[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..4a6137c5bf8c3fad34a5e12034114c71a8e7e4ea 100644
--- a/base/metrics/histogram_unittest.cc
+++ b/base/metrics/histogram_unittest.cc
@@ -401,6 +401,21 @@ TEST_P(HistogramTest, AddCount_LargeValuesDontOverflow) {
   EXPECT_EQ(19400000000LL, samples2->sum());
 }
 
+TEST_P(HistogramTest, AddCount_LargeCountsDontOverflow) {

[Ilya Sherman, 2017/05/09 22:15:32]

Please add a comment documenting what this is intending to test.  In particular,
it should be clear to someone reading this test that ten calls to AddCount(42, 1
<< 30) without an intervening call to SnapshotDelta() should be expected to
overflow.

[altimin, 2017/05/10 11:50:38]

Done.

+  const size_t kBucketCount = 50;
+  Histogram* histogram = static_cast<Histogram*>(
+      Histogram::FactoryGet("AddCountHistogram", 10, 1000000000, kBucketCount,

[Ilya Sherman, 2017/05/09 22:15:32]

nit: Do you need such a large max bucket value?  It's a bit hard to read and
distracting from the rest of the test.

[altimin, 2017/05/10 11:50:38]

Done.

+                            HistogramBase::kNoFlags));
+
+  // 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;