cmd/tsconnect: add basic panic handling

The go wasm process exiting is a sign of an unhandled panic. Also
add a explicit recover() call in the notify callback, that's where most
logic bugs are likely to happen (and they may not be fatal).

Also fixes the one panic that was encountered (nill pointer dereference
when generating the JS view of the netmap).

Fixes #5132

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

cmd/tsconnect/wasm/wasm_js.go

@@ -114,6 +114,7 @@ func newIPN(jsConfig js.Value) map[string]any {
 					notifyState(state: int): void,
 					notifyNetMap(netMap: object): void,
 					notifyBrowseToURL(url: string): void,
+					notifyPanicRecover(err: string): void,
 				})`)
 				return nil
 			}
@@ -166,6 +167,16 @@ func (i *jsIPN) run(jsCallbacks js.Value) {
 	notifyState(ipn.NoState)
 
 	i.lb.SetNotifyCallback(func(n ipn.Notify) {
+		// Panics in the notify callback are likely due to be due to bugs in
+		// this bridging module (as opposed to actual bugs in Tailscale) and
+		// thus may be recoverable. Let the UI know, and allow the user to
+		// choose if they want to reload the page.
+		defer func() {
+			if r := recover(); r != nil {
+				fmt.Println("Panic recovered:", r)
+				jsCallbacks.Call("notifyPanicRecover", fmt.Sprint(r))
+			}
+		}()
 		log.Printf("NOTIFY: %+v", n)
 		if n.State != nil {
 			notifyState(*n.State)
@@ -189,7 +200,7 @@ func (i *jsIPN) run(jsCallbacks js.Value) {
 							MachineKey: p.Machine.String(),
 							NodeKey:    p.Key.String(),
 						},
-						Online:              *p.Online,
+						Online:              p.Online,
 						TailscaleSSHEnabled: p.Hostinfo.TailscaleSSHEnabled(),
 					}
 				}),
@@ -352,8 +363,8 @@ type jsNetMapSelfNode struct {
 
 type jsNetMapPeerNode struct {
 	jsNetMapNode
-	Online              bool `json:"online"`
-	TailscaleSSHEnabled bool `json:"tailscaleSSHEnabled"`
+	Online              *bool `json:"online,omitempty"`
+	TailscaleSSHEnabled bool  `json:"tailscaleSSHEnabled"`
 }
 
 type jsStateStore struct {