save_variables: Support saving variables to a disk file

Signed-off-by: Dushyant Ahuja dusht.ahuja@gmail.com
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

docs/Command_Templates.md

@@ -396,3 +396,38 @@ gcode:
 ```
 UPDATE_DELAYED_GCODE ID=report_temp DURATION=0
 ```
+
+### Save Variables to disk
+
+If a
+[save_variables config section](Config_Reference.md#save_variables)
+has been enabled, `SAVE_VARIABLE VARIABLE=<name> VALUE=<value>` can be
+used to save the variable to disk so that it can be used across
+restarts. All stored variables are loaded into the
+`printer.save_variables.variables` dict at startup and can be used in
+gcode macros. to avoid overly long lines you can add the following at
+the top of the macro:
+```
+{% set svv = printer.save_variables.variables %}
+```
+
+As an example, it could be used to save the state of 2-in-1-out hotend
+and when starting a print ensure that the active extruder is used,
+instead of T0:
+
+```
+[gcode_macro T1]
+gcode:
+  ACTIVATE_EXTRUDER extruder=extruder1
+  SAVE_VARIABLE VARIABLE=currentextruder VALUE='"extruder1"'
+
+[gcode_macro T0]
+gcode:
+  ACTIVATE_EXTRUDER extruder=extruder
+  SAVE_VARIABLE VARIABLE=currentextruder VALUE='"extruder"'
+
+[gcode_macro START_GCODE]
+gcode:
+  {% set svv = printer.save_variables.variables %}
+  ACTIVATE_EXTRUDER extruder={svv.currentextruder}
+```

docs/Config_Reference.md

@@ -1182,6 +1182,20 @@ gcode:
 #   Default is 0.
 ```
 
+## [save_variables]
+
+Support saving variables to disk so that they are retained across
+restarts. See
+[command templates](Command_Templates.md#save-variables-to-disk) and
+[G-Code reference](G-Codes.md#save-variables) for further information.
+
+```
+[save_variables]
+filename:
+#   Required - provide a filename that would be used to save the
+#   variables to disk e.g. ~/variables.cfg
+```
+
 ## [idle_timeout]
 
 Idle timeout. An idle timeout is automatically enabled - add an

docs/G-Codes.md

@@ -647,6 +647,17 @@ been enabled (also see the
   for gcode execution.  A value of 0 will cancel a pending delayed gcode
   from executing.
 
+## Save Variables
+
+The following command is enabled if a
+[save_variables config section](Config_Reference.md#save_variables)
+has been enabled:
+- `SAVE_VARIABLE VARIABLE=<name> VALUE=<value>`: Saves the variable to
+  disk so that it can be used across restarts. All stored variables
+  are loaded into the `printer.save_variables.variables` dict at
+  startup and can be used in gcode macros. The provided VALUE is
+  parsed as a Python literal.
+
 ## Resonance compensation
 
 The following command is enabled if an