driver.go

func New(root string, options []string) (driver Driver, err error) {
  for _, name := range []string{os.Getenv("DOCKER_DRIVER"), DefaultDriver} {
    if name != "" {
      return GetDriver(name, root, options)
    }
  }

  // Check for priority drivers first
  for _, name := range priority {
    driver, err = GetDriver(name, root, options)
    if err != nil {
      if err == ErrNotSupported || err == ErrPrerequisites || err == ErrIncompatibleFS {
        continue
      }
      return nil, err
    }
    checkPriorDriver(name, root)
    return driver, nil
  }

  // Check all registered drivers if no priority driver is found
  for name, initFunc := range drivers {
    if driver, err = initFunc(root, options); err != nil {
      if err == ErrNotSupported || err == ErrPrerequisites || err == ErrIncompatibleFS {
        continue
      }
      return nil, err
    }
    checkPriorDriver(name, root)
    return driver, nil
  }
  return nil, fmt.Errorf("No supported storage backend found")
}