clean up ship status after mission

code/parse/sexp.cpp

@@ -12806,7 +12806,7 @@ int sexp_is_iff_or_species(int n, bool iff)
 					// ship is in the EXITED state but probably in the process of exploding
 					else if (oswpt.has_shipp())
 					{
-						UNREACHABLE("With the addition of the ShipStatus::DEATH_ROLL state, this shouldn't happen");
+						Assertion(false, "With the addition of the ShipStatus::DEATH_ROLL state, this shouldn't happen");
 						return SEXP_KNOWN_FALSE;
 					}
 					// ship has vanished

code/ship/ship.cpp

@@ -6914,6 +6914,25 @@ void ship_level_init()
 	Man_thruster_reset_timestamp = timestamp(0);
 }
 
+void ship_level_close()
+{
+	// at this point ships have all gone through ship_delete;
+	// clean up any ships that are still present so we don't
+	// leave any stray references behind (e.g. for sexps)
+	for (auto &ship_entry : Ship_registry)
+	{
+		if (ship_entry.status == ShipStatus::PRESENT || ship_entry.status == ShipStatus::DEATH_ROLL)
+		{
+			ship_entry.cleanup_mode = (ship_entry.status == ShipStatus::DEATH_ROLL) ? SHIP_DESTROYED : SHIP_PRESENT_AT_MISSION_END;
+			ship_entry.status = ShipStatus::EXITED;
+			ship_entry.objnum = -1;
+			ship_entry.shipnum = -1;
+		}
+	}
+
+	ship_close_cockpit_displays(Player_ship);
+}
+
 /**
  * Add a ship onto the exited ships list.
  *
@@ -8745,6 +8764,7 @@ void ship_close_cockpit_displays(ship* shipp)
 {
 	if (shipp && shipp->cockpit_model_instance >= 0) {
 		model_delete_instance(shipp->cockpit_model_instance);
+		shipp->cockpit_model_instance = -1;
 	}
 
 	for ( int i = 0; i < (int)Player_displays.size(); i++ ) {

code/ship/ship.h

@@ -1688,6 +1688,7 @@ extern SCP_vector<wing_formation> Wing_formations;
 
 extern void ship_init();				// called once	at game start
 extern void ship_level_init();		// called before the start of each level
+extern void ship_level_close();		// called after the end of each level
 
 //returns -1 if failed
 extern int ship_create(matrix* orient, vec3d* pos, int ship_type, const char* ship_name = nullptr, bool standalone_ship = false);
@@ -1700,13 +1701,14 @@ extern void ship_delete( object * objp );
 extern int ship_check_collision_fast( object * obj, object * other_obj, vec3d * hitpos );
 extern int ship_get_num_ships();
 
-#define SHIP_VANISHED			(1<<0)
-#define SHIP_DESTROYED			(1<<1)
-#define SHIP_DEPARTED_WARP		(1<<2)
-#define SHIP_DEPARTED_BAY		(1<<3)
-#define SHIP_DEPARTED			( SHIP_DEPARTED_BAY | SHIP_DEPARTED_WARP )
-#define SHIP_DESTROYED_REDALERT	(1<<4)
-#define SHIP_DEPARTED_REDALERT	(1<<5)
+#define SHIP_VANISHED               (1<<0)
+#define SHIP_DESTROYED              (1<<1)
+#define SHIP_DEPARTED_WARP          (1<<2)
+#define SHIP_DEPARTED_BAY           (1<<3)
+#define SHIP_DEPARTED               ( SHIP_DEPARTED_BAY | SHIP_DEPARTED_WARP )
+#define SHIP_DESTROYED_REDALERT     (1<<4)
+#define SHIP_DEPARTED_REDALERT      (1<<5)
+#define SHIP_PRESENT_AT_MISSION_END (1<<6)
 
 /**
  * @brief Deletes and de-inits a ship.

freespace2/freespace.cpp

@@ -984,7 +984,7 @@ void game_level_close()
 		particle::ParticleManager::get()->clearSources();
 		particle::close();
 		trail_level_close();
-		ship_close_cockpit_displays(Player_ship);
+		ship_level_close();
 		hud_level_close();
 		hud_escort_clear_all();
 		model_instance_free_all();