Expose mqtt.mailbox_soft_limit via Cuttlefish

Author: ansd

deps/rabbitmq_mqtt/priv/schema/rabbitmq_mqtt.schema

@@ -266,3 +266,21 @@ end}.
 
 {mapping, "mqtt.durable_queue_type", "rabbitmq_mqtt.durable_queue_type",
     [{datatype, {enum, [classic, quorum]}}]}.
+
+%% If feature flag rabbit_mqtt_qos0_queue is enabled, a pseudo queue of type rabbit_mqtt_qos0_queue is
+%% created for an MQTT "Clean Session" that subscribes with QoS 0. This queue type holds MQTT messages
+%% directly in the MQTT connection process mailbox. This queue type has no flow control which
+%% means MQTT messages might arrive faster in the Erlang mailbox than being sent to the subscribing MQTT
+%% client. To protect against high memory usage, RabbitMQ intentionally drops QoS 0 messages if both
+%% 1.) the number of messages in the MQTT connection process mailbox exceeds mailbox_soft_limit, and
+%% 2.) the socket sending to the MQTT client is busy (i.e. not sending fast enough).
+%% Note that there can be other messages in the mailbox (e.g. MQTT messages sent from the MQTT client to
+%% RabbitMQ or confirmations from another queue type to the MQTT connection process) which are
+%% obviously not dropped. However, these other messages also contribute to the `mailbox_soft_limit`.
+%% Setting mailbox_soft_limit to 0 disables the overload protection mechanism to drop QoS 0 messages.
+%% Setting mailbox_soft_limit to a very high value decreases the likelihood of intentionally dropping
+%% QoS 0 messages while increasing the risk of causing a cluster wide memory alarm.
+%%
+%% {mailbox_soft_limit, 30},
+{mapping, "mqtt.mailbox_soft_limit", "rabbitmq_mqtt.mailbox_soft_limit",
+    [{datatype, integer}]}.

deps/rabbitmq_mqtt/test/config_schema_SUITE_data/rabbitmq_mqtt.snippets

@@ -15,7 +15,8 @@
    mqtt.tcp_listen_options.backlog = 128
    mqtt.tcp_listen_options.nodelay = true
    mqtt.durable_queue_type = classic
-   mqtt.proxy_protocol = false",
+   mqtt.proxy_protocol = false
+   mqtt.mailbox_soft_limit = 200",
   [{rabbit,[{tcp_listeners,[5672]}]},
    {rabbitmq_mqtt,
        [{default_user,<<"guest">>},
@@ -30,7 +31,8 @@
         {tcp_listeners,[1883]},
         {tcp_listen_options,[{backlog,128},{nodelay,true}]},
         {durable_queue_type,classic},
-        {proxy_protocol,false}]}],
+        {proxy_protocol,false},
+        {mailbox_soft_limit,200}]}],
   [rabbitmq_mqtt]},
 
  {listener_tcp_options,
@@ -143,4 +145,11 @@
         {retained_message_store_dets_sync_interval,2000},
         {ssl_listeners,[]},
         {tcp_listeners,[1883]}]}],
-  [rabbitmq_mqtt]}].
+  [rabbitmq_mqtt]},
+
+ {mailbox_soft_limit,
+  "mqtt.mailbox_soft_limit = 30",
+  [{rabbitmq_mqtt,[
+      {mailbox_soft_limit, 30}]}],
+   [rabbitmq_mqtt]}
+].