issue #0002902: PyObjectBase notification chain can lead to unexpected changes to document

src/Base/PyObjectBase.cpp

@@ -181,6 +181,17 @@ int PyObjectBase::__setattr(PyObject *obj, char *attr, PyObject *value)
         return -1;
     }
 
+    // If an attribute references this as parent then reset it
+    // before setting the new attribute
+    PyObject* cur = static_cast<PyObjectBase*>(obj)->_getattr(attr);
+    if (cur) {
+        if (PyObject_TypeCheck(cur, &(PyObjectBase::Type))) {
+            PyObjectBase* base = static_cast<PyObjectBase*>(cur);
+            base->resetAttribute();
+        }
+        Py_DECREF(cur);
+    }
+
     int ret = static_cast<PyObjectBase*>(obj)->_setattr(attr, value);
 #if 1
     if (ret == 0) {
@@ -265,6 +276,18 @@ PyObject *PyObjectBase::_repr(void)
     return Py_BuildValue("s", a.str().c_str());
 }
 
+void PyObjectBase::resetAttribute()
+{
+    if (this->attribute) {
+        free(this->attribute);
+        this->attribute = 0;
+    }
+    if (this->parent) {
+        Py_DECREF(this->parent);
+        this->parent = 0;
+    }
+}
+
 void PyObjectBase::setAttributeOf(const char* attr, const PyObjectBase* par)
 {
     if (this->parent != par) {

src/Base/PyObjectBase.h

@@ -196,6 +196,7 @@ class BaseExport PyObjectBase : public PyObject
 protected:
     /// destructor
     virtual ~PyObjectBase();
+    void resetAttribute();
 
 public:  
     /** Constructor

src/Mod/Test/Document.py

@@ -342,6 +342,18 @@ class DocumentBasicCases(unittest.TestCase):
     self.Doc.removeObject(L7.Name)
     self.Doc.removeObject(L8.Name)
     
+  def testPropertyLink(self):
+    o1 = self.Doc.addObject("App::FeatureTest","test1")
+    o2 = self.Doc.addObject("App::FeatureTest","test2")
+    o3 = self.Doc.addObject("App::FeatureTest","test3")
+
+    o1.Link=o2
+    self.assertEqual(o1.Link, o2)
+    o1.Link=o3
+    self.assertEqual(o1.Link, o3)
+    o2.Placement = FreeCAD.Placement()
+    self.assertEqual(o1.Link, o3)
+
   def tearDown(self):
     #closing doc
     FreeCAD.closeDocument("CreateTest")