cryptonote_core: warn when the block rate deviates from expectations

The warning threshold is set to allow a false positive every
ten days on average.

src/cryptonote_core/blockchain.cpp

@@ -880,6 +880,15 @@ difficulty_type Blockchain::get_difficulty_for_next_block()
   return diff;
 }
 //------------------------------------------------------------------
+std::vector<time_t> Blockchain::get_last_block_timestamps(unsigned int blocks) const
+{
+  std::vector<time_t> timestamps(blocks);
+  uint64_t height = m_db->height();
+  while (blocks--)
+    timestamps[blocks] = m_db->get_block_timestamp(height - blocks - 1);
+  return timestamps;
+}
+//------------------------------------------------------------------
 // This function removes blocks from the blockchain until it gets to the
 // position where the blockchain switch started and then re-adds the blocks
 // that had been removed.

src/cryptonote_core/blockchain.h

@@ -943,6 +943,11 @@ namespace cryptonote
      */
     void on_new_tx_from_block(const cryptonote::transaction &tx);
 
+    /**
+     * @brief returns the timestamps of the last N blocks
+     */
+    std::vector<time_t> get_last_block_timestamps(unsigned int blocks) const;
+
   private:
 
     // TODO: evaluate whether or not each of these typedefs are left over from blockchain_storage

src/cryptonote_core/cryptonote_core.cpp

@@ -1471,6 +1471,7 @@ namespace cryptonote
     m_txpool_auto_relayer.do_call(boost::bind(&core::relay_txpool_transactions, this));
     m_check_updates_interval.do_call(boost::bind(&core::check_updates, this));
     m_check_disk_space_interval.do_call(boost::bind(&core::check_disk_space, this));
+    m_block_rate_interval.do_call(boost::bind(&core::check_block_rate, this));
     m_miner.on_idle();
     m_mempool.on_idle();
     return true;
@@ -1655,6 +1656,52 @@ namespace cryptonote
     return true;
   }
   //-----------------------------------------------------------------------------------------------
+  double factorial(unsigned int n)
+  {
+    if (n <= 1)
+      return 1.0;
+    double f = n;
+    while (n-- > 1)
+      f *= n;
+    return f;
+  }
+  //-----------------------------------------------------------------------------------------------
+  static double probability(unsigned int blocks, unsigned int expected)
+  {
+    // https://www.umass.edu/wsp/resources/poisson/#computing
+    return pow(expected, blocks) / (factorial(blocks) * exp(expected));
+  }
+  //-----------------------------------------------------------------------------------------------
+  bool core::check_block_rate()
+  {
+    if (m_offline || m_target_blockchain_height > get_current_blockchain_height())
+    {
+      MDEBUG("Not checking block rate, offline or syncing");
+      return true;
+    }
+
+    static constexpr double threshold = 1. / (864000 / DIFFICULTY_TARGET_V2); // one false positive every 10 days
+
+    const time_t now = time(NULL);
+    const std::vector<time_t> timestamps = m_blockchain_storage.get_last_block_timestamps(60);
+
+    static const unsigned int seconds[] = { 5400, 1800, 600 };
+    for (size_t n = 0; n < sizeof(seconds)/sizeof(seconds[0]); ++n)
+    {
+      unsigned int b = 0;
+      for (time_t ts: timestamps) b += ts >= now - seconds[n];
+      const double p = probability(b, seconds[n] / DIFFICULTY_TARGET_V2);
+      MDEBUG("blocks in the last " << seconds[n] / 60 << " minutes: " << b << " (probability " << p << ")");
+      if (p < threshold)
+      {
+        MWARNING("There were " << b << " blocks in the last " << seconds[n] / 60 << " minutes, there might be large hash rate changes, or we might be partitioned, cut off from the Monero network or under attack. Or it could be just sheer bad luck.");
+        break; // no need to look further
+      }
+    }
+
+    return true;
+  }
+  //-----------------------------------------------------------------------------------------------
   void core::set_target_blockchain_height(uint64_t target_blockchain_height)
   {
     m_target_blockchain_height = target_blockchain_height;

src/cryptonote_core/cryptonote_core.h

@@ -935,6 +935,13 @@ namespace cryptonote
       */
      bool check_disk_space();
 
+     /**
+      * @brief checks block rate, and warns if it's too slow
+      *
+      * @return true on success, false otherwise
+      */
+     bool check_block_rate();
+
      bool m_test_drop_download = true; //!< whether or not to drop incoming blocks (for testing)
 
      uint64_t m_test_drop_download_height = 0; //!< height under which to drop incoming blocks, if doing so
@@ -959,6 +966,7 @@ namespace cryptonote
      epee::math_helper::once_a_time_seconds<60*2, false> m_txpool_auto_relayer; //!< interval for checking re-relaying txpool transactions
      epee::math_helper::once_a_time_seconds<60*60*12, true> m_check_updates_interval; //!< interval for checking for new versions
      epee::math_helper::once_a_time_seconds<60*10, true> m_check_disk_space_interval; //!< interval for checking for disk space
+     epee::math_helper::once_a_time_seconds<90, false> m_block_rate_interval; //!< interval for checking block rate
 
      std::atomic<bool> m_starter_message_showed; //!< has the "daemon will sync now" message been shown?