linux-ramdump-parser-v2: Detect corruption in rb_tree

Red/Black trees are supposed to be trees, not directed cyclic graphs.
Sometimes trees in ramdumps may become corrupted though, resulting in
cycles. Detect these cycles in the tree to avoid looping forever.

Change-Id: I68cf018c6786b3c6947789a18c63c7a3a1f8d7bc

linux-ramdump-parser-v2/rb_tree.py

-# Copyright (c) 2012-2013, The Linux Foundation. All rights reserved.
+# Copyright (c) 2012-2014, The Linux Foundation. All rights reserved.
 #
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License version 2 and
@@ -30,14 +30,21 @@ class RbTreeWalker(object):
         self.left_offset = self.ram_dump.field_offset(
             'struct rb_node', 'rb_left')
 
-    def walk(self, node, func):
+    def _walk(self, node, func, seen):
         if node != 0:
             left_node_addr = node + self.left_offset
             left_node = self.ram_dump.read_word(left_node_addr)
-            self.walk(left_node, func)
+            if left_node not in seen:
+                seen.append(left_node)
+                self._walk(left_node, func, seen)
 
             func(node)
 
             right_node_addr = node + self.right_offset
             right_node = self.ram_dump.read_word(right_node_addr)
-            self.walk(right_node, func)
+            if right_node not in seen:
+                seen.append(right_node)
+                self._walk(right_node, func, seen)
+
+    def walk(self, node, func):
+        self._walk(node, func, [])