nixos/tests: add non-default-filesystems test

nixos/tests/all-tests.nix

···
   nixpkgs = pkgs.callPackage ../modules/misc/nixpkgs/test.nix { inherit evalMinimalConfig; };
   node-red = handleTest ./node-red.nix {};
   nomad = handleTest ./nomad.nix {};
+  non-default-filesystems = handleTest ./non-default-filesystems.nix {};
   noto-fonts = handleTest ./noto-fonts.nix {};
   novacomd = handleTestOn ["x86_64-linux"] ./novacomd.nix {};
   nsd = handleTest ./nsd.nix {};

nixos/tests/non-default-filesystems.nix

···
+import ./make-test-python.nix ({ lib, pkgs, ... }:
+{
+  name = "non-default-filesystems";
+
+  nodes.machine =
+    { config, pkgs, lib, ... }:
+    let
+      disk = config.virtualisation.bootDevice;
+    in
+    {
+      virtualisation.useDefaultFilesystems = false;
+
+      boot.initrd.availableKernelModules = [ "btrfs" ];
+      boot.supportedFilesystems = [ "btrfs" ];
+
+      boot.initrd.postDeviceCommands = ''
+        FSTYPE=$(blkid -o value -s TYPE ${disk} || true)
+        if test -z "$FSTYPE"; then
+          modprobe btrfs
+          ${pkgs.btrfs-progs}/bin/mkfs.btrfs ${disk}
+
+          mkdir /nixos
+          mount -t btrfs ${disk} /nixos
+
+          ${pkgs.btrfs-progs}/bin/btrfs subvolume create /nixos/root
+          ${pkgs.btrfs-progs}/bin/btrfs subvolume create /nixos/home
+
+          umount /nixos
+        fi
+      '';
+
+      virtualisation.fileSystems = {
+        "/" = {
+          device = disk;
+          fsType = "btrfs";
+          options = [ "subvol=/root" ];
+        };
+
+        "/home" = {
+          device = disk;
+          fsType = "btrfs";
+          options = [ "subvol=/home" ];
+        };
+      };
+    };
+
+  testScript = ''
+    machine.wait_for_unit("multi-user.target")
+
+    with subtest("BTRFS filesystems are mounted correctly"):
+      machine.succeed("grep -E '/dev/vda / btrfs rw,relatime,space_cache=v2,subvolid=[0-9]+,subvol=/root 0 0' /proc/mounts")
+      machine.succeed("grep -E '/dev/vda /home btrfs rw,relatime,space_cache=v2,subvolid=[0-9]+,subvol=/home 0 0' /proc/mounts")
+  '';
+})