[histogram] Make histograms more resistant to overflows.

base/metrics/histogram_unittest.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/metrics/histogram.h"
 
 #include <limits.h>
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 383 matching lines @@
   histogram->AddCount(200000000, 25);
   histogram->AddCount(300000000, 24);
 
   std::unique_ptr<HistogramSamples> samples2 = histogram->SnapshotSamples();
   EXPECT_EQ(78, samples2->TotalCount());
   EXPECT_EQ(40, samples2->GetCount(200000000));
   EXPECT_EQ(38, samples2->GetCount(300000000));
   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;
   Histogram* histogram = static_cast<Histogram*>(
       Histogram::FactoryGet("Bounded", 10, 100, kBucketCount,
                             HistogramBase::kNoFlags));
 
   // Put two samples "out of bounds" above and below.
   histogram->Add(5);
   histogram->Add(-50);
@@ skipping 329 matching lines @@
   // CustomHistogram needs at least 1 valid range.
   custom_ranges.clear();
   custom_ranges.push_back(0);
   EXPECT_DEATH_IF_SUPPORTED(
       CustomHistogram::FactoryGet("BadRangesCustom3", custom_ranges,
                                   HistogramBase::kNoFlags),
                "");
 }
 
 }  // namespace base