Knit inventory corrupt / wrong sha1

[Aaron Bentley]

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John Arbash Meinel wrote:
> I've been helping Mariano debug this problem, and it boils down to
> hardware problems.

Hmm.

I think it's worth sharing a possible cause I came up with, just so we
can be aware of it: recursive search and replace.

If a search & replace entered .bzr and actually found something to
replace, the results could be quite bad.  I guess this isn't a new
problem, and many of our files are gzipped, so their contents are
unlikely to match anyway.

We probably don't need to take extra measures.  If we did, the simplest
option that comes to mind is to XOR anything that might match.


> It could be because inventory.knit has 8-bit numbers, and the errors are
> always in the high bit. (So plain text files never have that bit set,
> and thus you don't get the error).
> 
> Regardless, if we can't guarantee that a plain read works, I don't think
> bzr can really function.

CRCs can be used to detect single-bit errors and also to correct them.
But we probably don't want that level of robustness.

Aaron

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