auth.go: find tsnet.Listener using reflection

Signed-off-by: hello <[email protected]>

auth.go

@@ -7,7 +7,9 @@ package tscaddy
 
 import (
 	"fmt"
+	"net"
 	"net/http"
+	"reflect"
 	"strings"
 
 	"github.com/caddyserver/caddy/v2"
@@ -40,6 +42,39 @@ func (Auth) CaddyModule() caddy.ModuleInfo {
 	}
 }
 
+// findTsnetListener recursively searches ln for embedded child net.Listener structs
+// until it finds the tsnetListener or runs out.
+// the 2nd return value is true if it was found, false if it wasn't.
+func findTsnetListener(ln net.Listener) (tsnetListener, bool) {
+	// if the input is a tsnetListener, return it.
+	if tsn, ok := ln.(tsnetListener); ok {
+		return tsn, true
+	}
+
+	s := reflect.ValueOf(ln)
+	// make sure s is a struct instead of a pointer to one
+	if s.Kind() == reflect.Ptr {
+		s = s.Elem()
+	}
+
+	if s.Kind() != reflect.Struct {
+		return nil, false
+	}
+
+	innerLn := s.FieldByName("Listener")
+	if innerLn.IsZero() {
+		// no more child/embedded listeners left
+		return nil, false
+	}
+
+	// if the "Listener" child is a net.Listener, run the function again on its child.
+	if child, ok := innerLn.Interface().(net.Listener); ok {
+		return findTsnetListener(child)
+	}
+
+	return nil, false
+}
+
 // client returns the tailscale LocalClient for the TailscaleAuth module.
 // If the LocalClient has not already been configured, the provided request will be used to
 // lookup the tailscale node that serviced the request, and get the associated LocalClient.
@@ -52,13 +87,11 @@ func (ta *Auth) client(r *http.Request) (*tailscale.LocalClient, error) {
 	// server.
 	server := r.Context().Value(caddyhttp.ServerCtxKey).(*caddyhttp.Server)
 	for _, listener := range server.Listeners() {
-		if tsServerListener, ok := listener.(*tsnetServerListener); ok {
-			if tsl, ok := tsServerListener.Listener.(tsnetListener); ok {
-				var err error
-				ta.localclient, err = tsl.Server().LocalClient()
-				if err != nil {
-					return nil, err
-				}
+		if tsl, ok := findTsnetListener(listener); ok {
+			var err error
+			ta.localclient, err = tsl.Server().LocalClient()
+			if err != nil {
+				return nil, err
 			}
 		}
 	}