blockchain: cache difficulty for next block

Takes about 10 ms, which takes pretty much all of the get_info
RPC, which is called pretty often from wallets.

Also add a new lock so we don't need to lock the blockchain lock,
which will avoid blocking for a long time when calling the getinfo
RPC while syncing. Users of get_difficulty_for_next_block who need
the lock will have locked it already.

src/cryptonote_core/blockchain.cpp

@@ -156,7 +156,9 @@ static const struct {
 //------------------------------------------------------------------
 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_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)
 {
   LOG_PRINT_L3("Blockchain::" << __func__);
 }
@@ -751,7 +753,17 @@ bool Blockchain::get_block_by_hash(const crypto::hash &h, block &blk, bool *orph
 difficulty_type Blockchain::get_difficulty_for_next_block()
 {
   LOG_PRINT_L3("Blockchain::" << __func__);
-  CRITICAL_REGION_LOCAL(m_blockchain_lock);
+
+  CRITICAL_REGION_LOCAL(m_difficulty_lock);
+  // we can call this without the blockchain lock, it might just give us
+  // something a bit out of date, but that's fine since anything which
+  // requires the blockchain lock will have acquired it in the first place,
+  // and it will be unlocked only when called from the getinfo RPC
+  crypto::hash top_hash = get_tail_id();
+  if (top_hash == m_difficulty_for_next_block_top_hash)
+    return m_difficulty_for_next_block;
+
+  CRITICAL_REGION_LOCAL1(m_blockchain_lock);
   std::vector<uint64_t> timestamps;
   std::vector<difficulty_type> difficulties;
   auto height = m_db->height();
@@ -794,7 +806,10 @@ difficulty_type Blockchain::get_difficulty_for_next_block()
     m_difficulties = difficulties;
   }
   size_t target = get_difficulty_target();
-  return next_difficulty(timestamps, difficulties, target);
+  difficulty_type diff = next_difficulty(timestamps, difficulties, target);
+  m_difficulty_for_next_block_top_hash = top_hash;
+  m_difficulty_for_next_block = diff;
+  return diff;
 }
 //------------------------------------------------------------------
 // This function removes blocks from the blockchain until it gets to the

src/cryptonote_core/blockchain.h

@@ -1006,6 +1006,10 @@ namespace cryptonote
     std::vector<difficulty_type> m_difficulties;
     uint64_t m_timestamps_and_difficulties_height;
 
+    epee::critical_section m_difficulty_lock;
+    crypto::hash m_difficulty_for_next_block_top_hash;
+    difficulty_type m_difficulty_for_next_block;
+
     boost::asio::io_service m_async_service;
     boost::thread_group m_async_pool;
     std::unique_ptr<boost::asio::io_service::work> m_async_work_idle;