# Calculate packfile and idxfile checksums 3 messages from 2019-01-18 to 2019-01-18. Participants: Farhan Khan, Ævar Arnfjörð Bjarmason. Thread: https://gitlist.dev/t/50257 ## Farhan Khan, 2019-01-18 07:24 Subject: Calculate packfile and idxfile checksums Message-ID: URL: https://gitlist.dev/e/CAFd4kYAbrUcV2U1u6Fz3VgZDN_YcpLrSymeA5bvvLy1LY98aHA%40mail.gmail.com ``` Hi all, I am trying to understand how git's index-pack works, particularly how it calculates the packfile checksum and idxfile checksum. 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 determine where this value is set. 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. Thanks! -- Farhan Khan PGP Fingerprint: B28D 2726 E2BC A97E 3854 5ABE 9A9F 00BC D525 16EE ``` ## Ævar Arnfjörð Bjarmason, 2019-01-18 08:36 Subject: Re: Calculate packfile and idxfile checksums Message-ID: <87lg3i4b15.fsf@evledraar.gmail.com> URL: https://gitlist.dev/e/87lg3i4b15.fsf%40evledraar.gmail.com In-Reply-To: ``` 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? ``` ## Farhan Khan, 2019-01-18 23:51 Subject: Re: Calculate packfile and idxfile checksums Message-ID: URL: https://gitlist.dev/e/CAFd4kYBo7HCBD56KddWdHH_E%3DnToQcCuAmBpWw7P6A3O2eYtcA%40mail.gmail.com In-Reply-To: <87lg3i4b15.fsf@evledraar.gmail.com> ``` On Fri, Jan 18, 2019 at 3:36 AM Ævar Arnfjörð Bjarmason wrote: > > > 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? I apologize for my poor email and appreciate your willingness to help. Let me try that again. My objective is to identify how to calculate the packfile and idxfile checksums and/or where this is performed in the git source code. You said that this value is initialized in builtin/index-pack.c:cmd_index_pack(). Is it correct that this value is the 'pack_hash' variable? At a high level, how is this value calculated? That seems to be the packfile index, how and where is that idxfile hash calculated? Thanks! ```