Report histogram creation results.

base/metrics/histogram_base.h

Index: base/metrics/histogram_base.h
diff --git a/base/metrics/histogram_base.h b/base/metrics/histogram_base.h
index 6817629bf8c080dd9a3fe259d96ff586c7ccebc2..fb63935863341d80ed97da60f28bcb2b5d66576f 100644
--- a/base/metrics/histogram_base.h
+++ b/base/metrics/histogram_base.h
@@ -30,7 +30,7 @@ class Pickle;
 class PickleIterator;
 
 ////////////////////////////////////////////////////////////////////////////////
-// These enums are used to facilitate deserialization of histograms from other
+// This enum is used to facilitate deserialization of histograms from other
 // processes into the browser. If you create another class that inherits from
 // HistogramBase, add new histogram types and names below.
 
@@ -44,6 +44,39 @@ enum HistogramType {
 
 std::string HistogramTypeToString(HistogramType type);
 
+// This enum is used for reporting how many histograms and of what types and
+// variations are being created. It has to be in the main .h file so it is
+// visible to files that define the various histogram types.
+enum HistogramReport {
+  // Count the number of reports created. The other counts divided by this
+  // number will give the average per run of the program.
+  HISTOGRAM_REPORT_CREATED,
+
+  // Count the total number of histograms created. It is the limit against
+  // which all others are compared.
+  HISTOGRAM_REPORT_HISTOGRAM_CREATED,
+
+  // Count the total number of histograms looked-up. It's better to cache
+  // the result of a single lookup rather than do it repeatedly.
+  HISTOGRAM_REPORT_HISTOGRAM_LOOKUP,
+
+  // These count the individual histogram types. This must follow the order
+  // of HistogramType above.
+  HISTOGRAM_REPORT_TYPE_GENERAL,
+  HISTOGRAM_REPORT_TYPE_LINEAR,
+  HISTOGRAM_REPORT_TYPE_BOOLEAN,
+  HISTOGRAM_REPORT_TYPE_CUSTOM,
+  HISTOGRAM_REPORT_TYPE_SPARSE,
+
+  // These indicate the individual flags that were set.
+  HISTOGRAM_REPORT_FLAG_UMA_TARGETED,
+  HISTOGRAM_REPORT_FLAG_UMA_STABILITY,
+  HISTOGRAM_REPORT_FLAG_PERSISTENT,
+
+  // This must be last.
+  HISTOGRAM_REPORT_MAX
+};
+
 // Create or find existing histogram that matches the pickled info.
 // Returns NULL if the pickled data has problems.
 BASE_EXPORT HistogramBase* DeserializeHistogramInfo(base::PickleIterator* iter);
@@ -178,6 +211,14 @@ class BASE_EXPORT HistogramBase {
   // customize the output.
   void WriteJSON(std::string* output) const;
 
+  // This enables a histogram that reports the what types of histograms are
+  // created and their flags. It must be called while still single-threaded.
+  //
+  // IMPORTANT: Callers must update tools/metrics/histograms/histograms.xml
+  // with the following histogram:
+  //    UMA.Histograms.process_type.Creations
+  static void EnableCreationReportHistogram(StringPiece process_type);
+
  protected:
   // Subclasses should implement this function to make SerializeInfo work.
   virtual bool SerializeInfoImpl(base::Pickle* pickle) const = 0;
@@ -210,7 +251,15 @@ class BASE_EXPORT HistogramBase {
   // passing |sample| as the parameter.
   void FindAndRunCallback(Sample sample) const;
 
+  // Update the creation (or lookup) report for a histogram.

[Alexei Svitkine (slow), 2016/03/01 16:41:34]

Document the |created| param, please. Also I think it's slightly confusing that
the function is called "UpdateReportHistogram" and takes "histogram" as a param
- at first read it would sound like that's the histogram being updated. Can you
rework things to avoid this possible confusion?

[bcwhite, 2016/03/02 19:14:19]

Done.

+  static void UpdateReportHistogram(HistogramBase* histogram, bool created);
+
+  // Retrieves the global histogram reporting what histograms are created.
+  static HistogramBase* report_histogram_;
+
  private:
+  friend class HistogramBaseTest;
+
   const std::string histogram_name_;
   AtomicCount flags_;