Re: pack corruption post-mortem

On Sat, Oct 19, 2013 at 7:41 AM, Nicolas Pitre [off-list ref] wrote:

On Sat, 19 Oct 2013, Duy Nguyen wrote:

quoted

On Wed, Oct 16, 2013 at 3:34 PM, Jeff King [off-list ref] wrote:

quoted

I was recently presented with a repository with a corrupted packfile,
and was asked if the data was recoverable. This post-mortem describes
the steps I took to investigate and fix the problem. I thought others
might find the process interesting, and it might help somebody in the
same situation.

I started with an fsck, which found a problem with exactly one object
(I've used $pack and $obj below to keep the output readable, and also
because I'll refer to them later):

    $ git fsck
    error: $pack SHA1 checksum mismatch
    error: index CRC mismatch for object $obj from $pack at offset 51653873
    error: inflate: data stream error (incorrect data check)
    error: cannot unpack $obj from $pack at offset 51653873

I wonder if we should protect the sha-1 and pathname tables in packv4
with CRC too. A bit flipped in there could cause stream of corrupt
objects and make it hard to pinpoint the corrupt location..

It turns out that we already have this covered.

The SHA1 used in the name of the pack file is actually the SHA1 checksum
of the SHA1 table.

I continue to believe this naming is wrong. The pack file name should
be the SHA1 checksum of the pack data stream, but the SHA1 table. This
would allow cleaner update of a repository that was repacked with
different compression settings, but identical objects.