Report histogram creation results.

base/metrics/histogram_base.cc

Index: base/metrics/histogram_base.cc
diff --git a/base/metrics/histogram_base.cc b/base/metrics/histogram_base.cc
index 3a40e6813fc6f5c7669c627e4bbc007e6d7184d3..ffcae1986435d99123951a97c143412448172485 100644
--- a/base/metrics/histogram_base.cc
+++ b/base/metrics/histogram_base.cc
@@ -62,6 +62,7 @@ HistogramBase* DeserializeHistogramInfo(PickleIterator* iter) {
 }
 
 const HistogramBase::Sample HistogramBase::kSampleType_MAX = INT_MAX;
+HistogramBase* HistogramBase::report_histogram_ = nullptr;
 
 HistogramBase::HistogramBase(const std::string& name)
     : histogram_name_(name),
@@ -122,6 +123,23 @@ void HistogramBase::WriteJSON(std::string* output) const {
   serializer.Serialize(root);
 }
 
+// static
+void HistogramBase::EnableCreationReportHistogram(StringPiece process_type) {
+  DCHECK(!report_histogram_);
+  size_t existing = StatisticsRecorder::NumberOfHistograms();
+  if (existing) {

[Alexei Svitkine (slow), 2016/03/01 16:41:33]

Nit: != 0

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

Done.

+    DLOG(WARNING) << existing
+                  << " histograms were created before reporting was enabled.";
+  }
+
+  std::string name =
+      "UMA.Histograms." + process_type.as_string() + ".Creations";
+  report_histogram_ = LinearHistogram::FactoryGet(
+      name, 1, HISTOGRAM_REPORT_MAX, HISTOGRAM_REPORT_MAX + 1,
+      kUmaTargetedHistogramFlag);
+  report_histogram_->Add(HISTOGRAM_REPORT_CREATED);
+}
+
 void HistogramBase::FindAndRunCallback(HistogramBase::Sample sample) const {
   if ((flags() & kCallbackExists) == 0)
     return;
@@ -163,4 +181,26 @@ void HistogramBase::WriteAsciiBucketValue(Count current,
   StringAppendF(output, " (%d = %3.1f%%)", current, current/scaled_sum);
 }
 
+// static
+void HistogramBase::UpdateReportHistogram(HistogramBase* histogram,
+                                          bool created) {
+  if (!report_histogram_)
+    return;
+
+  if (created) {
+    report_histogram_->Add(HISTOGRAM_REPORT_HISTOGRAM_CREATED);
+    report_histogram_->Add(HISTOGRAM_REPORT_TYPE_GENERAL +
+                           histogram->GetHistogramType());

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

This is a bit fragile - if the histogram type ever changes (a new value is
added), then this histogram gets broken.

I suggest just doing a switch on the histogram type and mapping to the value.
This way, if a new type is added, then there would be a compile error if there's
a new enum that's not handled in the switch - which ensures that the histogram
will be updated.

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

Done.

+    const int32_t flags = histogram->flags_;
+    if (flags & kIsPersistent)
+      report_histogram_->Add(HISTOGRAM_REPORT_FLAG_PERSISTENT);
+    if (flags & (kUmaStabilityHistogramFlag & ~kUmaTargetedHistogramFlag))
+      report_histogram_->Add(HISTOGRAM_REPORT_FLAG_UMA_STABILITY);
+    else if (flags & kUmaTargetedHistogramFlag)
+      report_histogram_->Add(HISTOGRAM_REPORT_FLAG_UMA_TARGETED);
+  } else {
+    report_histogram_->Add(HISTOGRAM_REPORT_HISTOGRAM_LOOKUP);
+  }
+}
+
 }  // namespace base