Dak Extensions

One of the challenges maintaining the Debian archive kit (dak) is dealing with Debian-specific requirements: fundamentally because there are a lot of them, and they can get quite hairy — yet at the same time, you want to keep them as separate as possible both so dak can be used elsewhere, and just so you can keep your head around what’s going on. You can always add in hooks, but that tends to make the code even harder to understand, and it doesn’t really achieve much if you hadn’t already added the hook.

However, dak’s coded in python, and being an interpreted language with lots of support for introspection, that more or less means there’s already hooks in place just about everywhere. For example, if you don’t like the way some function in some other module/class works, you can always change it (other_module.function = my_better_function).

Thus, with some care and a bit of behind the scenes kludging, you can have python load a module from a file specified in dak.conf that can both override functions/variables in existing modules, and be called directly from other modules where you’ve already decided a configurable hook would be a good idea.

So, at the moment, as a pretty simple example there’s an init() hook invoked from the main dak.py script, which simply says if userext.init is not None: userext.init(cmdname).

But more nifty is the ability to replace functions, simply by writing something like:

# Replace process_unchecked.py's check_signed_by_key
@replace_dak_function("process-unchecked", "check_signed_by_key")
def check_signed_by_key(old_chk_key):
    changes = dak_module.changes
    reject = dak_module.reject

That’s made possible mostly by the magic of python decorators — the little @-sign basically passes the new check_signed_by_key function to replace_dak_function (or, more accurately, the function replace_dak_function(...) returns), which does the dirty work replacing the function in the real module. To be just a little bit cleverer, it doesn’t replace it with the function we define, but its own function with simply invokes our function with an additional argument to whatever the caller supplied, so we can invoke the original function if we choose (the old_chk_key parameter — the original function takes no arguments, so our function only takes one).

Right now, we don’t do much interesting with it; but that should change once Ganneff’s little patch is finished, which should be RSN…

Hopefully, this might start making it easier to keep dak maintained in a way that’s useful for non-Debian installs — particularly if we can get to the point where hacking it for Debian generally just implies changing configuration and extension stuff — then we can treat updating all the real scripts as a regular software upgrade, just like it is outside Debian.

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