rabbitmq_ct_helpers: Fix how we set $RABBITMQ_FEATURE_FLAGS in tests

[Why]
In order to make `khepri_db` the default in the future, the handling of
`$RABBITMQ_FEATURE_FLAGS` had to be adapted to be able to *disable*
Khepri instead.

Unfortunately I broke the behavior with stable feature flags that are
only available in the primary umbrella. In this case, they were
automatically enabled and thus, clustering with an old umbrella that did
not have these feature flags failed with `incompatible_feature_flags`.

[How]
The solution is to always use an absolute list of feature flags, not the
new relative list.

deps/rabbitmq_ct_helpers/src/rabbit_ct_broker_helpers.erl

@@ -762,17 +762,6 @@ do_start_rabbitmq_node(Config, NodeConfig, I) ->
                      false -> ExtraArgs3;
                      _     -> ["NOBUILD=1" | ExtraArgs3]
                  end,
-    %% TODO: When we start to do mixed-version testing against 4.1.x as the
-    %% secondary umbrella, we will need to stop setting
-    %% `$RABBITMQ_FEATURE_FLAGS'.
-    MetadataStore = rabbit_ct_helpers:get_config(Config, metadata_store),
-    SecFeatureFlags0 = case MetadataStore of
-                           mnesia -> ?REQUIRED_FEATURE_FLAGS;
-                           khepri -> [khepri_db | ?REQUIRED_FEATURE_FLAGS]
-                       end,
-    SecFeatureFlags = string:join(
-                        [atom_to_list(F) || F <- SecFeatureFlags0],
-                        ","),
     ExtraArgs = case UseSecondaryUmbrella of
                     true ->
                         DepsDir = ?config(erlang_mk_depsdir, Config),
@@ -802,8 +791,7 @@ do_start_rabbitmq_node(Config, NodeConfig, I) ->
                          {"RABBITMQ_SCRIPTS_DIR=~ts", [SecScriptsDir]},
                          {"RABBITMQ_SERVER=~ts/rabbitmq-server", [SecScriptsDir]},
                          {"RABBITMQCTL=~ts/rabbitmqctl", [SecScriptsDir]},
-                         {"RABBITMQ_PLUGINS=~ts/rabbitmq-plugins", [SecScriptsDir]},
-                         {"RABBITMQ_FEATURE_FLAGS=~ts", [SecFeatureFlags]}
+                         {"RABBITMQ_PLUGINS=~ts/rabbitmq-plugins", [SecScriptsDir]}
                          | ExtraArgs4];
                     false ->
                         case UseSecondaryDist of
@@ -824,8 +812,7 @@ do_start_rabbitmq_node(Config, NodeConfig, I) ->
                                  {"CLI_ESCRIPTS_DIR=~ts/escript", [SecondaryDist]},
                                  {"RABBITMQ_SCRIPTS_DIR=~ts/sbin", [SecondaryDist]},
                                  {"RABBITMQ_SERVER=~ts/sbin/rabbitmq-server", [SecondaryDist]},
-                                 {"RABBITMQ_ENABLED_PLUGINS=~ts", [SecondaryEnabledPlugins]},
-                                 {"RABBITMQ_FEATURE_FLAGS=~ts", [SecFeatureFlags]}
+                                 {"RABBITMQ_ENABLED_PLUGINS=~ts", [SecondaryEnabledPlugins]}
                                 | ExtraArgs4];
                             false ->
                                 ExtraArgs4
@@ -1066,60 +1053,22 @@ configured_metadata_store(Config) ->
 
 configure_metadata_store(Config) ->
     ct:log("Configuring metadata store..."),
-    Value = rabbit_ct_helpers:get_app_env(
-              Config, rabbit, forced_feature_flags_on_init, undefined),
     MetadataStore = configured_metadata_store(Config),
     Config1 = rabbit_ct_helpers:set_config(
                 Config, {metadata_store, MetadataStore}),
-    %% To enabled or disable `khepri_db', we use the relative forced feature
-    %% flags mechanism. This allows us to select the state of Khepri without
-    %% having to worry about other feature flags.
-    %%
-    %% However, RabbitMQ 4.0.x and older don't support it. See the
-    %% `uses_expected_metadata_store/2' check to see how Khepri is enabled in
-    %% this case.
-    %%
-    %% Note that this setting will be ignored by the secondary umbrella because
-    %% we set `$RABBITMQ_FEATURE_FLAGS' explisitly. In this case, we handle the
-    %% `khepri_db' feature flag when we compute the value of that variable.
-    %%
-    %% TODO: When we start to do mixed-version testing against 4.1.x as the
-    %% secondary umbrella, we will need to stop setting
-    %% `$RABBITMQ_FEATURE_FLAGS'.
-    case MetadataStore of
-        khepri ->
-            ct:log("Enabling Khepri metadata store"),
-            case Value of
-                undefined ->
-                    rabbit_ct_helpers:merge_app_env(
-                      Config1,
-                      {rabbit,
-                       [{forced_feature_flags_on_init,
-                         {rel, [khepri_db], []}}]});
-                _ ->
-                    rabbit_ct_helpers:merge_app_env(
-                      Config1,
-                      {rabbit,
-                       [{forced_feature_flags_on_init,
-                         [khepri_db | Value]}]})
-            end;
-        mnesia ->
-            ct:log("Enabling Mnesia metadata store"),
-            case Value of
-                undefined ->
-                    rabbit_ct_helpers:merge_app_env(
-                      Config1,
-                      {rabbit,
-                       [{forced_feature_flags_on_init,
-                         {rel, [], [khepri_db]}}]});
-                _ ->
-                    rabbit_ct_helpers:merge_app_env(
-                      Config1,
-                      {rabbit,
-                       [{forced_feature_flags_on_init,
-                         Value -- [khepri_db]}]})
-            end
-    end.
+    FeatureNames0 = case MetadataStore of
+                        mnesia ->
+                            ct:log("Enabling Mnesia metadata store"),
+                            ?REQUIRED_FEATURE_FLAGS;
+                        khepri ->
+                            ct:log("Enabling Khepri metadata store"),
+                            [khepri_db | ?REQUIRED_FEATURE_FLAGS]
+                    end,
+    OtherFeatureNames = rabbit_ct_helpers:get_app_env(
+                          Config, rabbit, forced_feature_flags_on_init, []),
+    FeatureNames1 = lists:usort(FeatureNames0 ++ OtherFeatureNames),
+    rabbit_ct_helpers:merge_app_env(
+      Config1, {rabbit, [{forced_feature_flags_on_init, FeatureNames1}]}).
 
 %% Waits until the metadata store replica on Node is up to date with the leader.
 await_metadata_store_consistent(Config, Node) ->