Fix sd_daemon.Notify

I was working with net.UnixConn.WriteMsgUnix() incorrectly.  Literally the
only example of this I can find in the wild is in CoreOS go-systemd, which
does the same thing I did.  Unfortunately go-systemd only hits that
codepath as a fall-back if the primary code path errors... and the test
suite only hits the primary code path.

1 files changed, 21 insertions, 3 deletions

diff --git a/sd_daemon/notify.go b/sd_daemon/notify.go
index 1d72cc3..8b813cc 100644
--- a/sd_daemon/notify.go
+++ b/sd_daemon/notify.go
@@ -70,7 +70,7 @@ func Notify(pid int, unsetEnv bool, state string, files []*os.File) error {
 		return ErrDisabled
 	}
 
-	conn, err := net.DialUnix(socketAddr.Net, nil, socketAddr)
+	conn, err := socketUnixgram(socketAddr.Name)
 	if err != nil {
 		return err
 	}
@@ -98,7 +98,7 @@ func Notify(pid int, unsetEnv bool, state string, files []*os.File) error {
 	}
 
 	// If the 2nd argument is empty, this is equivalent to .Write([]byte(state))
-	_, _, err = conn.WriteMsgUnix([]byte(state), bytes.Join(cmsgs, nil), nil)
+	_, _, err = conn.WriteMsgUnix([]byte(state), bytes.Join(cmsgs, nil), socketAddr)
 
 	if err != nil && havePid {
 		// Maybe it failed because we don't have privileges to
@@ -108,8 +108,26 @@ func Notify(pid int, unsetEnv bool, state string, files []*os.File) error {
 		// notifying the user, but that's what
 		// sd_pid_notify_with_fds does.
 		cmsgs = cmsgs[:len(cmsgs)-1]
-		_, _, err = conn.WriteMsgUnix([]byte(state), bytes.Join(cmsgs, nil), nil)
+		_, _, err = conn.WriteMsgUnix([]byte(state), bytes.Join(cmsgs, nil), socketAddr)
 	}
 
 	return err
 }
+
+// socketUnixgram wraps socket(2), but doesn't bind(2) to anything.
+// This is an ugly hack because none of the functions in "net"
+// actually allow you to get a AF_UNIX socket not bound to anything (I
+// think that sentence is still true if you take out "AF_UNIX", but
+// I'm not totally certain of that).
+func socketUnixgram(name string) (*net.UnixConn, error) {
+	fd, err := unix.Socket(unix.AF_UNIX, unix.SOCK_DGRAM|unix.SOCK_CLOEXEC, 0)
+	if err != nil {
+		return nil, err
+	}
+	conn, err := net.FileConn(os.NewFile(uintptr(fd), name))
+	if err != nil {
+		return nil, err
+	}
+	unixConn := conn.(*net.UnixConn)
+	return unixConn, nil
+}