Mutating history in Subversion and Bazaar

[Aaron Bentley]

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John Arbash Meinel wrote:
> Aaron Bentley wrote:
> ...
> 
> 
>>But if you use a clever fetcher that works by slinging knit deltas
>>around, then yes, it's conveivable to corrupt the knit.  Knits store
>>sha1 hashes, so the corruption would be easy to detect.
>>
>>I don't know whether we check sha-1's when copying deltas from one knit
>>to another.  We could do that, or we could make sure that the sha-1 of
>>the parent in the target knit matches the sha-1 of the parent in the
>>source knit.  So for knits, the knit itself contains enough data to
>>verify that you're not creating a version that cannot be constructed.
>>
> 
> 
> Unfortunately we don't. We check the sha-1 sums when we extract the
> texts, but to check them at 'join()' time means that we have to extract
> the content, apply it to the previous text in the case it is a delta,
> which may mean extracting all of the other previous texts, etc.

Yes.  I suggested we check the parent sha-1's because it's cheaper to do
that.  It provides a good guarantee against history mutation, but it
doesn't provide a good guarantee against broken knit deltas.

> Right now we do:
> 1) download inventories to find out what file ids are modified, keep
> these cached in ram.
> 
> 2) for each file id found in (1):
>     join the remote knit to the local knit (data + index),
>     for a given set of revisions
> 
> 3) join the remote inventory.knit and .kndx to the local inventory.knit
> (the downloaded contents should still be cached in ram)
> 
> 4) join the remote revisions.knit and .kndx to the local revisions.knit.
> 
> Now, if we made some steps more explicit, we could do a little bit more
> integrity checking. The first thing that I would add is
[snip]

Yes, that sounds like a possible improvement, if the overhead isn't too big.

Aaron
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