blob: b2d82e26c29c6b8890dd7e0590299179c4090486 [file] [log] [blame]
// 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.
// Package singleflight provides a duplicate function call suppression
// mechanism.
package singleflight
import "sync"
// call is an in-flight or completed singleflight.Do call
type call struct {
wg sync.WaitGroup
// These fields are written once before the WaitGroup is done
// and are only read after the WaitGroup is done.
val interface{}
err error
// These fields are read and written with the singleflight
// mutex held before the WaitGroup is done, and are read but
// not written after the WaitGroup is done.
dups int
chans []chan<- Result
// Group represents a class of work and forms a namespace in
// which units of work can be executed with duplicate suppression.
type Group struct {
mu sync.Mutex // protects m
m map[string]*call // lazily initialized
// Result holds the results of Do, so they can be passed
// on a channel.
type Result struct {
Val interface{}
Err error
Shared bool
// 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.
// The return value shared indicates whether v was given to multiple callers.
func (g *Group) Do(key string, fn func() (interface{}, error)) (v interface{}, err error, shared bool) {
if g.m == nil {
g.m = make(map[string]*call)
if c, ok := g.m[key]; ok {
return c.val, c.err, true
c := new(call)
g.m[key] = c
g.doCall(c, key, fn)
return c.val, c.err, c.dups > 0
// DoChan is like Do but returns a channel that will receive the
// results when they are ready. The second result is true if the function
// will eventually be called, false if it will not (because there is
// a pending request with this key).
func (g *Group) DoChan(key string, fn func() (interface{}, error)) (<-chan Result, bool) {
ch := make(chan Result, 1)
if g.m == nil {
g.m = make(map[string]*call)
if c, ok := g.m[key]; ok {
c.chans = append(c.chans, ch)
return ch, false
c := &call{chans: []chan<- Result{ch}}
g.m[key] = c
go g.doCall(c, key, fn)
return ch, true
// doCall handles the single call for a key.
func (g *Group) doCall(c *call, key string, fn func() (interface{}, error)) {
c.val, c.err = fn()
delete(g.m, key)
for _, ch := range c.chans {
ch <- Result{c.val, c.err, c.dups > 0}
// ForgetUnshared tells the singleflight to forget about a key if it is not
// shared with any other goroutines. Future calls to Do for a forgotten key
// will call the function rather than waiting for an earlier call to complete.
// Returns whether the key was forgotten or unknown--that is, whether no
// other goroutines are waiting for the result.
func (g *Group) ForgetUnshared(key string) bool {
c, ok := g.m[key]
if !ok {
return true
if c.dups == 0 {
delete(g.m, key)
return true
return false