2 files changed, 77 insertions, 3 deletions

diff --git a/src/pkg/net/lookup.go b/src/pkg/net/lookup.go
index bec93ec08..28d439d64 100644
--- a/src/pkg/net/lookup.go
+++ b/src/pkg/net/lookup.go
@@ -8,15 +8,36 @@ import (
 	"time"
 )
 
+var lookupGroup singleflight
+
+// lookupHostMerge wraps lookupHost, but makes sure that for any given
+// host, only one lookup is in-flight at a time. The returned memory
+// is always owned by the caller.
+func lookupHostMerge(host string) (addrs []string, err error) {
+	addrsi, err, shared := lookupGroup.Do(host, func() (interface{}, error) {
+		return lookupHost(host)
+	})
+	if err != nil {
+		return nil, err
+	}
+	addrs = addrsi.([]string)
+	if shared {
+		clone := make([]string, len(addrs))
+		copy(clone, addrs)
+		addrs = clone
+	}
+	return addrs, nil
+}
+
 // LookupHost looks up the given host using the local resolver.
 // It returns an array of that host's addresses.
 func LookupHost(host string) (addrs []string, err error) {
-	return lookupHost(host)
+	return lookupHostMerge(host)
 }
 
 func lookupHostDeadline(host string, deadline time.Time) (addrs []string, err error) {
 	if deadline.IsZero() {
-		return lookupHost(host)
+		return lookupHostMerge(host)
 	}
 
 	// TODO(bradfitz): consider pushing the deadline down into the
@@ -39,7 +60,7 @@ func lookupHostDeadline(host string, deadline time.Time) (addrs []string, err er
 	}
 	resc := make(chan res, 1)
 	go func() {
-		a, err := lookupHost(host)
+		a, err := lookupHostMerge(host)
 		resc <- res{a, err}
 	}()
 	select {
diff --git a/src/pkg/net/singleflight.go b/src/pkg/net/singleflight.go
new file mode 100644
index 000000000..dc58affda
--- /dev/null
+++ b/src/pkg/net/singleflight.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 net
+
+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
+}