== Excluding buggy RPMs from a _yum_ repository

We mirror the Fedora Core 4 updates area, and we use _yum_ to install
things from it, and Red Hat recently released an extremely unsuitable
(for us) update to _xorg-x11_ (as recounted in MoreFC4Problems). So we
needed to take those buggy RPMs out of circulation and keep them out.

Fortunately this turns out to be pretty easy, because not all RPMs in
a directory have to be in the repository metadata that _yum_
uses. This let us continue to pull a full mirror from Red Hat (keeping
the mirroring script simple) and just built our own metadata that
excludes the buggy RPMs we don't want.

Modern _yum_ repository metadata is created with the _createrepo_
command (*not* _yum-arch_, despite the latter being packaged with
_yum_ and the former not), which is in the _createrepo_ RPM.
_createrepo_ has a _-x_ option to exclude RPMs from the metadata it
makes; _-x_ even takes a glob pattern, like say
((xorg-x11-*6.8.2-37.FC4.48.1*)) (remember to quote it).

There are two drawbacks with this:

# _createrepo_ takes a while to run.
# We're now dependent on having a Fedora Core 4 machine around to run
  the mirroring on (because we probably need to use a _createrepo_
  version that matches the Fedora Core 4 _yum_, so they agree on the
  metadata format).

Hopefully someday Red Hat will fix the bugs and we can go back to
using all of the current updates and thus to just mirroring the
update repository metadata as well as all of the RPMs.

(Because we are only ignoring a specific version of _xorg-x11_, _yum_
will include the previous, working Fedora Core 4 _xorg-x11_ update in
the metadata. Which is what we want, because we definitely want *that*
update installed.)