ra_aux: Expose effective machine version

A machine might want this information to determine if it should emit
an effect like `{append, NewCmd}` that can only be handled once a
cluster reaches an effective machine version.

src/ra_aux.erl

@@ -7,6 +7,7 @@
 
 -export([
          machine_state/1,
+         effective_machine_version/1,
          leader_id/1,
          last_applied/1,
          members_info/1,
@@ -17,6 +18,7 @@
         ]).
 
 -include("ra.hrl").
+-include("ra_server.hrl").
 
 -opaque internal_state() :: ra_server:state().
 
@@ -26,6 +28,11 @@
 machine_state(State) ->
     maps:get(?FUNCTION_NAME, State).
 
+-spec effective_machine_version(ra_aux:internal_state()) ->
+    ra_machine:version().
+effective_machine_version(#{cfg := Cfg}) ->
+    Cfg#cfg.effective_machine_version.
+
 -spec leader_id(ra_aux:internal_state()) -> undefined | ra_server_id().
 leader_id(State) ->
     maps:get(?FUNCTION_NAME, State).