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: cache.go
    path of file: ./repos/k6/vendor/github.com/lestrrat-go/httprc
    
    the code of the file until where you have to start completion: package httprc

import (
	"context"
	"fmt"
	"net/http"
	"sync"
	"time"
)

// ErrSink is an abstraction that allows users to consume errors
// produced while the cache queue is running.
type HTTPClient interface {
	Get(string) (*http.Response, error)
}

// Cache represents a cache that stores resources locally, while
// periodically refreshing the contents based on HTTP header values
// and/or user-supplied hints.
//
// Refresh is performed _periodically_, and therefore the contents
// are not kept up-to-date in real time. The interval between checks
// for refreshes is called the refresh window.
//
// The default refresh windo
    
func (c *Cache) getOrFetch(ctx context.Context, u string, forceRefresh bool) (interface{}, error) {
	c.mu.RLock()
	e, ok := c.queue.getRegistered(u)
	if !ok {
		c.mu.RUnlock()
		return nil, fmt.Errorf(`url %q is not registered (did you make sure to call Register() first?)`, u)
	}
	c.mu.RUnlock()

	// Only one goroutine may enter this section.
	e.acquireSem()

	// has this entry been fetched? (but ignore and do a fetch
	// if forceRefresh is true)
	if forceRefresh || !e.hasBeenFetched() {
		if err := c.queue.fetchAndStore(ctx, e); err != nil {
			e.releaseSem()
			return nil, fmt.Errorf(`failed to fetch %q: %w`, u, err)
		}
	}

	e.releaseSem()

	e.mu.RLock()
	data := e.data
	e.mu.RUnlock()

	return data, nil
}