[MERGE][#230567] Faster (local) branch

[Aaron Bentley]

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Ian Clatworthy wrote:
> John Arbash Meinel wrote:
> 
>> I think the reason we had the slow route, is that people wanted to be
>> able to
>> 'bzr co URL .' into a directory that already had files. I don't know who
>> asked
>> for that, etc, but I remember a conversation about it.
> 
> Ah. It seems that is supported. 'branch' OTOH fails if the directory
> already exists.

Rob implemented it.

>> I can say that if it is costing us a lot, then we might want to
>> reconsider how
>> we do it.
> 
> On large trees like Mozilla and OOo, it certainly is costing a lot.
> The bigger the tree, the higher the cost.

Could you please explain *what* is costing us a lot?  Your patch is
fairly opaque.

>> Other comments, though...
>>
>> 1) You set the executability after renaming the files. It seems
>> backward, but
>> maybe it is only changing ones that already existed?
> 
> That's just a reflection of how TreeTransform works now. I'm pretty
> sure there's a TODO in there re setting the execute bit when the
> files are created (in limbo) rather than afterwards, but I was
> keeping the logic sequence as it is today.

That TODO is stale.  I and Rob worked on that once, but there was no
observable performance improvement, so I never merged it.

> This isn't an issue. One issue is walking 100K files to find that we
> only need to rename the top few from limbo across.

Okay, so how can we fix this in the general case?

Aaron
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