Add support for persistent sparse histograms.

base/metrics/histogram.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.
 
 // 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).
 // See header file for details and examples.
 
 #include "base/metrics/histogram.h"
@@ skipping 109 matching lines @@
     return ranges;
   }
 
   // Allocate the correct Histogram object off the heap (in case persistent
   // memory is not available).
   virtual scoped_ptr<HistogramBase> HeapAlloc(const BucketRanges* ranges) {
     return make_scoped_ptr(new Histogram(name_, minimum_, maximum_, ranges));
   }
 
   // Perform any required datafill on the just-created histogram.  If
-  // overridden, be sure to call the "super" version.
-  virtual void FillHistogram(HistogramBase* histogram) {
-    histogram->SetFlags(flags_);
-  }
+  // overridden, be sure to call the "super" version -- this method may not
+  // always remain empty.
+  virtual void FillHistogram(HistogramBase* histogram) {}
 
   // These values are protected (instead of private) because they need to
   // be accessible to methods of sub-classes in order to avoid passing
   // unnecessary parameters everywhere.
   const std::string& name_;
   const HistogramType histogram_type_;
   HistogramBase::Sample minimum_;
   HistogramBase::Sample maximum_;
   uint32_t bucket_count_;
   int32_t flags_;
@@ skipping 21 matching lines @@
     // cases (such as with a CustomHistogram), they are calculated dynamically
     // at run-time. In the latter case, those ctor parameters are zero and
     // the results extracted from the result of CreateRanges().
     if (bucket_count_ == 0) {
       bucket_count_ = static_cast<uint32_t>(registered_ranges->bucket_count());
       minimum_ = registered_ranges->range(1);
       maximum_ = registered_ranges->range(bucket_count_ - 1);
     }
 
     // Try to create the histogram using a "persistent" allocator. As of
-    // 2015-01-14, the availability of such is controlled by a base::Feature
+    // 2016-02-25, the availability of such is controlled by a base::Feature
     // that is off by default. If the allocator doesn't exist or if
     // allocating from it fails, code below will allocate the histogram from
     // the process heap.
     PersistentHistogramAllocator::Reference histogram_ref = 0;
     scoped_ptr<HistogramBase> tentative_histogram;
     PersistentHistogramAllocator* allocator =
         PersistentHistogramAllocator::GetGlobalAllocator();
     if (allocator) {
-      flags_ |= HistogramBase::kIsPersistent;
       tentative_histogram = allocator->AllocateHistogram(
           histogram_type_,
           name_,
           minimum_,
           maximum_,
           registered_ranges,
           flags_,
           &histogram_ref);
     }
 
     // Handle the case where no persistent allocator is present or the
     // persistent allocation fails (perhaps because it is full).
     if (!tentative_histogram) {
       DCHECK(!histogram_ref);  // Should never have been set.
       DCHECK(!allocator);  // Shouldn't have failed.
       flags_ &= ~HistogramBase::kIsPersistent;
       tentative_histogram = HeapAlloc(registered_ranges);
+      tentative_histogram->SetFlags(flags_);
     }
 
     FillHistogram(tentative_histogram.get());
 
     // Register this histogram with the StatisticsRecorder. Keep a copy of
     // the pointer value to tell later whether the locally created histogram
     // was registered or deleted. The type is "void" because it could point
     // to released memory after the following line.
     const void* tentative_histogram_ptr = tentative_histogram.get();
     histogram = StatisticsRecorder::RegisterOrDeleteDuplicate(
@@ skipping 957 matching lines @@
     Sample sample = custom_ranges[i];
     if (sample < 0 || sample > HistogramBase::kSampleType_MAX - 1)
       return false;
     if (sample != 0)
       has_valid_range = true;
   }
   return has_valid_range;
 }
 
 }  // namespace base