Tune socket receive buffer sizes to fix false positive backpressure

test/mtp_test_client.erl

@@ -19,6 +19,7 @@ connect(Host, Port, Secret, DcId, Protocol) ->
     Opts = [{packet, raw},
             {mode, binary},
             {active, false},
+            {buffer, 1024},
             {send_timeout, 5000}],
     {ok, Sock} = gen_tcp:connect(Host, Port, Opts, 1000),
     {Header, _, _, CryptoLayer} = mtp_obfuscated:client_create(Secret, Protocol, DcId),

test/single_dc_SUITE.erl

@@ -169,6 +169,10 @@ downstream_size_backpressure_case(Cfg) when is_list(Cfg) ->
 
 %% @doc test downstream backpressure when count of non-acknowledged packets grows above threshold
 downstream_qlen_backpressure_case({pre, Cfg}) ->
+    application:load(mtproto_proxy),
+    %% Reducing downstream socket buffer size. Otherwise we can get queue overflow from just single
+    %% socket data packet
+    application:set_env(mtproto_proxy, downstream_socket_buffer_size, 1024),
     Cfg1 = setup_single(?FUNCTION_NAME, ?LINE, #{rpc_handler => mtp_test_cmd_rpc}, Cfg),
     %% Disable upstream healthchecks
     application:set_env(?APP, upstream_healthchecks, []),