net/wsconn: accept a remote addr string and plumb it through

This makes wsconn.Conns somewhat present reasonably when they are
the client of an http.Request, rather than just put a placeholder
in that field.

Updates tailscale/corp#13777

Signed-off-by: David Anderson <danderson@tailscale.com>

cmd/derper/websocket.go

@@ -50,7 +50,7 @@ func addWebSocketSupport(s *derp.Server, base http.Handler) http.Handler {
 			return
 		}
 		counterWebSocketAccepts.Add(1)
-		wc := wsconn.NetConn(r.Context(), c, websocket.MessageBinary)
+		wc := wsconn.NetConn(r.Context(), c, websocket.MessageBinary, r.RemoteAddr)
 		brw := bufio.NewReadWriter(bufio.NewReader(wc), bufio.NewWriter(wc))
 		s.Accept(r.Context(), wc, brw, r.RemoteAddr)
 	})

control/controlhttp/client_js.go

@@ -51,7 +51,7 @@ func (d *Dialer) Dial(ctx context.Context) (*ClientConn, error) {
 	if err != nil {
 		return nil, err
 	}
-	netConn := wsconn.NetConn(context.Background(), wsConn, websocket.MessageBinary)
+	netConn := wsconn.NetConn(context.Background(), wsConn, websocket.MessageBinary, wsURL.String())
 	cbConn, err := cont(ctx, netConn)
 	if err != nil {
 		netConn.Close()

control/controlhttp/server.go

@@ -146,7 +146,7 @@ func acceptWebsocket(ctx context.Context, w http.ResponseWriter, r *http.Request
 		return nil, fmt.Errorf("decoding base64 handshake parameter: %v", err)
 	}
 
-	conn := wsconn.NetConn(ctx, c, websocket.MessageBinary)
+	conn := wsconn.NetConn(ctx, c, websocket.MessageBinary, r.RemoteAddr)
 	nc, err := controlbase.Server(ctx, conn, private, init)
 	if err != nil {
 		conn.Close()

derp/derphttp/websocket.go

@@ -27,6 +27,6 @@ func dialWebsocket(ctx context.Context, urlStr string) (net.Conn, error) {
 		return nil, err
 	}
 	log.Printf("websocket: connected to %v", urlStr)
-	netConn := wsconn.NetConn(context.Background(), c, websocket.MessageBinary)
+	netConn := wsconn.NetConn(context.Background(), c, websocket.MessageBinary, urlStr)
 	return netConn, nil
 }

net/wsconn/wsconn.go

@@ -48,10 +48,18 @@ import (
 //
 // A received StatusNormalClosure or StatusGoingAway close frame will be translated to
 // io.EOF when reading.
-func NetConn(ctx context.Context, c *websocket.Conn, msgType websocket.MessageType) net.Conn {
+//
+// The given remoteAddr will be the value of the returned conn's
+// RemoteAddr().String(). For best compatibility with consumers of
+// conns, the string should be an ip:port if available, but in the
+// absence of that it can be any string that describes the remote
+// endpoint, or the empty string to makes RemoteAddr() return a place
+// holder value.
+func NetConn(ctx context.Context, c *websocket.Conn, msgType websocket.MessageType, remoteAddr string) net.Conn {
 	nc := &netConn{
-		c:       c,
-		msgType: msgType,
+		c:          c,
+		msgType:    msgType,
+		remoteAddr: remoteAddr,
 	}
 
 	var writeCancel context.CancelFunc
@@ -82,8 +90,9 @@ func NetConn(ctx context.Context, c *websocket.Conn, msgType websocket.MessageTy
 }
 
 type netConn struct {
-	c       *websocket.Conn
-	msgType websocket.MessageType
+	c          *websocket.Conn
+	msgType    websocket.MessageType
+	remoteAddr string
 
 	writeTimer         *time.Timer
 	writeContext       context.Context
@@ -167,6 +176,7 @@ func (c *netConn) Read(p []byte) (int, error) {
 }
 
 type websocketAddr struct {
+	addr string
 }
 
 func (a websocketAddr) Network() string {
@@ -174,15 +184,18 @@ func (a websocketAddr) Network() string {
 }
 
 func (a websocketAddr) String() string {
+	if a.addr != "" {
+		return a.addr
+	}
 	return "websocket/unknown-addr"
 }
 
 func (c *netConn) RemoteAddr() net.Addr {
-	return websocketAddr{}
+	return websocketAddr{c.remoteAddr}
 }
 
 func (c *netConn) LocalAddr() net.Addr {
-	return websocketAddr{}
+	return websocketAddr{""}
 }
 
 func (c *netConn) SetDeadline(t time.Time) error {