nixos/klipper: preserve SAVE_CONFIG for nixos-managed config

Klipper macros that use `SAVE_CONFIG` (eg. bed mesh calibration, PID
tuning, ...) don't currently work with a nixos-managed config.

This can be worked around by using `services.klipper.mutableConfig =
true`, but then you lose out on being able to configure klipper from
NixOS.

This changes the default behaviour so that:

1. The config is stored in `services.klipper.configDir` instead of `/etc`
2. The config is copied instead of symlinked (keeping a timestamped backup of the existing config)
3. The `SAVE_CONFIG` section from the backup is copied over into the new config
4. The backup is deleted if the final config is identical

Author: kira-bruneau

nixos/modules/services/misc/klipper.nix

@@ -18,6 +18,13 @@ let
   };
 in
 {
+  imports = [
+    (lib.mkRenamedOptionModule
+      [ "services" "klipper" "mutableConfigFolder" ]
+      [ "services" "klipper" "configDir" ]
+    )
+  ];
+
   ##### interface
   options = {
     services.klipper = {
@@ -52,23 +59,22 @@ in
         default = false;
         example = true;
         description = ''
-          Whether to copy the config to a mutable directory instead of using the one directly from the nix store.
-          This will only copy the config if the file at `services.klipper.mutableConfigPath` doesn't exist.
+          Whether to manage the config outside of NixOS.
+
+          It will still be initialized with the defined NixOS config if the file doesn't already exist.
         '';
       };
 
-      mutableConfigFolder = lib.mkOption {
+      configDir = lib.mkOption {
         type = lib.types.path;
         default = "/var/lib/klipper";
-        description = "Path to mutable Klipper config file.";
+        description = "Path to Klipper config file.";
       };
 
       configFile = lib.mkOption {
         type = lib.types.nullOr lib.types.path;
         default = null;
-        description = ''
-          Path to default Klipper config.
-        '';
+        description = "Path to default Klipper config.";
       };
 
       octoprintIntegration = lib.mkOption {
@@ -162,11 +168,6 @@ in
       }
     ];
 
-    environment.etc = lib.mkIf (!cfg.mutableConfig) {
-      "klipper.cfg".source =
-        if cfg.settings != null then format.generate "klipper.cfg" cfg.settings else cfg.configFile;
-    };
-
     services.klipper = lib.mkIf cfg.octoprintIntegration {
       user = config.services.octoprint.user;
       group = config.services.octoprint.group;
@@ -178,29 +179,39 @@ in
           "--input-tty=${cfg.inputTTY}"
           + lib.optionalString (cfg.apiSocket != null) " --api-server=${cfg.apiSocket}"
           + lib.optionalString (cfg.logFile != null) " --logfile=${cfg.logFile}";
-        printerConfigPath =
-          if cfg.mutableConfig then cfg.mutableConfigFolder + "/printer.cfg" else "/etc/klipper.cfg";
-        printerConfigFile =
+        printerConfig =
           if cfg.settings != null then format.generate "klipper.cfg" cfg.settings else cfg.configFile;
       in
       {
         description = "Klipper 3D Printer Firmware";
         wantedBy = [ "multi-user.target" ];
         after = [ "network.target" ];
         preStart = ''
-          mkdir -p ${cfg.mutableConfigFolder}
-          ${lib.optionalString (cfg.mutableConfig) ''
-            [ -e ${printerConfigPath} ] || {
-              cp ${printerConfigFile} ${printerConfigPath}
-              chmod +w ${printerConfigPath}
+          mkdir -p ${cfg.configDir}
+          pushd ${cfg.configDir}
+          if [ -e printer.cfg ]; then
+            ${
+              if cfg.mutableConfig then
+                ":"
+              else
+                ''
+                  # Backup existing config using the same date format klipper uses for SAVE_CONFIG
+                  old_config="printer-$(date +"%Y%m%d_%H%M%S").cfg"
+                  mv printer.cfg "$old_config"
+                  # Preserve SAVE_CONFIG section from the existing config
+                  cat ${printerConfig} <(printf "\n") <(sed -n '/#*# <---------------------- SAVE_CONFIG ---------------------->/,$p' "$old_config") > printer.cfg
+                  ${pkgs.diffutils}/bin/cmp printer.cfg "$old_config" && rm "$old_config"
+                ''
             }
-          ''}
-          mkdir -p ${cfg.mutableConfigFolder}/gcodes
+          else
+            cat ${printerConfig} > printer.cfg
+          fi
+          popd
         '';
 
         serviceConfig =
           {
-            ExecStart = "${cfg.package}/bin/klippy ${klippyArgs} ${printerConfigPath}";
+            ExecStart = "${cfg.package}/bin/klippy ${klippyArgs} ${cfg.configDir}/printer.cfg";
             RuntimeDirectory = "klipper";
             StateDirectory = "klipper";
             SupplementaryGroups = [ "dialout" ];