Support "shared" histograms between processes.

base/metrics/statistics_recorder.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.
 
 // StatisticsRecorder holds all Histograms and BucketRanges that are used by
 // Histograms in the system. It provides a general place for
 // Histograms/BucketRanges to register, and supports a global API for accessing
 // (i.e., dumping, or graphing) the data.
 
 #ifndef BASE_METRICS_STATISTICS_RECORDER_H_
@@ skipping 12 matching lines @@
 #include "base/lazy_instance.h"
 #include "base/macros.h"
 #include "base/metrics/histogram_base.h"
 
 namespace base {
 
 class BucketRanges;
 class Lock;
 
 class BASE_EXPORT StatisticsRecorder {
+  // We keep all registered histograms in a map, indexed by the hash of the
+  // name of the histogram.
+  typedef std::map<uint64_t, HistogramBase*> HistogramMap;

[Alexei Svitkine (slow), 2016/01/18 19:25:36]

Nit: Move this below the public: block.

[bcwhite, 2016/01/22 15:24:53]

Done.

+
  public:
   typedef std::vector<HistogramBase*> Histograms;
 
+  // A class for iterating over the histograms held within this global resource.
+  class BASE_EXPORT HistogramIterator {
+   public:
+    HistogramIterator(const HistogramMap::iterator& iter,
+                      bool include_persistent);
+
+    // STL Iterators must be copyable.
+    HistogramIterator(const HistogramIterator& rhs);
+
+    HistogramIterator& operator++();
+    HistogramIterator operator++(int) {
+      HistogramIterator tmp(*this);
+      operator++();
+      return tmp;
+    }
+
+    bool operator==(const HistogramIterator& rhs) const {
+      return iter_ == rhs.iter_;
+    }
+    bool operator!=(const HistogramIterator& rhs) const {
+      return iter_ != rhs.iter_;
+    }
+    HistogramBase* operator*() { return iter_->second; }
+
+   private:
+    HistogramMap::iterator iter_;
+    const bool include_persistent_;
+  };
+
   // Initializes the StatisticsRecorder system. Safe to call multiple times.
   static void Initialize();
 
   // Find out if histograms can now be registered into our list.
   static bool IsActive();
 
   // Register, or add a new histogram to the collection of statistics. If an
   // identically named histogram is already registered, then the argument
   // |histogram| will deleted.  The returned value is always the registered
   // histogram (either the argument, or the pre-existing registered histogram).
@@ skipping 19 matching lines @@
   // Method for extracting histograms which were marked for use by UMA.
   static void GetHistograms(Histograms* output);
 
   // Method for extracting BucketRanges used by all histograms registered.
   static void GetBucketRanges(std::vector<const BucketRanges*>* output);
 
   // Find a histogram by name. It matches the exact name. This method is thread
   // safe.  It returns NULL if a matching histogram is not found.
   static HistogramBase* FindHistogram(const std::string& name);
 
+  // Support for iterating over known histograms.
+  static HistogramIterator begin(bool include_persistent);
+  static HistogramIterator end();
+
   // GetSnapshot copies some of the pointers to registered histograms into the
   // caller supplied vector (Histograms). Only histograms which have |query| as
   // a substring are copied (an empty string will process all registered
   // histograms).
   static void GetSnapshot(const std::string& query, Histograms* snapshot);
 
   typedef base::Callback<void(HistogramBase::Sample)> OnSampleCallback;
 
   // SetCallback sets the callback to notify when a new sample is recorded on
   // the histogram referred to by |histogram_name|. The call to this method can
   // be be done before or after the histogram is created. This method is thread
   // safe. The return value is whether or not the callback was successfully set.
   static bool SetCallback(const std::string& histogram_name,
                           const OnSampleCallback& callback);
 
   // ClearCallback clears any callback set on the histogram referred to by
   // |histogram_name|. This method is thread safe.
   static void ClearCallback(const std::string& histogram_name);
 
   // FindCallback retrieves the callback for the histogram referred to by
   // |histogram_name|, or a null callback if no callback exists for this
   // histogram. This method is thread safe.
   static OnSampleCallback FindCallback(const std::string& histogram_name);
 
  private:
-  // We keep all registered histograms in a map, indexed by the hash of the
-  // name of the histogram.
-  typedef std::map<uint64_t, HistogramBase*> HistogramMap;
-
   // We keep a map of callbacks to histograms, so that as histograms are
   // created, we can set the callback properly.
   typedef std::map<std::string, OnSampleCallback> CallbackMap;
 
   // We keep all |bucket_ranges_| in a map, from checksum to a list of
   // |bucket_ranges_|.  Checksum is calculated from the |ranges_| in
   // |bucket_ranges_|.
   typedef std::map<uint32_t, std::list<const BucketRanges*>*> RangesMap;
 
   friend struct DefaultLazyInstanceTraits<StatisticsRecorder>;
   friend class HistogramBaseTest;
   friend class HistogramSnapshotManagerTest;
   friend class HistogramTest;
   friend class JsonPrefStoreTest;
+  friend class SharedHistogramTest;
   friend class SparseHistogramTest;
   friend class StatisticsRecorderTest;
   FRIEND_TEST_ALL_PREFIXES(HistogramDeltaSerializationTest,
                            DeserializeHistogramAndAddSamples);
 
   // The constructor just initializes static members. Usually client code should
   // use Initialize to do this. But in test code, you can friend this class and
   // call destructor/constructor to get a clean StatisticsRecorder.
   StatisticsRecorder();
   ~StatisticsRecorder();
 
   static void DumpHistogramsToVlog(void* instance);
 
   static HistogramMap* histograms_;
   static CallbackMap* callbacks_;
   static RangesMap* ranges_;
 
   // Lock protects access to above maps.
   static base::Lock* lock_;
 
   DISALLOW_COPY_AND_ASSIGN(StatisticsRecorder);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_STATISTICS_RECORDER_H_