Add thread cleanup to avoid double destruction of shared XS data

lib/Slic3r.pm

@@ -87,11 +87,9 @@ sub parallelize {
         $q->enqueue(@items, (map undef, 1..$Config->threads));
         
         my $thread_cb = sub {
-            # prevent destruction of shared objects
-            no warnings 'redefine';
-            *Slic3r::ExPolygon::XS::DESTROY = sub {};
-            
-            return $params{thread_cb}->($q);
+            my $result = $params{thread_cb}->($q);
+            Slic3r::thread_cleanup();
+            return $result;
         };
             
         @_ = ();
@@ -103,6 +101,19 @@ sub parallelize {
     }
 }
 
+# call this at the very end of each thread (except the main one)
+# so that it does not try to free existing objects.
+# at that stage, existing objects are only those that we 
+# inherited at the thread creation (thus shared) and those 
+# that we are returning: destruction will be handled by the
+# main thread in both cases.
+sub thread_cleanup {
+    # prevent destruction of shared objects
+    no warnings 'redefine';
+    *Slic3r::ExPolygon::XS::DESTROY = sub {};
+    *Slic3r::Point::XS::DESTROY     = sub {};
+}
+
 sub encode_path {
     my ($filename) = @_;
     return encode('locale_fs', $filename);

lib/Slic3r/GUI/Plater.pm

@@ -570,6 +570,7 @@ sub export_gcode {
                     });
                 },
             );
+            Slic3r::thread_cleanup();
         });
         $self->statusbar->SetCancelCallback(sub {
             $self->{export_thread}->kill('KILL')->join;
@@ -744,6 +745,8 @@ sub make_thumbnail {
         } else {
             $self->on_thumbnail_made($obj_idx);
         }
+        
+        Slic3r::thread_cleanup() if $Slic3r::have_threads;
     };
     
     @_ = ();