Motivation for this change

This fixes a regression from bb649d9.
There were permission problems, when the preStart script tried to copy the smokeping.fcgi file over the old file.

834fi85dzixkalmk383gp0v9ps6rd4gy-unit-script-smokeping-pre-start[1545]: cp: cannot create regular file '/var/lib/smokeping/smokeping.fcgi': Permission denied

Things done

As I don't see any reason why smokeping.fcgi should be writable, I replaced it with a symlink from /nix/store.

Warning: If you experience this problem, applying this patch and rebuilding your system is not enough:

• You need to manually remove the old file with higher privileges than smokeping user has.
• I also needed to restart thttpd.service.

This needs to be backported to nixos-19.09.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

cc @cransom as they introduced the module.