Add a StatisticsDeltaReader class and use cases

This CL adds a utility class to streamline the task of testing that
metrics have been updated as expected. Specifically, the
StatisticsDeltaReader class allows a client to obtain the differential
value of a given histogram in the interval since the given
StatisticsDeltaReader instance was created.

This reverts commit 6a7dfb3cd638829a04e002ea9bc5b18ed3c9c03f.

BUG=232414
TBR=rsleevy, groby

Review URL: https://codereview.chromium.org/120753002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243303 0039d316-1c4b-4281-b951-d872f2087c98

5 files changed, 140 insertions, 3 deletions

diff --git a/base/base.gyp b/base/base.gyp
index 7c2b54d..96ed56f 100644
--- a/base/base.gyp
+++ b/base/base.gyp
@@ -608,6 +608,7 @@
         'template_util_unittest.cc',
         'test/expectations/expectation_unittest.cc',
         'test/expectations/parser_unittest.cc',
+        'test/statistics_delta_reader_unittest.cc',
         'test/test_reg_util_win_unittest.cc',
         'test/trace_event_analyzer_unittest.cc',
         'threading/non_thread_safe_unittest.cc',
@@ -914,6 +915,8 @@
         'test/simple_test_clock.h',
         'test/simple_test_tick_clock.cc',
         'test/simple_test_tick_clock.h',
+        'test/statistics_delta_reader.cc',
+        'test/statistics_delta_reader.h',
         'test/task_runner_test_template.cc',
         'test/task_runner_test_template.h',
         'test/test_file_util.cc',
diff --git a/base/metrics/statistics_recorder.h b/base/metrics/statistics_recorder.h
index 0716e80..5e53f274 100644
--- a/base/metrics/statistics_recorder.h
+++ b/base/metrics/statistics_recorder.h
@@ -70,9 +70,9 @@ class BASE_EXPORT StatisticsRecorder {
   static HistogramBase* FindHistogram(const std::string& name);
 
   // GetSnapshot copies some of the pointers to registered histograms into the
-  // caller supplied vector (Histograms).  Only histograms with names matching
-  // query are returned. The query must be a substring of histogram name for its
-  // pointer to be copied.
+  // caller supplied vector (Histograms). Only histograms which have |query| as
+  // a substring are copied (an empty string will process all registered
+  // histograms).
   static void GetSnapshot(const std::string& query, Histograms* snapshot);
 
  private:
@@ -88,6 +88,7 @@ class BASE_EXPORT StatisticsRecorder {
   friend class HistogramBaseTest;
   friend class HistogramTest;
   friend class SparseHistogramTest;
+  friend class StatisticsDeltaReaderTest;
   friend class StatisticsRecorderTest;
   FRIEND_TEST_ALL_PREFIXES(HistogramDeltaSerializationTest,
                            DeserializeHistogramAndAddSamples);
diff --git a/base/test/statistics_delta_reader.cc b/base/test/statistics_delta_reader.cc
new file mode 100644
index 0000000..9e77b8b
--- /dev/null
+++ b/base/test/statistics_delta_reader.cc
@@ -0,0 +1,41 @@
+// 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/statistics_delta_reader.h"
+
+#include "base/metrics/histogram.h"
+#include "base/metrics/statistics_recorder.h"
+#include "base/stl_util.h"
+
+namespace base {
+
+StatisticsDeltaReader::StatisticsDeltaReader() {
+  // Record any histogram data that exists when the object is created so it can
+  // be subtracted later.
+  StatisticsRecorder::Histograms histograms;
+  StatisticsRecorder::GetSnapshot(std::string(), &histograms);
+  for (size_t i = 0; i < histograms.size(); ++i) {
+    original_samples_[histograms[i]->histogram_name()] =
+        histograms[i]->SnapshotSamples().release();
+  }
+}
+
+StatisticsDeltaReader::~StatisticsDeltaReader() {
+  STLDeleteValues(&original_samples_);
+}
+
+scoped_ptr<HistogramSamples>
+    StatisticsDeltaReader::GetHistogramSamplesSinceCreation(
+        const std::string& histogram_name) {
+  HistogramBase* histogram = StatisticsRecorder::FindHistogram(histogram_name);
+  if (!histogram)
+    return scoped_ptr<HistogramSamples>();
+  scoped_ptr<HistogramSamples> named_samples(histogram->SnapshotSamples());
+  HistogramSamples* named_original_samples = original_samples_[histogram_name];
+  if (named_original_samples)
+    named_samples->Subtract(*named_original_samples);
+  return named_samples.Pass();
+}
+
+}  // namespace base
diff --git a/base/test/statistics_delta_reader.h b/base/test/statistics_delta_reader.h
new file mode 100644
index 0000000..c4e4e99
--- /dev/null
+++ b/base/test/statistics_delta_reader.h
@@ -0,0 +1,41 @@
+// 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.
+
+#ifndef BASE_TEST_STATISTICS_DELTA_READER_H_
+#define BASE_TEST_STATISTICS_DELTA_READER_H_
+
+#include <map>
+#include <string>
+
+#include "base/basictypes.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/metrics/histogram_samples.h"
+
+namespace base {
+
+// This class acts as a differential reader for histogram samples, enabling
+// tests to check that metrics were recorded as they should be.
+// Before using this class, StatisticsRecoder must be initialized.
+class StatisticsDeltaReader {
+ public:
+  StatisticsDeltaReader();
+  ~StatisticsDeltaReader();
+
+  // Returns the histogram data accumulated since this instance was created.
+  // Returns NULL if no samples are available.
+  scoped_ptr<HistogramSamples> GetHistogramSamplesSinceCreation(
+      const std::string& histogram_name);
+
+ private:
+  // Used to determine the histogram changes made during this instance's
+  // lifecycle. This instance takes ownership of the samples, which are deleted
+  // when the instance is destroyed.
+  std::map<std::string, HistogramSamples*> original_samples_;
+
+  DISALLOW_COPY_AND_ASSIGN(StatisticsDeltaReader);
+};
+
+}  // namespace base
+
+#endif  // BASE_TEST_STATISTICS_DELTA_READER_H_
diff --git a/base/test/statistics_delta_reader_unittest.cc b/base/test/statistics_delta_reader_unittest.cc
new file mode 100644
index 0000000..acf8d5a
--- /dev/null
+++ b/base/test/statistics_delta_reader_unittest.cc
@@ -0,0 +1,51 @@
+// 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/memory/scoped_ptr.h"
+#include "base/metrics/histogram.h"
+#include "base/metrics/histogram_samples.h"
+#include "base/metrics/statistics_recorder.h"
+#include "base/test/statistics_delta_reader.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace base {
+
+class StatisticsDeltaReaderTest : public testing::Test {
+ protected:
+  virtual void SetUp() OVERRIDE {
+    // Each test will have a clean state (no Histogram / BucketRanges
+    // registered).
+    statistics_recorder_ = new StatisticsRecorder();
+  }
+
+  virtual void TearDown() OVERRIDE {
+    delete statistics_recorder_;
+    statistics_recorder_ = NULL;
+  }
+
+  StatisticsRecorder* statistics_recorder_;
+};
+
+TEST_F(StatisticsDeltaReaderTest, Scope) {
+  // Record a histogram before the creation of the recorder.
+  UMA_HISTOGRAM_BOOLEAN("Test", true);
+
+  StatisticsDeltaReader reader;
+
+  // Verify that no histogram is recorded.
+  scoped_ptr<HistogramSamples> samples(
+      reader.GetHistogramSamplesSinceCreation("Test"));
+  EXPECT_TRUE(samples);
+  EXPECT_EQ(0, samples->TotalCount());
+
+  // Record a histogram after the creation of the recorder.
+  UMA_HISTOGRAM_BOOLEAN("Test", true);
+
+  // Verify that one histogram is recorded.
+  samples = reader.GetHistogramSamplesSinceCreation("Test");
+  EXPECT_TRUE(samples);
+  EXPECT_EQ(1, samples->TotalCount());
+}
+
+}  // namespace base