net: add support for collective TCP listeners

[mikioh]

Latest Linux and DragonFly BSD kernel support this feature. 

The SO_REUSEPORT socket option
https://lwn.net/Articles/542629/

Also see
torvalds/linux@c617f39
and
http://gitweb.dragonflybsd.org/dragonfly.git/commitdiff/740d1d9f7b7bf9c9c021abb8197718d7a2d441c9
for the detail.

[mikioh]

Comment 1:

Some experiment:
https://golang.org/cl/34580043/

[mikioh]

Comment 2:

Labels changed: added repo-main, release-none.

[mikioh]

Comment 4:

Status changed to Thinking.

[mikioh]

Comment 5:

And for FreeBSD
http://lists.freebsd.org/pipermail/freebsd-net/2013-December/037178.html

[gopherbot]

Comment 6 by elazarl:

Instead of chasing the kernel, and implementing various options the kernel adds, I think
it's better to add a callback to Dial, that would let you use setsockopt on the socket
fd/handle before using it.
Something along the lines of
    Dialer{Callback: func (fd uintptr) { syscalls.Setsockopts(SO_BINDTOADDR, fd) }}.Dial(...)

[mikioh]

Comment 7:

I don't understand #6. This feature is for endpoints on the passive open side, not
related to Dial.

[gopherbot]

Comment 8 by elazarl:

I meant that we can add a general setsockopts options, to set socket options before the
net package handle them. And while we at it, we can add this option to dial to.
This way, you don't have to do anything special for SO_REUSEPORT.

[mikioh]

Comment 9:

Please open a new issue for your purpose. This issue is focused on "collective TCP
listeners", also not stick to SO_REUSEPORT or other option because probably on FreeBSD,
the way using shared tcpcb and inpcbgrup inside the kernel would be a different kernel
API.

[kavu]

Comment 11:

To whom It may concern: I have implemented similar feature as separate package
https://github.com/kavu/go_reuseport. I'll be glad if you'll find it useful, and I may
improve it further and submit it as a patch if you wish.

[mikioh]

I'll merge this issue into #9661.