// Copyright 2013 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package singleflight provides a duplicate function call suppression
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// mechanism.
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package singleflight
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import "sync"
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// call is an in-flight or completed singleflight.Do call
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type call struct {
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wg sync.WaitGroup
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// These fields are written once before the WaitGroup is done
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// and are only read after the WaitGroup is done.
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val interface{}
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err error
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// These fields are read and written with the singleflight
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// mutex held before the WaitGroup is done, and are read but
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// not written after the WaitGroup is done.
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dups int
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chans []chan<- Result
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}
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// Group represents a class of work and forms a namespace in
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// which units of work can be executed with duplicate suppression.
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type Group struct {
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mu sync.Mutex // protects m
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m map[string]*call // lazily initialized
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}
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// Result holds the results of Do, so they can be passed
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// on a channel.
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type Result struct {
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Val interface{}
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Err error
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Shared bool
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}
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// Do executes and returns the results of the given function, making
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// sure that only one execution is in-flight for a given key at a
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// time. If a duplicate comes in, the duplicate caller waits for the
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// original to complete and receives the same results.
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// The return value shared indicates whether v was given to multiple callers.
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func (g *Group) Do(key string, fn func() (interface{}, error)) (v interface{}, err error, shared bool) {
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g.mu.Lock()
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if g.m == nil {
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g.m = make(map[string]*call)
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}
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if c, ok := g.m[key]; ok {
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c.dups++
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g.mu.Unlock()
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c.wg.Wait()
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return c.val, c.err, true
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}
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c := new(call)
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c.wg.Add(1)
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g.m[key] = c
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g.mu.Unlock()
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g.doCall(c, key, fn)
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return c.val, c.err, c.dups > 0
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}
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// DoChan is like Do but returns a channel that will receive the
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// results when they are ready. The second result is true if the function
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// will eventually be called, false if it will not (because there is
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// a pending request with this key).
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func (g *Group) DoChan(key string, fn func() (interface{}, error)) (<-chan Result, bool) {
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ch := make(chan Result, 1)
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g.mu.Lock()
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if g.m == nil {
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g.m = make(map[string]*call)
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}
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if c, ok := g.m[key]; ok {
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c.dups++
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c.chans = append(c.chans, ch)
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g.mu.Unlock()
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return ch, false
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}
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c := &call{chans: []chan<- Result{ch}}
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c.wg.Add(1)
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g.m[key] = c
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g.mu.Unlock()
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go g.doCall(c, key, fn)
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return ch, true
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}
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// doCall handles the single call for a key.
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func (g *Group) doCall(c *call, key string, fn func() (interface{}, error)) {
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c.val, c.err = fn()
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c.wg.Done()
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g.mu.Lock()
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delete(g.m, key)
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for _, ch := range c.chans {
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ch <- Result{c.val, c.err, c.dups > 0}
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}
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g.mu.Unlock()
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}
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// ForgetUnshared tells the singleflight to forget about a key if it is not
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// shared with any other goroutines. Future calls to Do for a forgotten key
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// will call the function rather than waiting for an earlier call to complete.
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// Returns whether the key was forgotten or unknown--that is, whether no
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// other goroutines are waiting for the result.
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func (g *Group) ForgetUnshared(key string) bool {
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g.mu.Lock()
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defer g.mu.Unlock()
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c, ok := g.m[key]
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if !ok {
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return true
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}
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if c.dups == 0 {
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delete(g.m, key)
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return true
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}
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return false
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}
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