received
 	cipherSuite      uint16
@@ -1699,3 +1700,12 @@ func (c *Conn) VerifyHostname(host string) error {
 	}
 	return c.peerCertificates[0].VerifyHostname(host)
 }
+
+// ConnectionMetrics returns basic metrics about the connection.
+func (c *Conn) ConnectionMetrics() ConnectionMetrics {
+	c.handshakeMutex.Lock()
+	defer c.handshakeMutex.Unlock()
+	var metrics ConnectionMetrics
+	metrics.ClientSentTicket = c.clientSentTicket
+	return metrics
+}

u_handshake_client.go

@@ -494,6 +494,11 @@ func (c *UConn) clientHandshake(ctx context.Context) (err error) {
 		return err
 	}
 
+	// Client sent a session ticket or PSK.
+	if session != nil {
+		c.clientSentTicket = true
+	}
+
 	if hello.earlyData {
 		suite := cipherSuiteTLS13ByID(session.cipherSuite)
 		transcript := suite.hash.New()