[RFC] proposed user doc for nested trees

[John Arbash Meinel]

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> I don't understand what makes them more fragile.  We had a guarantee
> that heads not referenced by the branches in a repository could safely
> be garbage-collected.  Stacking breaks that guarantee.  I don't see how
> it could be more broken.
> 
> Even if shared branches were required, we still could not restore that
> guarantee, because a historical revision of the containing tree might
> refer to a revision of the subbranch that was not in the ancestry of the
> subbranch's head.

But we *could* guarantee that when we fetch the historical revision of
the containing tree, will will also fetch the referenced revision of all
subbranches regardless of whether they are in the ancestry of any given
branch.

We already make the guaranteed that if we fetch revision X, we fetch the
inventory for X, and all chk pages touched in X, and all file texts
touched in X. It isn't particularly harder to include and now revisions
Y as well. (And if fact, I think we *should*.)

I also prefer an 'everything is in this repository over here' approach.
Whether that is just requiring you to be using a shared repository when
using subtrees... I'm okay with that.

> 
>> That's why I thought you addressed that by using a shared
>> repository between containing tree and local branches.
> 
> Sorry if I was misleading, but I said that if the containing branch's
> repository was shared, it would be good to be able to take advantage of
> that.  I also suggested that a subbranch might in some cases be a branch
> reference (as used by lightweight checkouts).
> 
>> If that's not the case, I think
>> http://bazaar-vcs.org/NestedTreesDesign#sub-branches should still
>> be clarified by using more positive definitions.
> 
> I have reorganized it to put the declarations first and the rationale
> afterward.  Does that help?
> 
> Aaron

John
=:->
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