[histogram] Make histograms more resistant to overflows.

base/metrics/histogram.h

 // 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.
 
 // Histogram is an object that aggregates statistics, and can summarize them in
 // various forms, including ASCII graphical, HTML, and numerically (as a
 // vector of numbers corresponding to each of the aggregating buckets).
 
 // It supports calls to accumulate either time intervals (which are processed
 // as integral number of milliseconds), or arbitrary integral units.
@@ skipping 249 matching lines @@
   friend class StatisticsRecorder;  // To allow it to delete duplicates.
   friend class StatisticsRecorderTest;
 
   friend BASE_EXPORT HistogramBase* DeserializeHistogramInfo(
       base::PickleIterator* iter);
   static HistogramBase* DeserializeInfoImpl(base::PickleIterator* iter);
 
   // Implementation of SnapshotSamples function.
   std::unique_ptr<SampleVector> SnapshotSampleVector() const;
 
+  // Create a copy of unlogged samples.
+  std::unique_ptr<SampleVector> SnapshotUnloggedSamples() const;
+
   //----------------------------------------------------------------------------
   // Helpers for emitting Ascii graphic.  Each method appends data to output.
 
   void WriteAsciiImpl(bool graph_it,
                       const std::string& newline,
                       std::string* output) const;
 
   // Find out how large (graphically) the largest bucket will appear to be.
   double GetPeakBucketSize(const SampleVectorBase& samples) const;
 
@@ skipping 16 matching lines @@
   void GetCountAndBucketData(Count* count,
                              int64_t* sum,
                              ListValue* buckets) const override;
 
   // Does not own this object. Should get from StatisticsRecorder.
   const BucketRanges* bucket_ranges_;
 
   Sample declared_min_;  // Less than this goes into the first bucket.
   Sample declared_max_;  // Over this goes into the last bucket.
 
-  // Finally, provide the state that changes with the addition of each new
-  // sample.
-  std::unique_ptr<SampleVectorBase> samples_;
+  // Samples that have not yet been logged with SnapshotDelta().
+  std::unique_ptr<HistogramSamples> unlogged_samples_;
 
-  // Also keep a previous uploaded state for calculating deltas.
+  // Accumulation of all samples that have logged with SnapshotDelta().
   std::unique_ptr<HistogramSamples> logged_samples_;
 
   // Flag to indicate if PrepareFinalDelta has been previously called. It is
   // used to DCHECK that a final delta is not created multiple times.
   mutable bool final_delta_created_ = false;
 
   DISALLOW_COPY_AND_ASSIGN(Histogram);
 };
 
 //------------------------------------------------------------------------------
@@ skipping 214 matching lines @@
   static HistogramBase* DeserializeInfoImpl(base::PickleIterator* iter);
 
   static bool ValidateCustomRanges(const std::vector<Sample>& custom_ranges);
 
   DISALLOW_COPY_AND_ASSIGN(CustomHistogram);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_HISTOGRAM_H_