Re: [JGIT PATCH 1/2] index-pack: Avoid disk corruption yielding a valid pack footer checksum

> When we are processing a thin pack and making it non-thin we need
> to update the header with a new object count.  That causes us to
> recompute the footer checksum for the entire pack, and the only
> way to do that is to re-read the data from disk.
> 
> If there was filesystem corruption in the process (e.g. a bad
> disk sector, or a kernel bug) we don't want to produce a valid
> pack at the end.  Instead we need to fail-fast with the error
> so the user is aware of the corruption.
> 
> We now keep track of where the end of the original data is and
> run two SHA-1 computations during the header-footer fixup.  If
> the original data region doesn't match the original footer we
> got over the network we know there was corruption and we just
> cannot trust this pack file.
> 
> Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
> ---
> 
>   This was inspired by the data corruption thread started
>   by J. Bruce Fields.  At some point in the thread Linus
>   pointed out the C Git index-pack isn't as safe as it can
>   be, and offered a strategy to fix it.