Retro68/gcc/libgo/go/internal/singleflight/singleflight.go

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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
// license that can be found in the LICENSE file.
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// Package singleflight provides a duplicate function call suppression
// mechanism.
package singleflight
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import "sync"
// call is an in-flight or completed singleflight.Do call
type call struct {
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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
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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
m map[string]*call // lazily initialized
}
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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 {
Val interface{}
Err error
Shared bool
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}
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// 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.
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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()
if g.m == nil {
g.m = make(map[string]*call)
}
if c, ok := g.m[key]; ok {
c.dups++
g.mu.Unlock()
c.wg.Wait()
return c.val, c.err, true
}
c := new(call)
c.wg.Add(1)
g.m[key] = c
g.mu.Unlock()
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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.
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func (g *Group) DoChan(key string, fn func() (interface{}, error)) <-chan Result {
ch := make(chan Result, 1)
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g.mu.Lock()
if g.m == nil {
g.m = make(map[string]*call)
}
if c, ok := g.m[key]; ok {
c.dups++
c.chans = append(c.chans, ch)
g.mu.Unlock()
return ch
}
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c := &call{chans: []chan<- Result{ch}}
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c.wg.Add(1)
g.m[key] = c
g.mu.Unlock()
go g.doCall(c, key, fn)
return ch
}
// 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()
c.wg.Done()
g.mu.Lock()
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
// to Do for this key will call the function rather than waiting for
// an earlier call to complete.
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func (g *Group) Forget(key string) {
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g.mu.Lock()
delete(g.m, key)
g.mu.Unlock()
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}