Merge pull request #12811 from rabbitmq/mergify/bp/v4.0.x/pr-12809

Peer discovery cleanup: be more defensive (backport #12809)

Author: michaelklishin

deps/rabbit_common/src/rabbit_data_coercion.erl

@@ -10,6 +10,7 @@
 -export([to_binary/1, to_list/1, to_atom/1, to_integer/1, to_proplist/1, to_map/1]).
 -export([to_atom/2, atomize_keys/1, to_list_of_binaries/1]).
 -export([to_utf8_binary/1, to_unicode_charlist/1]).
+-export([as_list/1]).
 
 -spec to_binary(Val :: binary() | list() | atom() | integer() | function()) -> binary().
 to_binary(Val) when is_list(Val)     -> list_to_binary(Val);
@@ -109,3 +110,9 @@ to_unicode_charlist(Val) ->
         UnicodeValue ->
             UnicodeValue
     end.
+
+-spec as_list(list() | any()) -> [any()].
+as_list(Nodes) when is_list(Nodes) ->
+    Nodes;
+as_list(Other) ->
+    [Other].

deps/rabbitmq_peer_discovery_common/src/rabbit_peer_discovery_cleanup.erl

@@ -28,6 +28,8 @@
 -compile(export_all).
 -endif.
 
+-import(rabbit_data_coercion, [as_list/1]).
+
 -define(CONFIG_MODULE, rabbit_peer_discovery_config).
 -define(CONFIG_KEY, node_cleanup).
 
@@ -241,7 +243,7 @@ maybe_cleanup(State, UnreachableNodes) ->
        "Peer discovery: cleanup discovered unreachable nodes: ~tp",
        [UnreachableNodes],
        #{domain => ?RMQLOG_DOMAIN_PEER_DIS}),
-    case lists:subtract(UnreachableNodes, service_discovery_nodes()) of
+    case lists:subtract(as_list(UnreachableNodes), as_list(service_discovery_nodes())) of
         [] ->
             ?LOG_DEBUG(
                "Peer discovery: all unreachable nodes are still "
@@ -304,7 +306,8 @@ unreachable_nodes() ->
 service_discovery_nodes() ->
     Module = rabbit_peer_discovery:backend(),
     case rabbit_peer_discovery:normalize(Module:list_nodes()) of
-        {ok, {Nodes, _Type}} ->
+        {ok, {OneOrMultipleNodes, _Type}} ->
+            Nodes = as_list(OneOrMultipleNodes),
             ?LOG_DEBUG(
                "Peer discovery cleanup: ~tp returned ~tp",
                [Module, Nodes],