proxy: remove assumption that frontend is a TCPListener

We require the frontend to be a `net.Listener` and the `net.Conn`
connection which are established must be type-switched to
`Conn` to support the `CloseRead` and `CloseWrite` methods.

Signed-off-by: David Scott <dave.scott@docker.com>

alpine/packages/proxy/vendor/libproxy/proxy.go

@@ -24,13 +24,19 @@ type Proxy interface {
 	BackendAddr() net.Addr
 }
 
+
+
 // NewProxy creates a Proxy according to the specified frontendAddr and backendAddr.
 func NewProxy(frontendAddr, backendAddr net.Addr) (Proxy, error) {
 	switch frontendAddr.(type) {
 	case *net.UDPAddr:
 		return NewUDPProxy(frontendAddr.(*net.UDPAddr), backendAddr.(*net.UDPAddr))
 	case *net.TCPAddr:
-		return NewTCPProxy(frontendAddr.(*net.TCPAddr), backendAddr.(*net.TCPAddr))
+		listener, err := net.Listen("tcp", frontendAddr.String())
+		if err != nil {
+			return nil, err
+		}
+		return NewTCPProxy(listener, backendAddr.(*net.TCPAddr))
 	default:
 		panic(fmt.Errorf("Unsupported protocol"))
 	}

alpine/packages/proxy/vendor/libproxy/tcp_proxy.go

@@ -19,22 +19,18 @@ type Conn interface {
 // TCPProxy is a proxy for TCP connections. It implements the Proxy interface to
 // handle TCP traffic forwarding between the frontend and backend addresses.
 type TCPProxy struct {
-	listener     *net.TCPListener
+	listener     net.Listener
-	frontendAddr *net.TCPAddr
+	frontendAddr net.Addr
 	backendAddr  *net.TCPAddr
 }
 
 // NewTCPProxy creates a new TCPProxy.
-func NewTCPProxy(frontendAddr, backendAddr *net.TCPAddr) (*TCPProxy, error) {
+func NewTCPProxy(listener net.Listener, backendAddr *net.TCPAddr) (*TCPProxy, error) {
-	listener, err := net.ListenTCP("tcp", frontendAddr)
-	if err != nil {
-		return nil, err
-	}
 	// If the port in frontendAddr was 0 then ListenTCP will have a picked
 	// a port to listen on, hence the call to Addr to get that actual port:
 	return &TCPProxy{
 		listener:     listener,
-		frontendAddr: listener.Addr().(*net.TCPAddr),
+		frontendAddr: listener.Addr(),
 		backendAddr:  backendAddr,
 	}, nil
 }