net: make sure failed Dial returns nil Conn

Fixes #6614.

R=golang-dev, bradfitz, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/14950045

src/pkg/net/dial.go

@@ -215,26 +215,30 @@ func dialMulti(net, addr string, la Addr, ras addrList, deadline time.Time) (Con
 
 // dialSingle attempts to establish and returns a single connection to
 // the destination address.
-func dialSingle(net, addr string, la, ra Addr, deadline time.Time) (Conn, error) {
+func dialSingle(net, addr string, la, ra Addr, deadline time.Time) (c Conn, err error) {
 	if la != nil && la.Network() != ra.Network() {
 		return nil, &OpError{Op: "dial", Net: net, Addr: ra, Err: errors.New("mismatched local address type " + la.Network())}
 	}
 	switch ra := ra.(type) {
 	case *TCPAddr:
 		la, _ := la.(*TCPAddr)
-		return dialTCP(net, la, ra, deadline)
+		c, err = dialTCP(net, la, ra, deadline)
 	case *UDPAddr:
 		la, _ := la.(*UDPAddr)
-		return dialUDP(net, la, ra, deadline)
+		c, err = dialUDP(net, la, ra, deadline)
 	case *IPAddr:
 		la, _ := la.(*IPAddr)
-		return dialIP(net, la, ra, deadline)
+		c, err = dialIP(net, la, ra, deadline)
 	case *UnixAddr:
 		la, _ := la.(*UnixAddr)
-		return dialUnix(net, la, ra, deadline)
+		c, err = dialUnix(net, la, ra, deadline)
 	default:
 		return nil, &OpError{Op: "dial", Net: net, Addr: ra, Err: &AddrError{Err: "unexpected address type", Addr: addr}}
 	}
+	if err != nil {
+		return nil, err // c is non-nil interface containing nil pointer
+	}
+	return c, nil
 }
 
 // Listen announces on the local network address laddr.
@@ -246,14 +250,19 @@ func Listen(net, laddr string) (Listener, error) {
 	if err != nil {
 		return nil, &OpError{Op: "listen", Net: net, Addr: nil, Err: err}
 	}
+	var l Listener
 	switch la := la.toAddr().(type) {
 	case *TCPAddr:
-		return ListenTCP(net, la)
+		l, err = ListenTCP(net, la)
 	case *UnixAddr:
-		return ListenUnix(net, la)
+		l, err = ListenUnix(net, la)
 	default:
 		return nil, &OpError{Op: "listen", Net: net, Addr: la, Err: &AddrError{Err: "unexpected address type", Addr: laddr}}
 	}
+	if err != nil {
+		return nil, err // l is non-nil interface containing nil pointer
+	}
+	return l, nil
 }
 
 // ListenPacket announces on the local network address laddr.
@@ -265,14 +274,19 @@ func ListenPacket(net, laddr string) (PacketConn, error) {
 	if err != nil {
 		return nil, &OpError{Op: "listen", Net: net, Addr: nil, Err: err}
 	}
+	var l PacketConn
 	switch la := la.toAddr().(type) {
 	case *UDPAddr:
-		return ListenUDP(net, la)
+		l, err = ListenUDP(net, la)
 	case *IPAddr:
-		return ListenIP(net, la)
+		l, err = ListenIP(net, la)
 	case *UnixAddr:
-		return ListenUnixgram(net, la)
+		l, err = ListenUnixgram(net, la)
 	default:
 		return nil, &OpError{Op: "listen", Net: net, Addr: la, Err: &AddrError{Err: "unexpected address type", Addr: laddr}}
 	}
+	if err != nil {
+		return nil, err // l is non-nil interface containing nil pointer
+	}
+	return l, nil
 }

src/pkg/net/net_test.go

@@ -218,3 +218,41 @@ func TestTCPClose(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func TestErrorNil(t *testing.T) {
+	c, err := Dial("tcp", "127.0.0.1:65535")
+	if err == nil {
+		t.Fatal("Dial 127.0.0.1:65535 succeeded")
+	}
+	if c != nil {
+		t.Fatalf("Dial returned non-nil interface %T(%v) with err != nil", c, c)
+	}
+
+	// Make Listen fail by relistening on the same address.
+	l, err := Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatal("Listen 127.0.0.1:0: %v", err)
+	}
+	defer l.Close()
+	l1, err := Listen("tcp", l.Addr().String())
+	if err == nil {
+		t.Fatal("second Listen %v: %v", l.Addr(), err)
+	}
+	if l1 != nil {
+		t.Fatalf("Listen returned non-nil interface %T(%v) with err != nil", l1, l1)
+	}
+
+	// Make ListenPacket fail by relistening on the same address.
+	lp, err := ListenPacket("udp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatal("Listen 127.0.0.1:0: %v", err)
+	}
+	defer lp.Close()
+	lp1, err := ListenPacket("udp", lp.LocalAddr().String())
+	if err == nil {
+		t.Fatal("second Listen %v: %v", lp.LocalAddr(), err)
+	}
+	if lp1 != nil {
+		t.Fatalf("ListenPacket returned non-nil interface %T(%v) with err != nil", lp1, lp1)
+	}
+}