Add explicit tests for histogram factory matching.

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.
 
 // For Histogram(exponential histogram), LinearHistogram and CustomHistogram,
 // the minimum for a declared range is 1 (instead of 0), while the maximum is
 // (HistogramBase::kSampleType_MAX - 1). Currently you can declare histograms
 // with ranges exceeding those limits (e.g. 0 as minimal or
 // HistogramBase::kSampleType_MAX as maximal), but those excesses will be
 // silently clamped to those limits (for backwards compatibility with existing
 // code). Best practice is to not exceed the limits.
 
+// Each use of a histogram with the same name will reference the same underlying
+// data, so it is safe to record to the same histogram from multiple locations
+// in the code. It is a runtime error if all uses of the same histogram do not
+// agree exactly in type, bucket size and range.
+
 // For Histogram and LinearHistogram, the maximum for a declared range should
 // always be larger (not equal) than minmal range. Zero and
 // HistogramBase::kSampleType_MAX are implicitly added as first and last ranges,
 // so the smallest legal bucket_count is 3. However CustomHistogram can have
 // bucket count as 2 (when you give a custom ranges vector containing only 1
 // range).
 // For these 3 kinds of histograms, the max bucket count is always
 // (Histogram::kBucketCount_MAX - 1).
 
 // The buckets layout of class Histogram is exponential. For example, buckets
@@ skipping 490 matching lines @@
   // Most commonly this is the numeric value, but in derived classes it may
   // be a name (or string description) given to the bucket.
   virtual const std::string GetAsciiBucketRange(size_t it) const;
 
  private:
   // Allow tests to corrupt our innards for testing purposes.
   FRIEND_TEST_ALL_PREFIXES(HistogramTest, BoundsTest);
   FRIEND_TEST_ALL_PREFIXES(HistogramTest, BucketPlacementTest);
   FRIEND_TEST_ALL_PREFIXES(HistogramTest, CorruptBucketBounds);
   FRIEND_TEST_ALL_PREFIXES(HistogramTest, CorruptSampleCounts);
+  FRIEND_TEST_ALL_PREFIXES(HistogramTest, NameMatchTest);
 
   friend class StatisticsRecorder;  // To allow it to delete duplicates.
   friend class StatisticsRecorderTest;
 
   // Implementation of SnapshotSamples function.
   scoped_ptr<SampleVector> SnapshotSampleVector() const;
 
   //----------------------------------------------------------------------------
   // Helpers for emitting Ascii graphic.  Each method appends data to output.
 
@@ skipping 163 matching lines @@
   static bool ValidateCustomRanges(const std::vector<Sample>& custom_ranges);
   static BucketRanges* CreateBucketRangesFromCustomRanges(
       const std::vector<Sample>& custom_ranges);
 
   DISALLOW_COPY_AND_ASSIGN(CustomHistogram);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_HISTOGRAM_H_