Add PRESUBMIT check if modified UMA histogram name can be found

PRESUBMIT.py

 # Copyright (c) 2012 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.
 
 """Top-level presubmit script for Chromium.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts
 for more details about the presubmit API built into gcl.
 """
 
@@ skipping 339 matching lines @@
 
     for line_num, line in f.ChangedContents():
       if 'UNIT_TEST ' in line or line.endswith('UNIT_TEST'):
         problems.append('    %s:%d' % (f.LocalPath(), line_num))
 
   if not problems:
     return []
   return [output_api.PresubmitPromptWarning('UNIT_TEST is only for headers.\n' +
       '\n'.join(problems))]
 
+def _CheckUmaHistogramChanges(input_api, output_api):
+  """Check that UMA histogram names in touched lines can still be found in other
+  lines of the patch or in histograms.xml. Note that this check would not catch
+  the reverse: changes in histograms.xml not matched in the code itself."""
+
+  touched_histograms = []
+  histograms_xml_modifications = []
+  pattern = input_api.re.compile('UMA_HISTOGRAM.*\("(.*)"')
+  for f in input_api.AffectedFiles():
+    # If histograms.xml itself is modified, keep the modified lines for later.
+    if (f.LocalPath().endswith(('histograms.xml'))):
+      histograms_xml_modifications = f.ChangedContents()
+      continue
+    if (not f.LocalPath().endswith(('cc', 'mm', 'cpp'))):
+      continue
+    for line_num, line in f.ChangedContents():
+      found = pattern.search(line)
+      if found:
+        touched_histograms.append([found.group(1), f, line_num])
+
+  # Search for the touched histogram names in the local modifications to
+  # histograms.xml, and if not found on the base file.
+  problems = []
+  for histogram_name, f, line_num in touched_histograms:
+    histogram_name_found = False
+    for line_num, line in histograms_xml_modifications:
+      if histogram_name in line:
+        histogram_name_found = True;
+        break;
+    if histogram_name_found:
+      continue
+
+    with open('tools/metrics/histograms/histograms.xml') as histograms_xml:
+      for line in histograms_xml:
+        if histogram_name in line:
+          histogram_name_found = True;
+          break;
+    if histogram_name_found:
+      continue
+    problems.append(' [%s:%d] %s' % (f.LocalPath(), line_num, histogram_name))
+
+  if not problems:
+    return []
+  return [output_api.PresubmitPromptWarning('Some UMA_HISTOGRAM lines have '
+    'been modified and the associated histogram name has no match in either '
+    'metrics/histograms.xml or the modifications of it:',  problems)]
+
 
 def _CheckNoNewWStrings(input_api, output_api):
   """Checks to make sure we don't introduce use of wstrings."""
   problems = []
   for f in input_api.AffectedFiles():
     if (not f.LocalPath().endswith(('.cc', '.h')) or
         f.LocalPath().endswith(('test.cc', '_win.cc', '_win.h')) or
         '/win/' in f.LocalPath()):
       continue
 
@@ skipping 1143 matching lines @@
         _IPC_ENUM_TRAITS_DEPRECATED, problems)]
   else:
     return []
 
 
 def CheckChangeOnUpload(input_api, output_api):
   results = []
   results.extend(_CommonChecks(input_api, output_api))
   results.extend(_CheckValidHostsInDEPS(input_api, output_api))
   results.extend(_CheckJavaStyle(input_api, output_api))
+  results.extend(_CheckUmaHistogramChanges(input_api, output_api))
   return results
 
 
 def GetTryServerMasterForBot(bot):
   """Returns the Try Server master for the given bot.
 
   It tries to guess the master from the bot name, but may still fail
   and return None.  There is no longer a default master.
   """
   # Potentially ambiguous bot names are listed explicitly.
@@ skipping 203 matching lines @@
   ]
 
   # Match things like path/aura/file.cc and path/file_aura.cc.
   # Same for chromeos.
   if any(re.search(r'[\\\/_](aura|chromeos)', f) for f in files):
     builders.extend([
         'linux_chromeos_asan',
     ])
 
   return GetDefaultTryConfigs(builders)