blah.txt

  fetchFromGitHub = {
    owner, repo, rev, name ? "source",
    fetchSubmodules ? false, private ? false,
    githubBase ? "github.com", varPrefix ? null,
    ... # For hash agility
  }@args: assert private -> !fetchSubmodules;
  let
    baseUrl = "https://${githubBase}/${owner}/${repo}";
    passthruAttrs = removeAttrs args [ "owner" "repo" "rev" "fetchSubmodules" "private" "githubBase" "varPrefix" ];
    varBase = "NIX${if varPrefix == null then "" else "_${varPrefix}"}_GITHUB_PRIVATE_";
    # We prefer fetchzip in cases we don't need submodules as the hash
    # is more stable in that case.
    fetcher = if fetchSubmodules then fetchgit else fetchzip;
    privateAttrs = lib.optionalAttrs private {
      netrcPhase = ''
        if [ -z "''$${varBase}USERNAME" -o -z "''$${varBase}PASSWORD" ]; then
          echo "Error: Private fetchFromGitHub requires the nix building process (nix-daemon in multi user mode) to have the ${varBase}USERNAME and ${varBase}PASSWORD env vars set." >&2
          exit 1
        fi
        cat > netrc <<EOF
        machine ${githubBase}
                login ''$${varBase}USERNAME
                password ''$${varBase}PASSWORD
        EOF
      '';
      netrcImpureEnvVars = [ "${varBase}USERNAME" "${varBase}PASSWORD" ];
    };
    fetcherArgs = (if fetchSubmodules
        then { inherit rev fetchSubmodules; url = "${baseUrl}.git"; }
        else ({ url = "${baseUrl}/archive/${rev}.tar.gz"; } // privateAttrs)
      ) // passthruAttrs // { inherit name; };
  in fetcher fetcherArgs // { meta.homepage = baseUrl; inherit rev; };