appc: fix a deadlock in route advertisements (#15031)

`routeAdvertiser` is the `iplocal.LocalBackend`. Calls to
`Advertise/UnadvertiseRoute` end up calling `EditPrefs` which in turn
calls `authReconfig` which finally calls `readvertiseAppConnectorRoutes`
which calls `AppConnector.DomainRoutes` and gets stuck on a mutex that
was already held when `routeAdvertiser` was called.

Make all calls to `routeAdvertiser` in `app.AppConnector` go through the
execqueue instead as a short-term fix.

Updates tailscale/corp#25965

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>
Co-authored-by: Irbe Krumina <irbe@tailscale.com>

appc/appctest/appctest.go

@@ -11,12 +11,22 @@ import (
 
 // RouteCollector is a test helper that collects the list of routes advertised
 type RouteCollector struct {
+	// AdvertiseCallback (optional) is called synchronously from
+	// AdvertiseRoute.
+	AdvertiseCallback func()
+	// UnadvertiseCallback (optional) is called synchronously from
+	// UnadvertiseRoute.
+	UnadvertiseCallback func()
+
 	routes        []netip.Prefix
 	removedRoutes []netip.Prefix
 }
 
 func (rc *RouteCollector) AdvertiseRoute(pfx ...netip.Prefix) error {
 	rc.routes = append(rc.routes, pfx...)
+	if rc.AdvertiseCallback != nil {
+		rc.AdvertiseCallback()
+	}
 	return nil
 }
 
@@ -30,6 +40,9 @@ func (rc *RouteCollector) UnadvertiseRoute(toRemove ...netip.Prefix) error {
 			rc.removedRoutes = append(rc.removedRoutes, r)
 		}
 	}
+	if rc.UnadvertiseCallback != nil {
+		rc.UnadvertiseCallback()
+	}
 	return nil
 }