Darwin builds forking off processes never finish

Describe the bug

I have some Nix build forking off a long-running process in the background. In that specific example, a postgresql database.

The build process starts the database, does some insertions and queries in it, then renders a document into $out (end of build script).

On Linux, after all steps in the build script have been finished, the build exits, and the output can be observed.

On Darwin, the build process gets stuck indefinitely.

A workaround is to manually register cleanup traps in bash.

Steps To Reproduce

Build the following on x86_64-linux vs aarch64-darwin:

let
  pkgs = import <nixpkgs> { };
in
pkgs.callPackage
  (
    with import <nixpkgs> { };
    { stdenv, ... }:

    stdenv.mkDerivation {
      pname = "foo";
      version = "1";
      buildCommand = ''
        sleep infinity &
        
        echo foo > $out
      '';
    }
  )
{ }

Expected behavior

I'd expect in both cases the build to end, and the sleep process to be killed at the end of the build script.

nix-env --version output

nix-env (Nix) 2.13.3

Priorities

Add 👍 to issues you find important.

Start by running the provided derivation on x86_64-linux and aarch64-darwin, then trace the platform-specific build-process cleanup behavior. Compare how the background sleep process is handled on each platform; done means the Darwin build exits and the background process is terminated without manual cleanup traps.