The current implementation fails to address a basic use case, that is, installing a package from a 3rd party repository.
Say, a user would like to install ceph from http://ceph.com/debian-firefly (no matter what its version is) and adds Inktank APT repo
deb http://ceph.com/debian-firefly trusty main
with a priority 1100.
The pinning rules produced by the current code (with this patch applied) are
Package: *
Pin: a=trusty, c=main
Pin-Priority: 1100
These rules match *both* ceph packages from Canonical repo (http://archive.ubuntu.com/ubuntu trusty main) and from Inktank one (http://ceph.com/debian-firefly trusty main). As a result the newest package will be installed, which is not what the user wants. In fact installing the newest available version of a package is a default behavior, it does not require any pinning at all.
The correct approach is to make use of the APT repo metadata (Origin, Label, Codename, etc) in the pinning rules. For instance for the above mentioned Inktank repo the following rules should be produced:
Package: *
Pin: release o=Inktank, n=trusty
Pin-Priority: 1100
Needs to be documented