Parallel import support [Re: Ubuntu/bzr stories update]

[Andrew Bennetts]

Andrew Bennetts wrote:

>  * Recurring “nice to have” features: parallel imports, nested trees.

On parallel imports, here are more notes I made:

Requirement, or just nice to have?  The main motivation for it (at least as
discussed in the call): as we import more history (more tarballs, debian
versions, then upstream's repo), we want to incorporate it into our existing
bzr branches of packages.  Support for parallel imports would certainly help
a lot here: a more complete history would be a parallel import w.r.t. the
earlier history.

On the other hand, a more limited solution might work well enough.  What's
needed is a repeatable (but not necessarily deterministic) way to rewrite
existing revisions to include the new history.  i.e. when the canonical
(lower-case C!) copy of the branch is rewritten to incorporate more history,
publish the mapping of old revision details to new revision details.  Other
copies of that branch can then use that mapping to apply the same update.
Possibly we can use revision properties on the new revisions to do this.
Jelmer has suggested that perhaps the existing rebase-foreign code might
suit.

We would certainly need to pay attention to edge cases and making sure the
occasional (frequent?) rewrite is not too painful for users.  There is a
danger that aiming for a simple, partial answer for parallel imports would
end up needing as much complexity as aiming for the general solution.

-Andrew.