History horizons: how hard can they be ?

[Robert Collins]

On Sun, 2009-11-15 at 18:54 +0100, Jelmer Vernooij wrote:
> Imports from foreign branches are currently quite slow. This has got to
> do with the fact that we actually have to process all of the data and
> can't just copy verbatim as we can when cloning between two repositories
> with the same format. In Subversion's case there is also the fact that
> fetching all data in a repository is relatively slow.
> 
> Stacking seems like a good solution here, but unfortunately we don't
> support inter-format stacking yet and it won't be trivial to implement
> inter-format stacking.
> 
> Another option is supporting history horizons - just letting everything
> except for the last X revisions on the mainline be a ghost. This would
> work well at least for bzr-svn. 
> 
> As far as I can tell the only things required to support history
> horizons properly are:
> 
>  * have a --horizon X option to 'bzr branch' that limits the revisions
> to fetch
>  * have a branch.conf option that limits the revisions to fetch
>  * support ghosts on the mainline well (there are some open bugs in this
> area)
> 
> Am I missing anything important?

Yes, stopping fetch ever grabbing those revisions again: history
horizons are _not_ the same thing as 'just download a bit less'. They
are intended as a 'hard stop' on history - the history behind the
horizon is forever inaccessible.

Beyond that conceptual thing, I assume you want something more like
stacking, where old content is still accessible, *if* you're online or
when you do a merge?

There, its easier, because you can allow 'merge' and other commands to
still work.

However, I would expect bugs in:
 - annotate
 - merge
 - log FILE
 - fetching to old repositories (which do not support ghosts at the
store level as cleanly)

Further to that revno calculation is still all-history, and that would
really have to be fixed as a necessary condition.

I'd love to see you work on this; I suggest a long lived pipe or loom
tracking your work and all the various weird and wonderful bugs you'll
find.

-Rob
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