Go, IPv6, and single binding machines

The current libraries for the Go language and their built in tests strongly believe that you can talk to IPv4 addresses through IPv6 sockets, which is not necessarily the case. This is a known issue (see also), and is more than somewhat inconvenient on a machine with dual binding turned off, such as my workstation, as Go will not install from source unless all its tests pass.

(Since Debian has apparently changed their minds about dual binding, this may not affect very many people. I maintain that it should, although it's now a quixotic battle that I am probably not going to win any time soon.)

If this affects you, the simple fix is probably to just apply the patch from Joerg Sonnenberger that's (currently) at the end of Go issue 679. I opted for a slightly different fix, because I wanted to force Go to use IPv4 sockets where possible. Thus, I forced preferIPv4 in src/pkg/net/ipsock.go to true and applied only his patch to src/pkg/net/sock.go to always turn off IPV6_V6ONLY on IPv6 sockets.

(A more thorough fix for preferIPv4 would be to test if the kernel let you bind IPv4 addresses to an IPv6 socket. But I didn't feel like going to that much effort for what is ultimately a quick hack that the Go maintainers are unlikely to support.)

While this is an incomplete hack with some limits, I think it is generally going to do what I want from Go even with servers, provided that I am careful (basically I can't mix an explicit IPv4 server with a Go-based IPv6 one). A better fix would be to change the code to explicitly force IPV6_V6ONLY only when we are using IPv6 sockets with IPv4 addresses, and I may try that fix at some point when I feel more ambitious about hacking up the innards of Go packages.

(One of the attraction of Go is that it looks familiar enough to me that I can fumble my way through this sort of chainsaw modifications and usually get them to work.)

As a side note: since OpenBSD doesn't allow dual binding under any circumstances, this is going to be a real issue if anyone ever attempts to port Go to OpenBSD. I suspect that the solution will be to turn off a bunch of tests.