syscall: handle getsockname for unix sockets on openbsd 5.2

On OpenBSD 5.2, calling getsockname on an unbound Unix domain socket
results in a successful syscall, however the AF is unset and the length
is returned as zero. This has been changed to more portable behaviour,
which will be included in the OpenBSD 5.3 release.

For now, work around this by treating a successful getsockname() call
that returns a family of AF_UNSPEC and length of zero as a AF_UNIX
socket.

Makes TestPassFD work on OpenBSD 5.2.

Fixes #4956.

R=golang-dev, minux.ma, rsc, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/7449046

src/pkg/syscall/passfd_test.go

@@ -13,7 +13,6 @@ import (
 	"net"
 	"os"
 	"os/exec"
-	"runtime"
 	"syscall"
 	"testing"
 	"time"
@@ -27,9 +26,6 @@ import (
 // "-test.run=^TestPassFD$" and an environment variable used to signal
 // that the test should become the child process instead.
 func TestPassFD(t *testing.T) {
-	if runtime.GOOS == "openbsd" {
-		t.Skip("issue 4956")
-	}
 	if os.Getenv("GO_WANT_HELPER_PROCESS") == "1" {
 		passFDChild()
 		return

src/pkg/syscall/syscall_bsd.go

@@ -327,6 +327,11 @@ func Getsockname(fd int) (sa Sockaddr, err error) {
 	if err = getsockname(fd, &rsa, &len); err != nil {
 		return
 	}
+	// TODO(jsing): Remove after OpenBSD 5.4 is released (see issue 3349).
+	if runtime.GOOS == "openbsd" && rsa.Addr.Family == AF_UNSPEC && rsa.Addr.Len == 0 {
+		rsa.Addr.Family = AF_UNIX
+		rsa.Addr.Len = SizeofSockaddrUnix
+	}
 	return anyToSockaddr(&rsa)
 }