> Let's start bikeshedding the calling signature.  I like having a
> single callback, since multiple callables are a nuisance to manage.

IMO the callback should have a second argument as a dict containing
various statistics that we can expand over time. The generation number,
for example, should be present.

As for the phase number, if 0 means start and 1 means end, you can't
decently add another phase anyway (having 2 mean "somewhere between 0
and 1" would be completely confusing).

PS : please don't use C++-style comments in your patch