From 638e3a4b08ab1c68f702e4ed1d4792ddd467e32a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jean-S=C3=A9bastien=20P=C3=A9dron?=
 <jean-sebastien.pedron@dumbbell.fr>
Date: Wed, 6 Nov 2024 17:40:50 +0100
Subject: [PATCH] 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.

References #12619.
---
 deps/rabbit/src/rabbit_amqqueue.erl | 15 +++++++++++-
 deps/rabbit/src/rabbit_db_queue.erl | 37 ++++++++++++++++++++++++++++-
 2 files changed, 50 insertions(+), 2 deletions(-)

diff --git a/deps/rabbit/src/rabbit_amqqueue.erl b/deps/rabbit/src/rabbit_amqqueue.erl
index 2ef86b0203da..411d688aa854 100644
--- a/deps/rabbit/src/rabbit_amqqueue.erl
+++ b/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]).
diff --git a/deps/rabbit/src/rabbit_db_queue.erl b/deps/rabbit/src/rabbit_db_queue.erl
index 30251f4d5598..1e64a7f78c08 100644
--- a/deps/rabbit/src/rabbit_db_queue.erl
+++ b/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 +966,40 @@ 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() ->
+    try
+        List = ets:match_object(
+                 ?KHEPRI_PROJECTION,
+                 amqqueue:pattern_match_on_durable(false)),
+        {ok, List}
+    catch
+        error:badarg ->
+            {error, {khepri_projection_missing, ?KHEPRI_WILDCARD_STAR}}
+    end.
+
 %% -------------------------------------------------------------------
 %% delete_transient().
 %% -------------------------------------------------------------------