Fix for garbage key with OnlineGenerator

Summary:
With D58703719, key in Request struct was changed from sting to string_view, but for OnlineGenerator, key string won't live enough and garbage key was being used.
This fixes the issue in simple way.
(Since this issue will be only for OnlineGenerator, one field was simply added just for OnlineGenerator use case, rather than changing key back to string type).

Reviewed By: therealgymmy

Differential Revision: D69500945

fbshipit-source-id: eef3cd974bcaca0a1050735187ccf99382621474

Author: byahn0996

cachelib/cachebench/util/Request.h

@@ -172,6 +172,11 @@ struct Request {
 
   std::string_view key;
 
+  // For online generator, key will be created on the fly and should be valid
+  // until this request is processed. Since 'key' is a string_view type, this
+  // field will store the actual string for the lifetime of this request.
+  std::string onlineKeyString;
+
   // size iterators in case this request is
   // deemed to be a chained item.
   // If not chained, the size is *sizeBegin

cachelib/cachebench/workload/OnlineGenerator.cpp

@@ -55,12 +55,12 @@ const Request& OnlineGenerator::getReq(uint8_t poolId,
                                        std::optional<uint64_t>) {
   size_t keyIdx = getKeyIdx(poolId, gen);
 
-  std::string key(req_->key);
-  generateKey(poolId, keyIdx, key);
+  generateKey(poolId, keyIdx, req_->onlineKeyString);
   auto sizes = generateSize(poolId, keyIdx);
   req_->sizeBegin = sizes->begin();
   req_->sizeEnd = sizes->end();
-  req_->key = key;
+  req_->key = req_->onlineKeyString;
+
   auto op =
       static_cast<OpType>(workloadDist_[workloadIdx(poolId)].sampleOpDist(gen));
   req_->setOp(op);