net/http: fix data race in test

I can't reproduce the race, but this should fix it.

Fixes #8483

LGTM=dvyukov
R=dvyukov
CC=golang-codereviews
https://golang.org/cl/126610043

src/pkg/net/http/export_test.go

@@ -70,3 +70,9 @@ func ResetCachedEnvironment() {
 }
 
 var DefaultUserAgent = defaultUserAgent
+
+// SetPendingDialHooks sets the hooks that run before and after handling
+// pending dials.
+func SetPendingDialHooks(before, after func()) {
+	prePendingDial, postPendingDial = before, after
+}

src/pkg/net/http/transport.go

@@ -444,6 +444,9 @@ func (t *Transport) dial(network, addr string) (c net.Conn, err error) {
 	return net.Dial(network, addr)
 }
 
+// Testing hooks:
+var prePendingDial, postPendingDial func()
+
 // getConn dials and creates a new persistConn to the target as
 // specified in the connectMethod.  This includes doing a proxy CONNECT
 // and/or setting up TLS.  If this doesn't return an error, the persistConn
@@ -460,9 +463,17 @@ func (t *Transport) getConn(req *Request, cm connectMethod) (*persistConn, error
 	dialc := make(chan dialRes)
 
 	handlePendingDial := func() {
-		if v := <-dialc; v.err == nil {
-			t.putIdleConn(v.pc)
+		if prePendingDial != nil {
+			prePendingDial()
 		}
+		go func() {
+			if v := <-dialc; v.err == nil {
+				t.putIdleConn(v.pc)
+			}
+			if postPendingDial != nil {
+				postPendingDial()
+			}
+		}()
 	}
 
 	cancelc := make(chan struct{})
@@ -484,10 +495,10 @@ func (t *Transport) getConn(req *Request, cm connectMethod) (*persistConn, error
 		// else's dial that they didn't use.
 		// But our dial is still going, so give it away
 		// when it finishes:
-		go handlePendingDial()
+		handlePendingDial()
 		return pc, nil
 	case <-cancelc:
-		go handlePendingDial()
+		handlePendingDial()
 		return nil, errors.New("net/http: request canceled while waiting for connection")
 	}
 }

src/pkg/net/http/transport_test.go

@@ -1063,20 +1063,18 @@ func TestTransportConcurrency(t *testing.T) {
 	var wg sync.WaitGroup
 	wg.Add(numReqs)
 
-	tr := &Transport{
-		Dial: func(netw, addr string) (c net.Conn, err error) {
-			// Due to the Transport's "socket late
-			// binding" (see idleConnCh in transport.go),
-			// the numReqs HTTP requests below can finish
-			// with a dial still outstanding.  So count
-			// our dials as work too so the leak checker
-			// doesn't complain at us.
-			wg.Add(1)
-			defer wg.Done()
-			return net.Dial(netw, addr)
-		},
-	}
+	// Due to the Transport's "socket late binding" (see
+	// idleConnCh in transport.go), the numReqs HTTP requests
+	// below can finish with a dial still outstanding.  To keep
+	// the leak checker happy, keep track of pending dials and
+	// wait for them to finish (and be closed or returned to the
+	// idle pool) before we close idle connections.
+	SetPendingDialHooks(func() { wg.Add(1) }, wg.Done)
+	defer SetPendingDialHooks(nil, nil)
+
+	tr := &Transport{}
 	defer tr.CloseIdleConnections()
+
 	c := &Client{Transport: tr}
 	reqs := make(chan string)
 	defer close(reqs)