Today I did something interesting with the Debian packaging for propellor, which seems like it could be a useful technique for other Debian packages as well.

Propellor is configured by a directory, which is maintained as a local git repository. In propellor's case, it's ~/.propellor/. This contains a lot of haskell files, in fact the entire source code of propellor! That's really unusual, but I think this can be generalized to any package whose configuration is maintained in its own git repository on the user's system. For now on, I'll refer to this as the config repo.

The config repo is set up the first time a user runs propellor. But, until now, I didn't provide an easy way to update the config repo when the propellor package was updated. Nothing would break, but the old version would be used until the user updated it themselves somehow (probably by pulling from a git repository over the network, bypassing apt's signature validation).

So, what I wanted was a way to update the config repo, merging in any changes from the new version of the Debian package, while preserving the user's local modifications. Ideally, the user could just run git merge upstream/master, where the upstream repo was included in the Debian package.

But, that can't work! The Debian package can't reasonably include the full git repository of propellor with all its history. So, any git repository included in the Debian binary package would need to be a synthetic one, that only contains probably one commit that is not connected to anything else. Which means that if the config repo was cloned from that repo in version 1.0, then when version 1.1 came around, git would see no common parent when merging 1.1 into the config repo, and the merge would fail horribly.

To solve this, let's assume that the config repo's master branch has a parent commit that can be identified, somehow, as coming from a past version of the Debian package. It doesn't matter which version, although the last one merged with will be best. (The easy way to do this is to set refs/heads/upstream/master to point to it when creating the config repo.)

Once we have that parent commit, we have three things:

  1. The current content of the config repo.
  2. The content from some old version of the Debian package.
  3. The new content of the Debian package.

Now git can be used to merge #3 onto #2, with -Xtheirs, so the result is a git commit with parents of #3 and #2, and content of #3. (This can be done using a temporary clone of the config repo to avoid touching its contents.)

Such a git commit can be merged into the config repo, without any conflicts other than those the user might have caused with their own edits.

So, propellor will tell the user when updates are available, and they can simply run git merge upstream/master to get them. The resulting history looks like this:

* Merge remote-tracking branch 'upstream/master'
| * merging upstream version
| |\  
| | * upstream version
* | user change
* upstream version

So, generalizing this, if a package has a lot of config files, and creates a git repository containing them when the user uses it (or automatically when it's installed), this method can be used to provide an easily mergable branch that tracks the files as distributed with the package.

It would perhaps not be hard to get from here to a full git-backed version of ucf. Note that the Debian binary package doesn't have to ship a git repisitory, it can just as easily ship the current version of the config files somewhere in /usr, and check them into a new empty repository as part of the generation of the upstream/master branch.