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
  # Add cffi for import _cffi_backend in `cryptography` to work.
  py = pkgs.python37.withPackages (ps: with ps; [ cffi ]);

# 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.
in stdenv.mkDerivation {
  name = "py-wrapper";
  version = "1.0";
  src = ./.;
  unpackPhase = "";
  buildPhase = ''
    mkdir -p $out
    cat > $out/python3 <<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 ${py}/bin/python3 "\$@"
EOF
  '';
  installPhase = ''
    chmod +x $out/python3
  '';
}