appc,ipn/ipnlocal: add a required event bus to the AppConnector type (#17390)

Require the presence of the bus, but do not use it yet.  Check for required
fields and update tests and production use to plumb the necessary arguments.

Updates #15160
Updates #17192

Change-Id: I8cefd2fdb314ca9945317d3320bd5ea6a92e8dcb
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>

appc/appconnector.go

@@ -22,6 +22,7 @@ import (
 	"tailscale.com/types/views"
 	"tailscale.com/util/clientmetric"
 	"tailscale.com/util/dnsname"
+	"tailscale.com/util/eventbus"
 	"tailscale.com/util/execqueue"
 	"tailscale.com/util/slicesx"
 )
@@ -136,7 +137,9 @@ type RouteInfo struct {
 // routes not yet served by the AppConnector the local node configuration is
 // updated to advertise the new route.
 type AppConnector struct {
+	// These fields are immutable after initialization.
 	logf            logger.Logf
+	eventBus        *eventbus.Bus
 	routeAdvertiser RouteAdvertiser
 
 	// storeRoutesFunc will be called to persist routes if it is not nil.
@@ -168,6 +171,10 @@ type Config struct {
 	// It must be non-nil.
 	Logf logger.Logf
 
+	// EventBus receives events when the collection of routes maintained by the
+	// connector is updated. It must be non-nil.
+	EventBus *eventbus.Bus
+
 	// RouteAdvertiser allows the connector to update the set of advertised routes.
 	// It must be non-nil.
 	RouteAdvertiser RouteAdvertiser
@@ -183,8 +190,18 @@ type Config struct {
 
 // NewAppConnector creates a new AppConnector.
 func NewAppConnector(c Config) *AppConnector {
+	switch {
+	case c.Logf == nil:
+		panic("missing logger")
+	case c.EventBus == nil:
+		panic("missing event bus")
+	case c.RouteAdvertiser == nil:
+		panic("missing route advertiser")
+	}
+
 	ac := &AppConnector{
 		logf:            logger.WithPrefix(c.Logf, "appc: "),
+		eventBus:        c.EventBus,
 		routeAdvertiser: c.RouteAdvertiser,
 		storeRoutesFunc: c.StoreRoutesFunc,
 	}