nixos/klipper: always update mutable config & deprecate immutable config

Klipper macros that use SAVE_CONFIG (eg. bed mesh calibration, PID
tuning, ...) don't work with an immutable config.

The mutableConfig option was added in #188273 to support SAVE_CONFIG,
but it only seeds the config if it doesn't already exist.

This refactors `mutableConfig` to always replace the
config (preserving the SAVE_CONFIG section) and deprecates immutable
configs.

nixos/doc/manual/release-notes/rl-2505.section.md

@@ -164,6 +164,8 @@
 
 - `python3Packages.jaeger-client` was removed because it was deprecated upstream. [OpenTelemetry](https://opentelemetry.io) is the recommended replacement.
 
+- Immutable `klipper` config is deprecated. Klipper macros that use SAVE_CONFIG (eg. bed mesh calibration, PID tuning, ...) don't work with an immutable config. Set `services.klipper.mutableConfig = true;`, or carefully update your `system.stateVersion` to `"25.05"`.
+
 - `nodePackages.meshcommander` has been removed, as the package was deprecated by Intel.
 
 - `kanata` was updated to v1.7.0, which introduces several breaking changes.

nixos/modules/services/misc/klipper.nix

@@ -49,11 +49,11 @@ in
 
       mutableConfig = lib.mkOption {
         type = lib.types.bool;
-        default = false;
-        example = true;
+        default = lib.versionAtLeast config.system.stateVersion "25.05";
+        example = !(lib.versionAtLeast config.system.stateVersion "25.05");
         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.
+          This will keep a backup if `services.klipper.mutableConfigPath` already exists and differs.
         '';
       };
 
@@ -136,7 +136,12 @@ in
   };
 
   ##### implementation
+
   config = lib.mkIf cfg.enable {
+    warnings =
+      lib.optional (!cfg.mutableConfig)
+        "Immutable klipper config is deprecated. Klipper macros that use SAVE_CONFIG (eg. bed mesh calibration, PID tuning, ...) don't work with an immutable config. Set services.klipper.mutableConfig = true, or carefully update your system.stateVersion to 25.05.";
+
     assertions = [
       {
         assertion = cfg.octoprintIntegration -> config.services.octoprint.enable;
@@ -189,13 +194,21 @@ in
         after = [ "network.target" ];
         preStart = ''
           mkdir -p ${cfg.mutableConfigFolder}
+          pushd ${cfg.mutableConfigFolder}
           ${lib.optionalString (cfg.mutableConfig) ''
-            [ -e ${printerConfigPath} ] || {
-              cp ${printerConfigFile} ${printerConfigPath}
-              chmod +w ${printerConfigPath}
-            }
+            # Backup existing config using the same date format klipper uses for SAVE_CONFIG
+            old_config="printer-$(date +"%Y%m%d_%H%M%S").cfg"
+            if [ -e printer.cfg ]; then
+              mv printer.cfg "$old_config"
+              # Preserve SAVE_CONFIG section from the existing config
+              cat ${printerConfigFile} <(printf "\n") <(sed -n '/#*# <---------------------- SAVE_CONFIG ---------------------->/,$p' "$old_config") > printer.cfg
+              ${pkgs.diffutils}/bin/cmp printer.cfg "$old_config" && rm "$old_config"
+            else
+              cat ${printerConfigFile} > printer.cfg
+            fi
           ''}
-          mkdir -p ${cfg.mutableConfigFolder}/gcodes
+          mkdir -p gcodes
+          popd
         '';
 
         serviceConfig =