how to customize nix package builder script - cmake

Question

The root problem is that nix uses autoconf to build libxml2-2.9.14 instead of cmake, and a consequence of this is that the cmake-configuration is missing (details like version number, platform specific dependencies like ws2_32 etc which are needed by my project cmake scripts). libxml2-2.9.14 already comes with cmake configuration and works nicely, except that nix does not use it (I guess they have their own reasons).
Therefore I would like to reuse the libxml2-2.9.14 nix package and override the builder script with my own (which is a trivial cmake dance).
Here is my attempt:
defaultPackage = forAllSystems (system:
let
pkgs = nixpkgsFor.${system};
cmakeLibxml = pkgs.libxml2.overrideAttrs( o: rec {
PROJECT_ROOT = builtins.getEnv "PWD";
builder = "${PROJECT_ROOT}/nix-libxml2-builder.sh";
});
in
Where nix-libxml2-builder.sh is my script calling cmake with all the options I need. It fails like this:
last 1 log lines:
> bash: /nix-libxml2-builder.sh: No such file or directory
For full logs, run 'nix log /nix/store/andvld0jy9zxrscxyk96psal631awp01-libxml2-2.9.14.drv'.
As you can see the issue is that PROJECT_ROOT does not get set (ignored) and I do not know how to feed my builder script.
What am I doing wrong?

Answer 1

Guessing from the use of defaultPackage in your snippet, you use flakes. Flakes are evaluated in pure evaluation mode, which means there is no way to influence the build from outside. Hence, getEnv always returns an empty string (unfortunately, this is not properly documented).
There is no need to refer to the builder script via $PWD. The whole flake is copied to the nix store so you can use your files directly. For example:
builder = ./nix-libxml2-builder.sh;
That said, the build will probably still fail, because cmake will not be available in the build environment. You would have to override nativeBuildInputs attribute to add cmake there.