Some work in outstanding query de-deplication

client.go

@@ -35,6 +35,7 @@ type Client struct {
 	ReadTimeout  time.Duration     // the net.Conn.SetReadTimeout value for new connections (ns), defaults to 2 * 1e9
 	WriteTimeout time.Duration     // the net.Conn.SetWriteTimeout value for new connections (ns), defaults to 2 * 1e9
 	TsigSecret   map[string]string // secret(s) for Tsig map[<zonename>]<base64 secret>, zonename must be fully qualified
+	Inflight     bool              // if true suppress multiple outstanding queries for the same Qname, Qtype and Qclass
 }
 
 // Exchange performs an synchronous query. It sends the message m to the address

singleinflight.go

@@ -0,0 +1,53 @@
+// 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.
+
+package dns
+
+import "sync"
+
+// call is an in-flight or completed singleflight.Do call
+type call struct {
+	wg   sync.WaitGroup
+	val  interface{}
+	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 {
+	mu sync.Mutex       // protects m
+	m  map[string]*call // lazily initialized
+}
+
+// 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() (interface{}, error)) (v interface{}, err error, shared bool) {
+	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()
+
+	c.val, c.err = fn()
+	c.wg.Done()
+
+	g.mu.Lock()
+	delete(g.m, key)
+	g.mu.Unlock()
+
+	return c.val, c.err, c.dups > 0
+}