code: public function join(): Promise
      {
          if (!$this->handle) {
              throw new StatusError("Process has not been started.");
          }
  
          return $this->processRunner->join($this->handle);
      }
docs: Wait for the process to end.
  
  @return Promise <int> Succeeds with process exit code or fails with a ProcessException if the process is killed.
  
  @throws StatusError If the process has already been started.