Add histograms presubmit check for UseCounter.h

tools/metrics/histograms/update_histogram_enum.py

 # Copyright 2014 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.
 
 """Updates enums in histograms.xml file with values read from provided C++ enum.
 
 If the file was pretty-printed, the updated version is pretty-printed too.
 """
 
 import logging
 import os
 import re
 import sys
 
 from xml.dom import minidom
 
 sys.path.append(os.path.join(os.path.dirname(__file__), '..', 'common'))
 import diff_util
 import path_util
 
 import print_style
 
+
+HISTOGRAMS_PATH = path_util.GetHistogramsFile()
+
+
 class UserError(Exception):
   def __init__(self, message):
     Exception.__init__(self, message)
 
   @property
   def message(self):
     return self.args[0]
 
 
 def Log(message):
@@ skipping 102 matching lines @@
 
   # Add comments at the top.
   for comment in new_comments:
     enum_node.appendChild(comment)
 
   # Add in the new enums.
   for value in sorted(new_item_nodes.iterkeys()):
     enum_node.appendChild(new_item_nodes[value])
 
 
-def UpdateHistogramFromDict(histogram_enum_name, source_enum_values,
-                            source_enum_path):
-  """Updates |histogram_enum_name| enum in histograms.xml file with values
-  from the {value: 'key'} dictionary |source_enum_values|. A comment is added
-  to histograms.xml citing that the values in |histogram_enum_name| were
-  sourced from |source_enum_path|.
+def _GetOldAndUpdatedXml(histogram_enum_name, source_enum_values,
+                         source_enum_path):
+  """Reads old histogram from |histogram_enum_name| from |HISTOGRAMS_PATH|, and
+  calculates new histogram from |source_enum_values| from |source_enum_path|,
+  and returns both in XML format.
   """
-  HISTOGRAMS_PATH = path_util.GetHistogramsFile()
-
   Log('Reading existing histograms from "{0}".'.format(HISTOGRAMS_PATH))
   with open(HISTOGRAMS_PATH, 'rb') as f:
     histograms_doc = minidom.parse(f)
     f.seek(0)
     xml = f.read()
 
   Log('Comparing histograms enum with new enum definition.')
   UpdateHistogramDefinitions(histogram_enum_name, source_enum_values,
                              source_enum_path, histograms_doc)
 
-  Log('Writing out new histograms file.')
   new_xml = print_style.GetPrintStyle().PrettyPrintNode(histograms_doc)
+  return (xml, new_xml)
+
+
+def HistogramNeedsUpdate(histogram_enum_name, source_enum_path, start_marker,
+                         end_marker):
+  """Reads a C++ enum from a .h file and does a dry run of updating
+  histograms.xml to match. Returns true if the histograms.xml file would be
+  changed.
+
+  Args:
+      histogram_enum_name: The name of the XML <enum> attribute to update.
+      source_enum_path: A unix-style path, relative to src/, giving
+          the C++ header file from which to read the enum.
+      start_marker: A regular expression that matches the start of the C++ enum.
+      end_marker: A regular expression that matches the end of the C++ enum.
+  """
+  Log('Reading histogram enum definition from "{0}".'.format(source_enum_path))
+  source_enum_values = ReadHistogramValues(source_enum_path, start_marker,
+                                           end_marker)
+
+  (xml, new_xml) = _GetOldAndUpdatedXml(histogram_enum_name, source_enum_values,
+                                        source_enum_path)
+  return xml != new_xml
+
+
+def UpdateHistogramFromDict(histogram_enum_name, source_enum_values,
+                            source_enum_path):
+  """Updates |histogram_enum_name| enum in histograms.xml file with values
+  from the {value: 'key'} dictionary |source_enum_values|. A comment is added
+  to histograms.xml citing that the values in |histogram_enum_name| were
+  sourced from |source_enum_path|.
+  """
+  (xml, new_xml) = _GetOldAndUpdatedXml(histogram_enum_name, source_enum_values,
+                                        source_enum_path)
   if not diff_util.PromptUserToAcceptDiff(
       xml, new_xml, 'Is the updated version acceptable?'):
     Log('Cancelled.')
     return
 
   with open(HISTOGRAMS_PATH, 'wb') as f:
     f.write(new_xml)
 
   Log('Done.')
 
 
 def UpdateHistogramEnum(histogram_enum_name, source_enum_path,
                         start_marker, end_marker):
   """Reads a C++ enum from a .h file and updates histograms.xml to match.
 
   Args:
       histogram_enum_name: The name of the XML <enum> attribute to update.
       source_enum_path: A unix-style path, relative to src/, giving
           the C++ header file from which to read the enum.
       start_marker: A regular expression that matches the start of the C++ enum.
       end_marker: A regular expression that matches the end of the C++ enum.
   """
 
   Log('Reading histogram enum definition from "{0}".'.format(source_enum_path))
   source_enum_values = ReadHistogramValues(source_enum_path, start_marker,
                                            end_marker)
 
   UpdateHistogramFromDict(histogram_enum_name, source_enum_values,
       source_enum_path)