appc: factor app connector arguments into a Config type (#17389)

Replace the positional arguments to NewAppConnector with a Config struct.
Update the existing uses. Other than the API change, there are no functional
changes in this commit.

Updates #15160
Updates #17192

Change-Id: Ibf37f021372155a4db8aaf738f4b4f2c746bf623
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>

appc/appconnector.go

@@ -162,17 +162,36 @@ type AppConnector struct {
 	writeRateDay    *rateLogger
 }
 
+// Config carries the settings for an [AppConnector].
+type Config struct {
+	// Logf is the logger to which debug logs from the connector will be sent.
+	// It must be non-nil.
+	Logf logger.Logf
+
+	// RouteAdvertiser allows the connector to update the set of advertised routes.
+	// It must be non-nil.
+	RouteAdvertiser RouteAdvertiser
+
+	// RouteInfo, if non-nil, use used as the initial set of routes for the
+	// connector.  If nil, the connector starts empty.
+	RouteInfo *RouteInfo
+
+	// StoreRoutesFunc, if non-nil, is called when the connector's routes
+	// change, to allow the routes to be persisted.
+	StoreRoutesFunc func(*RouteInfo) error
+}
+
 // NewAppConnector creates a new AppConnector.
-func NewAppConnector(logf logger.Logf, routeAdvertiser RouteAdvertiser, routeInfo *RouteInfo, storeRoutesFunc func(*RouteInfo) error) *AppConnector {
+func NewAppConnector(c Config) *AppConnector {
 	ac := &AppConnector{
-		logf:            logger.WithPrefix(logf, "appc: "),
-		routeAdvertiser: routeAdvertiser,
-		storeRoutesFunc: storeRoutesFunc,
+		logf:            logger.WithPrefix(c.Logf, "appc: "),
+		routeAdvertiser: c.RouteAdvertiser,
+		storeRoutesFunc: c.StoreRoutesFunc,
 	}
-	if routeInfo != nil {
-		ac.domains = routeInfo.Domains
-		ac.wildcards = routeInfo.Wildcards
-		ac.controlRoutes = routeInfo.Control
+	if c.RouteInfo != nil {
+		ac.domains = c.RouteInfo.Domains
+		ac.wildcards = c.RouteInfo.Wildcards
+		ac.controlRoutes = c.RouteInfo.Control
 	}
 	ac.writeRateMinute = newRateLogger(time.Now, time.Minute, func(c int64, s time.Time, l int64) {
 		ac.logf("routeInfo write rate: %d in minute starting at %v (%d routes)", c, s, l)