core: cache block template where possible

This avoids constant rechecking of the same things each time
a miner asks for the block template. The tx pool maintains
a cookie to allow users to detect when the pool state changed,
which means the block template needs rebuilding.

src/cryptonote_core/blockchain.cpp

@@ -158,7 +158,8 @@ Blockchain::Blockchain(tx_memory_pool& tx_pool) :
   m_db(), m_tx_pool(tx_pool), m_hardfork(NULL), m_timestamps_and_difficulties_height(0), m_current_block_cumul_sz_limit(0), m_current_block_cumul_sz_median(0),
   m_enforce_dns_checkpoints(false), m_max_prepare_blocks_threads(4), m_db_blocks_per_sync(1), m_db_sync_mode(db_async), m_db_default_sync(false), m_fast_sync(true), m_show_time_stats(false), m_sync_counter(0), m_cancel(false),
   m_difficulty_for_next_block_top_hash(crypto::null_hash),
-  m_difficulty_for_next_block(1)
+  m_difficulty_for_next_block(1),
+  m_btc_valid(false)
 {
   LOG_PRINT_L3("Blockchain::" << __func__);
 }
@@ -632,6 +633,7 @@ block Blockchain::pop_block_from_blockchain()
 
   update_next_cumulative_size_limit();
   m_tx_pool.on_blockchain_dec(m_db->height()-1, get_tail_id());
+  invalidate_block_template_cache();
 
   return popped_block;
 }
@@ -642,6 +644,7 @@ bool Blockchain::reset_and_set_genesis_block(const block& b)
   CRITICAL_REGION_LOCAL(m_blockchain_lock);
   m_timestamps_and_difficulties_height = 0;
   m_alternative_chains.clear();
+  invalidate_block_template_cache();
   m_db->reset();
   m_hardfork->init();
 
@@ -1212,9 +1215,26 @@ bool Blockchain::create_block_template(block& b, const account_public_address& m
   LOG_PRINT_L3("Blockchain::" << __func__);
   size_t median_size;
   uint64_t already_generated_coins;
+  uint64_t pool_cookie;
 
   CRITICAL_REGION_BEGIN(m_blockchain_lock);
   height = m_db->height();
+  if (m_btc_valid) {
+    // The pool cookie is atomic. The lack of locking is OK, as if it changes
+    // just as we compare it, we'll just use a slightly old template, but
+    // this would be the case anyway if we'd lock, and the change happened
+    // just after the block template was created
+    if (!memcmp(&miner_address, &m_btc_address, sizeof(cryptonote::account_public_address)) && m_btc_nonce == ex_nonce && m_btc_pool_cookie == m_tx_pool.cookie()) {
+      MDEBUG("Using cached template");
+      m_btc.timestamp = time(NULL); // update timestamp unconditionally
+      b = m_btc;
+      diffic = m_btc_difficulty;
+      expected_reward = m_btc_expected_reward;
+      return true;
+    }
+    MDEBUG("Not using cached template: address " << (!memcmp(&miner_address, &m_btc_address, sizeof(cryptonote::account_public_address))) << ", nonce " << (m_btc_nonce == ex_nonce) << ", cookie " << (m_btc_pool_cookie == m_tx_pool.cookie()));
+    invalidate_block_template_cache();
+  }
 
   b.major_version = m_hardfork->get_current_version();
   b.minor_version = m_hardfork->get_ideal_version();
@@ -1241,6 +1261,7 @@ bool Blockchain::create_block_template(block& b, const account_public_address& m
   {
     return false;
   }
+  pool_cookie = m_tx_pool.cookie();
 #if defined(DEBUG_CREATE_BLOCK_TEMPLATE)
   size_t real_txs_size = 0;
   uint64_t real_fee = 0;
@@ -1355,6 +1376,8 @@ bool Blockchain::create_block_template(block& b, const account_public_address& m
     MDEBUG("Creating block template: miner tx size " << coinbase_blob_size <<
         ", cumulative size " << cumulative_size << " is now good");
 #endif
+
+    cache_block_template(b, miner_address, ex_nonce, diffic, expected_reward, pool_cookie);
     return true;
   }
   LOG_ERROR("Failed to create_block_template with " << 10 << " tries");
@@ -3697,6 +3720,7 @@ leave:
   // appears to be a NOP *and* is called elsewhere.  wat?
   m_tx_pool.on_blockchain_inc(new_height, id);
   get_difficulty_for_next_block(); // just to cache it
+  invalidate_block_template_cache();
 
   return true;
 }
@@ -4666,6 +4690,24 @@ bool Blockchain::for_all_outputs(uint64_t amount, std::function<bool(uint64_t he
   return m_db->for_all_outputs(amount, f);;
 }
 
+void Blockchain::invalidate_block_template_cache()
+{
+  MDEBUG("Invalidating block template cache");
+  m_btc_valid = false;
+}
+
+void Blockchain::cache_block_template(const block &b, const cryptonote::account_public_address &address, const blobdata &nonce, const difficulty_type &diff, uint64_t expected_reward, uint64_t pool_cookie)
+{
+  MDEBUG("Setting block template cache");
+  m_btc = b;
+  m_btc_address = address;
+  m_btc_nonce = nonce;
+  m_btc_difficulty = diff;
+  m_btc_expected_reward = expected_reward;
+  m_btc_pool_cookie = pool_cookie;
+  m_btc_valid = true;
+}
+
 namespace cryptonote {
 template bool Blockchain::get_transactions(const std::vector<crypto::hash>&, std::vector<transaction>&, std::vector<crypto::hash>&) const;
 template bool Blockchain::get_transactions_blobs(const std::vector<crypto::hash>&, std::vector<cryptonote::blobdata>&, std::vector<crypto::hash>&, bool) const;

src/cryptonote_core/blockchain.h

@@ -1052,6 +1052,15 @@ namespace cryptonote
 
     std::atomic<bool> m_cancel;
 
+    // block template cache
+    block m_btc;
+    account_public_address m_btc_address;
+    blobdata m_btc_nonce;
+    difficulty_type m_btc_difficulty;
+    uint64_t m_btc_pool_cookie;
+    uint64_t m_btc_expected_reward;
+    bool m_btc_valid;
+
     /**
      * @brief collects the keys for all outputs being "spent" as an input
      *
@@ -1407,5 +1416,17 @@ namespace cryptonote
      * that implicit data.
      */
     bool expand_transaction_2(transaction &tx, const crypto::hash &tx_prefix_hash, const std::vector<std::vector<rct::ctkey>> &pubkeys);
+
+    /**
+     * @brief invalidates any cached block template
+     */
+    void invalidate_block_template_cache();
+
+    /**
+     * @brief stores a new cached block template
+     *
+     * At some point, may be used to push an update to miners
+     */
+    void cache_block_template(const block &b, const cryptonote::account_public_address &address, const blobdata &nonce, const difficulty_type &diff, uint64_t expected_reward, uint64_t pool_cookie);
   };
 }  // namespace cryptonote

src/cryptonote_core/tx_pool.cpp

@@ -102,7 +102,7 @@ namespace cryptonote
   }
   //---------------------------------------------------------------------------------
   //---------------------------------------------------------------------------------
-  tx_memory_pool::tx_memory_pool(Blockchain& bchs): m_blockchain(bchs), m_txpool_max_size(DEFAULT_TXPOOL_MAX_SIZE), m_txpool_size(0)
+  tx_memory_pool::tx_memory_pool(Blockchain& bchs): m_blockchain(bchs), m_txpool_max_size(DEFAULT_TXPOOL_MAX_SIZE), m_txpool_size(0), m_cookie(0)
   {
 
   }
@@ -306,6 +306,8 @@ namespace cryptonote
     tvc.m_verifivation_failed = false;
     m_txpool_size += blob_size;
 
+    ++m_cookie;
+
     MINFO("Transaction added to pool: txid " << id << " bytes: " << blob_size << " fee/byte: " << (fee / (double)blob_size));
 
     prune(m_txpool_max_size);
@@ -341,6 +343,7 @@ namespace cryptonote
       bytes = m_txpool_max_size;
     CRITICAL_REGION_LOCAL1(m_blockchain);
     LockedTXN lock(m_blockchain);
+    bool changed = false;
 
     // this will never remove the first one, but we don't care
     auto it = --m_txs_by_fee_and_receive_time.end();
@@ -377,6 +380,7 @@ namespace cryptonote
         remove_transaction_keyimages(tx);
         MINFO("Pruned tx " << txid << " from txpool: size: " << it->first.second << ", fee/byte: " << it->first.first);
         m_txs_by_fee_and_receive_time.erase(it--);
+        changed = true;
       }
       catch (const std::exception &e)
       {
@@ -384,6 +388,8 @@ namespace cryptonote
         return;
       }
     }
+    if (changed)
+      ++m_cookie;
     if (m_txpool_size > bytes)
       MINFO("Pool size after pruning is larger than limit: " << m_txpool_size << "/" << bytes);
   }
@@ -401,6 +407,7 @@ namespace cryptonote
       auto ins_res = kei_image_set.insert(id);
       CHECK_AND_ASSERT_MES(ins_res.second, false, "internal error: try to insert duplicate iterator in key_image set");
     }
+    ++m_cookie;
     return true;
   }
   //---------------------------------------------------------------------------------
@@ -435,6 +442,7 @@ namespace cryptonote
       }
 
     }
+    ++m_cookie;
     return true;
   }
   //---------------------------------------------------------------------------------
@@ -480,6 +488,7 @@ namespace cryptonote
     }
 
     m_txs_by_fee_and_receive_time.erase(sorted_it);
+    ++m_cookie;
     return true;
   }
   //---------------------------------------------------------------------------------
@@ -553,6 +562,7 @@ namespace cryptonote
           // ignore error
         }
       }
+      ++m_cookie;
     }
     return true;
   }
@@ -1051,6 +1061,7 @@ namespace cryptonote
   {
     CRITICAL_REGION_LOCAL(m_transactions_lock);
     CRITICAL_REGION_LOCAL1(m_blockchain);
+    bool changed = false;
     LockedTXN lock(m_blockchain);
     for(size_t i = 0; i!= tx.vin.size(); i++)
     {
@@ -1071,6 +1082,7 @@ namespace cryptonote
           {
             MDEBUG("Marking " << txid << " as double spending " << itk.k_image);
             meta.double_spend_seen = true;
+            changed = true;
             try
             {
               m_blockchain.update_txpool_tx(txid, meta);
@@ -1084,6 +1096,8 @@ namespace cryptonote
         }
       }
     }
+    if (changed)
+      ++m_cookie;
   }
   //---------------------------------------------------------------------------------
   std::string tx_memory_pool::print_pool(bool short_format) const
@@ -1305,6 +1319,8 @@ namespace cryptonote
         }
       }
     }
+    if (n_removed > 0)
+      ++m_cookie;
     return n_removed;
   }
   //---------------------------------------------------------------------------------
@@ -1361,6 +1377,10 @@ namespace cryptonote
         }
       }
     }
+
+    m_cookie = 0;
+
+    // Ignore deserialization error
     return true;
   }
 

src/cryptonote_core/tx_pool.h

@@ -362,6 +362,13 @@ namespace cryptonote
      */
     size_t validate(uint8_t version);
 
+     /**
+      * @brief return the cookie
+      *
+      * @return the cookie
+      */
+    uint64_t cookie() const { return m_cookie; }
+
     /**
      * @brief get the cumulative txpool size in bytes
      *
@@ -549,6 +556,8 @@ private:
     //!< container for transactions organized by fee per size and receive time
     sorted_tx_container m_txs_by_fee_and_receive_time;
 
+    std::atomic<uint64_t> m_cookie; //!< incremented at each change
+
     /**
      * @brief get an iterator to a transaction in the sorted container
      *