cmd/tsconnect: make terminal resizable

Makes the terminal container DOM node as large as the window (except for
the header) via flexbox. The xterm.js terminal is then sized to fit via
xterm-addon-fit. Once we have a computed rows/columns size, and we can
tell the SSH session of the computed size.

Required introducing an IPNSSHSession type to allow the JS to control
the SSH session once opened. That alse allows us to programatically
close it, which we do when the user closes the window with the session
still active.

I initially wanted to open the terminal in a new window instead (so that
it could be resizable independently of the main window), but xterm.js
does not appear to work well in that mode (possibly because it adds an
IntersectionObserver to pause rendering when the window is not visible,
and it ends up doing that when the parent window is hidden -- see
xtermjs/xterm.js@87dca56dee)

Fixes #5150

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

cmd/tsconnect/wasm/wasm_js.go

@@ -142,11 +142,10 @@ func newIPN(jsConfig js.Value) map[string]any {
 				log.Printf("Usage: ssh(hostname, userName, termConfig)")
 				return nil
 			}
-			go jsIPN.ssh(
+			return jsIPN.ssh(
 				args[0].String(),
 				args[1].String(),
 				args[2])
-			return nil
 		}),
 	}
 }
@@ -256,13 +255,42 @@ func (i *jsIPN) logout() {
 	go i.lb.Logout()
 }
 
-func (i *jsIPN) ssh(host, username string, termConfig js.Value) {
-	writeFn := termConfig.Get("writeFn")
-	setReadFn := termConfig.Get("setReadFn")
-	rows := termConfig.Get("rows").Int()
-	cols := termConfig.Get("cols").Int()
-	onDone := termConfig.Get("onDone")
+func (i *jsIPN) ssh(host, username string, termConfig js.Value) map[string]any {
+	jsSSHSession := &jsSSHSession{
+		jsIPN:      i,
+		host:       host,
+		username:   username,
+		termConfig: termConfig,
+	}
 
+	go jsSSHSession.Run()
+
+	return map[string]any{
+		"close": js.FuncOf(func(this js.Value, args []js.Value) interface{} {
+			return jsSSHSession.Close() != nil
+		}),
+		"resize": js.FuncOf(func(this js.Value, args []js.Value) interface{} {
+			rows := args[0].Int()
+			cols := args[1].Int()
+			return jsSSHSession.Resize(rows, cols) != nil
+		}),
+	}
+}
+
+type jsSSHSession struct {
+	jsIPN      *jsIPN
+	host       string
+	username   string
+	termConfig js.Value
+	session    *ssh.Session
+}
+
+func (s *jsSSHSession) Run() {
+	writeFn := s.termConfig.Get("writeFn")
+	setReadFn := s.termConfig.Get("setReadFn")
+	rows := s.termConfig.Get("rows").Int()
+	cols := s.termConfig.Get("cols").Int()
+	onDone := s.termConfig.Get("onDone")
 	defer onDone.Invoke()
 
 	write := func(s string) {
@@ -274,7 +302,7 @@ func (i *jsIPN) ssh(host, username string, termConfig js.Value) {
 
 	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 	defer cancel()
-	c, err := i.dialer.UserDial(ctx, "tcp", net.JoinHostPort(host, "22"))
+	c, err := s.jsIPN.dialer.UserDial(ctx, "tcp", net.JoinHostPort(s.host, "22"))
 	if err != nil {
 		writeError("Dial", err)
 		return
@@ -283,10 +311,10 @@ func (i *jsIPN) ssh(host, username string, termConfig js.Value) {
 
 	config := &ssh.ClientConfig{
 		HostKeyCallback: ssh.InsecureIgnoreHostKey(),
-		User:            username,
+		User:            s.username,
 	}
 
-	sshConn, _, _, err := ssh.NewClientConn(c, host, config)
+	sshConn, _, _, err := ssh.NewClientConn(c, s.host, config)
 	if err != nil {
 		writeError("SSH Connection", err)
 		return
@@ -302,6 +330,7 @@ func (i *jsIPN) ssh(host, username string, termConfig js.Value) {
 		writeError("SSH Session", err)
 		return
 	}
+	s.session = session
 	write("Session Established\r\n")
 	defer session.Close()
 
@@ -338,11 +367,19 @@ func (i *jsIPN) ssh(host, username string, termConfig js.Value) {
 
 	err = session.Wait()
 	if err != nil {
-		writeError("Exit", err)
+		writeError("Wait", err)
 		return
 	}
 }
 
+func (s *jsSSHSession) Close() error {
+	return s.session.Close()
+}
+
+func (s *jsSSHSession) Resize(rows, cols int) error {
+	return s.session.WindowChange(rows, cols)
+}
+
 type termWriter struct {
 	f js.Value
 }