fix: don't crash retainer api when retainer is disabled

apps/emqx_retainer/src/emqx_retainer.erl

@@ -326,28 +326,43 @@ dispatch(Context, Topic) ->
 
 -spec delete_message(context(), topic()) -> ok.
 delete_message(Context, Topic) ->
-    Mod = backend_module(Context),
-    BackendState = backend_state(Context),
-    Mod:delete_message(BackendState, Topic).
+    with_backend_module(
+        Context,
+        fun(Mod, BackendState) ->
+            Mod:delete_message(BackendState, Topic)
+        end,
+        ok
+    ).
 
 -spec read_message(context(), topic()) -> {ok, list(message())}.
 read_message(Context, Topic) ->
-    Mod = backend_module(Context),
-    BackendState = backend_state(Context),
-    Mod:read_message(BackendState, Topic).
+    with_backend_module(
+        Context,
+        fun(Mod, BackendState) -> Mod:read_message(BackendState, Topic) end,
+        {ok, []}
+    ).
 
 -spec page_read(context(), emqx_maybe:t(topic()), deadline(), non_neg_integer(), non_neg_integer()) ->
     {ok, has_next(), list(message())}.
 page_read(Context, Topic, Deadline, Page, Limit) ->
-    Mod = backend_module(Context),
-    BackendState = backend_state(Context),
-    Mod:page_read(BackendState, Topic, Deadline, Page, Limit).
+    with_backend_module(
+        Context,
+        fun(Mod, BackendState) -> Mod:page_read(BackendState, Topic, Deadline, Page, Limit) end,
+        {ok, false, []}
+    ).
 
 -spec count(context()) -> non_neg_integer().
 count(Context) ->
-    Mod = backend_module(Context),
-    BackendState = backend_state(Context),
-    Mod:size(BackendState).
+    with_backend_module(Context, fun(Mod, BackendState) -> Mod:size(BackendState) end, 0).
+
+with_backend_module(Context, Fun, Default) ->
+    case backend_module(Context) of
+        undefined ->
+            Default;
+        Mod ->
+            BackendState = backend_state(Context),
+            Fun(Mod, BackendState)
+    end.
 
 -spec start_clear_expired(context()) -> ok.
 start_clear_expired(Context) ->
@@ -371,16 +386,16 @@ store_retained(Context, #message{topic = Topic, payload = Payload} = Msg) ->
                 limit => Limit
             });
         _ ->
-            Mod = backend_module(Context),
-            BackendState = backend_state(Context),
-            Mod:store_retained(BackendState, Msg)
+            with_backend_module(
+                Context,
+                fun(Mod, BackendState) -> Mod:store_retained(BackendState, Msg) end,
+                ok
+            )
     end.
 
 -spec clean(context()) -> ok.
 clean(Context) ->
-    Mod = backend_module(Context),
-    BackendState = backend_state(Context),
-    Mod:clean(BackendState).
+    with_backend_module(Context, fun(Mod, BackendState) -> Mod:clean(BackendState) end, ok).
 
 -spec update_config(state(), hocon:config(), hocon:config()) -> state().
 update_config(State, NewConfig, OldConfig) ->
@@ -512,8 +527,7 @@ create(Cfg) ->
 
 -spec close(context()) -> ok | {error, term()}.
 close(Context) ->
-    Mod = backend_module(Context),
-    Mod:close(Context).
+    with_backend_module(Context, fun(Mod, BackendState) -> Mod:close(BackendState) end, ok).
 
 -spec load(context()) -> ok.
 load(Context) ->