Add support for obsoleting individual histogram suffixes.

tools/metrics/histograms/extract_histograms.py

 # Copyright 2013 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.
 
 """Extract histogram names from the description XML file.
 
 For more information on the format of the XML file, which is self-documenting,
 see histograms.xml; however, here is a simple example to get you started. The
 XML below will generate the following five histograms:
 
@@ skipping 273 matching lines @@
       enum_name = histogram.getAttribute('enum')
       if enum_name not in enums:
         logging.error('Unknown enum %s in histogram %s', enum_name, name)
         have_errors = True
       else:
         histogram_entry['enum'] = enums[enum_name]
 
   return histograms, have_errors
 
 
+# Finds an <obsolete> node amongst |node|'s immediate children and returns its
+# content as a string. Returns None if no such node exists.
+def _GetObsoleteReason(node):
+  for child in node.childNodes:
+    if child.localName == 'obsolete':
+      # There can be at most 1 obsolete element per node.
+      return _JoinChildNodes(child)
+  return None
+
+
 def _UpdateHistogramsWithSuffixes(tree, histograms):
   """Process <histogram_suffixes> tags and combine with affected histograms.
 
   The histograms dictionary will be updated in-place by adding new histograms
   created by combining histograms themselves with histogram_suffixes targeting
   these histograms.
 
   Args:
     tree: XML dom tree.
     histograms: a dictionary of histograms previously extracted from the tree;
@@ skipping 44 matching lines @@
       if reprocess_count < MAX_HISTOGRAM_SUFFIX_DEPENDENCY_DEPTH:
         reprocess_queue.append((reprocess_count + 1, histogram_suffixes))
         continue
       else:
         logging.error('histogram_suffixes %s is missing its dependency %s',
                       histogram_suffixes.getAttribute('name'),
                       missing_dependency)
         have_errors = True
         continue
 
-    obsolete_nodes = histogram_suffixes.getElementsByTagName('obsolete')
-    obsolete_reason = None
-    if obsolete_nodes:
-      # There can be at most 1 obsolete element per histogram_suffixes node
-      obsolete_reason = _JoinChildNodes(obsolete_nodes[0])
+    # If the suffix group has an obsolete tag, all suffixes it generates inherit
+    # its reason.
+    group_obsolete_reason = _GetObsoleteReason(histogram_suffixes)
 
     name = histogram_suffixes.getAttribute('name')
     suffix_nodes = histogram_suffixes.getElementsByTagName(suffix_tag)
     suffix_labels = {}
     for suffix in suffix_nodes:
       suffix_labels[suffix.getAttribute('name')] = suffix.getAttribute('label')
     # Find owners list under current histogram_suffixes tag.
     owners = _ExtractOwners(histogram_suffixes)
 
     last_histogram_name = None
@@ skipping 36 matching lines @@
           if 'fieldtrial_labels' not in histograms[new_histogram_name]:
             histograms[new_histogram_name]['fieldtrial_labels'] = []
           histograms[new_histogram_name]['fieldtrial_labels'].append(
               suffix_label)
 
           # If no owners are added for this histogram-suffixes, it inherits the
           # owners of its parents.
           if owners:
             histograms[new_histogram_name]['owners'] = owners
 
+          # If a suffix has an obsolete node, it's marked as obsolete for the
+          # specified reason, overwriting its group's obsoletion reason if the
+          # group itself was obsolete as well.
+          obsolete_reason = _GetObsoleteReason(suffix)
+          if not obsolete_reason:
+            obsolete_reason = group_obsolete_reason
+
           # If the suffix has an obsolete tag, all histograms it generates
           # inherit it.
           if obsolete_reason:
             histograms[new_histogram_name]['obsolete'] = obsolete_reason
 
         except Error:
           have_errors = True
 
   return have_errors
 
@@ skipping 34 matching lines @@
   with open(filename, 'r') as f:
     histograms, had_errors = ExtractHistogramsFromFile(f)
     if had_errors:
       logging.error('Error parsing %s', filename)
       raise Error()
     return histograms
 
 
 def ExtractNames(histograms):
   return sorted(histograms.keys())