Add rolled back event

nano/lib/stats_enums.hpp

@@ -101,6 +101,7 @@ enum class detail : uint8_t
 	old,
 	gap_previous,
 	gap_source,
+	rollback,
 	rollback_failed,
 	progress,
 	bad_signature,
@@ -112,7 +113,7 @@ enum class detail : uint8_t
 	representative_mismatch,
 	block_position,
 
-	// blockprocessor result
+	// block source
 	live,
 	bootstrap,
 	bootstrap_legacy,

nano/node/blockprocessor.cpp

@@ -128,11 +128,15 @@ void nano::block_processor::rollback_competitor (store::write_transaction const
 		}
 		else
 		{
+			node.stats.inc (nano::stat::type::ledger, nano::stat::detail::rollback);
 			node.logger.debug (nano::log::type::blockprocessor, "Blocks rolled back: {}", rollback_list.size ());
 		}
+
 		// Deleting from votes cache, stop active transaction
 		for (auto & i : rollback_list)
 		{
+			rolled_back.notify (i);
+
 			node.history.erase (i->root ());
 			// Stop all rolled back active transactions except initial
 			if (i->hash () != successor->hash ())

nano/node/blockprocessor.hpp

@@ -90,6 +90,7 @@ class block_processor final
 	// The batch observer feeds the processed observer
 	nano::observer_set<nano::process_return const &, std::shared_ptr<nano::block> const &, context const &> block_processed;
 	nano::observer_set<processed_batch_t const &> batch_processed;
+	nano::observer_set<std::shared_ptr<nano::block> const &> rolled_back;
 
 private:
 	// Roll back block in the ledger that conflicts with 'block'