use c++17's try_emplace if available (#5696)

Summary:
This avoids rehashing the key in TrackKey() in case the key is not already
in the map of tracked keys, which will happen at least once per key used in a
transaction.

Additionally fix two typos.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5696

Differential Revision: D17210178

Pulled By: lth

fbshipit-source-id: 7e2c28e9e505c1d1c1535d435250cf2b191a6fdf

utilities/transactions/transaction_base.cc

@@ -633,18 +633,30 @@ void TransactionBaseImpl::TrackKey(TransactionKeyMap* key_map, uint32_t cfh_id,
                                    const std::string& key, SequenceNumber seq,
                                    bool read_only, bool exclusive) {
   auto& cf_key_map = (*key_map)[cfh_id];
+#ifdef __cpp_lib_unordered_map_try_emplace
+  // use c++17's try_emplace if available, to avoid rehashing the key
+  // in case it is not already in the map
+  auto result = cf_key_map.try_emplace(key, seq);
+  auto iter = result.first;
+  if (!result.second && 
+      seq < iter->second.seq) {
+    // Now tracking this key with an earlier sequence number
+    iter->second.seq = seq;
+  }
+#else
   auto iter = cf_key_map.find(key);
   if (iter == cf_key_map.end()) {
-    auto result = cf_key_map.emplace(key, TransactionKeyMapInfo(seq));
+    auto result = cf_key_map.emplace(key, seq);
     iter = result.first;
   } else if (seq < iter->second.seq) {
     // Now tracking this key with an earlier sequence number
     iter->second.seq = seq;
   }
+#endif
   // else we do not update the seq. The smaller the tracked seq, the stronger it
   // the guarantee since it implies from the seq onward there has not been a
   // concurrent update to the key. So we update the seq if it implies stronger
-  // guarantees, i.e., if it is smaller than the existing trakced seq.
+  // guarantees, i.e., if it is smaller than the existing tracked seq.
 
   if (read_only) {
     iter->second.num_reads++;

utilities/transactions/transaction_util.cc

@@ -75,8 +75,8 @@ Status TransactionUtil::CheckKey(DBImpl* db_impl, SuperVersion* sv,
 
     if (cache_only) {
       result = Status::TryAgain(
-          "Transaction ould not check for conflicts as the MemTable does not "
+          "Transaction could not check for conflicts as the MemTable does not "
-          "countain a long enough history to check write at SequenceNumber: ",
+          "contain a long enough history to check write at SequenceNumber: ",
           ToString(snap_seq));
     }
   } else if (snap_seq < earliest_seq || min_uncommitted <= earliest_seq) {