nixos/stage-1: support bind mounts of files

On my system, / is tmpfs, and /etc/machine-id is bind mounted from
/persist.

nixos/modules/system/boot/stage-1-init.sh

@@ -114,6 +114,28 @@ waitDevice() {
     done
 }
 
+# Create the mount point if required.
+makeMountPoint() {
+    local device="$1"
+    local mountPoint="$2"
+    local options="$3"
+
+    local IFS=,
+
+    # If we're bind mounting a file, the mount point should also be a file.
+    if ! [ -d "$device" ]; then
+        for opt in $options; do
+            if [ "$opt" = bind ] || [ "$opt" = rbind ]; then
+                mkdir -p "$(dirname "/mnt-root$mountPoint")"
+                touch "/mnt-root$mountPoint"
+                return
+            fi
+        done
+    fi
+
+    mkdir -m 0755 -p "/mnt-root$mountPoint"
+}
+
 # Mount special file systems.
 specialMount() {
   local device="$1"
@@ -400,7 +422,7 @@ mountFS() {
 
     info "mounting $device on $mountPoint..."
 
-    mkdir -p "/mnt-root$mountPoint"
+    makeMountPoint "$device" "$mountPoint" "$optionsPrefixed"
 
     # For ZFS and CIFS mounts, retry a few times before giving up.
     # We do this for ZFS as a workaround for issue NixOS/nixpkgs#25383.

nixos/tests/non-default-filesystems.nix

@@ -6,6 +6,31 @@
 with import ../lib/testing-python.nix { inherit system pkgs; };
 with pkgs.lib;
 {
+  bind = makeTest {
+    name = "non-default-filesystem-bind";
+
+    nodes.machine = { ... }: {
+      virtualisation.writableStore = false;
+
+      virtualisation.fileSystems."/test-bind-dir/bind" = {
+        device = "/";
+        neededForBoot = true;
+        options = [ "bind" ];
+      };
+
+      virtualisation.fileSystems."/test-bind-file/bind" = {
+        depends = [ "/nix/store" ];
+        device = builtins.toFile "empty" "";
+        neededForBoot = true;
+        options = [ "bind" ];
+      };
+    };
+
+    testScript = ''
+      machine.wait_for_unit("multi-user.target")
+    '';
+  };
+
   btrfs = makeTest
     {
       name = "non-default-filesystems-btrfs";