PFW-1522 Fix an issue where MMU unloads while nozzle is cold if print is stopped

In the case of a Fan Error, the print is paused. If the nozzle is allowed to cool for a while before the print is stopped via the LCD, then the MMU will try to unload with a cold nozzle.

This can happen with ANY recoverable thermal errors.

In this commit I attempt to fix all scenarios. Including PFW-1544 where the nozzle is cooling down when the print has finished normally.

PFW-1544
PFW-1552

Change in memory:
Flash: +74 bytes
SRAM: 0 bytes

Firmware/Marlin.h

@@ -197,6 +197,7 @@ void kill(const char *full_screen_message = NULL);
 void finishAndDisableSteppers();
 
 void UnconditionalStop();                   // Stop heaters, motion and clear current print status
+void ConditionalStop();                     // Similar to UnconditionalStop, but doesn't disable heaters
 void ThermalStop(bool allow_pause = false); // Emergency stop used by overtemp functions which allows
                                             // recovery (with pause=true)
 bool IsStopped();                           // Returns true if the print has been stopped

Firmware/Marlin_main.cpp

@@ -9755,6 +9755,28 @@ void UnconditionalStop()
 
     st_reset_timer();
     CRITICAL_SECTION_END;
+
+    // clear paused state immediately
+    did_pause_print = false;
+    print_job_timer.stop();
+}
+
+void ConditionalStop()
+{
+    CRITICAL_SECTION_START;
+
+    // Clear any saved printing state
+    cancel_saved_printing();
+
+    // Abort the planner
+    planner_abort_hard();
+    
+    // Reset the queue
+    cmdqueue_reset();
+    cmdqueue_serial_disabled = false;
+
+    st_reset_timer();
+    CRITICAL_SECTION_END;
 }
 
 // Emergency stop used by overtemp functions which allows recovery
@@ -9789,7 +9811,7 @@ void ThermalStop(bool allow_recovery)
             }
         } else {
             // We got a hard thermal error and/or there is no print going on. Just stop.
-            print_stop();
+            print_stop(false, true);
         }
 
         // Report the error on the serial

Firmware/ultralcd.cpp

@@ -5656,6 +5656,7 @@ void retract_for_ooze_prevention() {
 // continue stopping the print from the main loop after lcd_print_stop() is called
 void lcd_print_stop_finish()
 {
+    print_job_timer.stop();
     save_statistics();
 
     // lift Z
@@ -5688,26 +5689,40 @@ void lcd_print_stop_finish()
 
     if (MMU2::mmu2.Enabled() && MMU2::mmu2.FindaDetectsFilament())
     {
+        // The print was aborted while when the nozzle was cold:
+        //     1. in a paused state                      => a partial backup in RAM is always available
+        //     2. after a recoverable thermal/fan error had paused the print => only extruder temperature is saved to RAM
         if (printingIsPaused())
         {
             // Restore temperature saved in ram after pausing print
             restore_extruder_temperature_from_ram();
         }
-        MMU2::mmu2.unload(); // M702
+
+        // If the pause state was cleared previously or the target temperature is 0°C in the case
+        // of an unconditional stop. In that scenario we do not want to unload.
+        if (target_temperature[0] >= extrude_min_temp) {
+            MMU2::mmu2.unload(); // M702
+        }
     }
 
     lcd_cooldown(); //turns off heaters and fan; goes to status screen.
 
     finishAndDisableSteppers(); //M84
     axis_relative_modes = E_AXIS_MASK; //XYZ absolute, E relative
+    did_pause_print = false; // Clear pause state in case the print was aborted while paused
 }
 
-void print_stop(bool interactive)
+void print_stop(bool interactive, bool unconditional_stop)
 {
     // UnconditionalStop() will internally cause planner_abort_hard(), meaning we _cannot_ plan any
     // more move in this call! Any further move must happen inside lcd_print_stop_finish(), which is
     // called by the main loop one iteration later.
-    UnconditionalStop();
+    if (unconditional_stop) {
+        UnconditionalStop();
+    } else {
+        // Allow lcd_print_stop_finish() to use the heaters when it is safe
+        ConditionalStop();
+    }
 
     if (card.sdprinting) {
         // Reset the sd status
@@ -5721,10 +5736,6 @@ void print_stop(bool interactive)
     mbl.active = false;
 #endif
 
-    // clear any pending paused state immediately
-    did_pause_print = false;
-    print_job_timer.stop();
-
     if (interactive) {
         // acknowledged by the user from the LCD: resume processing USB commands again
         Stopped = false;

Firmware/ultralcd.h

@@ -60,7 +60,11 @@ void lcd_pause_usb_print();
 void lcd_send_action_start();
 void lcd_resume_print();
 void lcd_print_stop(); // interactive print stop
-void print_stop(bool interactive=false);
+
+/// @brief Stop the print immediately
+/// @param interactive True if the user acknowledged the action from the LCD: resume processing USB commands again
+/// @param unconditional_stop True when the print is stopped by a serious error condition e.g. Thermal Runaway. False otherwise.
+void print_stop(bool interactive=false, bool unconditional_stop=false);
 #ifdef THERMAL_MODEL
 void lcd_thermal_model_cal();
 #endif //THERMAL_MODEL