@@ -40,10 +40,16 @@ func lookupIPMerge(host string) (addrs []IP, err error) {
 	addrsi, err, shared := lookupGroup.Do(host, func() (interface{}, error) {
 		return lookupIP(host)
 	})
+	return lookupIPReturn(addrsi, err, shared)
+}
+
+// lookupIPReturn turns the return values from singleflight.Do into
+// the return values from LookupIP.
+func lookupIPReturn(addrsi interface{}, err error, shared bool) ([]IP, error) {
 	if err != nil {
 		return nil, err
 	}
-	addrs = addrsi.([]IP)
+	addrs := addrsi.([]IP)
 	if shared {
 		clone := make([]IP, len(addrs))
 		copy(clone, addrs)
@@ -52,41 +58,40 @@ func lookupIPMerge(host string) (addrs []IP, err error) {
 	return addrs, nil
 }
 
+// lookupIPDeadline looks up a hostname with a deadline.
 func lookupIPDeadline(host string, deadline time.Time) (addrs []IP, err error) {
 	if deadline.IsZero() {
 		return lookupIPMerge(host)
 	}
 
-	// TODO(bradfitz): consider pushing the deadline down into the
-	// name resolution functions. But that involves fixing it for
-	// the native Go resolver, cgo, Windows, etc.
-	//
-	// In the meantime, just use a goroutine. Most users affected
-	// by http://golang.org/issue/2631 are due to TCP connections
-	// to unresponsive hosts, not DNS.
+	// We could push the deadline down into the name resolution
+	// functions.  However, the most commonly used implementation
+	// calls getaddrinfo, which has no timeout.
+
 	timeout := deadline.Sub(time.Now())
 	if timeout <= 0 {
-		err = errTimeout
-		return
+		return nil, errTimeout
 	}
 	t := time.NewTimer(timeout)
 	defer t.Stop()
-	type res struct {
-		addrs []IP
-		err   error
-	}
-	resc := make(chan res, 1)
-	go func() {
-		a, err := lookupIPMerge(host)
-		resc <- res{a, err}
-	}()
+
+	ch := lookupGroup.DoChan(host, func() (interface{}, error) {
+		return lookupIP(host)
+	})
+
 	select {
 	case <-t.C:
-		err = errTimeout
-	case r := <-resc:
-		addrs, err = r.addrs, r.err
+		// The DNS lookup timed out for some reason.  Force
+		// future requests to start the DNS lookup again
+		// rather than waiting for the current lookup to
+		// complete.  See issue 8602.
+		lookupGroup.Forget(host)
+
+		return nil, errTimeout
+
+	case r := <-ch:
+		return lookupIPReturn(r.v, r.err, r.shared)
 	}
-	return
 }
 
 // LookupPort looks up the port for the given network and service.
 
@@ -8,10 +8,18 @@ import "sync"
 
 // call is an in-flight or completed singleflight.Do call
 type call struct {
-	wg   sync.WaitGroup
-	val  interface{}
-	err  error
-	dups int
+	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
+	chans []chan<- singleflightResult
 }
 
 // singleflight represents a class of work and forms a namespace in
@@ -21,6 +29,14 @@ type singleflight struct {
 	m  map[string]*call // lazily initialized
 }
 
+// singleflightResult holds the results of Do, so they can be passed
+// on a channel.
+type singleflightResult struct {
+	v      interface{}
+	err    error
+	shared bool
+}
+
 // 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
@@ -42,12 +58,52 @@ func (g *singleflight) Do(key string, fn func() (interface{}, error)) (v interfa
 	g.m[key] = c
 	g.mu.Unlock()
 
+	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.
+func (g *singleflight) DoChan(key string, fn func() (interface{}, error)) <-chan singleflightResult {
+	ch := make(chan singleflightResult, 1)
+	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
+	}
+	c := &call{chans: []chan<- singleflightResult{ch}}
+	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.
+func (g *singleflight) doCall(c *call, key string, fn func() (interface{}, error)) {
 	c.val, c.err = fn()
 	c.wg.Done()
 
 	g.mu.Lock()
 	delete(g.m, key)
+	for _, ch := range c.chans {
+		ch <- singleflightResult{c.val, c.err, c.dups > 0}
+	}
 	g.mu.Unlock()
+}
 
-	return c.val, c.err, c.dups > 0
+// 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.
+func (g *singleflight) Forget(key string) {
+	g.mu.Lock()
+	delete(g.m, key)
+	g.mu.Unlock()
 }