cmd/tailscaled, ipn/ipnlocal, wgengine: shutdown tailscaled if wgdevice is closed

Tailscaled becomes inoperative if the Tailscale Tunnel wintun adapter is abruptly removed.
wireguard-go closes the device in case of a read error, but tailscaled keeps running.
This adds detection of a closed WireGuard device, triggering a graceful shutdown of tailscaled.
It is then restarted by the tailscaled watchdog service process.

Fixes #11222

Signed-off-by: Nick Khyl <nickk@tailscale.com>

wgengine/userspace.go

@@ -425,6 +425,21 @@ func NewUserspaceEngine(logf logger.Logf, conf Config) (_ Engine, reterr error)
 		}
 	}()
 
+	go func() {
+		select {
+		case <-e.wgdev.Wait():
+			e.mu.Lock()
+			closing := e.closing
+			e.mu.Unlock()
+			if !closing {
+				e.logf("Closing the engine because the WireGuard device has been closed...")
+				e.Close()
+			}
+		case <-e.waitCh:
+			// continue
+		}
+	}()
+
 	e.logf("Bringing WireGuard device up...")
 	if err := e.wgdev.Up(); err != nil {
 		return nil, fmt.Errorf("wgdev.Up: %w", err)
@@ -1112,8 +1127,8 @@ func (e *userspaceEngine) Close() {
 	}
 }
 
-func (e *userspaceEngine) Wait() {
-	<-e.waitCh
+func (e *userspaceEngine) Done() <-chan struct{} {
+	return e.waitCh
 }
 
 func (e *userspaceEngine) linkChange(delta *netmon.ChangeDelta) {

wgengine/watchdog.go

@@ -150,8 +150,8 @@ func (e *watchdogEngine) PeerForIP(ip netip.Addr) (ret PeerForIP, ok bool) {
 	return ret, ok
 }
 
-func (e *watchdogEngine) Wait() {
-	e.wrap.Wait()
+func (e *watchdogEngine) Done() <-chan struct{} {
+	return e.wrap.Done()
 }
 
 func (e *watchdogEngine) InstallCaptureHook(cb capture.Callback) {

wgengine/wgengine.go

@@ -89,10 +89,11 @@ type Engine interface {
 	// new Engine.
 	Close()
 
-	// Wait waits until the Engine's Close method is called or the
-	// engine aborts with an error. You don't have to call this.
-	// TODO: return an error?
-	Wait()
+	// Done returns a channel that is closed when the Engine's
+	// Close method is called, the engine aborts with an error,
+	// or it shuts down due to the closure of the underlying device.
+	// You don't have to call this.
+	Done() <-chan struct{}
 
 	// SetNetworkMap informs the engine of the latest network map
 	// from the server. The network map's DERPMap field should be