A new crasher.

Lib/test/crashers/mutation_inside_cyclegc.py

+
+# The cycle GC collector can be executed when any GC-tracked object is
+# allocated, e.g. during a call to PyList_New(), PyDict_New(), ...
+# Moreover, it can invoke arbitrary Python code via a weakref callback.
+# This means that there are many places in the source where an arbitrary
+# mutation could unexpectedly occur.
+
+# The example below shows list_slice() not expecting the call to
+# PyList_New to mutate the input list.  (Of course there are many
+# more examples like this one.)
+
+
+import weakref
+
+class A(object):
+    pass
+
+def callback(x):
+    del lst[:]
+
+
+keepalive = []
+
+for i in range(100):
+    lst = [str(i)]
+    a = A()
+    a.cycle = a
+    keepalive.append(weakref.ref(a, callback))
+    del a
+    while lst:
+        keepalive.append(lst[:])