Properly re-initialize witness scheduler state after a long block gap

libraries/chain/db_witness_schedule.cpp

@@ -106,6 +106,8 @@ void database::update_witness_schedule(signed_block next_block)
    // triggering FC_ASSERT elsewhere
 
    assert( schedule_slot > 0 );
+   witness_id_type first_witness;
+   bool slot_is_near = wso.scheduler.get_slot( schedule_slot-1, first_witness );
 
    witness_id_type wit;
 
@@ -119,13 +121,21 @@ void database::update_witness_schedule(signed_block next_block)
       witness_scheduler_rng rng(wso.rng_seed.data, _wso.slots_since_genesis);
 
       _wso.scheduler._min_token_count = std::max(int(gpo.active_witnesses.size()) / 2, 1);
-      uint32_t drain = schedule_slot;
-      while( drain > 0 )
+
+      if( slot_is_near )
       {
-         if( _wso.scheduler.size() == 0 )
-            break;
-         _wso.scheduler.consume_schedule();
-         --drain;
+         uint32_t drain = schedule_slot;
+         while( drain > 0 )
+         {
+            if( _wso.scheduler.size() == 0 )
+               break;
+            _wso.scheduler.consume_schedule();
+            --drain;
+         }
+      }
+      else
+      {
+         _wso.scheduler.reset_schedule( first_witness );
       }
       while( !_wso.scheduler.get_slot(schedule_needs_filled, wit) )
       {

libraries/chain/include/graphene/chain/witness_scheduler.hpp

@@ -313,6 +313,40 @@ class generic_witness_scheduler
          return true;
       }
 
+      /**
+       * Reset the schedule, then re-schedule the given witness as the
+       * first witness.
+       */
+      void reset_schedule( WitnessID first_witness )
+      {
+         _schedule.clear();
+         for( const WitnessID& wid : _ineligible_no_turn )
+         {
+            _eligible.push_back( wid );
+         }
+         _turns += _ineligible_no_turn.size();
+         _ineligible_no_turn.clear();
+         for( const auto& item : _ineligible_waiting_for_token )
+         {
+            _eligible.push_back( item.first );
+            _turns += (item.second ? 0 : 1);
+         }
+         _tokens += _ineligible_waiting_for_token.size();
+         _ineligible_waiting_for_token.clear();
+         if( debug ) check_invariant();
+
+         auto it = std::find( _eligible.begin(), _eligible.end(), first_witness );
+         assert( it != _eligible.end() );
+
+         _schedule.push_back( *it );
+         _ineligible_waiting_for_token.emplace_back( *it, false );
+         _eligible.erase( it );
+         _turns--;
+         _tokens--;
+         if( debug ) check_invariant();
+         return;
+      }
+
       // keep track of total turns / tokens in existence
       CountType _turns = 0;
       CountType _tokens = 0;