Add SendMessagesScatter using conn.SendmsgBuffers

[Cellebyte]

Add conn.SendmsgBuffers additionally of conn.Sendmsg

The library is used in a project which applies netfilter rules into network namespaces of containers.
Some customers use very huge netfilter configurations which cause a default socket configuration on linux to not accept the message. The project currently increases the socket buffer dynamically, based on the message size.

There was already #254 which got implemented here.
It is a separate function as suggested in #254, due to issues I had during a one to one replacement in gooogle/nftables.

This can only be merged after mdlayher/socket#18.

[nickgarlis]

Do you have an example or test case that demonstrates the issue this change is intended to fix?
From what I understand, scatter/gather may reduce user-space copying or memory usage, but it doesn’t prevent the socket send buffer from filling up. If the buffer runs out of space, we could still see an ENOBUFS error unless the socket memory limits are increased.

[Cellebyte]

No the change does not help with the issue alone.
As I still get the message too long error, but I only use it in the nftables implementation.

[nickgarlis]

If you're using google/nftables and are trying to solve ENOBUFS or EMSGSIZE the solution might be to use the latest committed change (not the latest tagged one). There have been changes to address this issue.

This PR is still useful IMO (not a maintainer) but I just didn't think it was the solution to the issue described in the title.

[Cellebyte]

Ok, I just saw your Issue and suggestion to add this method. I already use the latest commits in nftables:main on a cluster. Maybe we move the discussion to the right place and I add infos about the use-case and my Issue there as well as the reproducer and maybe some hint how I can debug it.

[Cellebyte]

Changed the subject of this PR and implemented it as a separate function due to the mentioned issue above.

[nickgarlis]

I believe the issue discussed in google/nftables#350 now points to these buffers instead of the socket ones.

I think there is a limit on how many of these buffers you can use. Check getconf IOV_MAX