net: use original raddr if getpeername fails

Fixes #3838.

R=dave, mikioh.mikioh, rsc
CC=golang-dev
https://golang.org/cl/7511043

src/pkg/net/ipraw_test.go

@@ -337,3 +337,19 @@ func TestIPConnLocalName(t *testing.T) {
 		}
 	}
 }
+
+func TestIPConnRemoteName(t *testing.T) {
+	if os.Getuid() != 0 {
+		t.Skip("skipping test; must be root")
+	}
+
+	raddr := &IPAddr{IP: IPv4(127, 0, 0, 10).To4()}
+	c, err := DialIP("ip:tcp", &IPAddr{IP: IPv4(127, 0, 0, 1)}, raddr)
+	if err != nil {
+		t.Fatalf("DialIP failed: %v", err)
+	}
+	defer c.Close()
+	if !reflect.DeepEqual(raddr, c.RemoteAddr()) {
+		t.Fatalf("got %#v, expected %#v", c.RemoteAddr(), raddr)
+	}
+}

src/pkg/net/sock_posix.go

@@ -67,5 +67,8 @@ func socket(net string, f, t, p int, ipv6only bool, ulsa, ursa syscall.Sockaddr,
 	rsa, _ := syscall.Getpeername(s)
 	raddr := toAddr(rsa)
 	fd.setAddr(laddr, raddr)
+	if fd.raddr == nil {
+		fd.raddr = toAddr(ursa)
+	}
 	return fd, nil
 }