Fix auth.go not finding the tsnet.Listener.

auth.go

@@ -7,7 +7,9 @@ package tscaddy
 
 import (
 	"fmt"
+	"net"
 	"net/http"
+	"reflect"
 	"strings"
 
 	"github.com/caddyserver/caddy/v2"
@@ -40,6 +42,54 @@ func (Auth) CaddyModule() caddy.ModuleInfo {
 	}
 }
 
+// findTsnetListener recursively searches ln for wrapped or embedded net.Listeners
+// until it finds a tsnetListener or runs out.
+// ok indicates if a tsnetListener was found.
+//
+// In the future consider alternative approach if Caddy supports unwrapping listeners.
+// See discussion in https://github.com/tailscale/caddy-tailscale/pull/70
+func findTsnetListener(ln net.Listener) (_ tsnetListener, ok bool) {
+	if ln == nil {
+		return nil, false
+	}
+
+	// if ln is a tsnetListener, return it.
+	if tsn, ok := ln.(tsnetListener); ok {
+		return tsn, true
+	}
+
+	// if ln is a wrappedListener, unwrap it.
+	if wl, ok := ln.(wrappedListener); ok {
+		return findTsnetListener(wl.Unwrap())
+	}
+
+	// if ln has an embedded net.Listener field, unwrap it.
+	s := reflect.ValueOf(ln)
+	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" field is a net.Listener, use it.
+	if wl, ok := innerLn.Interface().(net.Listener); ok {
+		return findTsnetListener(wl)
+	}
+	return nil, false
+}
+
+// wrappedListener is implemented by types that wrap net.Listeners.
+type wrappedListener interface {
+	Unwrap() net.Listener
+}
+
 // 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.
@@ -52,7 +102,7 @@ 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 tsl, ok := listener.(tsnetListener); ok {
+		if tsl, ok := findTsnetListener(listener); ok {
 			var err error
 			ta.localclient, err = tsl.Server().LocalClient()
 			if err != nil {

module.go

@@ -306,6 +306,13 @@ type tsnetServerListener struct {
 	net.Listener
 }
 
+func (t *tsnetServerListener) Unwrap() net.Listener {
+	if t == nil {
+		return nil
+	}
+	return t.Listener
+}
+
 func (t *tsnetServerListener) Close() error {
 	if err := t.Listener.Close(); err != nil {
 		return err