Re: Calculate packfile and idxfile checksums

>
>
> 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?