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: singleinflight.go
    path of file: ./repos/docker-ce/components/engine/vendor/github.com/miekg/dns
    
    the code of the file until where you have to start completion: // Copyright 2013 The Go Authors.  All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Adapted for dns package usage by Miek Gieben.

package dns

import "sync"
import "time"

// call is an in-flight or completed singleflight.Do call
type call struct {
	wg   sync.WaitGroup
	val  *Msg
	rtt  time.Duration
	err  error
	dups int
}

// singleflight represents a 
    
func (g *singleflight) Do(key string, fn func() (*Msg, time.Duration, error)) (v *Msg, rtt time.Duration, err error, shared bool) {
	g.Lock()
	if g.m == nil {
		g.m = make(map[string]*call)
	}
	if c, ok := g.m[key]; ok {
		c.dups++
		g.Unlock()
		c.wg.Wait()
		return c.val, c.rtt, c.err, true
	}
	c := new(call)
	c.wg.Add(1)
	g.m[key] = c
	g.Unlock()

	c.val, c.rtt, c.err = fn()
	c.wg.Done()

	if !g.dontDeleteForTesting {
		g.Lock()
		delete(g.m, key)
		g.Unlock()
	}

	return c.val, c.rtt, c.err, c.dups > 0
}