Improve UMA tests; improve no-op reconcilor; new reconcilor test.

chrome/test/base/uma_histogram_helper.h

 // 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.
 
 #ifndef CHROME_TEST_BASE_UMA_HISTOGRAM_HELPER_H_
 #define CHROME_TEST_BASE_UMA_HISTOGRAM_HELPER_H_
 
 #include "base/metrics/histogram.h"
 #include "base/metrics/histogram_base.h"
 #include "base/metrics/histogram_samples.h"
 
 // UMAHistogramHelper provides a simple interface for examining UMA histograms.
 // Tests can use this interface to verify that UMA data is getting logged as
 // intended.
 class UMAHistogramHelper {
  public:
+  // To be called before the body of the test code.

[Alexei Svitkine (slow), 2014/06/12 17:09:14]

Comment is confusing since you can't generally call a constructor by itself.
Maybe expand to say that a new UMAHistogramHelper should be created for each
test.

[Mike Lerman, 2014/06/12 18:12:16]

Done.

   UMAHistogramHelper();
 

[Alexei Svitkine (slow), 2014/06/12 17:09:14]

Add a destructor declaration and put its implementation into the .cc file (so an
implicit one isn't generated into each object whose cc file include this
header).

[Mike Lerman, 2014/06/12 18:12:15]

Done.

+  // Parameters should be string literals of all histograms to snapshot.
+  // Call this before executing the test code.
+  void PrepareSnapshot(const char**, int num_histograms);
+
   // Each child process may have its own histogram data, make sure this data
   // gets accumulated into the browser process before we examine the histograms.
   void Fetch();
 
   // We know the exact number of samples in a bucket, and that no other bucket
-  // should have samples.
+  // should have samples. If |PrepareSnapshot| was called for |name| histogram,
+  // then the |expected_count| is the diff from the snapshot.
   void ExpectUniqueSample(const std::string& name,
                           base::HistogramBase::Sample sample,
                           base::HistogramBase::Count expected_count);
 
   // We know the exact number of samples in a bucket, but other buckets may
-  // have samples as well.
+  // have samples as well. If |PrepareSnapshot| was called for |name| histogram,
+  // then the |expected_count| is the diff from the snapshot.
   void ExpectBucketCount(const std::string& name,
                           base::HistogramBase::Sample sample,
                           base::HistogramBase::Count expected_count);
 
-  // We don't know the values of the samples, but we know how many there are.
+  // We don't know the values of the samples, but we know how many there are. If
+  // |PrepareSnapshot| was called for |name| histogram, then the
+  // |count| is the diff from the snapshot.
   void ExpectTotalCount(const std::string& name,
                         base::HistogramBase::Count count);
 
  private:
   void FetchCallback();
 
   void CheckBucketCount(const std::string& name,
                         base::HistogramBase::Sample sample,
                         base::Histogram::Count expected_count,
                         base::HistogramSamples& samples);
 
   void CheckTotalCount(const std::string& name,
                        base::Histogram::Count expected_count,
                        base::HistogramSamples& samples);
 
   DISALLOW_COPY_AND_ASSIGN(UMAHistogramHelper);
+
+  // The map from histogram names to their snapshots
+  std::map<std::string, scoped_ptr<base::HistogramSamples> >
+      histogram_snapshots;
 };
 
 #endif  // CHROME_TEST_BASE_UMA_HISTOGRAM_HELPER_H_