DNF vs YUM, $pkg, $pkg-mpi, $pkg-openmpi having same provides

[Kalev Lember]

On 06/12/2015 10:28 AM, Radek Holy wrote:
> If a package "Requires: foo" and both "bar" and "barbaz" "Provides:
> foo", they are handled as being equally suitable. DNF/libsolv is not
> going to prefer packages with shorter names.

Yes, very much agreed here. Please don't add the yum shortest name hack
to DNF. This and the rest of the depsolving quirks that choose one
provider over the other depending on what else happens to be installed,
have made my life quite hard over the years :)

Let me try to elaborate.

The issue I've had with yum's virtual provider depsolving is that there
is currently no way to set a default. A default where it would by
default install the package that the maintainer has chosen, but would
also allow swapping it out if the user needs to.

An example here would be an app that has two backends. One of those
backends would be the preferred backend and other one experimantel, and
maybe a third one that is for a special use case backend that isn't
interesting for the general audience. The way to express this with rpm
deps would be to have a virtual provide that all of the backends
packages provide, so that the user could choose which one to install.

An example here would be PackageKit with different backends. It used to
have 2 backends, PackageKit-hawkey and PackageKit-yum (we dropped the
different backends from Fedora now, but it was a valid issue in last
release). Both of them would have a common virtual provide that
PackageKit itself would depend on, to make sure at least one of the
backends is installed.

With the yum system, yum would pretty much always just pick the shortest
name. It _happened_ to be PackageKit-yum, but that was totally an
accident. And it made it impossible to change the default without
renaming the packages - so in a new release that was supposed to use
hawkey, we'd have very little options besides renaming PackageKit-hawkey
to PackageKit-h for example to make sure it wins over -yum.

What I feel would be a good solution to the problem above would be to
have a way to specify the default. I believe this problem is already
solved in apt-get with a very nice syntax: the OR syntax:

Requires: PackageKit-hawkey | PackageKit-backend

... where PackageKit-backend is a virtual provide that both of the
backends satisfy.

With the requires above, any of the two would solve the requires, but
dnf could use the information to choose the first one as the default
when it doesn't have any of the backends already installed.

-- 
Kalev