nixos: systemd doesn't respect fileSystems.<name>.depends

[ElvishJerricco]

Describe the bug

systemd doesn't respect the depends option for file systems. Usually this is ok because systemd will mount file systems in the correct topological ordering. The depends option is useful because it allows you to express dependencies on e.g. the file systems that an overlay FS's lower/upper/work directories exist on.

Steps To Reproduce

This test should hang forever. Instead, it exits cleanly.

import ./nixos/tests/make-test-python.nix ({ lib, pkgs, ... }: {
  name = "should-hang";

  nodes.machine = {
    systemd.services.block-foo = {
      unitConfig.DefaultDependencies = false;
      serviceConfig = {
        ExecStart = "${pkgs.coreutils}/bin/sleep infinity";
        Type = "oneshot";
      };
    };

    virtualisation.fileSystems = {
      "/foo" = {
        device = "tmpfs";
        fsType = "tmpfs";
        options = ["nofail" "x-systemd.requires=block-foo.service" "x-systemd.after=block-foo.service"];
      };

      "/bar" = {
        device = "tmpfs";
        fsType = "tmpfs";
        depends = ["/foo"];
        # This would fix it:
        # options = ["x-systemd.requires-mounts-for=/foo"];                                                                                                                                   
      };
    };
  };

  testScript = ''                                                                                                                                                                             
    machine.wait_for_unit("bar.mount")                                                                                                                                                        
    machine.fail("mount | grep foo")                                                                                                                                                          
  '';
})

Expected behavior

The block-foo service never exits, so /foo is never mounted, and therefore /bar should never be mounted. So the test should hang.

Additional context

This is also a problem for systemd stage 1, but it's worse because we also have to account for the /sysroot prefix. We can fix this by adding x-systemd.requires-mounts-for= options for every dependency, and prefixing /sysroot for the systemd stage 1 file systems.

[nikstur]

Do we even need the depends option? I don't think it's particularly useful. The overlay also shouldn't be a concern since #286176 anymore.

[ElvishJerricco]

@nikstur Well, currently depends literally only does anything in scripted stage 1. In stage 2 and in systemd stage 1, you have to use x-systemd.requires-mounts-for=, which is what the overlay stuff does. That's fine, but it's an incompatibility between scripted and systemd stage 1. So we need to either deprecate it or implement it, and it's really easy to implement (I just need to fixup #233707). It's a handy convenience for people I guess, though far from a necessary one.