nixos/systemd-initrd: skip activation when init= is not a NixOS system

initrd-nixos-activation ran the closure's prepare-root unconditionally. For a
non-NixOS init= there is no prepare-root, so the service failed, and since
initrd-switch-root.service requires it, switch-root never ran and the machine
dropped to the emergency shell. This broke init=/bin/sh recovery, and microVMs
that serve /nix/store over virtiofs and boot an arbitrary binary as init=.

initrd-find-nixos-closure already detects this and writes a non-empty NEW_INIT
to /etc/switch-root.conf (empty for a NixOS init). Read it via the same
EnvironmentFile= initrd-switch-root uses and skip activation when it's set, so
a non-NixOS init= switch-roots into its target directly. The NixOS path is
unchanged.

Also add a test booting a non-NixOS init=. It uses a store path rather than
/bin/sh: a real root already has /bin/sh and an os-release, but a fresh test
root has neither, so the test uses a tmpfs root and writes os-release first.

Assisted-by: Claude:claude-opus-4-8

nixos/modules/system/boot/systemd/initrd.nix

@@ -763,6 +763,7 @@ in
           ];
         };
         serviceConfig.Type = "oneshot";
+        serviceConfig.EnvironmentFile = "-/etc/switch-root.conf";
         description = "NixOS Activation";
 
         script = # bash
@@ -770,6 +771,14 @@ in
             set -uo pipefail
             export PATH="/bin:${cfg.package.util-linux}/bin"
 
+            # A non-NixOS closure (e.g. init=/bin/sh) has no prepare-root;
+            # initrd-find-nixos-closure records this as a non-empty NEW_INIT.
+            # Skip activation and let initrd-switch-root hand over to it directly.
+            if [ -n "''${NEW_INIT:-}" ]; then
+              echo "$NEW_INIT is not a NixOS system - not activating"
+              exit 0
+            fi
+
             closure="$(realpath /nixos-closure)"
 
             # Initialize the system

nixos/tests/all-tests.nix

@@ -1628,6 +1628,7 @@ in
     "i686-linux"
   ] ./initrd-network-openvpn { systemdStage1 = true; };
   systemd-initrd-networkd-ssh = runTest ./systemd-initrd-networkd-ssh.nix;
+  systemd-initrd-non-nixos = runTest ./systemd-initrd-non-nixos.nix;
   systemd-initrd-shutdown = runTest {
     imports = [ ./systemd-shutdown.nix ];
     _module.args.systemdStage1 = true;

nixos/tests/systemd-initrd-non-nixos.nix

@@ -0,0 +1,53 @@
+{ lib, pkgs, ... }:
+let
+  marker = "REACHED NON-NIXOS INIT AS PID 1";
+
+  # A non-NixOS init (no prepare-root). We use a store path, not literal
+  # /bin/sh: a fresh disk has no /bin/sh yet (it is created by the activation a
+  # non-NixOS init skips), while the store is always mounted; init=/bin/sh works
+  # the same on a real system. Writes a marker, then stays alive so PID 1 lives.
+  nonNixosInit = pkgs.writeShellScriptBin "non-nixos" ''
+    echo "${marker}" > /dev/console
+    exec ${pkgs.coreutils}/bin/sleep infinity
+  '';
+in
+{
+  name = "systemd-initrd-non-nixos";
+
+  nodes.machine = {
+    boot.initrd.systemd.enable = true;
+
+    virtualisation = {
+      # tmpfs root, like real non-NixOS closure init= microvm consumers.
+      diskImage = null;
+
+      graphics = false;
+    };
+
+    # Speed up wait_for_console.
+    boot.consoleLogLevel = lib.mkForce 3;
+    boot.initrd.systemd.managerEnvironment.SYSTEMD_LOG_LEVEL = "warning";
+
+    # switch-root needs an os-release on the target root. A real system has one
+    # on disk; our fresh tmpfs does not, so create it.
+    boot.initrd.systemd.tmpfiles.settings."10-os-release"."/sysroot/etc/os-release".f = {
+      mode = "0644";
+      argument = "ID=test-non-nixos";
+    };
+  };
+
+  testScript = ''
+    import os
+
+    # The last init= on the cmdline wins; QEMU_KERNEL_PARAMS is appended after
+    # the default one, so this boots our non-NixOS init.
+    os.environ["QEMU_KERNEL_PARAMS"] = "init=${lib.getExe nonNixosInit}"
+
+    machine.start()
+
+    # If activation does not skip the non-NixOS init, switch-root is blocked and
+    # the machine drops to emergency mode: the marker never appears and this
+    # times out.
+    machine.wait_for_console_text("${marker}", timeout=300)
+  '';
+}