rabbit_amqqueue: Add is_feature_used callback to transient_nonexcl_queues depr. feature

[Why]
Without this callback, the deprecated features subsystem can't report if
the feature is used or not.

This reduces the usefulness of the HTTP API endpoint or the CLI command
that help verify if a cluster is using deprecated features.

[How]
The callback counts transient non-exclusive queues and return `true` if
there are one or more of them.

Author: dumbbell

deps/rabbit/src/rabbit_amqqueue.erl

@@ -76,6 +76,9 @@
 -export([internal_declare/2, internal_delete/2, run_backing_queue/3,
          emit_consumers_local/3, internal_delete/3]).
 
+%% Deprecated feature callback.
+-export([are_transient_nonexcl_used/1]).
+
 -include_lib("rabbit_common/include/rabbit.hrl").
 -include_lib("stdlib/include/qlc.hrl").
 -include("amqqueue.hrl").
@@ -110,9 +113,19 @@
 -rabbit_deprecated_feature(
    {transient_nonexcl_queues,
     #{deprecation_phase => permitted_by_default,
-      doc_url => "https://blog.rabbitmq.com/posts/2021/08/4.0-deprecation-announcements/#removal-of-transient-non-exclusive-queues"
+      doc_url => "https://blog.rabbitmq.com/posts/2021/08/4.0-deprecation-announcements/#removal-of-transient-non-exclusive-queues",
+      callbacks => #{is_feature_used => {?MODULE, are_transient_nonexcl_used}}
      }}).
 
+are_transient_nonexcl_used(_) ->
+    case rabbit_db_queue:list_transient() of
+        {ok, Queues} ->
+            NonExclQueues = [Q || Q <- Queues, not is_exclusive(Q)],
+            length(NonExclQueues) > 0;
+        {error, _} ->
+            undefined
+    end.
+
 -define(CONSUMER_INFO_KEYS,
         [queue_name, channel_pid, consumer_tag, ack_required, prefetch_count,
          active, activity_status, arguments]).

deps/rabbit/src/rabbit_db_queue.erl

@@ -46,7 +46,8 @@
 
 %% Used by on_node_up and on_node_down.
 %% Can be deleted once transient entities/mnesia are removed.
--export([foreach_transient/1,
+-export([list_transient/0,
+         foreach_transient/1,
          delete_transient/1]).
 
 %% Only used by rabbit_amqqueue:forget_node_for_queue, which is only called
@@ -965,6 +966,36 @@ set_in_khepri(Q) ->
     Path = khepri_queue_path(amqqueue:get_name(Q)),
     rabbit_khepri:put(Path, Q).
 
+%% -------------------------------------------------------------------
+%% list_transient().
+%% -------------------------------------------------------------------
+
+-spec list_transient() -> {ok, Queues} | {error, any()} when
+      Queues :: [amqqueue:amqqueue()].
+%% @doc Applies `UpdateFun' to all transient queue records.
+%%
+%% @private
+
+list_transient() ->
+    rabbit_khepri:handle_fallback(
+      #{mnesia => fun() -> list_transient_in_mnesia() end,
+        khepri => fun() -> list_transient_in_khepri() end
+       }).
+
+list_transient_in_mnesia() ->
+    Pattern = amqqueue:pattern_match_all(),
+    AllQueues = mnesia:dirty_match_object(
+                  ?MNESIA_TABLE,
+                  Pattern),
+    {ok, AllQueues}.
+
+list_transient_in_khepri() ->
+    PathPattern = khepri_queue_path(
+                    ?KHEPRI_WILDCARD_STAR,
+                    #if_data_matches{
+                       pattern = amqqueue:pattern_match_on_durable(false)}),
+    rabbit_khepri:get_many(PathPattern).
+
 %% -------------------------------------------------------------------
 %% delete_transient().
 %% -------------------------------------------------------------------