nixos/qemu-vm: optionally disable overriding fileSystems

Allow the user to disable overriding the fileSystems option with
virtualisation.fileSystems by setting
`virtualisation.fileSystems = lib.mkForce { };`.

With this change you can use the qemu-vm module to boot from an external
image that was not produced by the qemu-vm module itself. The user can
now re-use the modularly set fileSystems option instead of having to
reproduce it in virtualisation.fileSystems.

4 files changed, 86 insertions, 5 deletions

diff --git a/nixos/doc/manual/release-notes/rl-2311.section.md b/nixos/doc/manual/release-notes/rl-2311.section.md
index 2ab28b898d7f1..f235bca8e7ce5 100644
--- a/nixos/doc/manual/release-notes/rl-2311.section.md
+++ b/nixos/doc/manual/release-notes/rl-2311.section.md
@@ -405,4 +405,10 @@ The module update takes care of the new config syntax and the data itself (user
 
 - `python3.pkgs.flitBuildHook` has been removed. Use `flit-core` and `format = "pyproject"` instead.
 
+- The `qemu-vm.nix` module now supports disabling overriding `fileSystems` with
+  `virtualisation.fileSystems`. This enables the user to boot VMs from
+  "external" disk images not created by the qemu-vm module. You can stop the
+  qemu-vm module from overriding `fileSystems` by setting
+  `virtualisation.fileSystems = lib.mkForce { };`.
+
 - The `electron` packages now places its application files in `$out/libexec/electron` instead of `$out/lib/electron`. Packages using electron-builder will fail to build and need to be adjusted by changing `lib` to `libexec`.
diff --git a/nixos/modules/virtualisation/qemu-vm.nix b/nixos/modules/virtualisation/qemu-vm.nix
index 74c3e1ecd03f2..e0004df6f6b2f 100644
--- a/nixos/modules/virtualisation/qemu-vm.nix
+++ b/nixos/modules/virtualisation/qemu-vm.nix
@@ -1121,11 +1121,12 @@ in
       }) cfg.emptyDiskImages)
     ];
 
-    # Use mkVMOverride to enable building test VMs (e.g. via `nixos-rebuild
-    # build-vm`) of a system configuration, where the regular value for the
-    # `fileSystems' attribute should be disregarded (since those filesystems
-    # don't necessarily exist in the VM).
-    fileSystems = mkVMOverride cfg.fileSystems;
+    # By default, use mkVMOverride to enable building test VMs (e.g. via
+    # `nixos-rebuild build-vm`) of a system configuration, where the regular
+    # value for the `fileSystems' attribute should be disregarded (since those
+    # filesystems don't necessarily exist in the VM). You can disable this
+    # override by setting `virtualisation.fileSystems = lib.mkForce { };`.
+    fileSystems = lib.mkIf (cfg.fileSystems != { }) (mkVMOverride cfg.fileSystems);
 
     virtualisation.fileSystems = let
       mkSharedDir = tag: share:
diff --git a/nixos/tests/all-tests.nix b/nixos/tests/all-tests.nix
index 92389fdad55aa..05dfaa2507497 100644
--- a/nixos/tests/all-tests.nix
+++ b/nixos/tests/all-tests.nix
@@ -676,6 +676,7 @@ in {
   qboot = handleTestOn ["x86_64-linux" "i686-linux"] ./qboot.nix {};
   qemu-vm-restrictnetwork = handleTest ./qemu-vm-restrictnetwork.nix {};
   qemu-vm-volatile-root = runTest ./qemu-vm-volatile-root.nix;
+  qemu-vm-external-disk-image = runTest ./qemu-vm-external-disk-image.nix;
   qgis = handleTest ./qgis.nix { qgisPackage = pkgs.qgis; };
   qgis-ltr = handleTest ./qgis.nix { qgisPackage = pkgs.qgis-ltr; };
   qownnotes = handleTest ./qownnotes.nix {};
diff --git a/nixos/tests/qemu-vm-external-disk-image.nix b/nixos/tests/qemu-vm-external-disk-image.nix
new file mode 100644
index 0000000000000..a229fc5e39633
--- /dev/null
+++ b/nixos/tests/qemu-vm-external-disk-image.nix
@@ -0,0 +1,73 @@
+# Tests that you can boot from an external disk image with the qemu-vm module.
+# "External" here means that the image was not produced within the qemu-vm
+# module and relies on the fileSystems option also set outside the qemu-vm
+# module. Most notably, this tests that you can stop the qemu-vm module from
+# overriding fileSystems with virtualisation.fileSystems so you don't have to
+# replicate the previously set fileSystems in virtualisation.fileSystems.
+
+{ lib, ... }:
+
+let
+  rootFslabel = "external";
+  rootFsDevice = "/dev/disk/by-label/${rootFslabel}";
+
+  externalModule = { config, lib, pkgs, ... }: {
+    boot.loader.systemd-boot.enable = true;
+
+    fileSystems = {
+      "/".device = rootFsDevice;
+    };
+
+    system.build.diskImage = import ../lib/make-disk-image.nix {
+      inherit config lib pkgs;
+      label = rootFslabel;
+      partitionTableType = "efi";
+      format = "qcow2";
+      bootSize = "32M";
+      additionalSpace = "0M";
+      copyChannel = false;
+    };
+  };
+in
+{
+  name = "qemu-vm-external-disk-image";
+
+  meta.maintainers = with lib.maintainers; [ nikstur ];
+
+  nodes.machine = { config, lib, pkgs, ... }: {
+    virtualisation.directBoot.enable = false;
+    virtualisation.mountHostNixStore = false;
+    virtualisation.useEFIBoot = true;
+
+    # This stops the qemu-vm module from overriding the fileSystems option
+    # with virtualisation.fileSystems.
+    virtualisation.fileSystems = lib.mkForce { };
+
+    imports = [ externalModule ];
+  };
+
+  testScript = { nodes, ... }: ''
+    import os
+    import subprocess
+    import tempfile
+
+    tmp_disk_image = tempfile.NamedTemporaryFile()
+
+    subprocess.run([
+      "${nodes.machine.virtualisation.qemu.package}/bin/qemu-img",
+      "create",
+      "-f",
+      "qcow2",
+      "-b",
+      "${nodes.machine.system.build.diskImage}/nixos.qcow2",
+      "-F",
+      "qcow2",
+      tmp_disk_image.name,
+    ])
+
+    # Set NIX_DISK_IMAGE so that the qemu script finds the right disk image.
+    os.environ['NIX_DISK_IMAGE'] = tmp_disk_image.name
+
+    machine.succeed("findmnt --kernel --source ${rootFsDevice} --target /")
+  '';
+}