Telemetry: Don't silently compare histograms from different processes.

tools/perf/perf_tools/histogram.py

 # Copyright (c) 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.
 import json
 import logging
 
 def SubtractHistogram(histogram_json, start_histogram_json):
   """Subtracts a previous histogram from a histogram. Both parameters are json
   serializations of histograms."""
   start_histogram = json.loads(start_histogram_json)
   # It's ok if the start histogram is empty (we had no data, maybe even no
   # histogram at all, at the start of the test).
   if 'buckets' not in start_histogram:
     return histogram_json
 
+  histogram = json.loads(histogram_json)
+  if ('pid' in start_histogram and 'pid' in histogram
+      and start_histogram['pid'] != histogram['pid']):
+    raise Exception(
+        'Trying to compare histograms from different processes (%d and %d)'
+        % (start_histogram['pid'], histogram['pid']))
+
   start_histogram_buckets = dict()
   for b in start_histogram['buckets']:
     start_histogram_buckets[b['low']] = b['count']
 
   new_buckets = []
-  histogram = json.loads(histogram_json)
   for b in histogram['buckets']:
     new_bucket = b
     low = b['low']
     if low in start_histogram_buckets:
       new_bucket['count'] = b['count'] - start_histogram_buckets[low]
       if new_bucket['count'] < 0:
         logging.error('Histogram subtraction error, starting histogram most '
                       'probably invalid.')
     if new_bucket['count']:
       new_buckets.append(new_bucket)
   histogram['buckets'] = new_buckets
   histogram['count'] -= start_histogram['count']
 
   return json.dumps(histogram)