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 HistogramBase* HeapAlloc(const BucketRanges* ranges) {
     return new Histogram(name_, minimum_, maximum_, ranges);
   }
 
   // Perform any required datafill on the just-created histogram.  If
   // overridden, be sure to call the "super" version.

[Alexei Svitkine (slow), 2016/03/14 18:48:59]

Nit: Remove second sentence, since super version is a no-op.

[bcwhite, 2016/03/15 00:00:21]

It is, but there is no guarantee it will always remain that way, or be true if
it were to be derived from a derived class.

-  virtual void FillHistogram(HistogramBase* histogram) {
-    histogram->SetFlags(flags_);
-  }
+  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.
     PersistentMemoryAllocator::Reference histogram_ref = 0;
     HistogramBase* tentative_histogram = nullptr;
     PersistentMemoryAllocator* allocator =
         GetPersistentHistogramMemoryAllocator();
     if (allocator) {
-      flags_ |= HistogramBase::kIsPersistent;
       tentative_histogram = AllocatePersistentHistogram(
           allocator,
           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_);

[Alexei Svitkine (slow), 2016/03/14 18:49:00]

Nit: Put this inside HeapAlloc()

[bcwhite, 2016/03/15 00:00:21]

It would have to be done inside every HeapAlloc and the purpose of those many 
methods is only to allocate and return; no modifications.

[Alexei Svitkine (slow), 2016/03/16 15:25:43]

To me, it seem a small implementation detail that flags_ is set via a separate
call vs. passed to the ctor. As such, it would be better to abstract it into
HeapAlloc() - since I see that function as allocating the object with the state
specified in member fields (i.e. flags_ is no different than name_ or maximum_).
So I think it's better to have SetFlags() be in HeapAlloc(). And maybe in a
future CL to event make it a ctor param.

[bcwhite, 2016/03/16 15:42:48]

I was thinking the same: It would be best as a ctor param.  Will do.

     }
 
     FillHistogram(tentative_histogram);
     histogram =
         StatisticsRecorder::RegisterOrDeleteDuplicate(tentative_histogram);
 
     // Persistent histograms need some follow-up processing.
     if (histogram_ref) {
       FinalizePersistentHistogram(histogram_ref,
                                   histogram == tentative_histogram);
@@ skipping 942 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