Bound the growth of election_winner_details set

nano/core_test/active_elections.cpp

@@ -1388,3 +1388,51 @@ TEST (active_elections, limit_vote_hinted_elections)
 	// Ensure there was no overflow of elections
 	ASSERT_EQ (0, node.stats.count (nano::stat::type::active_elections_dropped, nano::stat::detail::priority));
 }
+
+/*
+ * Ensures that election winners set won't grow without bounds when cementing is slower that the rate of confirming new elections
+ */
+TEST (active_elections, bound_election_winners)
+{
+	nano::test::system system;
+	nano::node_config config = system.default_config ();
+	// Set election winner limit to a low value
+	config.active_elections.max_election_winners = 5;
+	auto & node = *system.add_node (config);
+
+	// Start elections for a couple of blocks, number of elections is larger than the election winner set limit
+	auto blocks = nano::test::setup_independent_blocks (system, node, 10);
+	ASSERT_TIMELY (5s, std::all_of (blocks.begin (), blocks.end (), [&node] (auto & block) {
+		return node.active.active (*block);
+	}));
+
+	// Prevent cementing of confirmed blocks
+	auto guard = node.ledger.tx_begin_write ({}, nano::store::writer::testing);
+
+	// Ensure that when the number of election winners reaches the limit, AEC vacancy reflects that
+	ASSERT_TRUE (node.active.vacancy (nano::election_behavior::priority) > 0);
+
+	int index = 0;
+	for (; index < config.active_elections.max_election_winners; ++index)
+	{
+		auto election = node.vote_router.election (blocks[index]->hash ());
+		ASSERT_TRUE (election);
+		election->force_confirm ();
+	}
+
+	ASSERT_TIMELY_EQ (5s, node.active.vacancy (nano::election_behavior::priority), 0);
+
+	// Confirming more elections should make the vacancy negative
+	for (; index < blocks.size (); ++index)
+	{
+		auto election = node.vote_router.election (blocks[index]->hash ());
+		ASSERT_TRUE (election);
+		election->force_confirm ();
+	}
+
+	ASSERT_TIMELY (5s, node.active.vacancy (nano::election_behavior::priority) < 0);
+
+	// Release the guard to allow cementing, there should be some vacancy now
+	guard.commit ();
+	ASSERT_TIMELY (5s, node.active.vacancy (nano::election_behavior::priority) > 0);
+}

nano/node/active_elections.cpp

@@ -243,19 +243,27 @@ int64_t nano::active_elections::limit (nano::election_behavior behavior) const
 
 int64_t nano::active_elections::vacancy (nano::election_behavior behavior) const
 {
-	nano::lock_guard<nano::mutex> guard{ mutex };
-	switch (behavior)
-	{
-		case nano::election_behavior::manual:
-			return std::numeric_limits<int64_t>::max ();
-		case nano::election_behavior::priority:
-			return limit (nano::election_behavior::priority) - static_cast<int64_t> (roots.size ());
-		case nano::election_behavior::hinted:
-		case nano::election_behavior::optimistic:
-			return limit (behavior) - count_by_behavior[behavior];
-	}
-	debug_assert (false); // Unknown enum
-	return 0;
+	auto election_vacancy = [this] (nano::election_behavior behavior) {
+		nano::lock_guard<nano::mutex> guard{ mutex };
+		switch (behavior)
+		{
+			case nano::election_behavior::manual:
+				return std::numeric_limits<int64_t>::max ();
+			case nano::election_behavior::priority:
+				return limit (nano::election_behavior::priority) - static_cast<int64_t> (roots.size ());
+			case nano::election_behavior::hinted:
+			case nano::election_behavior::optimistic:
+				return limit (behavior) - count_by_behavior[behavior];
+		}
+		debug_assert (false); // Unknown enum
+		return 0ll;
+	};
+
+	auto election_winners_vacancy = [this] () {
+		return static_cast<int64_t> (config.max_election_winners) - static_cast<int64_t> (election_winner_details_size ());
+	};
+
+	return std::min (election_vacancy (behavior), election_winners_vacancy ());
 }
 
 void nano::active_elections::request_confirm (nano::unique_lock<nano::mutex> & lock_a)
@@ -557,7 +565,7 @@ bool nano::active_elections::publish (std::shared_ptr<nano::block> const & block
 	return result;
 }
 
-std::size_t nano::active_elections::election_winner_details_size ()
+std::size_t nano::active_elections::election_winner_details_size () const
 {
 	nano::lock_guard<nano::mutex> guard{ election_winner_details_mutex };
 	return election_winner_details.size ();

nano/node/active_elections.hpp

@@ -64,6 +64,8 @@ class active_elections_config final
 	std::size_t confirmation_history_size{ 2048 };
 	// Maximum cache size for recently_confirmed
 	std::size_t confirmation_cache{ 65536 };
+	// Maximum size of election winner details set
+	std::size_t max_election_winners{ 1024 * 16 };
 };
 
 /**
@@ -138,7 +140,7 @@ class active_elections final
 	int64_t vacancy (nano::election_behavior behavior) const;
 	std::function<void ()> vacancy_update{ [] () {} };
 
-	std::size_t election_winner_details_size ();
+	std::size_t election_winner_details_size () const;
 	void add_election_winner_details (nano::block_hash const &, std::shared_ptr<nano::election> const &);
 	std::shared_ptr<nano::election> remove_election_winner_details (nano::block_hash const &);
 
@@ -170,7 +172,7 @@ class active_elections final
 	mutable nano::mutex mutex{ mutex_identifier (mutexes::active) };
 
 private:
-	nano::mutex election_winner_details_mutex{ mutex_identifier (mutexes::election_winner_details) };
+	mutable nano::mutex election_winner_details_mutex{ mutex_identifier (mutexes::election_winner_details) };
 	std::unordered_map<nano::block_hash, std::shared_ptr<nano::election>> election_winner_details;
 
 	// Maximum time an election can be kept active if it is extending the container