{stdenv, git}:
{url, rev ? "HEAD", md5 ? "", sha256 ? ""}:

/* NOTE:
   fetchgit has one problem: git fetch only works for refs.
   This is because fetching arbitrary (maybe dangling) commits may be a security risk
   and checking whether a commit belongs to a ref is expensive. This may
   change in the future when some caching is added to git (?)
   Usually refs are either tags (refs/tags/*) or branches (refs/heads/*)
   Cloning branches will make the hash check fail when there is an update.
   But not all patches we want can be accessed by tags.

   The workaround is getting the last n commits so that it's likly that they
   still contain the hash we want.

   for now : increase depth iteratively (TODO)

   real fix: ask git folks to add a
   git fetch $HASH contained in $BRANCH
   facility because checking that $HASH is contained in $BRANCH is less
   expensive than fetching --depth $N.
   Even if git folks implemented this feature soon it may take years until
   server admins start using the new version?
*/

stdenv.mkDerivation {
  name = "git-export";
  builder = ./builder.sh;
  buildInputs = [git];

  outputHashAlgo = if sha256 == "" then "md5" else "sha256";
  outputHashMode = "recursive";
  outputHash = if sha256 == "" then md5 else sha256;

  inherit url rev;

  impureEnvVars = [
    # We borrow these environment variables from the caller to allow
    # easy proxy configuration.  This is impure, but a fixed-output
    # derivation like fetchurl is allowed to do so since its result is
    # by definition pure.
    "http_proxy" "https_proxy" "ftp_proxy" "all_proxy" "no_proxy"
    ];
}