Add PRESUBMIT check if modified UMA histogram name can be found

PRESUBMIT.py

Index: PRESUBMIT.py
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 90d1df4e286eaad7dac54b35f082b83609c7b5bc..6762e2775e461be1d8d90598d060d252ea7168c8 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -357,6 +357,53 @@ def _CheckNoUNIT_TESTInSourceFiles(input_api, output_api):
   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."""
@@ -1520,6 +1567,7 @@ def CheckChangeOnUpload(input_api, output_api):
   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