Use a persistent term in rabbit_federation_app_state

michaelklishin · michaelklishin · commit a8347b24eefa · 2026-01-14T07:03:57.000-08:00
This avoids a classic deadlock in Erlang: when
an application_controller (AC) invokes a callback,
such as pre_stop/1, the function invoked cannot
use any OTP functions that would ultimately require
an AC response.

application:set_env/2 is one of such functions,
so with this commit we switch to a persistent term.
diff --git a/deps/rabbitmq_federation_common/src/rabbit_federation_app_state.erl b/deps/rabbitmq_federation_common/src/rabbit_federation_app_state.erl
@@ -7,20 +7,24 @@
 
 %% Tracks transient application state for federation plugins.
 %% Used to prevent link restarts during node shutdown.
+%%
+%% Uses persistent_term for storage because prep_stop/1 is called by the
+%% application_controller, and application:set_env/3 would deadlock.
 -module(rabbit_federation_app_state).
 
 -export([is_shutting_down/0, mark_as_shutting_down/0, reset_shutting_down_marker/0]).
 
--define(APP, rabbitmq_federation_common).
+-define(KEY, {?MODULE, shutting_down}).
 
 -spec is_shutting_down() -> boolean().
 is_shutting_down() ->
-    application:get_env(?APP, shutting_down, false).
+    persistent_term:get(?KEY, false).
 
 -spec mark_as_shutting_down() -> ok.
 mark_as_shutting_down() ->
-    application:set_env(?APP, shutting_down, true).
+    persistent_term:put(?KEY, true).
 
 -spec reset_shutting_down_marker() -> ok.
 reset_shutting_down_marker() ->
-    application:unset_env(?APP, shutting_down).
+    _ = persistent_term:erase(?KEY),
+    ok.
diff --git a/deps/rabbitmq_federation_common/test/unit_SUITE.erl b/deps/rabbitmq_federation_common/test/unit_SUITE.erl
@@ -68,16 +68,16 @@ obfuscate_upstream_params_network_with_char_list_password_value(_Config) ->
     ok.
 
 shutdown_flag_defaults_to_false(_Config) ->
-    application:unset_env(rabbitmq_federation_common, shutting_down),
+    ok = rabbit_federation_app_state:reset_shutting_down_marker(),
     ?assertEqual(false, rabbit_federation_app_state:is_shutting_down()),
     ok.
 
 shutdown_flag_can_be_set(_Config) ->
-    application:unset_env(rabbitmq_federation_common, shutting_down),
+    ok = rabbit_federation_app_state:reset_shutting_down_marker(),
     ?assertEqual(false, rabbit_federation_app_state:is_shutting_down()),
     ok = rabbit_federation_app_state:mark_as_shutting_down(),
     ?assertEqual(true, rabbit_federation_app_state:is_shutting_down()),
-    application:unset_env(rabbitmq_federation_common, shutting_down),
+    ok = rabbit_federation_app_state:reset_shutting_down_marker(),
     ok.
 
 shutdown_flag_can_be_cleared(_Config) ->
