[rfc] more control over activating fallback repositories (aka what gets stacked on)

[Martin Pool]

On Thu, Oct 9, 2008 at 10:29 AM, Robert Collins
<robertc at robertcollins.net> wrote:
> On Wed, 2008-10-08 at 18:21 +1300, Michael Hudson wrote:
>> Currently, opening a branch also opens any stacked on branch specified
>> in the branch's branch.conf file.  This is inconvenient some of the
>> time, particularly in the innards of the codehosting system on Launchpad
>> where we want to be able to inspect and maybe interfere with the stacked
>> on URL before opening what it points at.

At the moment we have the invariant that if you open a branch, its
repository is always opened and configured with a fallback repository.
 Allowing you to have a branch without those things being true will
probably break code that expects them to be true, and I'm not sure
it'd be generally desirable to change them to cope without it.

However, we do have the shortcoming that you can't really even think
about a branch without opening it, and you can't determine it's
stacked on url without opening it.  One way to put this in would be to
give the branch format the method to get the stacked-on url, and have
the branch instance call that.

This approach means that Launchpad won't be able to pull or scan
branches stacked on branches not located on Launchpad, as we don't
want it making arbitrary outgoing connections.  That may be
reasonable.  I suppose you could automatically or manually mirror that
branch and then transform it as described.

-- 
Martin <http://launchpad.net/~mbp/>