RFC: rollback commit in some situations

[Aaron Bentley]

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Robert Collins wrote:
> On Thu, 2008-08-14 at 01:00 -0400, Aaron Bentley wrote:

>> It seems like it would be better to not reference the commit until we
>> know it can't raise these exceptions.  Is that possible?
> 
> In general no (svn), but for bzr it seems possible - put it in the repo,
> update the tree, update the branch tip. It does raise a different race
> condition though - which is that if the tree change succeeds but the
> branch change fails, you have a tree that is ahead of the branch
> (equivalent to someone doing 'branch.uncommit()' while bound, without
> your tree there to update in parallel).
> 
> I'm not sure which race is better :/.

I would think that a race that messes up a tree is better, because
branches have more visibility than trees.  But I can live with either
solution.

Do bear in mind that TreeTransform has some nice facilities for rolling
back tree changes.

Aaron
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