tx_pool: speed up take_tx for transactions from blocks

This happens for every historical tx when syncing, and the
unnecessary parsing is actually showing up on profile.
Since these are kept cached for just one block, this does
not increase memory usage after syncing.

src/cryptonote_core/tx_pool.cpp

@@ -247,6 +247,8 @@ namespace cryptonote
         memset(meta.padding, 0, sizeof(meta.padding));
         try
         {
+          if (kept_by_block)
+            m_parsed_tx_cache.insert(std::make_pair(id, tx));
           CRITICAL_REGION_LOCAL1(m_blockchain);
           LockedTXN lock(m_blockchain);
           m_blockchain.add_txpool_tx(id, blob, meta);
@@ -288,6 +290,8 @@ namespace cryptonote
 
       try
       {
+        if (kept_by_block)
+          m_parsed_tx_cache.insert(std::make_pair(id, tx));
         CRITICAL_REGION_LOCAL1(m_blockchain);
         LockedTXN lock(m_blockchain);
         m_blockchain.remove_txpool_tx(id);
@@ -468,7 +472,12 @@ namespace cryptonote
         return false;
       }
       cryptonote::blobdata txblob = m_blockchain.get_txpool_tx_blob(id);
-      if (!parse_and_validate_tx_from_blob(txblob, tx))
+      auto ci = m_parsed_tx_cache.find(id);
+      if (ci != m_parsed_tx_cache.end())
+      {
+        tx = ci->second;
+      }
+      else if (!parse_and_validate_tx_from_blob(txblob, tx))
       {
         MERROR("Failed to parse tx from txpool");
         return false;
@@ -911,6 +920,7 @@ namespace cryptonote
   {
     CRITICAL_REGION_LOCAL(m_transactions_lock);
     m_input_cache.clear();
+    m_parsed_tx_cache.clear();
     return true;
   }
   //---------------------------------------------------------------------------------
@@ -918,6 +928,7 @@ namespace cryptonote
   {
     CRITICAL_REGION_LOCAL(m_transactions_lock);
     m_input_cache.clear();
+    m_parsed_tx_cache.clear();
     return true;
   }
   //---------------------------------------------------------------------------------

src/cryptonote_core/tx_pool.h

@@ -584,6 +584,8 @@ private:
     size_t m_txpool_weight;
 
     mutable std::unordered_map<crypto::hash, std::tuple<bool, tx_verification_context, uint64_t, crypto::hash>> m_input_cache;
+
+    std::unordered_map<crypto::hash, transaction> m_parsed_tx_cache;
   };
 }