fix gc

de.churl.simple/garbagecollection.py

@@ -1,12 +1,14 @@
 def mark(w_context):
+    if w_context.mark:  # skip cycles
+        return
+
     w_context.mark = True
 
     if not hasattr(w_context, "slots"):  # skip primitive objects
         return
 
     for name, obj in w_context.slots.items():
-        if name != "__parent__":  # only descent
-            mark(obj)
+        mark(obj)
 
 
 def sweep(objects):

de.churl.simple/objspace.py

@@ -44,7 +44,8 @@ class ObjectSpace(object):
         else:
             slots = {'__parent__': self.getbuiltins()}
 
-        return W_NormalObject(name=name, slots=slots)  # lobby isn't collected
+        self.objects.append(W_NormalObject(name=name, slots=slots))  # needed, otherwise marking skips the root
+        return self.objects[-1]
 
     def newobject(self, name, slots, parentnames):
         self.objects.append(W_NormalObject(space=self, name=name,

mytests/test_gc.py

@@ -4,7 +4,6 @@ from interpreter import Interpreter
 
 def test_reassignment_gc():
     ast = parse("""
-true
 x = 2
 y = 3
 """)
@@ -66,6 +65,45 @@ z = y
     assert x not in interpreter.space.objects
 
 
+def test_cycle_gc():
+    ast = parse("""
+object a:
+    x = 1
+    
+object b:
+    x = a
+    
+a x = b
+""")
+    interpreter = Interpreter()
+    w_model = interpreter.make_module()
+    interpreter.eval(ast, w_model)
+    interpreter.space.gc(w_model)
+
+    a = w_model.getvalue("a")
+    b = w_model.getvalue("b")
+    assert a in interpreter.space.objects
+    assert b in interpreter.space.objects
+
+    ast = parse("""
+a = 0
+""")
+    interpreter.eval(ast, w_model)
+    interpreter.space.gc(w_model)
+
+    assert a in interpreter.space.objects
+    assert b in interpreter.space.objects
+
+    ast = parse("""
+b = 0
+""")
+    interpreter.eval(ast, w_model)
+    interpreter.space.gc(w_model)
+
+    assert a not in interpreter.space.objects
+    assert b not in interpreter.space.objects
+
+
 def test_object_gc():
     ast = parse("""
 object x: