net/http: fix crash with Transport.CloseIdleConnections

Thanks Michael Lore for the bug report!

Fixes #3266

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5754068

src/pkg/net/http/transport.go

@@ -196,7 +196,7 @@ func (t *Transport) CloseIdleConnections() {
 			pconn.close()
 		}
 	}
-	t.idleConn = nil
+	t.idleConn = make(map[string][]*persistConn)
 }
 
 //

src/pkg/net/http/transport_test.go

@@ -698,6 +698,32 @@ func TestTransportPersistConnLeak(t *testing.T) {
 	}
 }
 
+// This used to crash; http://golang.org/issue/3266
+func TestTransportIdleConnCrash(t *testing.T) {
+	tr := &Transport{}
+	c := &Client{Transport: tr}
+
+	unblockCh := make(chan bool, 1)
+	ts := httptest.NewServer(HandlerFunc(func(w ResponseWriter, r *Request) {
+		<-unblockCh
+		tr.CloseIdleConnections()
+	}))
+	defer ts.Close()
+
+	didreq := make(chan bool)
+	go func() {
+		res, err := c.Get(ts.URL)
+		if err != nil {
+			t.Error(err)
+		} else {
+			res.Body.Close() // returns idle conn
+		}
+		didreq <- true
+	}()
+	unblockCh <- true
+	<-didreq
+}
+
 type fooProto struct{}
 
 func (fooProto) RoundTrip(req *Request) (*Response, error) {