Lazily registering hooks?

[Robert Collins]

A couple of things - we already have a registry of hooks, so there
isn't a  big conceptual change there, but I would be sad to see the
details of hooks centralised - if feels bad.

Another thing we could do is supply a hook to trigger that calls back
when some hook point becomes available.

You wouldn't bother with it for most things: effectively all
operations need Tree, Branch, Repository, Lock, Command etc. But to
lazy handle info and rio:

def hook_when_used(modulename, attrname, hooks):
    """Called when a hooks object becomes available.

    :param modulename: The module the hooks are defined in. e.g. bzrlib.smart
    :param attrname: The attribute within the module. e.g. SmartTCPServer.hooks
    :param hooks: The hooks object itself
    """
    if (modulename, attrname) == ('bzrlib.msgeditor', 'hooks'):
        hooks.install_named_hook(....)
    elif ...


And finally, when using this api, you'd get called immediately for
already-present hooks, so you wouldn't need to care about what order
things happen in.

-Rob