1// Copyright 2013 The Go Authors.  All rights reserved.
2// Use of this source code is governed by a BSD-style
3// license that can be found in the LICENSE file.
4
5// Adapted for dns package usage by Miek Gieben.
6
7package dns
8
9import "sync"
10import "time"
11
12// call is an in-flight or completed singleflight.Do call
13type call struct {
14	wg   sync.WaitGroup
15	val  *Msg
16	rtt  time.Duration
17	err  error
18	dups int
19}
20
21// singleflight represents a class of work and forms a namespace in
22// which units of work can be executed with duplicate suppression.
23type singleflight struct {
24	sync.Mutex                  // protects m
25	m          map[string]*call // lazily initialized
26}
27
28// Do executes and returns the results of the given function, making
29// sure that only one execution is in-flight for a given key at a
30// time. If a duplicate comes in, the duplicate caller waits for the
31// original to complete and receives the same results.
32// The return value shared indicates whether v was given to multiple callers.
33func (g *singleflight) Do(key string, fn func() (*Msg, time.Duration, error)) (v *Msg, rtt time.Duration, err error, shared bool) {
34	g.Lock()
35	if g.m == nil {
36		g.m = make(map[string]*call)
37	}
38	if c, ok := g.m[key]; ok {
39		c.dups++
40		g.Unlock()
41		c.wg.Wait()
42		return c.val, c.rtt, c.err, true
43	}
44	c := new(call)
45	c.wg.Add(1)
46	g.m[key] = c
47	g.Unlock()
48
49	c.val, c.rtt, c.err = fn()
50	c.wg.Done()
51
52	g.Lock()
53	delete(g.m, key)
54	g.Unlock()
55
56	return c.val, c.rtt, c.err, c.dups > 0
57}
58