// 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. // Adapted for dns package usage by Miek Gieben. package dns import "sync" import "time" // call is an in-flight or completed singleflight.Do call type call struct { wg sync.WaitGroup val *Msg rtt time.Duration err error dups int } // singleflight represents a class of work and forms a namespace in // which units of work can be executed with duplicate suppression. type singleflight struct { sync.Mutex // protects m m map[string]*call // lazily initialized dontDeleteForTesting bool // this is only to be used by TestConcurrentExchanges } // 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 *singleflight) Do(key string, fn func() (*Msg, time.Duration, error)) (v *Msg, rtt time.Duration, err error, shared bool) { g.Lock() if g.m == nil { g.m = make(map[string]*call) } if c, ok := g.m[key]; ok { c.dups++ g.Unlock() c.wg.Wait() return c.val, c.rtt, c.err, true } c := new(call) c.wg.Add(1) g.m[key] = c g.Unlock() c.val, c.rtt, c.err = fn() c.wg.Done() if !g.dontDeleteForTesting { g.Lock() delete(g.m, key) g.Unlock() } return c.val, c.rtt, c.err, c.dups > 0 }