// 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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// Adapted for dns package usage by Miek Gieben.
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package dns
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import "sync"
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import "time"
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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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val *Msg
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rtt time.Duration
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err error
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dups int
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}
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// singleflight 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 singleflight struct {
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sync.Mutex // protects m
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m map[string]*call // lazily initialized
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dontDeleteForTesting bool // this is only to be used by TestConcurrentExchanges
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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 *singleflight) Do(key string, fn func() (*Msg, time.Duration, error)) (v *Msg, rtt time.Duration, err error, shared bool) {
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g.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.Unlock()
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c.wg.Wait()
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return c.val, c.rtt, 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.Unlock()
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c.val, c.rtt, c.err = fn()
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c.wg.Done()
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if !g.dontDeleteForTesting {
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g.Lock()
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delete(g.m, key)
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g.Unlock()
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}
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return c.val, c.rtt, c.err, c.dups > 0
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}
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