[rfc] bzr-colo into core

[Matthew D. Fuller]

On Fri, Mar 25, 2011 at 06:42:35PM +1100 I heard the voice of
Martin Pool, and lo! it spake thus:
> 
> One point is that it makes it more dangerous to garbage-collect the
> origin branch; but that is not something people tend to do often in
> Bazaar, and the same kind of danger is already present with
> stacking.  We could make the referring branches add a note that they
> exist (either a full url or just a flag.)

Well, people don't GC often because we don't have a command to do it.
That's kinda strawman   :p

I think it's much MORE dangerous than stacking, for several reasons.

One is that stacking is generally something you do "on top of", then
move forward in your branch.  So all the stacked-on revs are already
part of the history of that origin branch, so no GC would remove them
anyway.  Contrarily, with a repository reference, you're presumably
going to be storing in NEW revs which aren't in those existing
branches.  So you're storing up attractive targets for GC.

A second is that this sounds like something that will wind up being
"recommended".  We've never recommended stacked branches; even shy of
the various bugs that afflict them, they're still a "there may be
cases where you want to do this, but".  Heck, if you'd ask me about
them, I'd say "this is a primitive that could be useful for several
things, and was developed just far enough for Launchpad to use"...
But here you're talking referenced repositories as a standard
workflow.

And by being single-branch on single-branch, you're limiting the scope
of potential damage.  But doing whole repositories on one or both
sides opens things pretty wide.

Generally, I'm OK with a CVS/SVN/mtn/etc setup where you can have a
repo over THERE, and checkouts over HERE, and the repo knows nothing
about the checkouts, and the two parts depend on each other but aren't
physically connected.  And it's OK to have a traditional-bzr
everything together, or a shared-repo setup where they're split but
still physically connected (by having one under the other).  But
trying to blend the two makes me all kinds of antsy.  We should
aboslutely let people shoot themselves in the foot, but we don't need
to ramp up a R&D program for homing bullets   :)


-- 
Matthew Fuller     (MF4839)   |  fullermd at over-yonder.net
Systems/Network Administrator |  http://www.over-yonder.net/~fullermd/
           On the Internet, nobody can hear you scream.