code: func (d *driver) URLFor(ctx context.Context, path string, options map[string]interface{}) (string, error) {
  	methodString := "GET"
  	method, ok := options["method"]
  	if ok {
  		methodString, ok = method.(string)
  		if !ok || (methodString != "GET" && methodString != "HEAD") {
  			return "", storagedriver.ErrUnsupportedMethod{}
  		}
  	}
  
  	expiresIn := 20 * time.Minute
  	expires, ok := options["expiry"]
  	if ok {
  		et, ok := expires.(time.Time)
  		if ok {
  			expiresIn = time.Until(et)
  		}
  	}
  
  	var req *request.Request
  
  	switch methodString {
  	case "GET":
  		req, _ = d.S3.GetObjectRequest(&s3.GetObjectInput{
  			Bucket: aws.String(d.Bucket),
  			Key:    aws.String(d.s3Path(path)),
  		})
  	case "HEAD":
  		req, _ = d.S3.HeadObjectRequest(&s3.HeadObjectInput{
  			Bucket: aws.String(d.Bucket),
  			Key:    aws.String(d.s3Path(path)),
  		})
  	default:
  		panic("unreachable")
  	}
  
  	return req.Presign(expiresIn)
  }
docs: // URLFor returns a URL which may be used to retrieve the content stored at the given path.
  // May return an UnsupportedMethodErr in certain StorageDriver implementations.