You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: slowmessage.go
    path of file: ./repos/terraform/internal/helper/slowmessage
    
    the code of the file until where you have to start completion: // Copyright (c) HashiCorp, Inc.
// SPDX-License-Identifier: BUSL-1.1

package slowmessage

import (
	"time"
)

// SlowFunc is the function that could be slow. Usually, you'll have to
// wrap an existing function in 
    
func Do(threshold time.Duration, sf SlowFunc, cb CallbackFunc) error {
	// Call the slow function
	errCh := make(chan error, 1)
	go func() {
		errCh <- sf()
	}()

	// Wait for it to complete or the threshold to pass
	select {
	case err := <-errCh:
		return err
	case <-time.After(threshold):
		// Threshold reached, call the callback
		cb()
	}

	// Wait an indefinite amount of time for it to finally complete
	return <-errCh
}