[Libvir] RPM upgrades interaction with default networking

[Daniel P. Berrange]

I've been thinking about the tricky issue of RPM upgrades and how they
interact with the new networking stuff and don't think we've currently
got quite the optimal setup.

The current approach is that 'make install' puts a default.xml file
into /etc/libvirt/qemu/networks and also symlinks that file into the
autostart directory. So if someone builds from tar.gz and does make
install they'll get a default network config installed & turned on
out of the box. This is fine for tar.gz scenario.

When we build an RPM we also include the default.xml file in the 
/etc/libvirt/qemu/networks directory, as well as the autostart symlink.
So anyone installing the libvirt RPM gets the default network, whether
they're doing an upgrade or fresh install.  This is reasonable for the
new install, or the first time you upgrade to a new neworking-enabled
libvirt. If you subsequently delete the default network, or turn off
autostarting, then along comes the next libvirt RPM update and autostart
gets turned back on, and/or the default network recreated. This will be
considered to be rather unpleasant by many people because no matter
what they do, they will always be given a virbr0, a dnsmasq process
and a bunch of extra iptables rules whether they want them or not.

So I think we need to figure out a way to deploy a default network out
of the box, but at the same time ensure that if the turn it off / delete
it, upgrades won't re-introduce it.  Ideally if someone upgrades from
FC6 -> FC7 they will also get the default network created (once only).

One way we can address this is to put the default.xml into the docs
directory /usr/share/doc/libvirt-X.Y.Z (or perhaps stuff is into a dir
like /usr/share/libvirt instead) and then have a RPM %post script which
copies it into /etc/libvirt/qemu/networks. If we make the %post script
conditional on '$1 == 1' then it will only be run for completely new
libvirt installs. This doesn't address the upgrade question though -
so someone updating from FC6 -> FC7 won't get the default network. Perhaps
we shouldn't worry about them ?

Dan.
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