[RFC] Caching in Stacked Branches

[John Arbash Meinel]

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At the moment stacked branches only contain what you have pushed into
them. Which means they work fairly well for "publishing" (bzr push
- --stacked).

There is another use for stacked branches, which is the "get me the tip
of a project so I can make a quick patch". As an example, to branch
"python/trunk" takes about 200+ MB to download. By comparison, doing
"bzr branch --stacked" only really needs to download about 21MB to get
all of the working files.

The downside, though, is that you download that 21MB, and then have to
download it *again* whenever doing any more work. (Such as during,
commit, diff, etc.)

One fairly obvious possibility, would be to have Stacked branches cache
any data that gets requested from a fallback.

I was looking into it in the past, and the problem is that stuff like
"diff" only has a read lock, and to insert things into the repo, you
really need a write_lock() + start_commit_group().

We could add a new locking, which would be aware of wanting to cache
things in the stacked branch.

Any thoughts? It would be pretty slick if doing:

 bzr branch --stacked $BIG_PROJECT work
 cd work
 < change stuff >
 bzr diff
 bzr commit -m "my local change"
 bzr send

Could download just the content for the working files, and then not have
to download anything extra.

(I was able to hack this together as a test, but it seems like something
we would really want to support.)

John
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