Reland "Add a HistogramRecorder 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 HistogramRecorder class allows a client to obtain the differential value of a given histogram in the interval since the given HistogramRecorder instance was created.

This reverts commit b957026f43ef9464a8cdd5a240867c807b2281da.

BUG=232414
TBR=rsleevi, mark, groby, thakis

Review URL: https://codereview.chromium.org/19866004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242121 0039d316-1c4b-4281-b951-d872f2087c98

3 files changed, 137 insertions, 0 deletions

diff --git a/base/test/histogram_recorder.cc b/base/test/histogram_recorder.cc
new file mode 100644
index 0000000..ee852f7
--- /dev/null
+++ b/base/test/histogram_recorder.cc
@@ -0,0 +1,52 @@
+// 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.
+
+#include "base/test/histogram_recorder.h"
+
+#include "base/metrics/histogram.h"
+#include "base/metrics/statistics_recorder.h"
+#include "base/stl_util.h"
+
+namespace base {
+
+// static
+void HistogramRecorder::Initialize() {
+  // Ensure that StatisticsRecorder is initialized.
+  StatisticsRecorder::Initialize();
+}
+
+HistogramRecorder::HistogramRecorder() {
+  // Record any histogram data that exists when the object is created so it can
+  // be subtracted later.
+  StatisticsRecorder::Histograms histograms;
+  StatisticsRecorder::GetSnapshot("", &histograms);
+  for (size_t i = 0; i < histograms.size(); i++) {
+    original_samples_[histograms[i]->histogram_name()] =
+        histograms[i]->SnapshotSamples().release();
+  }
+}
+
+HistogramRecorder::~HistogramRecorder() {
+  STLDeleteValues(&original_samples_);
+}
+
+// static
+bool HistogramRecorder::IsActive() {
+  return StatisticsRecorder::IsActive();
+}
+
+scoped_ptr<HistogramSamples>
+    HistogramRecorder::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/histogram_recorder.h b/base/test/histogram_recorder.h
new file mode 100644
index 0000000..f867b1e
--- /dev/null
+++ b/base/test/histogram_recorder.h
@@ -0,0 +1,44 @@
+// 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.
+
+#ifndef BASE_TEST_HISTOGRAM_RECORDER_H_
+#define BASE_TEST_HISTOGRAM_RECORDER_H_
+
+#include <map>
+
+#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.
+class HistogramRecorder {
+ public:
+  // Initializes the HistogramRecorder system.
+  static void Initialize();
+  HistogramRecorder();
+  virtual ~HistogramRecorder();
+
+  // Returns whether the HistogramRecorder has been initialized.
+  static bool IsActive();
+
+  // 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 isntance takes ownership of the samples, which are deleted
+  // when the instance is destroyed.
+  std::map<std::string, HistogramSamples*> original_samples_;
+
+  DISALLOW_COPY_AND_ASSIGN(HistogramRecorder);
+};
+
+}  // namespace base
+
+#endif  // BASE_TEST_HISTOGRAM_RECORDER_H_
diff --git a/base/test/histogram_recorder_unittest.cc b/base/test/histogram_recorder_unittest.cc
new file mode 100644
index 0000000..f4cdf49
--- /dev/null
+++ b/base/test/histogram_recorder_unittest.cc
@@ -0,0 +1,41 @@
+// 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.
+
+#include "base/memory/scoped_ptr.h"
+#include "base/metrics/histogram.h"
+#include "base/metrics/histogram_samples.h"
+#include "base/test/histogram_recorder.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace base {
+
+class HistogramRecorderTest : public testing::Test {
+ public:
+  static void SetUpTestCase() {
+    HistogramRecorder::Initialize();
+  }
+};
+
+TEST_F(HistogramRecorderTest, Scope) {
+  // Record a histogram before the creation of the recorder.
+  UMA_HISTOGRAM_BOOLEAN("Test", true);
+
+  HistogramRecorder recorder;
+
+  // Verify that no histogram is recorded.
+  scoped_ptr<HistogramSamples> samples(
+      recorder.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 = recorder.GetHistogramSamplesSinceCreation("Test");
+  EXPECT_TRUE(samples);
+  EXPECT_EQ(1, samples->TotalCount());
+}
+
+}  // namespace base