Helper function to time Merges

Summary: Remove duplicate code.  If this diff looks good, I will cleanup other call sites as well.

Test Plan: unit tests

Reviewers: rven, yhchiang, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D37761

db/merge_helper.cc

@@ -16,6 +16,40 @@
 
 namespace rocksdb {
 
+// TODO(agiardullo): Clean up merge callsites to use this func
+Status MergeHelper::TimedFullMerge(const Slice& key, const Slice* value,
+                                   const std::deque<std::string>& operands,
+                                   const MergeOperator* merge_operator,
+                                   Statistics* statistics, Env* env,
+                                   Logger* logger, std::string* result) {
+  if (operands.size() == 0) {
+    result->assign(value->data(), value->size());
+    return Status::OK();
+  }
+
+  if (merge_operator == nullptr) {
+    return Status::NotSupported("Provide a merge_operator when opening DB");
+  }
+
+  // Setup to time the merge
+  StopWatchNano timer(env, statistics != nullptr);
+  PERF_TIMER_GUARD(merge_operator_time_nanos);
+
+  // Do the merge
+  bool success =
+      merge_operator->FullMerge(key, value, operands, result, logger);
+
+  RecordTick(statistics, MERGE_OPERATION_TOTAL_TIME,
+             env != nullptr ? timer.ElapsedNanos() : 0);
+
+  if (!success) {
+    RecordTick(statistics, NUMBER_MERGE_FAILURES);
+    return Status::Corruption("Error: Could not perform merge.");
+  }
+
+  return Status::OK();
+}
+
 // PRE:  iter points to the first merge type entry
 // POST: iter points to the first entry beyond the merge process (or the end)
 //       keys_, operands_ are updated to reflect the merge result.
@@ -81,25 +115,20 @@ void MergeHelper::MergeUntil(Iterator* iter, SequenceNumber stop_before,
       //   => store result in operands_.back() (and update keys_.back())
       //   => change the entry type to kTypeValue for keys_.back()
       // We are done! Return a success if the merge passes.
-      {
-        StopWatchNano timer(env_, stats != nullptr);
-        PERF_TIMER_GUARD(merge_operator_time_nanos);
-        success_ = user_merge_operator_->FullMerge(
-            ikey.user_key, nullptr, operands_, &merge_result, logger_);
-        RecordTick(stats, MERGE_OPERATION_TOTAL_TIME,
-                   env_ != nullptr ? timer.ElapsedNanos() : 0);
-      }
+
+      Status s = TimedFullMerge(ikey.user_key, nullptr, operands_,
+                                user_merge_operator_, stats, env_, logger_,
+                                &merge_result);
+
       // We store the result in keys_.back() and operands_.back()
       // if nothing went wrong (i.e.: no operand corruption on disk)
-      if (success_) {
+      if (s.ok()) {
         std::string& original_key =
             keys_.back();  // The original key encountered
         orig_ikey.type = kTypeValue;
         UpdateInternalKey(&original_key[0], original_key.size(),
                           orig_ikey.sequence, orig_ikey.type);
         swap(operands_.back(), merge_result);
-      } else {
-        RecordTick(stats, NUMBER_MERGE_FAILURES);
       }
 
       // move iter to the next entry (before doing anything else)
@@ -117,25 +146,19 @@ void MergeHelper::MergeUntil(Iterator* iter, SequenceNumber stop_before,
       //   => change the entry type to kTypeValue for keys_.back()
       // We are done! Success!
       const Slice val = iter->value();
-      {
-        StopWatchNano timer(env_, stats != nullptr);
-        PERF_TIMER_GUARD(merge_operator_time_nanos);
-        success_ = user_merge_operator_->FullMerge(
-            ikey.user_key, &val, operands_, &merge_result, logger_);
-        RecordTick(stats, MERGE_OPERATION_TOTAL_TIME,
-                   env_ != nullptr ? timer.ElapsedNanos() : 0);
-      }
+      Status s =
+          TimedFullMerge(ikey.user_key, &val, operands_, user_merge_operator_,
+                         stats, env_, logger_, &merge_result);
+
       // We store the result in keys_.back() and operands_.back()
       // if nothing went wrong (i.e.: no operand corruption on disk)
-      if (success_) {
+      if (s.ok()) {
         std::string& original_key =
             keys_.back();  // The original key encountered
         orig_ikey.type = kTypeValue;
         UpdateInternalKey(&original_key[0], original_key.size(),
                           orig_ikey.sequence, orig_ikey.type);
         swap(operands_.back(), merge_result);
-      } else {
-        RecordTick(stats, NUMBER_MERGE_FAILURES);
       }
 
       // move iter to the next entry

db/merge_helper.h

@@ -35,6 +35,15 @@ class MergeHelper {
         operands_(),
         success_(false) {}
 
+  // Wrapper around MergeOperator::FullMerge() that records perf statistics.
+  // Result of merge will be written to result if status returned is OK.
+  // If operands is empty, the value will simply be copied to result.
+  static Status TimedFullMerge(const Slice& key, const Slice* value,
+                               const std::deque<std::string>& operands,
+                               const MergeOperator* merge_operator,
+                               Statistics* statistics, Env* env, Logger* logger,
+                               std::string* result);
+
   // Merge entries until we hit
   //     - a corrupted key
   //     - a Put/Delete,