[histogram] Make histograms more resistant to overflows.

base/metrics/histogram_unittest.cc

Index: base/metrics/histogram_unittest.cc
diff --git a/base/metrics/histogram_unittest.cc b/base/metrics/histogram_unittest.cc
index d89e7e93247d8b60896aa46b6bd4fa3dafe03b39..8d8d6aaac7e84f1926176abbd7ddae9f31978bc2 100644
--- a/base/metrics/histogram_unittest.cc
+++ b/base/metrics/histogram_unittest.cc
@@ -401,6 +401,22 @@ TEST_P(HistogramTest, AddCount_LargeValuesDontOverflow) {
   EXPECT_EQ(19400000000LL, samples2->sum());
 }
 
+// Make sure that counts returned by Histogram::SnapshotDelta do not overflow
+// even when a total count (returned by Histogram::SnapshotSample) does.
+TEST_P(HistogramTest, AddCount_LargeCountsDontOverflow) {
+  const size_t kBucketCount = 50;
+  Histogram* histogram = static_cast<Histogram*>(Histogram::FactoryGet(
+      "AddCountHistogram", 10, 50, kBucketCount, HistogramBase::kNoFlags));

[bcwhite, 2017/05/10 12:40:33]

You have 50 buckets for a range of 40.

[altimin, 2017/05/10 13:08:55]

Done.

+
+  // Repeat N times to make sure that there is no internal value overflow.
+  for (int i = 0; i < 10; ++i) {
+    histogram->AddCount(42, 1 << 30);
+    std::unique_ptr<HistogramSamples> samples = histogram->SnapshotDelta();
+    EXPECT_EQ(1 << 30, samples->TotalCount());
+    EXPECT_EQ(1 << 30, samples->GetCount(42));
+  }
+}
+
 // Make sure histogram handles out-of-bounds data gracefully.
 TEST_P(HistogramTest, BoundsTest) {
   const size_t kBucketCount = 50;