kernel: fix Julia GC code

src/julia_gc.c

@@ -64,6 +64,13 @@ static inline Bag * DATA(BagHeader * bag)
 }
 
 
+static TNumExtraMarkFuncBags ExtraMarkFuncBags;
+void SetExtraMarkFuncBags(TNumExtraMarkFuncBags func)
+{
+    ExtraMarkFuncBags = func;
+}
+
+
 /****************************************************************************
 **
 *F  InitFreeFuncBag(<type>,<free-func>)
@@ -498,6 +505,13 @@ void GapRootScanner(int full)
     // types, which thus also will not be collected prematurely)
     JMark(Module);
 
+    // allow installing a custom marking function. This is used for
+    // integrating GAP (possibly linked as a shared library) with other code
+    // bases which use their own form of garbage collection. For example,
+    // with Python (for SageMath).
+    if (ExtraMarkFuncBags)
+        (*ExtraMarkFuncBags)();
+
     // scan the stack for further object references, and mark them
     syJmp_buf registers;
     sySetjmp(registers);

src/weakptr.c

@@ -147,12 +147,12 @@ static inline void SET_ELM_WPOBJ(Obj list, UInt pos, Obj val)
         ptr[pos] = val;
         return;
     }
-    jl_weakref_t * wref = (jl_weakref_t *)(ptr[pos]);
-    if (!IS_BAG_REF(wref)) {
+    if (!IS_BAG_REF(ptr[pos])) {
         ptr[pos] = (Bag)jl_gc_new_weakref((jl_value_t *)val);
         jl_gc_wb_back(BAG_HEADER(list));
     }
     else {
+        jl_weakref_t * wref = (jl_weakref_t *)(ptr[pos]);
         wref->value = (jl_value_t *)val;
         jl_gc_wb(wref, BAG_HEADER(val));
     }