fix: throttle ar_nonce_limiter_client:maybe_request_sessions

During block validation ar_nonce_limiter can trigger 2 full session queries via
ar_nonce_limiter_client:maybe_request_sessions/1. A full session query is slow whic
can cause a cascading backlog of requests. To avoid this, we'll throttle the full session
queries more severely than other VDF queries.

apps/arweave/src/ar_nonce_limiter_client.erl

@@ -17,6 +17,11 @@
 -define(PULL_FREQUENCY_MS, 800).
 -define(NO_UPDATE_PULL_FREQUENCY_MS, 200).
 -define(PULL_THROTTLE_MS, 200).
+%% During block validation ar_nonce_limiter can trigger 2 full session queries via
+%% ar_nonce_limiter_client:maybe_request_sessions/1. A full session query is slow whic
+%% can cause a cascading backlog of requests. To avoid this, we'll throttle the full session
+%% queries more severely than other VDF queries.
+-define(FULL_SESSION_PULL_THROTTLE_MS, 2000).
 
 %%%===================================================================
 %%% Public interface.
@@ -148,24 +153,31 @@ handle_cast(pull, State) ->
 
 handle_cast({maybe_request_sessions, SessionKey}, State) ->
 	#state{ remote_servers = Q } = State,
-	{{value, {RawPeer, _Timestamp}}, Q2} = queue:out(Q),
+	{{value, {RawPeer, Timestamp}}, Q2} = queue:out(Q),
 	Now = erlang:system_time(millisecond),
-	State2 = State#state{ remote_servers = queue:in({RawPeer, Now}, Q2) },
-	case ar_peers:resolve_and_cache_peer(RawPeer, vdf_server_peer) of
-		{error, _} ->
-			%% Push the peer to the back of the queue.
-			{noreply, State2};
-		{ok, Peer} ->
-			case get_latest_session_key(Peer, State) of
-				SessionKey ->
-					%% No reason to make extra requests.
-					{noreply, State};
-				_ ->
-					case fetch_and_apply_session_and_previous_session(Peer) of
-						{error, _} ->
-							{noreply, State2};
+	case Now < Timestamp + ?FULL_SESSION_PULL_THROTTLE_MS of
+		true ->
+			%% Just skip the request - if needed ar_nonce_limiter will trigger it again the
+			%% next time it tries to validate a block.
+			{noreply, State};
+		false ->
+			State2 = State#state{ remote_servers = queue:in({RawPeer, Now}, Q2) },
+			case ar_peers:resolve_and_cache_peer(RawPeer, vdf_server_peer) of
+				{error, _} ->
+					%% Push the peer to the back of the queue.
+					{noreply, State2};
+				{ok, Peer} ->
+					case get_latest_session_key(Peer, State) of
+						SessionKey ->
+							%% No reason to make extra requests.
+							{noreply, State};
 						_ ->
-							{noreply, State}
+							case fetch_and_apply_session_and_previous_session(Peer) of
+								{error, _} ->
+									{noreply, State2};
+								_ ->
+									{noreply, State}
+							end
 					end
 			end
 	end;