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	dontDeleteForTesting bool // this is only to be used by TestConcurrentExchanges
28}
29
30// Do executes and returns the results of the given function, making
31// sure that only one execution is in-flight for a given key at a
32// time. If a duplicate comes in, the duplicate caller waits for the
33// original to complete and receives the same results.
34// The return value shared indicates whether v was given to multiple callers.
35func (g *singleflight) Do(key string, fn func() (*Msg, time.Duration, error)) (v *Msg, rtt time.Duration, err error, shared bool) {
36	g.Lock()
37	if g.m == nil {
38		g.m = make(map[string]*call)
39	}
40	if c, ok := g.m[key]; ok {
41		c.dups++
42		g.Unlock()
43		c.wg.Wait()
44		return c.val, c.rtt, c.err, true
45	}
46	c := new(call)
47	c.wg.Add(1)
48	g.m[key] = c
49	g.Unlock()
50
51	c.val, c.rtt, c.err = fn()
52	c.wg.Done()
53
54	if !g.dontDeleteForTesting {
55		g.Lock()
56		delete(g.m, key)
57		g.Unlock()
58	}
59
60	return c.val, c.rtt, c.err, c.dups > 0
61}
62