Introduce "plugins_data_dir" which is preserved during khepri migration

This is mainly for community plugins which want to migrate away from
Mnesia into a new subdir. (They might not even use Khepri, or use
Khepri but a different store id.)

Fixes #11304

Author: gomoripeti

deps/rabbit/src/rabbit_mnesia.erl

@@ -1066,7 +1066,8 @@ mnesia_and_msg_store_files() ->
              rabbit_node_monitor:classic_filename(),
              rabbit_node_monitor:quorum_filename(),
              rabbit_feature_flags:enabled_feature_flags_list_file(),
-             rabbit_khepri:dir()],
+             rabbit_khepri:dir(),
+             rabbit_plugins:plugins_data_dir()],
             IgnoredFiles = [filename:basename(File) || File <- IgnoredFiles0],
             ?LOG_DEBUG("Files and directories found in node's data directory: ~ts, of them to be ignored: ~ts",
                             [string:join(lists:usort(List0), ", "), string:join(lists:usort(IgnoredFiles), ", ")]),

deps/rabbit/src/rabbit_plugins.erl

@@ -13,6 +13,7 @@
 -export([validate_plugins/1, format_invalid_plugins/1]).
 -export([is_strictly_plugin/1, strictly_plugins/2, strictly_plugins/1]).
 -export([plugins_dir/0, plugin_names/1, plugins_expand_dir/0, enabled_plugins_file/0]).
+-export([plugins_data_dir/0]).
 -export([is_enabled/1, is_enabled_on_node/2, enabled_plugins/0]).
 -export([which_plugin/1]).
 
@@ -93,6 +94,15 @@ plugins_dir() ->
             filename:join([rabbit:data_dir(), "plugins_dir_stub"])
     end.
 
+plugins_data_dir() ->
+    case application:get_env(rabbit, plugins_data_dir) of
+        {ok, PluginsDataDir} ->
+            PluginsDataDir;
+        _ ->
+            DataDir = rabbit_mnesia:dir(),
+            filename:join([DataDir, "plugins_data_dir"])
+    end.
+
 -spec enabled_plugins_file() -> file:filename().
 enabled_plugins_file() ->
      case application:get_env(rabbit, enabled_plugins_file) of

deps/rabbit/test/metadata_store_migration_SUITE.erl

@@ -124,6 +124,11 @@ from_mnesia_to_khepri(Config) ->
     Maintenance = rabbit_ct_broker_helpers:rpc(Config, 0, rabbit_db_maintenance, get, [Server]),
     ?assertNot(undefined == Maintenance),
 
+    PluginsDataDir = rabbit_ct_broker_helpers:rpc(Config, 0, rabbit_plugins, plugins_data_dir, []),
+    ok = file:make_dir(PluginsDataDir),
+    PluginDataFile = filename:join(PluginsDataDir, "test.txt"),
+    ok = file:write_file(PluginDataFile, "test content"),
+
     %% 2)
     Servers = rabbit_ct_broker_helpers:get_node_configs(Config, nodename),
     ok = rabbit_ct_broker_helpers:enable_feature_flag(Config, Servers, khepri_db),
@@ -156,5 +161,8 @@ from_mnesia_to_khepri(Config) ->
     MaintenanceK = rabbit_ct_broker_helpers:rpc(Config, 0, rabbit_db_maintenance, get, [Server]),
     ?assertEqual(MaintenanceK, Maintenance),
 
+    %% plugin data file should be preserved
+    ?assertEqual({ok, <<"test content">>}, file:read_file(PluginDataFile)),
+
     ok.