[Predator] Make ``CallStackDetector`` return a namedtuple.

appengine/findit/crash/stacktrace.py

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 from collections import namedtuple
 import copy
 import logging
 import math
 import re
 
@@ skipping 257 matching lines @@
     """Converts ``CallStackBuffer`` object to ``CallStack`` object.
 
     Note, some metadata will be discarded after the conversion.
     """
     if not self:
       return None
 
     return CallStack(self.priority, tuple(self.frames),
                      self.format_type, self.language_type)
 
+  @staticmethod
+  def FromStartOfCallStack(start_of_callstack):
+    """Constructs a ``CallStackBuffer`` from a ``StartOfCallStack``."""
+    if not start_of_callstack:
+      return None
+
+    return CallStackBuffer(
+        priority=start_of_callstack.priority,
+        format_type=start_of_callstack.format_type,
+        language_type=start_of_callstack.language_type,
+        metadata=start_of_callstack.metadata)
+
 
 # N.B., because ``list`` is mutable it isn't hashable, thus cannot be
 # used as a key in a dict. Because we want to usecallstacks as keys (for
 # memoization) we has-a tuple rather than is-a list.
 # TODO(http://crbug.com/644476): this class needs a better name.
 class Stacktrace(namedtuple('Stacktrace', ['stacks', 'crash_stack'])):
   """A collection of callstacks which together provide a trace of what happened.
 
   For instance, when doing memory debugging we will have callstacks for
   (1) when the crash occurred, (2) when the object causing the crash
@@ skipping 99 matching lines @@
                   for stack_buffer in self.stacks]
 
     if crash_stack_index is None:
       # If there is no signature callstack, fall back to set crash stack using
       # the first least priority callstack.
       crash_stack = min(callstacks, key=lambda stack: stack.priority)
     else:
       crash_stack = callstacks[crash_stack_index]
 
     return Stacktrace(tuple(callstacks), crash_stack)