Re: Calculate packfile and idxfile checksums

From: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Date: Jan 18, 2019, 08:36 UTC
Message-ID: <87lg3i4b15.fsf@evledraar.gmail.com>
In-Reply-To: <CAFd4kYAbrUcV2U1u6Fz3VgZDN_YcpLrSymeA5bvvLy1LY98aHA@mail.gmail.com>

On Fri, Jan 18 2019, Farhan Khan wrote:

> Hi all,
>
> I am trying to understand how git's index-pack works, particularly how
> it calculates the packfile checksum and idxfile checksum.

It's unclear if this is what you're asking, but the pack checksum is just a 20 bytes of SHA1 of the preceding contents at the end for *.idx and *.pack. As seen with this program:

    $ perl -MFile::Slurp=slurp -MDigest::SHA=sha1 -wE 'my $f = shift; my $c = slurp($f); my $cp = $c; $cp =~ s/.{20}$//s; my $n = $cp . sha1($cp); if ($n eq $c) { say "Computed checksum trailer for $f" } else { say "Failed trailer for new content for $f is different" }' pack-79c2ccce950e6676452dc9f0473f80003e7ccdef.idx
    Computed checksum trailer for pack-79c2ccce950e6676452dc9f0473f80003e7ccdef.idx

You can also feed it *.pack files.

> I traced back the packfile checksum in the source to the value char
> *sha1 that is utilized in write_idx_file() in pack_file.c:45. However,
> I cannot

It seems you mean pack-write.c not pack_file.c

> determine where this value is set.

It's initialized in cmd_index_pack() and then passed down to that function.

> My printf() debugging has it set at pack-write.c:171 (right before the
> hashwrite call) but it does not seem to be utilized prior to that
> point. Please assist.

I'm happy to help, but still not quite sure what the source of the confusion is, maybe that the variable in index-pack.c has a different name and is passed down to pack-write.c's function as a pointer?