appc: add flag shouldStoreRoutes and controlknob for it

When an app connector is reconfigured and domains to route are removed,
we would like to no longer advertise routes that were discovered for
those domains. In order to do this we plan to store which routes were
discovered for which domains.

Add a controlknob so that we can enable/disable the new behavior.

Updates #11008
Signed-off-by: Fran Bull <fran@tailscale.com>

appc/appconnector.go

@@ -62,6 +62,9 @@ type AppConnector struct {
 	logf            logger.Logf
 	routeAdvertiser RouteAdvertiser
 
+	// storeRoutesFunc will be called to persist routes if it is not nil.
+	storeRoutesFunc func(*RouteInfo) error
+
 	// mu guards the fields that follow
 	mu sync.Mutex
 
@@ -80,11 +83,24 @@ type AppConnector struct {
 }
 
 // NewAppConnector creates a new AppConnector.
-func NewAppConnector(logf logger.Logf, routeAdvertiser RouteAdvertiser) *AppConnector {
-	return &AppConnector{
+func NewAppConnector(logf logger.Logf, routeAdvertiser RouteAdvertiser, routeInfo *RouteInfo, storeRoutesFunc func(*RouteInfo) error) *AppConnector {
+	ac := &AppConnector{
 		logf:            logger.WithPrefix(logf, "appc: "),
 		routeAdvertiser: routeAdvertiser,
+		storeRoutesFunc: storeRoutesFunc,
 	}
+	if routeInfo != nil {
+		ac.domains = routeInfo.Domains
+		ac.wildcards = routeInfo.Wildcards
+		ac.controlRoutes = routeInfo.Control
+	}
+	return ac
+}
+
+// ShouldStoreRoutes returns true if the appconnector was created with the controlknob on
+// and is storing its discovered routes persistently.
+func (e *AppConnector) ShouldStoreRoutes() bool {
+	return e.storeRoutesFunc != nil
 }
 
 // UpdateDomainsAndRoutes starts an asynchronous update of the configuration