Add a presubmit checker for duplicated enum values in UseCounter::Feature

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
@@ skipping 20 matching lines @@
   @property
   def message(self):
     return self.args[0]
 
 
 def Log(message):
   logging.info(message)
 
 
 def ReadHistogramValues(filename, start_marker, end_marker):
-  """Returns a dictionary of enum values, read from a C++ file.
+  """Returns a dictionary of enum values and a pair of labels that have the same

[Ilya Sherman, 2017/04/13 01:12:49]

Please clarify this comment.  The function returns a pair of labels, or None if
there are no such elements.  (It's probably clearer to write that as two
sentences, though.)

+     enum values, read from a C++ file.
 
   Args:
       filename: The unix-style path (relative to src/) of the file to open.
       start_marker: A regex that signifies the start of the enum values.
       end_marker: A regex that signifies the end of the enum values.
   """
   # Read the file as a list of lines
   with open(path_util.GetInputFile(filename)) as f:
     content = f.readlines()
 
@@ skipping 16 matching lines @@
         m = ITEM_REGEX_WITH_INIT.match(line)
         if m:
           enum_value = int(m.group(2))
           label = m.group(1)
         else:
           m = ITEM_REGEX.match(line)
           if m:
             label = m.group(1)
           else:
             continue
+        # If two enum labels have the same value
+        if enum_value in result:
+          return result, (result[enum_value], label)

[Ilya Sherman, 2017/04/13 01:12:49]

Hmm, this is an early return.  Should the result be None if there is an error? 
In general, I wonder whether it makes more sense to implement this by throwing
an exception, rather than returning a second result value.

         result[enum_value] = label
         enum_value += 1
     else:
       if START_REGEX.match(line):
         inside_enum = True
         enum_value = 0
-  return result
+  return result, None
 
 
 def CreateEnumItemNode(document, value, label):
   """Creates an int element to append to an enum."""
   item_node = document.createElement('int')
   item_node.attributes['value'] = str(value)
   item_node.attributes['label'] = label
   return item_node
 
 
@@ skipping 68 matching lines @@
                              source_enum_path, histograms_doc)
 
   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.

[Ilya Sherman, 2017/04/13 01:12:49]

Please update the documentation around the return value (or, perhaps, the thrown
exception).

 
   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)
+  source_enum_values, duplicated_values = ReadHistogramValues(
+      source_enum_path, start_marker, end_marker)
+  if duplicated_values:
+    return False, duplicated_values
 
   (xml, new_xml) = _GetOldAndUpdatedXml(histogram_enum_name, source_enum_values,
                                         source_enum_path)
-  return xml != new_xml
+  return xml != new_xml, None
 
 
 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,
@@ skipping 20 matching lines @@
       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)