Relax feature flag compat check during join cluster

deps/rabbit/src/rabbit_db_cluster.erl

@@ -57,7 +57,7 @@ can_join(RemoteNode) ->
        "DB: checking if `~ts` can join cluster using remote node `~ts`",
        [node(), RemoteNode],
        #{domain => ?RMQLOG_DOMAIN_DB}),
-    case rabbit_feature_flags:check_node_compatibility(RemoteNode) of
+    case rabbit_feature_flags:check_node_compatibility(RemoteNode, true) of
         ok ->
             case rabbit_khepri:is_enabled(RemoteNode) of
                 true  -> can_join_using_khepri(RemoteNode);

deps/rabbit/src/rabbit_feature_flags.erl

@@ -103,7 +103,7 @@
          init/0,
          get_state/1,
          get_stability/1,
-         check_node_compatibility/1,
+         check_node_compatibility/1, check_node_compatibility/2,
          sync_feature_flags_with_cluster/2,
          refresh_feature_flags_after_app_load/0,
          enabled_feature_flags_list_file/0
@@ -1302,7 +1302,9 @@ does_node_support(Node, FeatureNames, Timeout) ->
             false
     end.
 
--spec check_node_compatibility(node()) -> ok | {error, any()}.
+-spec check_node_compatibility(RemoteNode) -> Ret when
+      RemoteNode :: node(),
+      Ret :: ok | {error, any()}.
 %% @doc
 %% Checks if a node is compatible with the local node.
 %%
@@ -1314,11 +1316,40 @@ does_node_support(Node, FeatureNames, Timeout) ->
 %%   local node</li>
 %% </ol>
 %%
-%% @param Node the name of the remote node to test.
+%% @param RemoteNode the name of the remote node to test.
+%% @returns `ok' if they are compatible, `{error, Reason}' if they are not.
+
+check_node_compatibility(RemoteNode) ->
+    check_node_compatibility(RemoteNode, false).
+
+-spec check_node_compatibility(RemoteNode, LocalNodeAsVirgin) -> Ret when
+      RemoteNode :: node(),
+      LocalNodeAsVirgin :: boolean(),
+      Ret :: ok | {error, any()}.
+%% @doc
+%% Checks if a node is compatible with the local node.
+%%
+%% To be compatible, the following two conditions must be met:
+%% <ol>
+%% <li>feature flags enabled on the local node must be supported by the
+%%   remote node</li>
+%% <li>feature flags enabled on the remote node must be supported by the
+%%   local node</li>
+%% </ol>
+%%
+%% Unlike {@link check_node_compatibility/1}, the local node's feature flags
+%% inventory is evaluated as if the node was virgin if `LocalNodeAsVirgin' is
+%% true. This is useful if the local node will be reset as part of joining a
+%% remote cluster for instance.
+%%
+%% @param RemoteNode the name of the remote node to test.
+%% @param LocalNodeAsVirgin flag to indicate if the local node should be
+%% evaluated as if it was virgin.
 %% @returns `ok' if they are compatible, `{error, Reason}' if they are not.
 
-check_node_compatibility(Node) ->
-    rabbit_ff_controller:check_node_compatibility(Node).
+check_node_compatibility(RemoteNode, LocalNodeAsVirgin) ->
+    rabbit_ff_controller:check_node_compatibility(
+      RemoteNode, LocalNodeAsVirgin).
 
 run_feature_flags_mod_on_remote_node(Node, Function, Args, Timeout) ->
     rabbit_ff_controller:rpc_call(Node, ?MODULE, Function, Args, Timeout).

deps/rabbit/src/rabbit_ff_controller.erl

@@ -36,7 +36,7 @@
 -export([is_supported/1, is_supported/2,
          enable/1,
          enable_default/0,
-         check_node_compatibility/1,
+         check_node_compatibility/2,
          sync_cluster/1,
          refresh_after_app_load/0,
          get_forced_feature_flag_names/0]).
@@ -134,20 +134,30 @@ enable_default() ->
             Ret
     end.
 
-check_node_compatibility(RemoteNode) ->
+check_node_compatibility(RemoteNode, LocalNodeAsVirgin) ->
     ThisNode = node(),
-    ?LOG_DEBUG(
-       "Feature flags: CHECKING COMPATIBILITY between nodes `~ts` and `~ts`",
-       [ThisNode, RemoteNode],
-       #{domain => ?RMQLOG_DOMAIN_FEAT_FLAGS}),
+    case LocalNodeAsVirgin of
+        true ->
+            ?LOG_DEBUG(
+               "Feature flags: CHECKING COMPATIBILITY between nodes `~ts` "
+               "and `~ts`; consider node `~ts` as virgin",
+               [ThisNode, RemoteNode, ThisNode],
+               #{domain => ?RMQLOG_DOMAIN_FEAT_FLAGS});
+        false ->
+            ?LOG_DEBUG(
+               "Feature flags: CHECKING COMPATIBILITY between nodes `~ts` "
+               "and `~ts`",
+               [ThisNode, RemoteNode],
+               #{domain => ?RMQLOG_DOMAIN_FEAT_FLAGS})
+    end,
     %% We don't go through the controller process to check nodes compatibility
     %% because this function is used while `rabbit' is stopped usually.
     %%
     %% There is no benefit in starting a controller just for this check
     %% because it would not guaranty that the compatibility remains true after
     %% this function finishes and before the node starts and synchronizes
     %% feature flags.
-    check_node_compatibility_task(ThisNode, RemoteNode).
+    check_node_compatibility_task(ThisNode, RemoteNode, LocalNodeAsVirgin).
 
 sync_cluster(Nodes) ->
     ?LOG_DEBUG(
@@ -382,12 +392,14 @@ notify_waiting_controller({ControlerPid, _} = From) ->
 %% Code to check compatibility between nodes.
 %% --------------------------------------------------------------------
 
--spec check_node_compatibility_task(Node, Node) -> Ret when
-      Node :: node(),
+-spec check_node_compatibility_task(NodeA, NodeB, NodeAAsVirigin) -> Ret when
+      NodeA :: node(),
+      NodeB :: node(),
+      NodeAAsVirigin :: boolean(),
       Ret :: ok | {error, Reason},
       Reason :: incompatible_feature_flags.
 
-check_node_compatibility_task(NodeA, NodeB) ->
+check_node_compatibility_task(NodeA, NodeB, NodeAAsVirigin) ->
     ?LOG_NOTICE(
        "Feature flags: checking nodes `~ts` and `~ts` compatibility...",
        [NodeA, NodeB],
@@ -400,7 +412,8 @@ check_node_compatibility_task(NodeA, NodeB) ->
                 _ when is_list(NodesB) ->
                     check_node_compatibility_task1(
                       NodeA, NodesA,
-                      NodeB, NodesB);
+                      NodeB, NodesB,
+                      NodeAAsVirigin);
                 Error ->
                     ?LOG_WARNING(
                        "Feature flags: "
@@ -419,10 +432,12 @@ check_node_compatibility_task(NodeA, NodeB) ->
             {error, {aborted_feature_flags_compat_check, Error}}
     end.
 
-check_node_compatibility_task1(NodeA, NodesA, NodeB, NodesB)
+check_node_compatibility_task1(NodeA, NodesA, NodeB, NodesB, NodeAAsVirigin)
   when is_list(NodesA) andalso is_list(NodesB) ->
     case collect_inventory_on_nodes(NodesA) of
-        {ok, InventoryA} ->
+        {ok, InventoryA0} ->
+            InventoryA = virtually_reset_inventory(
+                           InventoryA0, NodeAAsVirigin),
             ?LOG_DEBUG(
                "Feature flags: inventory of node `~ts`:~n~tp",
                [NodeA, InventoryA],
@@ -488,6 +503,42 @@ list_nodes_clustered_with(Node) ->
         ListOrError -> ListOrError
     end.
 
+virtually_reset_inventory(
+  #{feature_flags := FeatureFlags,
+    states_per_node := StatesPerNode} = Inventory,
+  true = _NodeAsVirgin) ->
+    [Node | _] = maps:keys(StatesPerNode),
+    FeatureStates0 = maps:get(Node, StatesPerNode),
+    FeatureStates1 = maps:map(
+                       fun(FeatureName, _FeatureState) ->
+                               FeatureProps = maps:get(
+                                                FeatureName, FeatureFlags),
+                               state_after_virtual_state(
+                                 FeatureName, FeatureProps)
+                       end, FeatureStates0),
+    StatesPerNode1 = maps:map(
+                       fun(_Node, _FeatureStates) ->
+                               FeatureStates1
+                       end, StatesPerNode),
+    Inventory1 = Inventory#{states_per_node => StatesPerNode1},
+    Inventory1;
+virtually_reset_inventory(
+  Inventory,
+  false = _NodeAsVirgin) ->
+    Inventory.
+
+state_after_virtual_state(_FeatureName, FeatureProps)
+  when ?IS_FEATURE_FLAG(FeatureProps) ->
+    Stability = rabbit_feature_flags:get_stability(FeatureProps),
+    case Stability of
+        required -> true;
+        _        -> false
+    end;
+state_after_virtual_state(FeatureName, FeatureProps)
+  when ?IS_DEPRECATION(FeatureProps) ->
+    not rabbit_deprecated_features:should_be_permitted(
+          FeatureName, FeatureProps).
+
 -spec are_compatible(Inventory, Inventory) -> AreCompatible when
       Inventory :: rabbit_feature_flags:cluster_inventory(),
       AreCompatible :: boolean().

deps/rabbit/src/rabbit_khepri.erl

@@ -871,10 +871,7 @@ check_cluster_consistency(Node, CheckNodesConsistency) ->
                     Error
             end;
         {_OTP, _Rabbit, {ok, Status}} ->
-            case rabbit_db_cluster:check_compatibility(Node) of
-                ok    -> {ok, Status};
-                Error -> Error
-            end
+            {ok, Status}
     end.
 
 remote_node_info(Node) ->

deps/rabbit/src/rabbit_mnesia.erl

@@ -407,7 +407,24 @@ cluster_nodes(WhichNodes) -> cluster_status(WhichNodes).
 cluster_status_from_mnesia() ->
     case is_running() of
         false ->
-            {error, mnesia_not_running};
+            case rabbit_khepri:get_feature_state() of
+                enabled ->
+                    %% To keep this API compatible with older remote nodes who
+                    %% don't know about Khepri, we take the cluster status
+                    %% from `rabbit_khepri' and reformat the return value to
+                    %% ressemble the node from this module.
+                    %%
+                    %% Both nodes won't be compatible, but let's leave that
+                    %% decision to the Feature flags subsystem.
+                    case rabbit_khepri:cluster_status_from_khepri() of
+                        {ok, {All, Running}} ->
+                            {ok, {All, All, Running}};
+                        {error, _} = Error ->
+                            Error
+                    end;
+                _ ->
+                    {error, mnesia_not_running}
+            end;
         true ->
             %% If the tables are not present, it means that
             %% `init_db/3' hasn't been run yet. In other words, either
@@ -475,8 +492,23 @@ members() ->
     end.
 
 node_info() ->
+    %% Once Khepri is enabled, the Mnesia protocol is irrelevant obviously.
+    %%
+    %% That said, older remote nodes who don't known about Khepri will request
+    %% this information anyway as part of calling `node_info/0'. Here, we
+    %% simply return `unsupported' as the Mnesia protocol. Older versions of
+    %% RabbitMQ will skip the protocol negotiation and use other ways.
+    %%
+    %% The goal is mostly to let older nodes which check Mnesia before feature
+    %% flags to reach the feature flags check. This one will correctly
+    %% indicate that they are incompatible. That's why we return `unsupported'
+    %% here, even if we could return the actual Mnesia protocol.
+    MnesiaProtocol = case rabbit_khepri:get_feature_state() of
+                         enabled -> unsupported;
+                         _       -> mnesia:system_info(protocol_version)
+                     end,
     {rabbit_misc:otp_release(), rabbit_misc:version(),
-     mnesia:system_info(protocol_version),
+     MnesiaProtocol,
      cluster_status_from_mnesia()}.
 
 -spec node_type() -> rabbit_db_cluster:node_type().
@@ -694,10 +726,7 @@ check_cluster_consistency(Node, CheckNodesConsistency) ->
                     Error
             end;
         {_OTP, _Rabbit, _Protocol, {ok, Status}} ->
-            case rabbit_db_cluster:check_compatibility(Node) of
-                ok    -> {ok, Status};
-                Error -> Error
-            end
+            {ok, Status}
     end.
 
 remote_node_info(Node) ->

deps/rabbit/test/feature_flags_v2_SUITE.erl

@@ -49,6 +49,7 @@
          failed_enable_feature_flag_with_post_enable/1,
          have_required_feature_flag_in_cluster_and_add_member_with_it_disabled/1,
          have_required_feature_flag_in_cluster_and_add_member_without_it/1,
+         have_unknown_feature_flag_in_cluster_and_add_member_with_it_enabled/1,
          error_during_migration_after_initial_success/1,
          controller_waits_for_own_task_to_finish_before_exiting/1,
          controller_waits_for_remote_task_to_finish_before_exiting/1
@@ -98,6 +99,7 @@ groups() ->
        failed_enable_feature_flag_with_post_enable,
        have_required_feature_flag_in_cluster_and_add_member_with_it_disabled,
        have_required_feature_flag_in_cluster_and_add_member_without_it,
+       have_unknown_feature_flag_in_cluster_and_add_member_with_it_enabled,
        error_during_migration_after_initial_success,
        controller_waits_for_own_task_to_finish_before_exiting,
        controller_waits_for_remote_task_to_finish_before_exiting
@@ -1506,6 +1508,53 @@ have_required_feature_flag_in_cluster_and_add_member_without_it(
          || Node <- AllNodes],
     ok.
 
+have_unknown_feature_flag_in_cluster_and_add_member_with_it_enabled(
+  Config) ->
+    [NewNode | [FirstNode | _] = Nodes] = ?config(nodes, Config),
+    connect_nodes(Nodes),
+    override_running_nodes([NewNode]),
+    override_running_nodes(Nodes),
+
+    FeatureName = ?FUNCTION_NAME,
+    FeatureFlags = #{FeatureName =>
+                     #{provided_by => rabbit,
+                       stability => stable}},
+    ?assertEqual(ok, inject_on_nodes([NewNode], FeatureFlags)),
+
+    ct:pal(
+      "Checking the feature flag is unsupported on the cluster but enabled on "
+      "the standalone node"),
+    ok = run_on_node(
+           NewNode,
+           fun() ->
+                   ?assertEqual(ok, rabbit_feature_flags:enable(FeatureName)),
+                   ?assert(rabbit_feature_flags:is_enabled(FeatureName)),
+                   ok
+           end,
+           []),
+    _ = [ok =
+         run_on_node(
+           Node,
+           fun() ->
+                   ?assertNot(rabbit_feature_flags:is_supported(FeatureName)),
+                   ?assertNot(rabbit_feature_flags:is_enabled(FeatureName)),
+                   ok
+           end,
+           [])
+         || Node <- Nodes],
+
+    %% Check compatibility between NewNodes and Nodes.
+    ok = run_on_node(
+           NewNode,
+           fun() ->
+                   ?assertEqual(
+                      ok,
+                      rabbit_feature_flags:check_node_compatibility(
+                        FirstNode, true)),
+                   ok
+           end, []),
+    ok.
+
 error_during_migration_after_initial_success(Config) ->
     AllNodes = [NewNode | [FirstNode | _] = Nodes] = ?config(nodes, Config),
     connect_nodes(Nodes),