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 package singleflight
6 7 import (
8 "bytes"
9 "errors"
10 "fmt"
11 "runtime"
12 "runtime/debug"
13 "sync"
14 )
15 16 // errGoexit indicates the runtime.Goexit was called in
17 // the user given function.
18 var errGoexit = errors.New("runtime.Goexit was called")
19 20 // A panicError is an arbitrary value recovered from a panic
21 // with the stack trace during the execution of given function.
22 type panicError struct {
23 value interface{}
24 stack []byte
25 }
26 27 // Error implements error interface.
28 func (p *panicError) Error() string {
29 return fmt.Sprintf("%v\n\n%s", p.value, p.stack)
30 }
31 32 func newPanicError(v interface{}) error {
33 stack := debug.Stack()
34 35 // The first line of the stack trace is of the form "goroutine N [status]:"
36 // but by the time the panic reaches Do the goroutine may no longer exist
37 // and its status will have changed. Trim out the misleading line.
38 if line := bytes.IndexByte(stack[:], '\n'); line >= 0 {
39 stack = stack[line+1:]
40 }
41 return &panicError{value: v, stack: stack}
42 }
43 44 // call is an in-flight or completed singleflight.Do call
45 type call struct {
46 wg sync.WaitGroup
47 48 // These fields are written once before the WaitGroup is done
49 // and are only read after the WaitGroup is done.
50 val interface{}
51 err error
52 53 // forgotten indicates whether Forget was called with this call's key
54 // while the call was still in flight.
55 forgotten bool
56 57 // These fields are read and written with the singleflight
58 // mutex held before the WaitGroup is done, and are read but
59 // not written after the WaitGroup is done.
60 dups int
61 chans []chan<- Result
62 }
63 64 // Group represents a class of work and forms a namespace in
65 // which units of work can be executed with duplicate suppression.
66 type Group struct {
67 mu sync.Mutex // protects m
68 m map[string]*call // lazily initialized
69 }
70 71 // Result holds the results of Do, so they can be passed
72 // on a channel.
73 type Result struct {
74 Val interface{}
75 Err error
76 Shared bool
77 }
78 79 // Do executes and returns the results of the given function, making
80 // sure that only one execution is in-flight for a given key at a
81 // time. If a duplicate comes in, the duplicate caller waits for the
82 // original to complete and receives the same results.
83 // The return value shared indicates whether v was given to multiple callers.
84 func (g *Group) Do(key string, fn func() (interface{}, error)) (v interface{}, err error, shared bool) {
85 g.mu.Lock()
86 if g.m == nil {
87 g.m = make(map[string]*call)
88 }
89 if c, ok := g.m[key]; ok {
90 c.dups++
91 g.mu.Unlock()
92 c.wg.Wait()
93 94 if e, ok := c.err.(*panicError); ok {
95 panic(e)
96 } else if c.err == errGoexit {
97 runtime.Goexit()
98 }
99 return c.val, c.err, true
100 }
101 c := new(call)
102 c.wg.Add(1)
103 g.m[key] = c
104 g.mu.Unlock()
105 106 g.doCall(c, key, fn)
107 return c.val, c.err, c.dups > 0
108 }
109 110 // DoChan is like Do but returns a channel that will receive the
111 // results when they are ready.
112 //
113 // The returned channel will not be closed.
114 func (g *Group) DoChan(key string, fn func() (interface{}, error)) <-chan Result {
115 ch := make(chan Result, 1)
116 g.mu.Lock()
117 if g.m == nil {
118 g.m = make(map[string]*call)
119 }
120 if c, ok := g.m[key]; ok {
121 c.dups++
122 c.chans = append(c.chans, ch)
123 g.mu.Unlock()
124 return ch
125 }
126 c := &call{chans: []chan<- Result{ch}}
127 c.wg.Add(1)
128 g.m[key] = c
129 g.mu.Unlock()
130 131 go g.doCall(c, key, fn)
132 133 return ch
134 }
135 136 // doCall handles the single call for a key.
137 func (g *Group) doCall(c *call, key string, fn func() (interface{}, error)) {
138 normalReturn := false
139 recovered := false
140 141 // use double-defer to distinguish panic from runtime.Goexit,
142 // more details see https://golang.org/cl/134395
143 defer func() {
144 // the given function invoked runtime.Goexit
145 if !normalReturn && !recovered {
146 c.err = errGoexit
147 }
148 149 c.wg.Done()
150 g.mu.Lock()
151 defer g.mu.Unlock()
152 if !c.forgotten {
153 delete(g.m, key)
154 }
155 156 if e, ok := c.err.(*panicError); ok {
157 // In order to prevent the waiting channels from being blocked forever,
158 // needs to ensure that this panic cannot be recovered.
159 if len(c.chans) > 0 {
160 go panic(e)
161 select {} // Keep this goroutine around so that it will appear in the crash dump.
162 } else {
163 panic(e)
164 }
165 } else if c.err == errGoexit {
166 // Already in the process of goexit, no need to call again
167 } else {
168 // Normal return
169 for _, ch := range c.chans {
170 ch <- Result{c.val, c.err, c.dups > 0}
171 }
172 }
173 }()
174 175 func() {
176 defer func() {
177 if !normalReturn {
178 // Ideally, we would wait to take a stack trace until we've determined
179 // whether this is a panic or a runtime.Goexit.
180 //
181 // Unfortunately, the only way we can distinguish the two is to see
182 // whether the recover stopped the goroutine from terminating, and by
183 // the time we know that, the part of the stack trace relevant to the
184 // panic has been discarded.
185 if r := recover(); r != nil {
186 c.err = newPanicError(r)
187 }
188 }
189 }()
190 191 c.val, c.err = fn()
192 normalReturn = true
193 }()
194 195 if !normalReturn {
196 recovered = true
197 }
198 }
199 200 // Forget tells the singleflight to forget about a key. Future calls
201 // to Do for this key will call the function rather than waiting for
202 // an earlier call to complete.
203 func (g *Group) Forget(key string) {
204 g.mu.Lock()
205 if c, ok := g.m[key]; ok {
206 c.forgotten = true
207 }
208 delete(g.m, key)
209 g.mu.Unlock()
210 }
211