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 ce4102b8dfa36408fe037bd6772184d759a48a4e..33ecdda693eb55677238a43bbcf585381f839b29 100644
--- a/components/metrics/metrics_service.h
+++ b/components/metrics/metrics_service.h
@@ -22,6 +22,7 @@
 #include "base/metrics/field_trial.h"
 #include "base/metrics/histogram_flattener.h"
 #include "base/metrics/histogram_snapshot_manager.h"
+#include "base/metrics/persistent_memory_allocator.h"
 #include "base/metrics/user_metrics.h"
 #include "base/observer_list.h"
 #include "base/time/time.h"
@@ -40,6 +41,7 @@ namespace base {
 class DictionaryValue;
 class HistogramSamples;
 class PrefService;
+class SharedMemoryAllocator;
 }
 
 namespace variations {
@@ -214,6 +216,16 @@ 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_; }
@@ -244,6 +256,7 @@ class MetricsService : public base::HistogramFlattener {
     UNSET
   };
 
+  typedef std::set<base::PersistentMemoryAllocator*> AllocatorSet;
   typedef std::vector<variations::SyntheticTrialGroup> SyntheticTrialGroups;
 
   // Registers a field trial name and group to be used to annotate a UMA report
@@ -256,6 +269,58 @@ class MetricsService : public base::HistogramFlattener {
   void RegisterSyntheticFieldTrial(
       const variations::SyntheticTrialGroup& trial_group);
 
+
+  // A class for iterating over the histograms in persistent memory segments.
+  // This is implemented as an STL-compatible iterator so it can be used in
+  // places that accept histograms both from here and from std containers.
+  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();
+    }
+
+   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_;
+  };
+
+
   // Calls into the client to initialize some system profile metrics.
   void StartInitTask();
 
@@ -394,6 +459,10 @@ 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_;
 
@@ -476,6 +545,7 @@ 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.
@@ -485,6 +555,9 @@ 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);
 };