Message pack and unpack fails with an empty name in a RR header

[qdm12]

I lost quite a few hours fighting this off, so I thought I would voice this concern over here 😉
TLDR is packing a message with an empty string as its RR header name works, but unpacking it will fail with the rather obscure message dns: overflowing header size.

I wrote a little test to show the problem:

func Test_MessagePackUnpack(t *testing.T) {
	response := &dns.Msg{
		Answer: []dns.RR{
			&dns.A{
				Hdr: dns.RR_Header{
					Name: "",
				},
				A: net.IPv4(1, 2, 3, 4),
			},
		},
	}

	b, err := response.Pack()
	if err != nil { // no error
		t.Fatal(err)
	}

	decoded := new(dns.Msg)
	err = decoded.Unpack(b)
	if err != nil { // dns: overflowing header size
		t.Fatal(err)
	}
}

The reason behind this is likely from commit fdcdc6d which I'm not too sure why it is there, but assuming there is a reason for it, I feel like the Unpack method should work with empty name strings? If not, at least return a more explicit error stating the RR header name cannot be empty??

Thanks!

[miekg]

some symmetry here would be nice