Nix - Avoid overrideAttrs when wrapping an executable

Posted on June 3, 2022 by Marijan

Let’s say you created a program which requires another executable to be present in your environment i.e. in the PATH variable.

Using Nix, there is a way to ensure that your program will only be distributed together with that executable by wrapping it.

E.g. we want a program named demo to be packaged such that hello is available in the PATH.

Wrong solution

What I used to do was the following:

demo = <some package>;
demo-wrapped = demo.overrideAttrs (oldAttrs: rec {
  buildInputs = oldAttrs.buildInputs or [] ++ [ pkgs.makeWrapper ];
  postInstall = oldAttrs.postInstall or "" + ''
    wrapProgram $out/bin/demo \
      --set PATH ${pkgs.lib.makeBinPath [ pkgs.hello ]}

But recently Matthew made me aware that my approach causes compilation of the wrapped executable (demo) to occur twice under certain circumstances.

The reason for that is that overrideAttrs per definition runs through all phases again just to run the new customized phase (in this example: postInstall).

If demo itself was not changed in any way but demo-wrapped was modified e.g. by logging a message to stdout, Nix would compile demo again just to run the updated postInstall phase instead of obtaining it either from a binary cache or from the local Nix store.

The reason I used wrapProgram inside overrideAttrs was that I found an usage example of makeWrapper written like this in nixpkgs but with a subtle difference I didn’t recognize. In the usage example from nixpkgs recompilation was inevitable because there was a change in overrideAttrs that caused the result of the compilation of the binary to change completely. So the author decided to add makeWrapper along the way. For me this was not the case, therefore one should not use overrideAttrs just to wrap an executable.

Better solution

To avoid the problem mentioned above, a much better approach is to do the following:

demo-wrapped-better = pkgs.runCommand "demo-wrapped-better" {
    buildInputs = [ pkgs.makeWrapper ];
      mkdir -p $out/bin
      makeWrapper ${demo}/bin/demo $out/bin/demo \
        --set PATH ${pkgs.lib.makeBinPath [ pkgs.hello ]}

I would like to thank Matthew for taking his time to explain me this subtle issue.