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// 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. |
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func (g *Group) DoChan(key string, fn func() (interface{}, error)) <-chan Result { |
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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 |
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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 |
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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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// Forget tells the singleflight to forget about a key. Future calls |
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// to Do for this key will call the function rather than waiting for |
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// an earlier call to complete. |
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func (g *Group) Forget(key string) { |
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g.mu.Lock() |
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delete(g.m, key) |
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g.mu.Unlock() |
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} |