net: avoid TCP self-connect

Fixes #2690.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5650071

src/pkg/net/dial_test.go

@@ -84,3 +84,34 @@ func TestDialTimeout(t *testing.T) {
 		}
 	}
 }
+
+func TestSelfConnect(t *testing.T) {
+	// Test that Dial does not honor self-connects.
+	// See the comment in DialTCP.
+
+	// Find a port that would be used as a local address.
+	l, err := Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	c, err := Dial("tcp", l.Addr().String())
+	if err != nil {
+		t.Fatal(err)
+	}
+	addr := c.LocalAddr().String()
+	c.Close()
+	l.Close()
+
+	// Try to connect to that address repeatedly.
+	n := 100000
+	if testing.Short() {
+		n = 1000
+	}
+	for i := 0; i < n; i++ {
+		c, err := Dial("tcp", addr)
+		if err == nil {
+			c.Close()
+			t.Errorf("#%d: Dial %q succeeded", i, addr)
+		}
+	}
+}

src/pkg/net/tcpsock_posix.go

@@ -227,13 +227,43 @@ func DialTCP(net string, laddr, raddr *TCPAddr) (*TCPConn, error) {
 	if raddr == nil {
 		return nil, &OpError{"dial", net, nil, errMissingAddress}
 	}
+
 	fd, err := internetSocket(net, laddr.toAddr(), raddr.toAddr(), syscall.SOCK_STREAM, 0, "dial", sockaddrToTCP)
+
+	// TCP has a rarely used mechanism called a 'simultaneous connection' in
+	// which Dial("tcp", addr1, addr2) run on the machine at addr1 can
+	// connect to a simultaneous Dial("tcp", addr2, addr1) run on the machine
+	// at addr2, without either machine executing Listen.  If laddr == nil,
+	// it means we want the kernel to pick an appropriate originating local
+	// address.  Some Linux kernels cycle blindly through a fixed range of
+	// local ports, regardless of destination port.  If a kernel happens to
+	// pick local port 50001 as the source for a Dial("tcp", "", "localhost:50001"),
+	// then the Dial will succeed, having simultaneously connected to itself.
+	// This can only happen when we are letting the kernel pick a port (laddr == nil)
+	// and when there is no listener for the destination address.
+	// It's hard to argue this is anything other than a kernel bug.  If we
+	// see this happen, rather than expose the buggy effect to users, we
+	// close the fd and try again.  If it happens twice more, we relent and
+	// use the result.  See also:
+	//	http://golang.org/issue/2690
+	//	http://stackoverflow.com/questions/4949858/
+	for i := 0; i < 2 && err == nil && laddr == nil && selfConnect(fd); i++ {
+		fd.Close()
+		fd, err = internetSocket(net, laddr.toAddr(), raddr.toAddr(), syscall.SOCK_STREAM, 0, "dial", sockaddrToTCP)
+	}
+
 	if err != nil {
 		return nil, err
 	}
 	return newTCPConn(fd), nil
 }
 
+func selfConnect(fd *netFD) bool {
+	l := fd.laddr.(*TCPAddr)
+	r := fd.raddr.(*TCPAddr)
+	return l.Port == r.Port && l.IP.Equal(r.IP)
+}
+
 // TCPListener is a TCP network listener.
 // Clients should typically use variables of type Listener
 // instead of assuming TCP.