HistogramTester::GetHistogramSamplesSinceCreation never returns null

base/test/histogram_tester.cc

 // Copyright 2014 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.
 
 #include "base/test/histogram_tester.h"
 
 #include "base/metrics/histogram.h"
 #include "base/metrics/histogram_samples.h"
+#include "base/metrics/sample_map.h"
 #include "base/metrics/statistics_recorder.h"
 #include "base/stl_util.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace base {
 
 HistogramTester::HistogramTester() {
   StatisticsRecorder::Initialize();  // Safe to call multiple times.
 
   // Record any histogram data that exists when the object is created so it can
@@ skipping 86 matching lines @@
     if (new_samples->TotalCount()) {
       result[histogram->histogram_name()] = new_samples->TotalCount();
     }
   }
   return result;
 }
 
 scoped_ptr<HistogramSamples> HistogramTester::GetHistogramSamplesSinceCreation(
     const std::string& histogram_name) const {
   HistogramBase* histogram = StatisticsRecorder::FindHistogram(histogram_name);
+  // Whether the histogram exists or not may not depend on the current test
+  // calling this method, but rather on which tests ran before and possibly
+  // generated a histogram or not (see http://crbug.com/473689). To provide a
+  // response which is independent of the previously run tests, this method
+  // creates empty samples in the absence of the histogram, rather than
+  // returning null.
   if (!histogram)
-    return scoped_ptr<HistogramSamples>();
+    return scoped_ptr<HistogramSamples>(new SampleMap);

[Lei Zhang, 2015/07/31 18:30:59]

GetHistogramSamplesSinceCreation() now always return a non-null scoped_ptr.
There's still places like sql/connection_unittest.cc that checks for the
possibility of a null return value. I'm ok with the other tests that
ASSERT_TRUE(returned_scoper);

[vabr (Chromium), 2015/07/31 18:49:45]

Good catch. I grepped for GetHistogramSamplesSinceCreation and verified that the
sql tests were the only place, where the EXPECT macros still allowed for samples
being null (and removed that). I left the places with ASSERTS untouched,
following your comment.

   scoped_ptr<HistogramSamples> named_samples(histogram->SnapshotSamples());
   auto original_samples_it = histograms_snapshot_.find(histogram_name);
   if (original_samples_it != histograms_snapshot_.end())
     named_samples->Subtract(*original_samples_it->second);
   return named_samples.Pass();
 }
 
 void HistogramTester::CheckBucketCount(
     const std::string& name,
     base::HistogramBase::Sample sample,
@@ skipping 30 matching lines @@
 
 bool Bucket::operator==(const Bucket& other) const {
   return min == other.min && count == other.count;
 }
 
 void PrintTo(const Bucket& bucket, std::ostream* os) {
   *os << "Bucket " << bucket.min << ": " << bucket.count;
 }
 
 }  // namespace base