Issue #24098: Fixed possible crash when AST is changed in process of

compiling it.

Misc/NEWS

@@ -10,6 +10,9 @@ What's New in Python 3.6.0 beta 2
 Core and Builtins
 -----------------
 
+- Issue #24098: Fixed possible crash when AST is changed in process of
+  compiling it.
+
 - Issue #28201: Dict reduces possibility of 2nd conflict in hash table when
   hashes have same lower bits.
 

Parser/asdl_c.py

@@ -526,6 +526,13 @@ class Obj2ModVisitor(PickleVisitor):
             self.emit("res = obj2ast_%s(PyList_GET_ITEM(tmp, i), &value, arena);" %
                       field.type, depth+2, reflow=False)
             self.emit("if (res != 0) goto failed;", depth+2)
+            self.emit("if (len != PyList_GET_SIZE(tmp)) {", depth+2)
+            self.emit("PyErr_SetString(PyExc_RuntimeError, \"%s field \\\"%s\\\" "
+                      "changed size during iteration\");" %
+                      (name, field.name),
+                      depth+3, reflow=False)
+            self.emit("goto failed;", depth+3)
+            self.emit("}", depth+2)
             self.emit("asdl_seq_SET(%s, i, value);" % field.name, depth+2)
             self.emit("}", depth+1)
         else: