singleflight

// Do executes and returns the results of the given function, making

// sure that only one execution is in-flight for a given key at a

// time. If a duplicate comes in, the duplicate caller waits for the

// original to complete and receives the same results.

func (g *Group) Do(key string, fn func() (interface{}, error)) (interface{}, error) {

	g.mu.Lock()

	if g.m == nil {

		g.m = make(map[string]*call)

	}

	if c, ok := g.m[key]; ok {

		g.mu.Unlock()

		c.wg.Wait()

		return c.val, c.err

	}

	c := new(call)

	c.wg.Add(1)

	g.m[key] = c

	g.mu.Unlock()



	c.val, c.err = fn()

	c.wg.Done()



	g.mu.Lock()

	delete(g.m, key)

	g.mu.Unlock()



	return c.val, c.err

}