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

appengine/findit/crash/chromecrash_parser.py

Index: appengine/findit/crash/chromecrash_parser.py
diff --git a/appengine/findit/crash/chromecrash_parser.py b/appengine/findit/crash/chromecrash_parser.py
index 408d1b7e642984e304c14d47f6bd85c30362e97a..395b2bf37870a648f1a664ea010df2f88dc4b07f 100644
--- a/appengine/findit/crash/chromecrash_parser.py
+++ b/appengine/findit/crash/chromecrash_parser.py
@@ -32,15 +32,15 @@ class ChromeCrashParser(StacktraceParser):
     # Initial background callstack which is not to be added into Stacktrace.
     stack_buffer = CallStackBuffer()
     for line in stacktrace_string.splitlines():
-      is_new_callstack, priority, format_type, language_type, metadata = (
-          stack_detector.IsStartOfNewCallStack(line))
+      start_of_callstack = stack_detector(line)
 
-      if is_new_callstack:
+      if start_of_callstack:
         stacktrace_buffer.AddFilteredStack(stack_buffer)
-        stack_buffer = CallStackBuffer(priority=priority,
-                                       format_type=format_type,
-                                       language_type=language_type,
-                                       metadata=metadata)
+        stack_buffer = 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)

[wrengr, 2016/12/29 20:13:02]

Since we're just passing all the parts through, why not have the stack_detector
return the new CallStackBuffer (or None) directly?

[Sharu Jiang, 2016/12/29 21:40:14]

I think it would be more clear that the ``stack_detector`` only detects the
``StartOfCallstack``, and let the ``Parser`` takes care of the whole
``stack_buffer`` (including creating the ``stack_buffer`` based
``StartOfCallStack`` and adding all the frames.)

       else:
         frame = StackFrame.Parse(stack_buffer.language_type,
                                  stack_buffer.format_type, line, deps,