syscall, net: Fix unix socket autobind on Linux.

R=rsc, iant, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/7300047

src/pkg/net/unix_test.go

@@ -10,6 +10,8 @@ import (
 	"bytes"
 	"io/ioutil"
 	"os"
+	"reflect"
+	"runtime"
 	"syscall"
 	"testing"
 	"time"
@@ -121,3 +123,35 @@ func TestReadUnixgramWithZeroBytesBuffer(t *testing.T) {
 		t.Errorf("peer adddress is %v", peer)
 	}
 }
+
+func TestUnixAutobind(t *testing.T) {
+	if runtime.GOOS != "linux" {
+		t.Skip("skipping: autobind is linux only")
+	}
+
+	laddr := &UnixAddr{Name: "", Net: "unixgram"}
+	c1, err := ListenUnixgram("unixgram", laddr)
+	if err != nil {
+		t.Fatalf("ListenUnixgram failed: %v", err)
+	}
+	defer c1.Close()
+
+	// retrieve the autobind address
+	autoAddr := c1.LocalAddr().(*UnixAddr)
+	if len(autoAddr.Name) <= 1 {
+		t.Fatalf("Invalid autobind address: %v", autoAddr)
+	}
+	if autoAddr.Name[0] != '@' {
+		t.Fatalf("Invalid autobind address: %v", autoAddr)
+	}
+
+	c2, err := DialUnix("unixgram", nil, autoAddr)
+	if err != nil {
+		t.Fatalf("DialUnix failed: %v", err)
+	}
+	defer c2.Close()
+
+	if !reflect.DeepEqual(c1.LocalAddr(), c2.RemoteAddr()) {
+		t.Fatalf("Expected autobind address %v, got %v", c1.LocalAddr(), c2.RemoteAddr())
+	}
+}

src/pkg/syscall/syscall_linux.go

@@ -279,15 +279,18 @@ type SockaddrUnix struct {
 func (sa *SockaddrUnix) sockaddr() (uintptr, _Socklen, error) {
 	name := sa.Name
 	n := len(name)
-	if n >= len(sa.raw.Path) || n == 0 {
+	if n >= len(sa.raw.Path) {
 		return 0, 0, EINVAL
 	}
 	sa.raw.Family = AF_UNIX
 	for i := 0; i < n; i++ {
 		sa.raw.Path[i] = int8(name[i])
 	}
 	// length is family (uint16), name, NUL.
-	sl := 2 + _Socklen(n) + 1
+	sl := _Socklen(2)
+	if n > 0 {
+		sl += _Socklen(n) + 1
+	}
 	if sa.raw.Path[0] == '@' {
 		sa.raw.Path[0] = 0
 		// Don't count trailing NUL for abstract address.