Cannot upgrade stacked branches from 1.9 to 2a on Launchpad

lp:bzr and lp:bzr/2.0 both upgraded from 1.9 format to 2a format yesterday. So I want to upgrade my 1.9 format branches that are stacked on those.

I can execute "bzr upgrade lp:~spiv/bzr/..." just fine, but this doesn't cause the branch to be updated. i.e. nothing visible happens on e.g. <https://code.edge.launchpad.net/~spiv/bzr/insert-stream-check-chk-root>, everything is exactly as it was before the upgrade. The mirrored branch is still in the old format. That's the first part of this bug.

The second part is that when I force Launchpad to rescan the branch with e.g. "bzr push --overwrite -r -2 && bzr push", Launchpad reports an error on the branch page:

"CHKInventoryRepository('lp-mirrored:///~bzr-pqm/bzr/2.0/.bzr/repository') is not compatible with KnitPackRepository('lp-mirrored:///~spiv/bzr/insert-stream-check-chk-root/.bzr/repository') different rich-root support"

But I upgraded precisely to fix that problem!

On #launchpad, Michael speculated that this is because Branch.open on the existing (un-upgraded) mirrored branch fails, and that occurs before comparing repository formats. The code either needs to check the repository format before the branch format, or pass ignore_fallbacks to BzrDir.open_branch, or something like that.

Any non-rich-root -> rich-root upgrade of a branch will cause this for all branches that stack on that branch.