[node-devel] Rethinking implementation for oVirt Node Plugins

[Perry Myers]

The current thinking/design around doing oVirt Node Plugins is here:
http://ovirt.org/wiki/Node_plugins

And is based mostly on the premise that:
* Plugins are self contained blobs of RPMs that are internally
  dependency complete
* Plugins are installed via smth like rpm -Uvh of a set of RPMs
  contained inside the blob (tarball likely)

As I was thinking about some additional use cases for plugins (like
including CIM/tog-pegasus and making vdsm a plugin), it seems like a lot
of redundancy to pull packages out of Fedora repos, and stick them in a
tarball when there are perfectly good yum mirrors that have those packages.

It's also a lot of overhead on the part of the plugin creators to be
doing dual updates: Update RPM in Fedora and simultaneously update and
publish a new plugin.

The core problem is: remote retrieval of packages and dependency
resolution... wait, doesn't yum solve that set of problems?

But there's no yum on oVirt Node...  The original reasons for excluding
yum were:
* No python on the node (but vdsm pulled python in, so that's moot now)
* Don't want folks running yum on a live oVirt Node image (we can
  address that by making yum impossible to run when the image is booted
  vs. offline)

So I'd like to rethink the plugins concept by first starting with
putting yum back on oVirt Node, and leveraging that for what it is good at.

If we put yum on the node, then plugin installation could be as simple as:

mount ISO
cp foo.repo /etc/yum.conf.d/
yum install foo --enablerepo=foo

If offline is desired, then the plugin is basically a repo inside a
tarball and you do

mount ISO
cp foo.repo /etc/yum.conf.d/
yum localinstall foo/repo/foo.rpm

In either case, we could enforce offline vs. online plugin integration
by always setting all repo files to disabled, and manually needing to
enable them with --enablerepo=* if the user is doing an online plugin

So a plugin could just simply be:
* repo file (with one or more repo definitions that are not included in
  the base distro)
* rpm list
* blacklisting info
* optionally a collection of RPMs with repo metadata

This means that we can let normal yum dep resolution work and plugins
essentially become dependent on things like 'what version of ovirt-node
is installed' or 'what version of the kernel is installed' and if
dependencies aren't met, the plugin installation should fail gracefully

We can prevent _core_ files from being upgraded (like ovirt-node,
kernel, etc) by adding explicit excludepkg directives so that if a
plugin tries to bring in a version of a package already core to oVirt
Node, it fails and reports "dude, you need a newer ISO already"

Thoughts?  This should make the plugin concept easier to implement and
also allow us to include support for plugins that pull packages from
remote repositories much easier.

Perry