Re: [libvirt PATCH 00/28] native support for nftables in virtual network driver

This patch series enables libvirt to use nftables rules rather than iptables *when setting up virtual networks* (it does *not* add nftables support to the nwfilter driver). It accomplishes this by abstracting several iptables functions (from viriptables.[ch] called by the virtual network driver into a rudimentary "virNetfilter API" (in virnetfilter.[ch], having the virtual network driver call the virNetFilter API rather than calling the existing iptables functions directly, and then finally adding an equivalent virNftables backend that can be used instead of iptables (selected manually via a network.conf setting, or automatically if iptables isn't found on the host). A first look at the result may have you thinking that it's filled with a lot of bad decisions. While I would agree with that in many cases, I think that overall they are the "least bad" decisions, or at least "bad within acceptable limits / no worse than something else", and point out that it's been done in a way that minimizes (actually eliminates) the need for immediate changes to nwfilter (the other consumer of iptables, which *also* needs to be updated to use native nftables), and makes it much easier to change our mind about the details in the future. When I first started on this (long, protracted, repeatedly interrupted for extended periods - many of these patches are > a year old) task, I considered doing an all-at-once complete replacement of iptables with nftables, since all the Linux distros we support have had nftables for several years, and I'm pretty sure nobody has it disabled (not even sure if it's possible to disable nftables while still enabling iptables, since they both use xtables in the kernel). But due to libvirt's use of "-t mangle -j CHECKSUM --checksum-fill" (see commit fd5b15ff all the way back in July 2010 for details) which has no equivalent in nftables rules (and we don't *want* it to!!), and the desire to be able to easily switch back to iptables in case of an unforeseen regression, we decided that both iptables and nftables need to be supported (for now), with the default (for now) remaining as iptables. Just allowing for dual backends complicated matters, since it means that we have to have a config file, a setting, detection of which backends are available, and of course some sort of concept of an abstracted frontend that can use either backend based on the config setting (and/or auto-detection). Combining that with the fact that it would just be "too big" of a project to switch over nwfilter's iptables usage at the same time means that we have to keep around a lot of existing code for compatibility's sake rather than just wiping it all away and starting over. So, what I've ended up with is: 1) a network.conf file (didn't exist before) with a single setting "firewall_backend". If unset, the network driver tries to use iptables on the backend, and if that's missing, then tries to use nftables.