What steps will reproduce the problem?

https://groups.google.com/d/topic/golang-nuts/PyOeimgyvFw/discussion

WriteMsgUnix always sends a dummy normal byte if one tries to send only
out-of-band/ancillary data (e.g. a file descriptor), because ReadMsgUnix doesn't receive
oob-only writes (it just blocks) on stream sockets.

This was done based on prior art:

https://git.gitorious.org/libancillary/libancillary.git

http://svn.python.org/projects/python/branches/release32-maint/Modules/_multiprocessing/multiprocessing.c

and discussions:

https://groups.google.com/d/msg/golang-dev/Rkwgwug2OpI/7YYunK4Z9NUJ

https://golang.org/cl/2331044#msg12

However, it turns out that you can send oob-only messages on a datagram socket.

systemd implements a datagram socket protocol for talking to its journal to exchange
file descriptors of temporary files containing messages that are too long to send via
the datagram socket directly.

systemd doesn't expect the dummy byte and returns an error if it is present.

Removing the dummy byte will cause this test with a stream socket to hang:

http://play.golang.org/p/ebLI7mwBPF

but this test with a datagram socket still works:

http://play.golang.org/p/q-eWEP3_96

which is expected, because systemd has a test for this scenario.

Which version are you using?  (run 'go version')

tip

Please provide any additional information below.

systemd code:

http://cgit.freedesktop.org/systemd/systemd/tree/src/journal/journal-send.c#n345
http://cgit.freedesktop.org/systemd/systemd/tree/src/journal/journald-server.c#n1197
http://cgit.freedesktop.org/systemd/systemd/tree/src/journal/test-journal-send.c#n48

systemd test output shows that the test passes:

https://gist.github.com/philips/1938c88d544e49f40889/raw/cf64b66c261aee63c0476b28a32e448ecb2d5b60/gistfile1.txt