Migrating to "rich-root"

[John Arbash Meinel]

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We've had a rich-root style format for quite some time now (in -subtree
at least). I think it is past time for us to migrate over to it.

I specifically ran into it with Monty's patch to bzr-email. bzr-email
itself is in plain pack-0.92 format. I assume Monty is using bzr-svn or
somehow has upgrade his repository.

I went to "bzr merge" his patch, and it came back with:
  bzr: ERROR: Revision is not compatible with KnitPackRepository('...')

Now, *I* know why, but I think this is very opaque for someone who is
just merging a contributor's patch.

I think switching to rich-root is going to be rather painful for us, as
if we make it the default, then some people start getting upgraded to
it, and when they try to send submissions to other people, suddenly they
have to upgrade.

We've been putting it off because of this watershed effect, but we have
to do it sometime. bzr-svn sort of tips our hand, because Jelmer needs
the new format. So anyone who wants to experiment with it suddenly has
already jumped the gap.

I wanted to start a thread of where people might run into it, and where
we might need a bit more polish to ease the transition.

For example, the error when trying to merge should give you an
indication of *why* it isn't compatible, as well as *what* you can do to
get to compatibility. (target repository does not support rich roots,
and bzr upgrade, respectively.)

John
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