Comment 13 for bug 388269

Vincent and I chatted about this, and I think we've settled that we need a new method on "ParentsProvider", which exposes the fact that some data may be cached, while other data may require an expensive call. This makes it a little bit risky for a stable series, but not unbearably so. I'll plan on progressing by starting with a 2.4 branch, land it in bzr.dev, smoke test it for a while, and then we can decide if we want to backport it.

Note that I think the patch I have already put together for this branch is still useful, it is orthogonal to the next patch.