[Nix-dev] ‘nix-prefetch-git’ optimizations

[Ludovic Courtès]

Hello,

Nicolas Pierron <nicolas.b.pierron at gmail.com> writes:

> Author: NicolasPierron
> Date: 2010-01-18 08:28:38 +0000 (Mon, 18 Jan 2010)
> New Revision: 19513
>
> You can view the changes in this commit at:
>    https://svn.nixos.org/viewvc/nix?rev=19513&view=rev
>
> Modified:
>    nixpkgs/trunk/pkgs/build-support/fetchgit/nix-prefetch-git
>
> Log:
> nix-prefetch-git:
> * Optimized download size in case of references.
> * Add support for submodules.

[...]

> +# Fetch only a branch/tag and checkout it.
> +checkout_ref(){
> +    local hash="$1";
> +    local ref="$2";
> +
> +    if test -z "$ref"; then
> +        ref=$(ref_from_hash $hash);
> +    fi;
> +
> +    if test -n "$ref"; then
> +        # --depth option is ignored on http repository.
> +        git fetch --depth 1 origin +"$ref" || return 1

People tried ‘--depth 1’ in the past (see r16047) and reported problems,
because the ‘--depth’ argument asks Git to fetch the last revisions of
the given on *any* branch, which is often useless (see r18438).  Does
your code specifically work around that?

Besides, this technique prevents “git describe” from working, probably
because it can’t access all the tags and history.  This causes problems
to most projects at http://hydra.nixos.org/project/gnu because they
derive their version string from the output of “git describe” – a very
useful feature because it leads to source tarballs with a git commit
hash in their names.  In some cases having a wrong version string can
even lead to test failures.

Any idea how to restore that functionality?

> +clone(){
> +    local top=$(pwd)
> +    local dir="$1"
> +    local url="$2"
> +    local hash="$3"
> +    local ref="$4"
> +
> +    cd $dir;
> +
> +    # Initialize the repository.
> +    init_remote "$url";
> +
> +    # Download data from the repository.
> +    checkout_ref "$hash" "$ref" ||
> +    checkout_hash "$hash" "$ref" || (
> +        echo 1>&2 "Unable to checkout $hash$ref from $url.";
> +        exit 1;
> +    )
> +
> +    # Checkout linked sources.
> +    init_submodules;
> +
> +    cd $top;
> +}

[...]

> -    git clone "$url" $tmpFile 1>&2
> -    if test -n "$rev"; then
> -      cd $tmpFile
> -      echo $tmpFile >&2
> -      git checkout $rev 1>&2
> -    fi
> +    case "$rev" in
> +        HEAD|refs/*)
> +            clone "$tmpFile" "$url" "" "$rev" 1>&2;;

Why not just use ‘git clone "$url" "$tmpFile"’ here?  I would find it
simpler and easier to understand.

Thanks,
Ludo’.