Add configuration setting to restore old machine upgrade behavior

src/ra_server.erl

@@ -194,6 +194,18 @@
 %% The simple machine config is version that can only be used for simple state
 %% machines that cannot access any of the advanced features.
 
+-type upgrade_machine_when() :: after_election | when_available_everywhere.
+%% When a new Ra machine version should be upgraded to.
+%%
+%% <ul>
+%% <li>`after_election': when a leader is elected and if it supports a newer
+%% machine version, it switches to that new version. This was the default and
+%% only possible behavior in Ra up to 2.15.</li>
+%% <li>`when_available_everywhere': a new machine version it used only when
+%% all members of the cluster support it. This is the default behavior
+%% starting with Ra 2.16.</li>
+%% </ul>
+
 -type ra_server_config() :: #{id := ra_server_id(),
                               uid := ra_uid(),
                               %% a friendly name to refer to a particular
@@ -218,7 +230,8 @@
                               counter => counters:counters_ref(),
                               membership => ra_membership(),
                               system_config => ra_system:config(),
-                              has_changed => boolean()
+                              has_changed => boolean(),
+                              upgrade_machine_when => upgrade_machine_when()
                              }.
 
 -type ra_server_info_key() :: machine_version | atom().
@@ -271,7 +284,8 @@
               command_reply_mode/0,
               ra_event_formatter_fun/0,
               effect/0,
-              effects/0
+              effects/0,
+              upgrade_machine_when/0
              ]).
 
 -spec name(ClusterName :: ra_cluster_name(), UniqueSuffix::string()) -> atom().
@@ -358,6 +372,10 @@ init(#{id := Id,
         end,
     MacMod = ra_machine:which_module(Machine, MacVer),
 
+    UpgradeMachineWhen = maps:get(
+                           upgrade_machine_when, Config,
+                           when_available_everywhere),
+
     CommitIndex = max(LastApplied, SnapshotIdx),
     Cfg = #cfg{id = Id,
                uid = UId,
@@ -371,6 +389,7 @@ init(#{id := Id,
                effective_handle_aux_fun = ra_machine:which_aux_fun(MacMod),
                max_pipeline_count = MaxPipelineCount,
                max_append_entries_rpc_batch_size = MaxAERBatchSize,
+               upgrade_machine_when = UpgradeMachineWhen,
                counter = maps:get(counter, Config, undefined),
                system_config = SystemConfig},
     put_counter(Cfg, ?C_RA_SVR_METRIC_COMMIT_INDEX, CommitIndex),
@@ -3445,19 +3464,25 @@ after_log_append_reply(Cmd, Idx, Term, Effects0) ->
 
 post_election_effects(
   #{cfg := #cfg{effective_machine_version = EffectiveMacVer,
-                machine = Mac}} = State) ->
+                machine = Mac,
+                upgrade_machine_when = UpgradeMachineWhen}} = State) ->
     Peers = peers(State),
     PeerIds = maps:keys(Peers),
-    case PeerIds of
-        [] ->
+
+    UpgradeMachineNow = (
+      PeerIds =:= [] orelse
+      UpgradeMachineWhen =:= after_election),
+
+    case UpgradeMachineNow of
+        true ->
             %% This node is alone in the cluster, we can send the `noop'
             %% command with the newer machine version right away.
             MacVer = ra_machine:version(Mac),
             Noop = {noop, #{ts => erlang:system_time(millisecond)},
                     MacVer},
             NoopEffect = {next_event, cast, {command, Noop}},
             [NoopEffect];
-        _ ->
+        false ->
             %% We continue to send the `noop' command immediately after
             %% becoming a leader, but compared to Ra 2.15 and older, we don't
             %% set the machine version to the latest: we keep the same

src/ra_server.hrl

@@ -23,6 +23,7 @@
          effective_handle_aux_fun :: undefined | {handle_aux, 5 | 6},
          max_pipeline_count = ?DEFAULT_MAX_PIPELINE_COUNT :: non_neg_integer(),
          max_append_entries_rpc_batch_size = ?AER_CHUNK_SIZE :: non_neg_integer(),
+         upgrade_machine_when = when_available_everywhere :: ra_server:upgrade_machine_when(),
          counter :: undefined | counters:counters_ref(),
          system_config :: ra_system:config()
         }).