Re: [PATCH] Fix packname hash generation.

Junio C Hamano <junkio@cox.net> writes:

In case it was not obvious, this is not a backward incompatible change. Your existing packs will be valid after this change.

What those 40-byte hashes were buying us was that we did not have to worry about name clashes. We could have said "these two packs have the same name so they must have the same set of objects", but there is no tool that relies on this fact. We could not even say "these two packs have different names so the set of objects contained by them must be different" -- the resulting pack name depended on the order of objects fed to git-pack-objects, even if you fed the same set of objects.

The really core part never cared about how packfiles and their indices are named. The only restrictions were that they live immediately under .git/objects/pack/, have .pack and .idx suffix respectively, and their basename match with each other.

The commit walkers (anything that link with fetch.c) impose another limitation that their basenames are "pack-" followed by 40-byte hexadecimal digits. But they do not check if the name is consistent with the set of objects in the pack (checking it was computationally infeasible for huge packs in the previous hashing mechanism -- you have to feed all permutations of objects contained in the pack to SHA1 hash and see if any produces the same hash as the pack name). We _could_ now do this additional check if we wanted to (the same goes to the really core part in sha1_file.c::check_packed_git_idx()).

In short, it does not matter if your existing packs are named using the old hashing mechanism. They will continue to be valid.

But if you really care about consistency, here is an easy way to rename your existing packs to their new names the new hashing scheme would produce.

#!/bin/sh

: ${GIT_DIR=.git} : ${GIT_OBJECT_DIRECTORY="${GIT_DIR}/objects"}

O="$GIT_OBJECT_DIRECTORY"
P="$GIT_OBJECT_DIRECTORY/pack"
for existing in `cd "$GIT_OBJECT_DIRECTORY" &&
		 find pack -name '*.pack' -print`
do
    idx=`expr "$existing" : '\(.*\)\.pack$'`.idx &&
    test -f "$O/$idx" || {
        echo >&2 "Missing idx $idx?"
        continue
    }
    new=`git-index-pack -o tmp-idx "$O/$existing"` || {
        echo >&2 "Corrupt pack $existing?"
        continue
    }           
    # index generated for an existing pack should match.
    cmp "$O/$idx" tmp-idx || {
        echo >&2 "Corrupt idx $idx?"
        continue
    }
    if test "pack/pack-$new.pack" = "$existing"
    then
        echo >&2 "Already converted $existing."
        continue
    fi
    if test -f "$P/pack-$new.pack" || test -f "$P/pack-$new.idx"
    then
        echo >&2 "Name clash! $new"
        continue
    fi
    mv "$O/$existing" "$P/pack-$new.pack" &&
    mv "$O/$idx" "$P/pack-$new.idx" || {
        echo >&2 "Cannot move $existing to $new"
        continue
    }
    echo >&2 "Renamed $existing -> $new"
done