Fix dirty modelcollide state in shipfx and harden with assert (#7390)

Author: BMagnu

code/model/modelcollide.cpp

@@ -1150,7 +1150,8 @@ int model_collide(mc_info *mc_info_obj)
 		Mc_pmi = NULL;
 	}
 
-	if (Mc_pmi && Mc->collision_checked.empty()) {
+	if (Mc_pmi && Mc->collision_checked.size() != static_cast<size_t>(Mc_pm->n_models)) {
+		Assertion(Mc->collision_checked.empty(), "model_collide was called with a dirty mc_info state! Please report to the SCP.");
 		Mc->collision_checked.resize(Mc_pm->n_models, 0);
 	}
 

code/ship/shipfx.cpp

@@ -791,13 +791,7 @@ bool shipfx_eye_in_shadow( vec3d *eye_pos, object * src_obj, int light_n )
 	vec3d rp0, rp1;
 	vec3d light_dir;
 
-	// The mc_info struct only needs to be initialized once for this entire function.  This is because
-	// every time the mc variable is reused, every parameter that model_collide reads from is reassigned.
-	// Therefore the stale fields in the rest of the struct do not matter because either a) they are never
-	// read from, or b) they are overwritten by the new collision calculation.
-	mc_info mc;
-
-	rp0 = *eye_pos;	
+	rp0 = *eye_pos;
 
 	// get the light dir
 	if(!light_get_global_dir(&light_dir, light_n)){
@@ -811,6 +805,8 @@ bool shipfx_eye_in_shadow( vec3d *eye_pos, object * src_obj, int light_n )
 			continue;
 
 		if ( src_obj != objp )	{
+			mc_info mc;
+
 			vm_vec_scale_add( &rp1, &rp0, &light_dir, objp->radius*10.0f );
 
 			mc.model_instance_num = Ships[objp->instance].model_instance_num;
@@ -834,6 +830,8 @@ bool shipfx_eye_in_shadow( vec3d *eye_pos, object * src_obj, int light_n )
 				continue;
 
 			if (src_obj != objp) {
+				mc_info mc;
+
 				vm_vec_scale_add(&rp1, &rp0, &light_dir, objp->radius * 10.0f);
 
 				mc.model_instance_num = p->model_instance_num;
@@ -861,6 +859,8 @@ bool shipfx_eye_in_shadow( vec3d *eye_pos, object * src_obj, int light_n )
 
 		vm_vec_scale_add( &rp1, &rp0, &light_dir, objp->radius*10.0f );
 
+		mc_info mc;
+
 		mc.model_instance_num = -1;
 		mc.model_num = db.model_num;	// Fill in the model to check
 		mc.submodel_num = db.submodel_num;
@@ -889,6 +889,8 @@ bool shipfx_eye_in_shadow( vec3d *eye_pos, object * src_obj, int light_n )
 				vm_vec_unrotate(&pos, &sip->cockpit_offset, &eye_ori);
 				vm_vec_add2(&pos, &eye_posi);
 
+				mc_info mc;
+
 				mc.model_instance_num = -1;
 				mc.model_num = sip->cockpit_model_num;
 				mc.submodel_num = -1;
@@ -938,6 +940,8 @@ bool shipfx_eye_in_shadow( vec3d *eye_pos, object * src_obj, int light_n )
 				&& (!Show_ship_only_if_cockpits_enabled || Cockpit_active) ) {
 				vm_vec_scale_add( &rp1, &rp0, &light_dir, Viewer_obj->radius*10.0f );
 
+				mc_info mc;
+
 				mc.model_instance_num = -1;
 				mc.model_num = sip->model_num;
 				mc.submodel_num = -1;
@@ -1001,6 +1005,8 @@ bool shipfx_eye_in_shadow( vec3d *eye_pos, object * src_obj, int light_n )
 
         vm_vec_scale_add( &rp1, &rp0, &light_dir, objp->radius*10.0f );
 
+    	mc_info mc;
+
 		mc.model_instance_num = -1;
 		mc.model_num = Asteroid_info[ast->asteroid_type].subtypes[ast->asteroid_subtype].model_number;	// Fill in the model to check
 		mc.submodel_num = -1;