Fix shadowtls server detection

transport/shadowtls/hash.go

@@ -34,19 +34,25 @@ func (c *HashReadConn) Sum() []byte {
 
 type HashWriteConn struct {
 	net.Conn
-	hmac hash.Hash
+	hmac       hash.Hash
+	hasContent bool
+	lastSum    []byte
 }
 
 func NewHashWriteConn(conn net.Conn, password string) *HashWriteConn {
 	return &HashWriteConn{
-		conn,
-		hmac.New(sha1.New, []byte(password)),
+		Conn: conn,
+		hmac: hmac.New(sha1.New, []byte(password)),
 	}
 }
 
 func (c *HashWriteConn) Write(p []byte) (n int, err error) {
 	if c.hmac != nil {
+		if c.hasContent {
+			c.lastSum = c.Sum()
+		}
 		c.hmac.Write(p)
+		c.hasContent = true
 	}
 	return c.Conn.Write(p)
 }
@@ -55,6 +61,14 @@ func (c *HashWriteConn) Sum() []byte {
 	return c.hmac.Sum(nil)[:8]
 }
 
+func (c *HashWriteConn) LastSum() []byte {
+	return c.lastSum
+}
+
 func (c *HashWriteConn) Fallback() {
 	c.hmac = nil
 }
+
+func (c *HashWriteConn) HasContent() bool {
+	return c.hasContent
+}