Merge multiple histogram snapshots into single one for reporting.

base/metrics/histogram_snapshot_manager.h

Index: base/metrics/histogram_snapshot_manager.h
diff --git a/base/metrics/histogram_snapshot_manager.h b/base/metrics/histogram_snapshot_manager.h
index 0d2f05bf6819e15f071e87d00eda93bb7c31906a..1f3e0330158bc46af54caa0f576594d877dfb5ac 100644
--- a/base/metrics/histogram_snapshot_manager.h
+++ b/base/metrics/histogram_snapshot_manager.h
@@ -24,6 +24,32 @@ class HistogramFlattener;
 // calling for the marginal change (a.k.a., delta) in a histogram to be
 // recorded.
 class BASE_EXPORT HistogramSnapshotManager {
+  struct SampleInfo {
+    // A histogram associated with this sample; it may be one of many if
+    // several have been aggregated into the same "accumulated" sample set.
+    // Ownership of the histogram remains elsewhere and this pointer is
+    // cleared by FinishDeltas() called at the end of PrepareDeltas().
+    const HistogramBase* histogram;
+
+    // The previous snapshot values that were logged to the server so can
+    // record only the delta with the next log.
+    scoped_ptr<HistogramSamples> logged;
+
+    // The current snapshot values being accumulated. There is no need for
+    // this to be a scoped_ptr<> because it is only used temporarily during
+    // a loop after which it replaces the "logged" value.
+    HistogramSamples* accumulated;
+
+    // The set of inconsistencies already seen for the histogram.
+    int inconsistencies;

[Alexei Svitkine (slow), 2015/12/04 18:30:11]

Nit: inconsistencies_count or num_inconsistencies

[bcwhite, 2015/12/08 17:32:18]

It's not a count but a set of flags.  I'll add to the comment.

+
+    SampleInfo() : histogram(nullptr),
+                   accumulated(nullptr),
+                   inconsistencies(0) {}
+  };
+
+  typedef std::map<uint64_t, SampleInfo> HashInfoMap;
+
  public:
   explicit HistogramSnapshotManager(HistogramFlattener* histogram_flattener);
   virtual ~HistogramSnapshotManager();
@@ -33,34 +59,43 @@ class BASE_EXPORT HistogramSnapshotManager {
   // |required_flags| is used to select histograms to be recorded.
   // Only histograms that have all the flags specified by the argument will be
   // chosen. If all histograms should be recorded, set it to
-  // |Histogram::kNoFlags|.
+  // |Histogram::kNoFlags|. Though any "forward" iterator will work, the
+  // histograms over which it iterates *must* remain valid until this method
+  // returns; the iterator cannot deallocate histograms once it iterates past
+  // them.
   template <class ForwardHistogramIterator>
   void PrepareDeltas(ForwardHistogramIterator begin,
                      ForwardHistogramIterator end,
                      HistogramBase::Flags flags_to_set,
                      HistogramBase::Flags required_flags) {
+    StartDeltas();
     for (ForwardHistogramIterator it = begin; it != end; ++it) {
       (*it)->SetFlags(flags_to_set);
       if (((*it)->flags() & required_flags) == required_flags)
-        PrepareDelta(**it);
+        PrepareDelta(*it);
     }
+    FinishDeltas();
   }
 
  private:
-  // Snapshot this histogram, and record the delta.
-  void PrepareDelta(const HistogramBase& histogram);
+  FRIEND_TEST_ALL_PREFIXES(HistogramSnapshotManagerTest, CheckMerge);
+
+  // Start delta accumulation.
+  void StartDeltas();
+
+  // Snapshot this histogram and record the delta.
+  void PrepareDelta(const HistogramBase* histogram);
+
+  // Finish all deltas and flush them to the Flattener.
+  void FinishDeltas();
 
   // Try to detect and fix count inconsistency of logged samples.
   void InspectLoggedSamplesInconsistency(
       const HistogramSamples& new_snapshot,
       HistogramSamples* logged_samples);
 
-  // For histograms, track what we've already recorded (as a sample for
-  // each histogram) so that we can record only the delta with the next log.
-  std::map<uint64_t, HistogramSamples*> logged_samples_;
-
-  // List of histograms found to be corrupt, and their problems.
-  std::map<uint64_t, int> inconsistencies_;
+  // For histograms, track what has been previously seen.
+  HashInfoMap known_histograms_;
 
   // |histogram_flattener_| handles the logistics of recording the histogram
   // deltas.