systemd: ensure fsck Requires/After links are created in mount units

systemd-fsck-generator only produces these lines if it can find the
necessary fsck executable in its PATH.

fixes #29139.

nixos/modules/system/boot/stage-2-init.sh

@@ -159,6 +159,6 @@ exec {logOutFd}>&- {logErrFd}>&-
 
 # Start systemd.
 echo "starting systemd..."
-PATH=/run/current-system/systemd/lib/systemd \
+PATH=/run/current-system/systemd/lib/systemd:@fsPackagesPath@ \
     LOCALE_ARCHIVE=/run/current-system/sw/lib/locale/locale-archive \
     exec systemd

nixos/modules/system/boot/stage-2.nix

@@ -17,6 +17,7 @@ let
       pkgs.utillinux
       pkgs.openresolv
     ];
+    fsPackagesPath = lib.makeBinPath config.system.fsPackages;
     postBootCommands = pkgs.writeText "local-cmds"
       ''
         ${config.boot.postBootCommands}

nixos/release.nix

@@ -291,6 +291,7 @@ in rec {
   tests.firefox = callTest tests/firefox.nix {};
   tests.flatpak = callTest tests/flatpak.nix {};
   tests.firewall = callTest tests/firewall.nix {};
+  tests.fsck = callTest tests/fsck.nix {};
   tests.fwupd = callTest tests/fwupd.nix {};
   tests.gdk-pixbuf = callTest tests/gdk-pixbuf.nix {};
   #tests.gitlab = callTest tests/gitlab.nix {};

nixos/tests/fsck.nix

@@ -0,0 +1,29 @@
+import ./make-test.nix {
+  name = "fsck";
+
+  machine = { lib, ... }: {
+    virtualisation.emptyDiskImages = [ 1 ];
+
+    fileSystems = lib.mkVMOverride {
+      "/mnt" = {
+        device = "/dev/vdb";
+        fsType = "ext4";
+        autoFormat = true;
+      };
+    };
+  };
+
+  testScript = ''
+    $machine->waitForUnit('default.target');
+
+    subtest "root fs is fsckd", sub {
+      $machine->succeed('journalctl -b | grep "fsck.ext4.*/dev/vda"');
+    };
+
+    subtest "mnt fs is fsckd", sub {
+      $machine->succeed('journalctl -b | grep "fsck.*/dev/vdb.*clean"');
+      $machine->succeed('grep "Requires=systemd-fsck@dev-vdb.service" /run/systemd/generator/mnt.mount');
+      $machine->succeed('grep "After=systemd-fsck@dev-vdb.service" /run/systemd/generator/mnt.mount');
+    };
+  '';
+}