Disallow retyping from type w/o free func to one with

src/julia_gc.c

@@ -78,6 +78,10 @@ void JFinalizer(jl_value_t * obj)
     BagHeader * hdr = (BagHeader *)obj;
     Bag         contents = (Bag)(hdr + 1);
     UInt        tnum = hdr->type;
+
+    // calling MakeImmutable on a weak pointer object can turn it into a
+    // plist, which no longer requires a free function; but we already
+    // scheduled one, so we must check TabFreeFuncBags here
     if (TabFreeFuncBags[tnum])
         TabFreeFuncBags[tnum]((Bag)&contents);
 }
@@ -648,8 +652,27 @@ void RetypeBag(Bag bag, UInt new_type)
     }
 #endif
 
-    if (!TabFreeFuncBags[old_type] && TabFreeFuncBags[new_type])
-        jl_gc_schedule_foreign_sweepfunc(JuliaTLS, (jl_value_t *)header);
+    if (!TabFreeFuncBags[old_type] && TabFreeFuncBags[new_type]) {
+        // calling MakeImmutable on a weak pointer object can turn it into a
+        // plist, which no longer requires a free functions; we allow this,
+        // but we disallow changing the other way. Otherwise, we might end up
+        // switch an object several times between types which require a free
+        // callback and types which don't.
+        //
+        // But then we either end up calling jl_gc_schedule_foreign_sweepfunc
+        // multiple times for the same object, which then can lead to Julia
+        // invoking the free function on that object multiple times, which in
+        // turn could have bad consequences.
+        //
+        // Or else we would have to write code to to track whether
+        // jl_gc_schedule_foreign_sweepfunc was called for an object (e.g. by
+        // using an object flag). But right now no GAP code needs to do this,
+        // and changing the type of an object to a completely different type
+        // is something better to be avoided anyway. So instead of supporting
+        // a feature nobody uses right now, we error out and wait to see if
+        // somebody complains.
+        Panic("cannot change bag type to one which requires a 'free' callback");
+    }
     header->type = new_type;
 }