Make classic_queue_consumer_unsent_message_limit configurable

Similar to other RabbitMQ internal credit flow configurations such as
`credit_flow_default_credit` and `msg_store_credit_disc_bound`, this
commit makes the `classic_queue_consumer_unsent_message_limit`
configurable via `advanced.config`.

See #11822 for the
original motivation to make this setting configurable.

(cherry picked from commit c771b24)

Author: ansd

deps/rabbit/src/rabbit_queue_consumers.erl

@@ -22,7 +22,8 @@
 
 -define(QUEUE, lqueue).
 
--define(UNSENT_MESSAGE_LIMIT,          200).
+-define(KEY_UNSENT_MESSAGE_LIMIT, classic_queue_consumer_unsent_message_limit).
+-define(DEFAULT_UNSENT_MESSAGE_LIMIT, 200).
 
 %% Utilisation average calculations are all in μs.
 -define(USE_AVG_HALF_LIFE, 1000000.0).
@@ -72,10 +73,15 @@
 
 -spec new() -> state().
 
-new() -> #state{consumers = priority_queue:new(),
-                use       = {active,
-                             erlang:monotonic_time(micro_seconds),
-                             1.0}}.
+new() ->
+    Val = application:get_env(rabbit,
+                              ?KEY_UNSENT_MESSAGE_LIMIT,
+                              ?DEFAULT_UNSENT_MESSAGE_LIMIT),
+    persistent_term:put(?KEY_UNSENT_MESSAGE_LIMIT, Val),
+    #state{consumers = priority_queue:new(),
+           use = {active,
+                  erlang:monotonic_time(microsecond),
+                  1.0}}.
 
 -spec max_active_priority(state()) -> integer() | 'infinity' | 'empty'.
 
@@ -286,7 +292,6 @@ deliver_to_consumer(FetchFun,
                     E = {ChPid, Consumer = #consumer{tag = CTag}},
                     QName) ->
     C = #cr{link_states = LinkStates} = lookup_ch(ChPid),
-    ChBlocked = is_ch_blocked(C),
     case LinkStates of
         #{CTag := #link_state{delivery_count = DeliveryCount0,
                               credit = Credit} = LinkState0} ->
@@ -308,22 +313,24 @@ deliver_to_consumer(FetchFun,
                     block_consumer(C, E),
                     undelivered
             end;
-        _ when ChBlocked ->
-            %% not a link credit consumer, use credit flow
-            block_consumer(C, E),
-            undelivered;
         _ ->
             %% not a link credit consumer, use credit flow
-            case rabbit_limiter:can_send(C#cr.limiter,
-                                         Consumer#consumer.ack_required,
-                                         CTag) of
-                {suspend, Limiter} ->
-                    block_consumer(C#cr{limiter = Limiter}, E),
+            case is_ch_blocked(C) of
+                true ->
+                    block_consumer(C, E),
                     undelivered;
-                {continue, Limiter} ->
-                    {delivered, deliver_to_consumer(
-                                  FetchFun, Consumer,
-                                  C#cr{limiter = Limiter}, QName)}
+                false ->
+                    case rabbit_limiter:can_send(C#cr.limiter,
+                                                 Consumer#consumer.ack_required,
+                                                 CTag) of
+                        {suspend, Limiter} ->
+                            block_consumer(C#cr{limiter = Limiter}, E),
+                            undelivered;
+                        {continue, Limiter} ->
+                            {delivered, deliver_to_consumer(
+                                          FetchFun, Consumer,
+                                          C#cr{limiter = Limiter}, QName)}
+                    end
             end
     end.
 
@@ -653,7 +660,8 @@ block_consumer(C = #cr{blocked_consumers = Blocked}, QEntry) ->
     update_ch_record(C#cr{blocked_consumers = add_consumer(QEntry, Blocked)}).
 
 is_ch_blocked(#cr{unsent_message_count = Count, limiter = Limiter}) ->
-    Count >= ?UNSENT_MESSAGE_LIMIT orelse rabbit_limiter:is_suspended(Limiter).
+    UnsentMessageLimit = persistent_term:get(?KEY_UNSENT_MESSAGE_LIMIT),
+    Count >= UnsentMessageLimit orelse rabbit_limiter:is_suspended(Limiter).
 
 tags(CList) -> [CTag || {_P, {_ChPid, #consumer{tag = CTag}}} <- CList].