Support "shared" histograms between processes.

components/metrics/metrics_service.h

Index: components/metrics/metrics_service.h
diff --git a/components/metrics/metrics_service.h b/components/metrics/metrics_service.h
index 234508327238273a11989328dd18d1ef99b617f3..8d81484da9896ad8c643930db52730da2b8b3a1b 100644
--- a/components/metrics/metrics_service.h
+++ b/components/metrics/metrics_service.h
@@ -37,6 +37,7 @@ namespace base {
 class DictionaryValue;
 class HistogramSamples;
 class PrefService;
+class SharedMemoryAllocator;
 }
 
 namespace variations {
@@ -253,6 +254,16 @@ class MetricsService : public base::HistogramFlattener {
   // Pushes a log that has been generated by an external component.
   void PushExternalLog(const std::string& log);
 
+  // Add a persistent memory segment that contains histograms. Ownership
+  // of this object remains with the caller; it must be removed from here
+  // before being destroyed.
+  void AddPersistentMemorySegment(base::PersistentMemoryAllocator* alloc);
+
+  // Remove a persistent memory segment. This should be done only after
+  // a final reporting has been done to avoid data loss and incorrect
+  // reporting.
+  void RemovePersistentMemorySegment(base::PersistentMemoryAllocator* alloc);
+
  protected:
   // Exposed for testing.
   MetricsLogManager* log_manager() { return &log_manager_; }
@@ -281,6 +292,56 @@ class MetricsService : public base::HistogramFlattener {
     UNSET
   };
 
+  typedef std::set<base::PersistentMemoryAllocator*> AllocatorSet;
+
+  // A class for iterating over the histograms in persistent memory segments.
+  class PersistentHistogramIterator {
+    // This class provides a reference-counted pointer to a histogram which
+    // is necessary for histogram objects that are dynamically generated
+    // (generally pointing to persistent memory) and to be owned by an STL
+    // iterator which, by necessity, must be copyable.
+    class HistogramPointer : public base::RefCounted<HistogramPointer> {
+     public:
+      HistogramPointer(base::HistogramBase* h) : histogram_(h) {}
+      base::HistogramBase* get() { return histogram_.get(); }
+
+     private:
+      scoped_ptr<base::HistogramBase> histogram_;
+    };
+
+   public:
+    PersistentHistogramIterator(AllocatorSet& allocators,
+                                AllocatorSet::iterator pos);
+
+    PersistentHistogramIterator& operator++();
+    PersistentHistogramIterator operator++(int) {
+      PersistentHistogramIterator tmp(*this);
+      operator++();
+      return tmp;
+    }
+
+    bool operator==(const PersistentHistogramIterator& rhs) const {
+      return allocator_iter_ == rhs.allocator_iter_ &&
+             histogram_iter_ == rhs.histogram_iter_;
+    }
+    bool operator!=(const PersistentHistogramIterator& rhs) const {
+      return allocator_iter_ != rhs.allocator_iter_ ||
+             histogram_iter_ != rhs.histogram_iter_;
+    }
+    base::HistogramBase* operator*() {
+      return current_histogram_->get();
+    }

[Alexei Svitkine (slow), 2015/12/15 22:48:52]

I think I mentioned this before, but my suggestion was to use our own interface
similar to:
https://code.google.com/p/chromium/codesearch#chromium/src/net/base/expiring_...

Rather than overloading operators, which is generally not done in Chromium code.

[bcwhite, 2015/12/16 13:54:29]

The code taking this iterator accepts standard STL iterators.  That could be
changed but then it wouldn't be possible to pass in iterators to STL containers
such as ::map or ::vector.

[Alexei Svitkine (slow), 2015/12/16 16:33:48]

Do we currently pass in other iterators? If indeed we do and we need to support
that use case, I'm ok with this, but it should be documented in a comment above
the class. If we don't, I think it would be better to simplify it.

[bcwhite, 2015/12/17 15:55:48]

Yes.  The StatisticsRecorder provides an iterator, too.  It isn't an STL
iterator at the moment but I hope to reduce it to that in later optimizations.

Comment added.

+
+   private:
+    AllocatorSet& allocators_;
+    AllocatorSet::iterator allocator_iter_;
+    base::PersistentMemoryAllocator::Iterator histogram_iter_;
+
+    // STL iterators must be copyable so a regular scoped_ptr is not
+    // sufficient as it doesn't allow such. A ref-counted one is needed.
+    scoped_refptr<HistogramPointer> current_histogram_;
+  };
+
   typedef std::vector<SyntheticTrialGroup> SyntheticTrialGroups;
 
   // Calls into the client to initialize some system profile metrics.
@@ -421,6 +482,10 @@ class MetricsService : public base::HistogramFlattener {
   // upload or intentional sampling of logs.
   void SkipAndDiscardUpload();
 
+  // Begin and End iterators for going through all the metrics under management.
+  PersistentHistogramIterator persistent_begin();
+  PersistentHistogramIterator persistent_end();
+
   // Manager for the various in-flight logs.
   MetricsLogManager log_manager_;
 
@@ -502,6 +567,7 @@ class MetricsService : public base::HistogramFlattener {
   FRIEND_TEST_ALL_PREFIXES(MetricsServiceTest,
                            PermutedEntropyCacheClearedWhenLowEntropyReset);
   FRIEND_TEST_ALL_PREFIXES(MetricsServiceTest, RegisterSyntheticTrial);
+  FRIEND_TEST_ALL_PREFIXES(MetricsServiceTest, MultiplePersistentAllocators);
 
   // Weak pointers factory used to post task on different threads. All weak
   // pointers managed by this factory have the same lifetime as MetricsService.
@@ -511,6 +577,9 @@ class MetricsService : public base::HistogramFlattener {
   // this factory are invalidated in ScheduleNextStateSave.
   base::WeakPtrFactory<MetricsService> state_saver_factory_;
 
+  // Persistent memory segments that contain histograms created elsewhere.
+  AllocatorSet allocators_;
+
   DISALLOW_COPY_AND_ASSIGN(MetricsService);
 };