[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..f234c7b8c0957838532c50e02c8b341bce0b3cf8 100644
--- a/base/metrics/histogram_unittest.cc
+++ b/base/metrics/histogram_unittest.cc
@@ -401,6 +401,24 @@ 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.

[Ilya Sherman, 2017/05/11 00:42:34]

nit: I'd phrase this comment as something like:

"Some metrics are designed so that they are guaranteed not to overflow between
snapshots, but could overflow over a long-running session.  Make sure that ..."

[altimin, 2017/05/11 12:38:33]

Done.

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