On client CloseWrite, do CloseWrite instead of Close for backend

Co-authored-by: Mathieu Lonjaret <mathieu.lonjaret@gmail.com>

pkg/tcp/handler.go

@@ -6,14 +6,23 @@ import (
 
 // Handler is the TCP Handlers interface
 type Handler interface {
-	ServeTCP(conn net.Conn)
+	ServeTCP(conn WriteCloser)
 }
 
 // The HandlerFunc type is an adapter to allow the use of
 // ordinary functions as handlers.
-type HandlerFunc func(conn net.Conn)
+type HandlerFunc func(conn WriteCloser)
 
 // ServeTCP serves tcp
-func (f HandlerFunc) ServeTCP(conn net.Conn) {
+func (f HandlerFunc) ServeTCP(conn WriteCloser) {
 	f(conn)
 }
+
+// WriteCloser describes a net.Conn with a CloseWrite method.
+type WriteCloser interface {
+	net.Conn
+	// CloseWrite on a network connection, indicates that the issuer of the call
+	// has terminated sending on that connection.
+	// It corresponds to sending a FIN packet.
+	CloseWrite() error
+}