fix: apply managed Nginx public entry without service restart

backend/internal/nginxgateway/gateway_test.go

@@ -0,0 +1,42 @@
+package nginxgateway
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestSwapManagedSiteRestoresPreviousOnFailure(t *testing.T) {
+	for _, failAt := range []string{"validate", "reload", "persist"} {
+		t.Run(failAt, func(t *testing.T) {
+			site := "old"
+			validations, reloads := 0, 0
+			write := func(content string) error { site = content; return nil }
+			validate := func() error {
+				validations++
+				if failAt == "validate" && site == "new" {
+					return errors.New("invalid nginx config")
+				}
+				return nil
+			}
+			reload := func() error {
+				reloads++
+				if failAt == "reload" && site == "new" {
+					return errors.New("reload failed")
+				}
+				return nil
+			}
+			persist := func() error {
+				if failAt == "persist" {
+					return errors.New("save failed")
+				}
+				return nil
+			}
+			if err := swapManagedSite("old", "new", persist, write, validate, reload); err == nil {
+				t.Fatal("expected failure")
+			}
+			if site != "old" || validations < 2 || reloads < 1 {
+				t.Fatalf("old site not restored: site=%s validations=%d reloads=%d", site, validations, reloads)
+			}
+		})
+	}
+}