With wheezy released, the floodgates are opened on a lot of debhelper changes that have been piling up. Most of these should be pretty minor, but I released one yesterday that will affect all users of dh. Hopefully in a good way.

I made dh smarter about selecting which debhelper commands it runs. It can tell when a package does not use the stuff done by a particular command, and skips running the command entirely.

So the debian/rules binary of a package using dh will now often look like this:

dh binary

Which is pretty close to the optimal hand-crafted debian/rules file (and just about as fast, too). But with the benefit that if you later add, say, cron job files, dh_installcron will automatically start being run too.

Hopefully this will not result in any behavior changes, other than packages building faster and with less noise. If there is a bug it'll probably be something missing in the specification of when a command needs to be run.

Beyond speed, I hope that this will help to lower the bar to adding new commands to debhelper, and to the default dh sequences. Before, every such new command slowed things down and was annoying. Now more special-purpose commands won't get in the way of packages that don't need them.

The way this works is that debhelper commands can include a "PROMISE" directive. An example from dh_installexamples


Mostly this specifies the files in debian/ that are used by the command, and whose presence triggers the command to run. There is also a syntax to specify items that can be present in the package build directory to trigger the command to run.

(Unfortunatly, dh_perl can't use this. There's no good way to specify when dh_perl needs to run, short of doing nearly as much work as dh_perl would do when run. Oh well.)

Note that third-party dh_ commands can include these directives too, if that makes sense.

I'm happy how this turned out, but I could be happier about the implementation. The PROMISE directives need to be maintained along with the code of the command. If another config file is added, they obviously must be updated. Other changes to a command can invalidate the PROMISE directive, and cause unexpected bugs.

What would be ideal is to not repeat the inputs of the command in these directives, but instead write the command such that its inputs can be automatically extracted. I played around with some code like this:

$behavior = main_behavior("docs tmp(usr/share/doc/)", sub {
       my $package=shift;
       my $docs=shift;
       my $docdir=shift;

       install($docs, $docdir);

But refactoring all debhelper commands to be written in this style would be a big job. And I was not happy enough with the flexability and expressiveness of this to continue with it.

I can however, dream about what this would look like if debhelper were written in Haskell. Then I would have a Debhelper a monad, within which each command executes.

main = runDebhelperIO installDocs

installDocs :: Monad a => Debhelper a
installDocs = do
    docs <- configFile "docs"
    docdir <- tmpDir "usr/share/doc"
    lift $ install docs docdir

To run the command, runDebhelperIO would loop over all the packages and run the action, in the Debhelper IO monad.

But, this also allows making an examineDebhelper that takes an action like installDocs, and runs it in a Debhelper Writer monad. That would accumulate a list of all the inputs used by the action, and return it, without performing any side effecting IO actions.

It's been 15 years since I last changed the language debhelper was written in. I did that for less gains than this, really. (The issue back then was that shell getopt sucked.) IIRC it was not very hard, and only took a few days. Still, I don't really anticipate reimplementing debhelper in Haskell any time soon.

For one thing, individual Haskell binaries are quite large, statically linking all Haskell libraries they use, and so the installed size of debhelper would go up quite a bit. I hope that forthcoming changes will move things toward dynamically linked haskell libraries, and make it more appealing for projects that involve a lot of small commands.

So, just a thought experiment for now..