pyqt cannot load plugins

[kirelagin]

This repository has a test that pyqt5 builds, but it does not test that it actually works. In reality, even a “hello world”, i.e. trying to show an empty window, fails with:

qt.qpa.plugin: Could not find the Qt platform plugin "xcb" in ""
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

This is probably somehow related to NixOS/nixpkgs#65399 and the solution is to set $QT_PLUGIN_PATH correctly. I did this by adding:

{
  postInstall = ''
    for i in "$out"/bin/*; do
      wrapProgram "$i" --prefix QT_PLUGIN_PATH : "${pkgs.qt5.qtbase}/${pkgs.qt5.qtbase.qtPluginPrefix}"
    done
  '';
}

Ideally, we’d want to make this happen automatically, and also somehow reuse wrapQtAppsHook. The problem with this hook is that it is too smart and it only wraps ELF binaries.