History horizons spec

[Aaron Bentley]

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John Arbash Meinel wrote:
>>I don't know about 'historical integrity'.  What would it allow us to do?

> Make sure that if we ever request an old revision, it actually matches
> what we said it was.

I suppose.  I tend to conflate integrity with authenticity, I guess.  So
 I was thinking that we should use an authenticity check to ensure
integrity.  But no, if you've got an inauthentic revision asd at mkfbf, you
want to diff against the inauthenic one, not the authentic one.

> It bothers me a lot right now that we don't
> actually verify most of the sha1 hashes. I put in the code to verify the
> Weave.get_lines() so we at least check that the text matches what the
> weave says it should. But we don't check that the inventory sha1 matches
> the Weave's sha1, and we certainly don't check that Revision points to a
> valid inventory.

I agree that it would make sense to check these things within a given
repository.

> I would really like to be able to say both that this revision id is my
> ancestor, and that it should have a hash of this. We got rid of the
> chained hashes, because they didn't work properly with ghosts, and they
> made upgrading difficult.
> 
> I would like to track the ancestors + their hash. I don't think it would
> add a lot of overhead (its not like the Arch patch log problem).

It would probably be something like 1 meg uncompressed for the bzr.dev
tree.  (Because it's 470k for half that information in the ancestry cache.)

It's important that retrieving such info should not be time-consuming;
otherwise, we lose the performance advantage that shallow branches
provide.  That suggests that all repositories should keep an up-to-date
version of this data.  Which means we're duplicating information, which
I really hate doing.  But maybe we should.

Aaron
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