daemon: guard against rare 'difficulty drift' bug with checkpoints and recalculation

On startup, it checks against the difficulty checkpoints, and if any mismatch is found, recalculates all the blocks with wrong difficulties. Additionally, once a week it recalculates difficulties of blocks after the last difficulty checkpoint.

src/cryptonote_core/blockchain.cpp

@@ -32,6 +32,7 @@
 #include <cstdio>
 #include <boost/filesystem.hpp>
 #include <boost/range/adaptor/reversed.hpp>
+#include <boost/format.hpp>
 
 #include "include_base_utils.h"
 #include "cryptonote_basic/cryptonote_basic_impl.h"
@@ -439,6 +440,15 @@ bool Blockchain::init(BlockchainDB* db, const network_type nettype, bool offline
     m_long_term_block_weights_cache_rolling_median = epee::misc_utils::rolling_median_t<uint64_t>(m_long_term_block_weights_window);
   }
 
+  bool difficulty_ok;
+  uint64_t difficulty_recalc_height;
+  std::tie(difficulty_ok, difficulty_recalc_height) = check_difficulty_checkpoints();
+  if (!difficulty_ok)
+  {
+    MERROR("Difficulty drift detected!");
+    recalculate_difficulties(difficulty_recalc_height);
+  }
+
   {
     db_txn_guard txn_guard(m_db, m_db->is_read_only());
     if (!update_next_cumulative_weight_limit())
@@ -888,6 +898,111 @@ difficulty_type Blockchain::get_difficulty_for_next_block()
   return diff;
 }
 //------------------------------------------------------------------
+std::pair<bool, uint64_t> Blockchain::check_difficulty_checkpoints() const
+{
+  uint64_t res = 0;
+  for (const std::pair<uint64_t, difficulty_type>& i : m_checkpoints.get_difficulty_points())
+  {
+    if (i.first >= m_db->height())
+      break;
+    if (m_db->get_block_cumulative_difficulty(i.first) != i.second)
+      return {false, res};
+    res = i.first;
+  }
+  return {true, res};
+}
+//------------------------------------------------------------------
+size_t Blockchain::recalculate_difficulties(boost::optional<uint64_t> start_height_opt)
+{
+  if (m_fixed_difficulty)
+  {
+    return 0;
+  }
+  LOG_PRINT_L3("Blockchain::" << __func__);
+  CRITICAL_REGION_LOCAL(m_blockchain_lock);
+
+  const uint64_t start_height = start_height_opt ? *start_height_opt : check_difficulty_checkpoints().second;
+  const uint64_t top_height = m_db->height() - 1;
+  MGINFO("Recalculating difficulties from height " << start_height << " to height " << top_height);
+
+  std::vector<uint64_t> timestamps;
+  std::vector<difficulty_type> difficulties;
+  timestamps.reserve(DIFFICULTY_BLOCKS_COUNT + 1);
+  difficulties.reserve(DIFFICULTY_BLOCKS_COUNT + 1);
+  if (start_height > 1)
+  {
+    for (uint64_t i = 0; i < DIFFICULTY_BLOCKS_COUNT; ++i)
+    {
+      uint64_t height = start_height - 1 - i;
+      if (height == 0)
+        break;
+      timestamps.insert(timestamps.begin(), m_db->get_block_timestamp(height));
+      difficulties.insert(difficulties.begin(), m_db->get_block_cumulative_difficulty(height));
+    }
+  }
+  difficulty_type last_cum_diff = start_height <= 1 ? start_height : difficulties.back();
+  uint64_t drift_start_height = 0;
+  std::vector<difficulty_type> new_cumulative_difficulties;
+  for (uint64_t height = start_height; height <= top_height; ++height)
+  {
+    size_t target = get_ideal_hard_fork_version(height) < 2 ? DIFFICULTY_TARGET_V1 : DIFFICULTY_TARGET_V2;
+    difficulty_type recalculated_diff = next_difficulty(timestamps, difficulties, target);
+
+    boost::multiprecision::uint256_t recalculated_cum_diff_256 = boost::multiprecision::uint256_t(recalculated_diff) + last_cum_diff;
+    CHECK_AND_ASSERT_THROW_MES(recalculated_cum_diff_256 <= std::numeric_limits<difficulty_type>::max(), "Difficulty overflow!");
+    difficulty_type recalculated_cum_diff = recalculated_cum_diff_256.convert_to<difficulty_type>();
+
+    if (drift_start_height == 0)
+    {
+      difficulty_type existing_cum_diff = m_db->get_block_cumulative_difficulty(height);
+      if (recalculated_cum_diff != existing_cum_diff)
+      {
+        drift_start_height = height;
+        new_cumulative_difficulties.reserve(top_height + 1 - height);
+        LOG_ERROR("Difficulty drift found at height:" << height << ", hash:" << m_db->get_block_hash_from_height(height) << ", existing:" << existing_cum_diff << ", recalculated:" << recalculated_cum_diff);
+      }
+    }
+    if (drift_start_height > 0)
+    {
+      new_cumulative_difficulties.push_back(recalculated_cum_diff);
+      if (height % 100000 == 0)
+        LOG_ERROR(boost::format("%llu / %llu (%.1f%%)") % height % top_height % (100 * (height - drift_start_height) / float(top_height - drift_start_height)));
+    }
+
+    if (height > 0)
+    {
+      timestamps.push_back(m_db->get_block_timestamp(height));
+      difficulties.push_back(recalculated_cum_diff);
+    }
+    if (timestamps.size() > DIFFICULTY_BLOCKS_COUNT)
+    {
+      CHECK_AND_ASSERT_THROW_MES(timestamps.size() == DIFFICULTY_BLOCKS_COUNT + 1, "Wrong timestamps size: " << timestamps.size());
+      timestamps.erase(timestamps.begin());
+      difficulties.erase(difficulties.begin());
+    }
+    last_cum_diff = recalculated_cum_diff;
+  }
+
+  if (drift_start_height > 0)
+  {
+    LOG_ERROR("Writing to the DB...");
+    try
+    {
+      m_db->correct_block_cumulative_difficulties(drift_start_height, new_cumulative_difficulties);
+    }
+    catch (const std::exception& e)
+    {
+      LOG_ERROR("Error correcting cumulative difficulties from height " << drift_start_height << ", what = " << e.what());
+    }
+    LOG_ERROR("Corrected difficulties for " << new_cumulative_difficulties.size() << " blocks");
+    // clear cache
+    m_difficulty_for_next_block_top_hash = crypto::null_hash;
+    m_timestamps_and_difficulties_height = 0;
+  }
+
+  return new_cumulative_difficulties.size();
+}
+//------------------------------------------------------------------
 std::vector<time_t> Blockchain::get_last_block_timestamps(unsigned int blocks) const
 {
   uint64_t height = m_db->height();