Refactor MergeHelper::MergeUntil a bit (#10943)

Summary:
The patch untangles some nested ifs in `MergeHelper::MergeUntil`. This will come in handy when extending the compaction logic to support `Merge` for wide-column entities, and also enables us to eliminate some repeated branching on value type and to decrease the scope of some variables.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10943

Test Plan: `make check`

Reviewed By: riversand963

Differential Revision: D41201946

Pulled By: ltamasi

fbshipit-source-id: 890bd3d4e31cdccadca614489a94686d76485ba9

db/merge_helper.cc

@@ -290,22 +290,28 @@ Status MergeHelper::MergeUntil(InternalIterator* iter,
       // (almost) silently dropping the put/delete. That's probably not what we
       // want. Also if we're in compaction and it's a put, it would be nice to
       // run compaction filter on it.
+      std::string merge_result;
+
+      if (range_del_agg &&
+          range_del_agg->ShouldDelete(
+              ikey, RangeDelPositioningMode::kForwardTraversal)) {
+        s = TimedFullMerge(user_merge_operator_, ikey.user_key, nullptr,
+                           merge_context_.GetOperands(), &merge_result, logger_,
+                           stats_, clock_,
+                           /* result_operand */ nullptr,
+                           /* update_num_ops_stats */ false);
+      } else if (ikey.type == kTypeValue) {
         const Slice val = iter->value();
-      PinnableSlice blob_value;
+
-      const Slice* val_ptr;
+        s = TimedFullMerge(user_merge_operator_, ikey.user_key, &val,
-      if ((kTypeValue == ikey.type || kTypeBlobIndex == ikey.type ||
+                           merge_context_.GetOperands(), &merge_result, logger_,
-           kTypeWideColumnEntity == ikey.type) &&
+                           stats_, clock_,
-          (range_del_agg == nullptr ||
+                           /* result_operand */ nullptr,
-           !range_del_agg->ShouldDelete(
+                           /* update_num_ops_stats */ false);
-               ikey, RangeDelPositioningMode::kForwardTraversal))) {
-        if (ikey.type == kTypeWideColumnEntity) {
-          // TODO: support wide-column entities
-          return Status::NotSupported(
-              "Merge currently not supported for wide-column entities");
       } else if (ikey.type == kTypeBlobIndex) {
         BlobIndex blob_index;
 
-          s = blob_index.DecodeFrom(val);
+        s = blob_index.DecodeFrom(iter->value());
         if (!s.ok()) {
           return s;
         }
@@ -319,30 +325,34 @@ Status MergeHelper::MergeUntil(InternalIterator* iter,
 
         assert(blob_fetcher);
 
-          s = blob_fetcher->FetchBlob(ikey.user_key, blob_index,
+        PinnableSlice blob_value;
-                                      prefetch_buffer, &blob_value,
+        s = blob_fetcher->FetchBlob(ikey.user_key, blob_index, prefetch_buffer,
-                                      &bytes_read);
+                                    &blob_value, &bytes_read);
         if (!s.ok()) {
           return s;
         }
 
-          val_ptr = &blob_value;
-
         if (c_iter_stats) {
           ++c_iter_stats->num_blobs_read;
           c_iter_stats->total_blob_bytes_read += bytes_read;
         }
+
+        s = TimedFullMerge(user_merge_operator_, ikey.user_key, &blob_value,
+                           merge_context_.GetOperands(), &merge_result, logger_,
+                           stats_, clock_,
+                           /* result_operand */ nullptr,
+                           /* update_num_ops_stats */ false);
+      } else if (ikey.type == kTypeWideColumnEntity) {
+        // TODO: support wide-column entities
+        return Status::NotSupported(
+            "Merge currently not supported for wide-column entities");
       } else {
-          val_ptr = &val;
+        s = TimedFullMerge(user_merge_operator_, ikey.user_key, nullptr,
-        }
+                           merge_context_.GetOperands(), &merge_result, logger_,
-      } else {
+                           stats_, clock_,
-        val_ptr = nullptr;
+                           /* result_operand */ nullptr,
+                           /* update_num_ops_stats */ false);
       }
-      std::string merge_result;
-      s = TimedFullMerge(
-          user_merge_operator_, ikey.user_key, val_ptr,
-          merge_context_.GetOperands(), &merge_result, logger_, stats_, clock_,
-          /* result_operand */ nullptr, /* update_num_ops_stats */ false);
 
       // We store the result in keys_.back() and operands_.back()
       // if nothing went wrong (i.e.: no operand corruption on disk)