Add 'tmp.nix'

tmp.nix

@@ -0,0 +1,66 @@
+#source of this code: https://github.com/NixOS/nixpkgs/blob/nixos-22.11/nixos/modules/system/boot/tmp.nix
+
+{ config, lib, ... }:
+
+with lib;
+
+let
+  cfg = config.boot;
+in
+{
+
+  ###### interface
+
+  options = {
+
+    boot.cleanTmpDir = mkOption {
+      type = types.bool;
+      default = true;
+      description = lib.mdDoc ''
+        Whether to delete all files in {file}`/tmp` during boot.
+      '';
+    };
+
+    boot.tmpOnTmpfs = mkOption {
+      type = types.bool;
+      default = true;
+      description = lib.mdDoc ''
+         Whether to mount a tmpfs on {file}`/tmp` during boot.
+      '';
+    };
+
+    boot.tmpOnTmpfsSize = mkOption {
+      type = types.oneOf [ types.str types.types.ints.positive ];
+      default = "50%";
+      description = lib.mdDoc ''
+        Size of tmpfs in percentage.
+        Percentage is defined by systemd.
+      '';
+    };
+
+  };
+
+  ###### implementation
+
+  config = {
+
+    # When changing remember to update /tmp mount in virtualisation/qemu-vm.nix
+    systemd.mounts = mkIf cfg.tmpOnTmpfs [
+      {
+        what = "tmpfs";
+        where = "/tmp";
+        type = "tmpfs";
+        mountConfig.Options = concatStringsSep "," [ "mode=1777"
+                                                     "strictatime"
+                                                     "rw"
+                                                     "nosuid"
+                                                     "nodev"
+                                                     "size=${toString cfg.tmpOnTmpfsSize}" ];
+      }
+    ];
+
+    systemd.tmpfiles.rules = optional config.boot.cleanTmpDir "D! /tmp 1777 root root";
+
+  };
+
+}