fix(core): take over agent, reuse callbacks

- Kill running agents before replace
- Retry copy with backoff while locked
- Reuse syscall callbacks at package scope
- Add callback regression test

lib/startup/startup.go

@@ -45,23 +45,24 @@ func SyncInstalledPath() error {
 }
 
 func Disable() error {
+	var regErr error
 	k, err := registry.OpenKey(registry.CURRENT_USER, config.StartupRunKey, registry.SET_VALUE)
 	if err != nil {
-		if errors.Is(err, registry.ErrNotExist) {
-			return nil
+		if !errors.Is(err, registry.ErrNotExist) {
+			regErr = err
+		}
+	} else {
+		err = k.DeleteValue(config.StartupValueName)
+		_ = k.Close()
+		if err != nil && !errors.Is(err, registry.ErrNotExist) {
+			regErr = err
 		}
-		return err
 	}
-	defer k.Close()
-	err = k.DeleteValue(config.StartupValueName)
-	if errors.Is(err, registry.ErrNotExist) {
-		_ = disableWatchdog()
-		return nil
+	taskErr := disableWatchdog()
+	if regErr != nil {
+		return regErr
 	}
-	if err != nil {
-		return err
-	}
-	return disableWatchdog()
+	return taskErr
 }
 
 func installedCommandLine() (string, error) {