Automatically set socket read & write buffer sizes

[nickgarlis]

This is an attempt to port some of the logic that nftables uses to automatically adjust the recvmsg & sendmsg buffer sizes. The implementation of setting sndmsg size is not the same as nftables due to some limitations in the underlying netlink & socket libraries.

You can compare it the behavior to nftables if you:

• strace -e trace=sendmsg,setsockopt,getsockopt -s 16384 -xx ./nftables.test -test.v -run_system_tests -test.run ^TestAutoBufferSize$
• strace -e trace=sendmsg,setsockopt,getsockopt -s 16384 -xx nft --echo -f ./test-rules where test-rules contains https://gist.github.com/nickgarlis/d31cf585b77eef0628118704db8e8310

In my system it's

getsockopt(7, SOL_SOCKET, SO_SNDBUF, [212992], [4]) = 0
setsockopt(7, SOL_SOCKET, SO_SNDBUFFORCE, [458868], 4) = 0
getsockopt(7, SOL_SOCKET, SO_RCVBUF, [212992], [4]) = 0
setsockopt(7, SOL_SOCKET, SO_RCVBUFFORCE, [4197376], 4) = 0 

and

getsockopt(3, SOL_SOCKET, SO_SNDBUF, [212992], [4]) = 0
setsockopt(3, SOL_SOCKET, SO_SNDBUF, [2097152], 4) = 0
setsockopt(3, SOL_SOCKET, SO_SNDBUFFORCE, [2097152], 4) = 0
getsockopt(3, SOL_SOCKET, SO_RCVBUF, [212992], [4]) = 0
setsockopt(3, SOL_SOCKET, SO_RCVBUFFORCE, [4197376], 4) = 0

respectively.

The change depends on https://github.com/mdlayher/netlink/pull/223

• https://git.netfilter.org/nftables/tree/src/mnl.c?id=713592c6008a8c589a00d3d3d2e49709ff2de62c#n262
• https://git.netfilter.org/libmnl/tree/include/libmnl/libmnl.h?id=03da98bcd284d55212bc79e91dfb63da0ef7b937#n20
• https://git.netfilter.org/nftables/tree/src/mnl.c?id=713592c6008a8c589a00d3d3d2e49709ff2de62c#n391
• https://git.netfilter.org/nftables/tree/src/mnl.c?id=713592c6008a8c589a00d3d3d2e49709ff2de62c#n426