Motivation for this change

When you have a NixOS configuration like the following:

{ config, ... }:

{
  boot.loader.grub.device = "nodev";
  fileSystems."/" = {
    device = "nodev";
    options = [ "uid=${toString config.users.users.root.uid}" ];
  };
  fileSystems."/foo" = {
    device = "nodev";
  };
}

And you test it like so:

NIXOS_CONFIG=debug-fs-user-recursion.nix nix-instantiate --eval \
  --expr '(import <nixpkgs/nixos> { }).config.services.rpcbind.enable'

You will get infinite recursion:

1. rpcbind.nix defines users.extraUsers.rpc
2. nfs.nix sets services.rpcbind.enable depending on config.boot.supportedFilesystems
3. filesystems.nix defines boot.supportedFilesystems based on toposort fsBefore (attrValues config.fileSystems)
4. utils.nix defines fsBefore and uses config.fileSystems.<name>.options to do so
5. this file defines fileSystems."/".options to refer to config.users.users (extraUsers is an alias for users)
6. causing infinite recursion

The original check had this:

((any (x: elem x [ "bind" "move" ]) b.options) && (a.mountPoint == b.device))

However any file system type that allows a device to be what is also valid in a mount point (i.e. normal directories), like "bind" and "move" do, are the only ones that will ever pass the a.mountPoint == b.device condition. Hence the precondition any (x: elem x [ "bind" "move" ]) b.options should be unnecessary. Removing the precondition solves the infinite recursion issue.

Then to address # FIXME: it's incorrect to simply use hasPrefix here: "/dev/a" is not a parent of "/dev/ab", we could just ensure that the prefix check always end with a "/", such that hasPrefix only succeeds if its an actual parent, like so:

hasPrefix "${a.mountPoint}${optionalString (!(hasSuffix "/" a.mountPoint)) "/"}" b.mountPoint

Things done

• 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 nox --run "nox-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)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.