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

chrome/test/base/uma_histogram_helper.cc

Index: chrome/test/base/uma_histogram_helper.cc
diff --git a/chrome/test/base/uma_histogram_helper.cc b/chrome/test/base/uma_histogram_helper.cc
index 9b5122974bb6cc4ede50a82bc235d7b83f0a3c45..37ba5ca8a19717c7b96ec037b9c6eb274a1514a8 100644
--- a/chrome/test/base/uma_histogram_helper.cc
+++ b/chrome/test/base/uma_histogram_helper.cc
@@ -11,6 +11,22 @@
 #include "content/public/browser/histogram_fetcher.h"
 
 UMAHistogramHelper::UMAHistogramHelper() {
+  base::StatisticsRecorder::Initialize();
+}
+
+void UMAHistogramHelper::PrepareSnapshot(const char** histogram_names,
+                                         int num_histograms) {
+  for (int i = 0; i < num_histograms; i++) {
+    std::string histogram_name = histogram_names[i];
+
+    base::HistogramBase* histogram =
+        base::StatisticsRecorder::FindHistogram(histogram_name);
+    // If there is no histogram present, then don't record a snapshot. The logic
+    // in the Expect* methods will act to treat no histogram equivalent to
+    // samples with zeros.
+    if (histogram)
+      histogram_snapshots[histogram_name] = histogram->SnapshotSamples();
+  }
 }
 
 void UMAHistogramHelper::Fetch() {
@@ -82,18 +98,33 @@ void UMAHistogramHelper::CheckBucketCount(
     base::HistogramBase::Sample sample,
     base::HistogramBase::Count expected_count,
     base::HistogramSamples& samples) {
-  EXPECT_EQ(expected_count, samples.GetCount(sample))
+  int actual_count;
+  if (histogram_snapshots.count(name)) {
+    actual_count = samples.GetCount(sample) -
+                   histogram_snapshots[name]->GetCount(sample);
+  } else {
+    actual_count = samples.GetCount(sample);
+  }
+  EXPECT_EQ(expected_count, actual_count)
       << "Histogram \"" << name
       << "\" does not have the right number of samples (" << expected_count
-      << ") in the expected bucket (" << sample << ").";
+      << ") in the expected bucket (" << sample << "). It has ("
+      << actual_count << ").";
 }
 
 void UMAHistogramHelper::CheckTotalCount(
     const std::string& name,
     base::HistogramBase::Count expected_count,
     base::HistogramSamples& samples) {
-  EXPECT_EQ(expected_count, samples.TotalCount())
+  int actual_count;
+  if (histogram_snapshots.count(name)) {

[Alexei Svitkine (slow), 2014/06/12 17:09:11]

Nit: How about:

int actual_count = samples.TotalCount();
if (histogram_snapshots.count(name))
  actual_count -= histogram_snapshots[name]->TotalCount();

Same above.

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

Slick. Done.

+    actual_count = samples.TotalCount() -
+                   histogram_snapshots[name]->TotalCount();
+  } else {
+    actual_count = samples.TotalCount();
+  }
+  EXPECT_EQ(expected_count, actual_count)
       << "Histogram \"" << name
       << "\" does not have the right total number of samples ("
-      << expected_count << ").";
+      << expected_count << "). It has (" << actual_count << ").";
 }