Fix for recursive call in onSettingsChanged.

[hangsman]

Summary

In gcode.c, gc_init chains the on_settings_changed callback by storing the previous handler into settings_changed and assigning onSettingsChanged to grbl.on_settings_changed. If grbl.on_settings_changed already references onSettingsChanged prior to chaining, settings_changed may end up pointing to onSettingsChanged itself, resulting in infinite recursion and a stack overflow when settings are updated.

Changes

Added a check settings_changed != onSettingsChanged before invoking settings_changed(settings, changed) to prevent the callback from recursively calling itself.

[terjeio]

When/how did the recursion happen?

[hangsman]

The recursion is caused by a self-referencing hook created during soft-reset:

1. On cold boot (grbl_enter -> gc_init), grbl.on_settings_changed is still NULL, so settings_changed is assigned NULL while grbl.on_settings_changed becomes onSettingsChanged (gcode.c:803-806).
2. When a sender connects and sends 0x18 (soft reset), protocol_exec_rt_system calls gc_init(true) again (protocol.c:586).
3. Since settings_changed is still NULL, if(settings_changed == NULL) passes again and executes settings_changed = grbl.on_settings_changed. But grbl.on_settings_changed is already onSettingsChanged, so settings_changed now points to itself.

Then any setting change calling settings_changed(settings, changed) enters an infinite recursion in onSettingsChanged (gcode.c:755).

[terjeio]

Thanks, I see where now - all the boards I am testing with have I/O ports enabled and this sets grbl.on_settings_changed during boot masking this bug.