third_party/nix/python.nix

# This is a Python interpreter wrapper that's passed to pip3_import under
# NixOS.
# It allows us to build some pip wheels under NixOS that require special
# system libraries. This is quite hacky, it would be much better if we could
# somehow tell pip3_import that a given package needs to be built within a
# given environment.

with import <nixpkgs> {};

let
  # We use mkDerivation instead of writeScript or writeScriptBin as we need a
  # derivation that both:
  # - has a directory structure (for rules_nixpkgs to be able to use it)
  # - has the Python interpreter directly in that structure and not in bin/, as
  #   rules_python's pip3_import interpreter_path requires a file target, and
  #   will not take an alias. Meanwhile, rules_nixpkgs only creates a BUILD file
  #   in the root path of the external repository (which is populated with a
  #   symlink tree from the nix derivation), so we can onlly directly reference
  #   file in the root of a Nix derivation.
  generic = package: binary:  stdenv.mkDerivation {
    name = "${binary}-wrapper";
    version = "1.0";
    src = ./.;
    unpackPhase = "";
    buildPhase = ''
      mkdir -p $out
      cat > $out/${binary} <<EOF
#!/bin/bash

# pyscopg wants libpq, and uses pg_config to find paths. Inject pg_config into
# the Python interpreter's path.
export PATH="${pkgs.postgresql}/bin:\$PATH"

# uWSGI has a truly cheese-grade build system, and this is the only way to let
# it know where to find ncurses.
export LDFLAGS="-L${pkgs.ncurses}/lib"
exec ${package}/bin/${binary} "\$@"
EOF
    '';
    installPhase = ''
      chmod +x $out/${binary}
    '';
  };

in {
  # Add cffi for import _cffi_backend in `cryptography` to work.
  python2 = generic (pkgs.python27.withPackages (ps: with ps; [ cffi ])) "python2";
  python3 = generic (pkgs.python37.withPackages (ps: with ps; [ cffi ])) "python3";
}