Relax mutex requirement for retrieving active difficulty (#2901)

* Make active multiplier an atomic to avoid an active mutex lock

* Add failing test to ensure the active mutex is not held in the difficulty observer update

* Unlock active mutex updating difficulty observers

nano/core_test/active_transactions.cpp

@@ -1434,3 +1434,16 @@ TEST (active_transactions, activate_inactive)
 	// The first block was not active so no activation takes place
 	ASSERT_FALSE (node.active.active (open->qualified_root ()) || node.block_confirmed_or_being_confirmed (node.store.tx_begin_read (), open->hash ()));
 }
+
+TEST (active_transactions, difficulty_update_observer)
+{
+	nano::system system (1);
+	auto & node (*system.nodes[0]);
+	std::atomic<bool> update_received (false);
+	node.observers.difficulty.add ([& mutex = node.active.mutex, &update_received](uint64_t difficulty_a) {
+		nano::unique_lock<std::mutex> lock (mutex, std::defer_lock);
+		EXPECT_TRUE (lock.try_lock ());
+		update_received = true;
+	});
+	ASSERT_TIMELY (3s, update_received);
+}

nano/node/active_transactions.cpp

@@ -882,7 +882,9 @@ void nano::active_transactions::update_active_multiplier (nano::unique_lock<std:
 	debug_assert (difficulty >= node.network_params.network.publish_thresholds.entry);
 
 	trended_active_multiplier = avg_multiplier;
+	lock_a.unlock ();
 	node.observers.difficulty.notify (difficulty);
+	lock_a.lock ();
 }
 
 uint64_t nano::active_transactions::active_difficulty ()
@@ -912,8 +914,7 @@ uint64_t nano::active_transactions::limited_active_difficulty (nano::work_versio
 
 double nano::active_transactions::active_multiplier ()
 {
-	nano::lock_guard<std::mutex> lock (mutex);
-	return trended_active_multiplier;
+	return trended_active_multiplier.load ();
 }
 
 // List of active blocks in elections

nano/node/active_transactions.hpp

@@ -199,7 +199,7 @@ public:
 	nano::node & node;
 	mutable std::mutex mutex;
 	boost::circular_buffer<double> multipliers_cb;
-	double trended_active_multiplier;
+	std::atomic<double> trended_active_multiplier;
 	size_t priority_cementable_frontiers_size ();
 	size_t priority_wallet_cementable_frontiers_size ();
 	boost::circular_buffer<double> difficulty_trend ();