Avoid possible creation of multiple references to the same capsule

Author: msimacek

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/objects/capsule/PyCapsule.java

@@ -77,6 +77,7 @@ public Object getDestructor() {
     }
 
     private final CapsuleData data;
+    private CApiTransitions.PyCapsuleReference reference;
 
     /**
      * (mq) We are forcing all PyCapsule objects to be of a builtin type
@@ -122,8 +123,8 @@ public Object getDestructor() {
 
     public void registerDestructor(Object destructor) {
         assert destructor == null || !InteropLibrary.getUncached().isNull(destructor);
-        if (data.destructor == null && destructor != null) {
-            CApiTransitions.registerPyCapsuleDestructor(this);
+        if (reference == null && destructor != null) {
+            reference = CApiTransitions.registerPyCapsuleDestructor(this);
         }
         data.destructor = destructor;
     }

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/objects/cext/capi/transitions/CApiTransitions.java

@@ -377,11 +377,12 @@ public String toString() {
     }
 
     @TruffleBoundary
-    public static void registerPyCapsuleDestructor(PyCapsule capsule) {
+    public static PyCapsuleReference registerPyCapsuleDestructor(PyCapsule capsule) {
         assert PythonContext.get(null).ownsGil();
         HandleContext handleContext = getContext();
         PyCapsuleReference ref = new PyCapsuleReference(handleContext, capsule);
         handleContext.pyCapsuleReferences.add(ref);
+        return ref;
     }
 
     @TruffleBoundary