Add Statistics::getAndResetTickerCount().

Summary: A convience method to atomically get and reset ticker count. I'm wanting to use it to have a thin wrapper to the statistics object to export ticker counts to ODS for LogDevice (since they don't even use fb303).

Test Plan:
test in LogDevice shadow cluster.
https://fburl.com/461868822

Reviewers: andrewkr, yhchiang, IslamAbdelRahman

Reviewed By: IslamAbdelRahman

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D64869

HISTORY.md

@@ -2,6 +2,7 @@
 ## Unreleased
 ### Public API Change
 * DB::GetOptions() reflect dynamic changed options (i.e. through DB::SetOptions()) and return copy of options instead of reference.
+* Added Statistics::getAndResetTickerCount().
 
 ## 4.12.0 (9/12/2016)
 ### Public API Change

include/rocksdb/statistics.h

@@ -416,6 +416,7 @@ class Statistics {
   virtual std::string getHistogramString(uint32_t type) const { return ""; }
   virtual void recordTick(uint32_t tickerType, uint64_t count = 0) = 0;
   virtual void setTickerCount(uint32_t tickerType, uint64_t count) = 0;
+  virtual uint64_t getAndResetTickerCount(uint32_t tickerType) = 0;
   virtual void measureTime(uint32_t histogramType, uint64_t time) = 0;
 
   // String representation of the statistic object.

util/statistics.cc

@@ -123,6 +123,30 @@ void StatisticsImpl::setTickerCount(uint32_t tickerType, uint64_t count) {
   }
 }
 
+uint64_t StatisticsImpl::getAndResetTickerCount(uint32_t tickerType) {
+  uint64_t sum = 0;
+  {
+    MutexLock lock(&aggregate_lock_);
+    assert(enable_internal_stats_ ? tickerType < INTERNAL_TICKER_ENUM_MAX
+                                  : tickerType < TICKER_ENUM_MAX);
+    if (tickerType < TICKER_ENUM_MAX || enable_internal_stats_) {
+      tickers_[tickerType].thread_value->Fold(
+          [](void* curr_ptr, void* res) {
+            auto* sum_ptr = static_cast<uint64_t*>(res);
+            *sum_ptr += static_cast<std::atomic<uint64_t>*>(curr_ptr)->exchange(
+                0, std::memory_order_relaxed);
+          },
+          &sum);
+      sum += tickers_[tickerType].merged_sum.exchange(
+          0, std::memory_order_relaxed);
+    }
+  }
+  if (stats_ && tickerType < TICKER_ENUM_MAX) {
+    stats_->setTickerCount(tickerType, 0);
+  }
+  return sum;
+}
+
 void StatisticsImpl::recordTick(uint32_t tickerType, uint64_t count) {
   assert(
     enable_internal_stats_ ?

util/statistics.h

@@ -41,6 +41,7 @@ class StatisticsImpl : public Statistics {
   std::string getHistogramString(uint32_t histogram_type) const override;
 
   virtual void setTickerCount(uint32_t ticker_type, uint64_t count) override;
+  virtual uint64_t getAndResetTickerCount(uint32_t ticker_type) override;
   virtual void recordTick(uint32_t ticker_type, uint64_t count) override;
   virtual void measureTime(uint32_t histogram_type, uint64_t value) override;