From 74a68ff60b97b2b8bfe9fedd1cb08af3a7f81420 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, 29 Oct 2025 15:36:59 +0100
Subject: [PATCH] WIP

---
 deps/rabbit/src/rabbit_db_cluster.erl | 88 +++++++++++++++++++++------
 1 file changed, 69 insertions(+), 19 deletions(-)

diff --git a/deps/rabbit/src/rabbit_db_cluster.erl b/deps/rabbit/src/rabbit_db_cluster.erl
index 1fd720e527f..f3245a693ff 100644
--- a/deps/rabbit/src/rabbit_db_cluster.erl
+++ b/deps/rabbit/src/rabbit_db_cluster.erl
@@ -9,6 +9,7 @@
 -module(rabbit_db_cluster).
 
 -include_lib("kernel/include/logger.hrl").
+-include_lib("stdlib/include/assert.hrl").
 
 -include_lib("rabbit_common/include/logging.hrl").
 
@@ -253,27 +254,30 @@ join_using_khepri(_ClusterNodes, ram = NodeType) ->
       RemoveWhenOffline :: boolean().
 %% @doc Removes `Node' from the cluster.
 
-forget_member(Node, RemoveWhenOffline) ->
-    case forget_member0(Node, RemoveWhenOffline) of
-        ok ->
-            rabbit_node_monitor:notify_left_cluster(Node);
-        Error ->
-            Error
+forget_member(Node, RemoveWhenOffline)
+  when is_atom(Node) andalso Node =/= node() ->
+    {ok, InitialState} = lock_cluster_changes(Node),
+    try
+        forget_member_locked(Node, RemoveWhenOffline)
+    after
+        unlock_cluster_changes(InitialState)
     end.
 
-forget_member0(Node, RemoveWhenOffline) ->
-    case rabbit:is_running(Node) of
-        false ->
-            ?LOG_DEBUG(
-               "DB: removing cluster member `~ts`", [Node],
-               #{domain => ?RMQLOG_DOMAIN_DB}),
-            case rabbit_khepri:is_enabled() of
-                true  -> forget_member_using_khepri(Node, RemoveWhenOffline);
-                false -> forget_member_using_mnesia(Node, RemoveWhenOffline)
-            end;
-        true ->
-            {error, {failed_to_remove_node, Node, rabbit_still_running}}
-    end.
+forget_member_locked(Node, RemoveWhenOffline) ->
+    ?LOG_DEBUG(
+       "DB: removing cluster member `~ts`", [Node],
+       #{domain => ?RMQLOG_DOMAIN_DB}),
+    ?assertNot(rabbit:is_running(Node)),
+    case rabbit_khepri:is_enabled() of
+        true  -> forget_member_using_khepri(Node, RemoveWhenOffline);
+        false -> forget_member_using_mnesia(Node, RemoveWhenOffline)
+    end,
+
+    rabbit_amqqueue:forget_all(Node),
+    rabbit_quorum_queue:shrink_all(Node),
+    rabbit_stream_queue:delete_all_replicas(Node),
+    rabbit_stream_coordinator:forget_node(Node),
+    rabbit_node_monitor:notify_left_cluster(Node).
 
 forget_member_using_mnesia(Node, RemoveWhenOffline) ->
     rabbit_mnesia:forget_cluster_node(Node, RemoveWhenOffline).
@@ -287,6 +291,52 @@ forget_member_using_khepri(_Node, true) ->
 forget_member_using_khepri(Node, false = _RemoveWhenOffline) ->
     rabbit_khepri:remove_member(Node).
 
+lock_cluster_changes(ChangingNode) ->
+    RabbitWasRunning = stop_rabbit_if_running(ChangingNode),
+    InitialState = #{changing_node => ChangingNode,
+                     rabbit_was_running => RabbitWasRunning},
+
+    %% We acquire the feature flags registry reload lock because between
+    %% the time we reset the registry (as part of `rabbit_db:reset/0' and
+    %% the states copy from the remote node, there could be a concurrent
+    %% reload of the registry (for instance because of peer discovery on
+    %% another node) with the default/empty states.
+    %%
+    %% To make this work, the lock is also acquired from
+    %% `rabbit_ff_registry_wrapper'.
+    rabbit_ff_registry_factory:acquire_state_change_lock(),
+    {ok, InitialState}.
+
+stop_rabbit_if_running(ThisNode) when ThisNode =:= node() ->
+    RabbitWasRunning = rabbit:is_running(),
+    case RabbitWasRunning of
+        true  -> ok = rabbit:stop();
+        false -> ok
+    end,
+    RabbitWasRunning;
+stop_rabbit_if_running(RemoteNode) when is_atom(RemoteNode) ->
+    RabbitWasRunning = erpc:call(RemoteNode, rabbit, is_running, []),
+    case RabbitWasRunning of
+        true  -> ok = erpc:call(RemoteNode, rabbit, stop, []);
+        false -> ok
+    end,
+    RabbitWasRunning.
+
+unlock_cluster_changes(
+  #{changing_node := ChangingNode,
+    rabbit_was_running := RabbitWasRunning}) ->
+    rabbit_ff_registry_factory:release_state_change_lock(),
+    start_rabbit_if_was_running(ChangingNode, RabbitWasRunning),
+    ok.
+
+start_rabbit_if_was_running(_ChangingNode, false = _RabbitWasRunning) ->
+    ok;
+start_rabbit_if_was_running(ThisNode, true = _RabbitWasRunning)
+  when ThisNode =:= node() ->
+    rabbit:start();
+start_rabbit_if_was_running(RemoteNode, true = _RabbitWasRunning) ->
+    erpc:call(RemoteNode, rabbit, start, []).
+
 %% -------------------------------------------------------------------
 %% Cluster update.
 %% -------------------------------------------------------------------