tsconnect: add flag to specify control server (#5294)

To improve the local development experience, this change allows a
control url to be passed in with the `--dev-control=` flag.

If the flag is passed in when not specifying dev, an error is returned.

If no flag is passed, the default remains the Tailscale controlled
control server set by `ipn.DefaultControlURL`.

Co-authored-by: Maisem Ali <maisem@tailscale.com>
Signed-off-by: Charlotte Brandhorst-Satzkorn <charlotte@tailscale.com>

cmd/tsconnect/wasm/wasm_js.go

@@ -42,6 +42,9 @@ import (
 	"tailscale.com/words"
 )
 
+// ControlURL defines the URL to be used for connection to Control.
+var ControlURL = ipn.DefaultControlURL
+
 func main() {
 	js.Global().Set("newIPN", js.FuncOf(func(this js.Value, args []js.Value) interface{} {
 		if len(args) != 1 {
@@ -244,7 +247,7 @@ func (i *jsIPN) run(jsCallbacks js.Value) {
 		err := i.lb.Start(ipn.Options{
 			StateKey: "wasm",
 			UpdatePrefs: &ipn.Prefs{
-				ControlURL:       ipn.DefaultControlURL,
+				ControlURL:       ControlURL,
 				RouteAll:         false,
 				AllowSingleHosts: true,
 				WantRunning:      true,