[histogram] Make histograms more resistant to overflows.

base/metrics/sparse_histogram_unittest.cc

Index: base/metrics/sparse_histogram_unittest.cc
diff --git a/base/metrics/sparse_histogram_unittest.cc b/base/metrics/sparse_histogram_unittest.cc
index 40e6fbf644bdf4ac8024259534fed9b01126c70d..a6fb3efea730797c050bce0a1864f8b138ff4f2d 100644
--- a/base/metrics/sparse_histogram_unittest.cc
+++ b/base/metrics/sparse_histogram_unittest.cc
@@ -149,6 +149,25 @@ TEST_P(SparseHistogramTest, AddCount_LargeValuesDontOverflow) {
   EXPECT_EQ(55250000000LL, snapshot2->sum());
 }
 
+// Make sure that counts returned by Histogram::SnapshotDelta do not overflow
+// even when a total count (returned by Histogram::SnapshotSample) does.
+TEST_P(SparseHistogramTest, AddCount_LargeCountsDontOverflow) {
+  std::unique_ptr<SparseHistogram> histogram(NewSparseHistogram("Sparse"));
+  std::unique_ptr<HistogramSamples> snapshot(histogram->SnapshotSamples());
+  EXPECT_EQ(0, snapshot->TotalCount());
+  EXPECT_EQ(0, snapshot->sum());
+
+  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));
+  }
+}
+
 TEST_P(SparseHistogramTest, MacroBasicTest) {
   UMA_HISTOGRAM_SPARSE_SLOWLY("Sparse", 100);
   UMA_HISTOGRAM_SPARSE_SLOWLY("Sparse", 200);