rabbit_feature_flags: Drop the -on_load() directive

We used it to prevent a node breakage when RabbitMQ was updated from
e.g. 3.6.x to 3.7.0 when the package manager uncompresses the files of
the package while the service is running, overwriting existing files. In
this situation, a 3.6.x node could be queried for its feature flags
state and the `rabbit_feature_flags` module would be loaded (because it
appeared during the upgrade but before the restart) and this would fail
because the node really didn't support feature flags.

With some feature flags being required as of RabbitMQ 3.11.0, upgrading
directly from e.g. 3.6.x is unsupported now. Therefore, we can drop this
`-on_load()` failsafe.

Author: dumbbell

deps/rabbit/src/rabbit_feature_flags.erl

@@ -360,8 +360,6 @@
               inventory/0,
               cluster_inventory/0]).
 
--on_load(on_load/0).
-
 -spec list() -> feature_flags().
 %% @doc
 %% Lists all supported feature flags.
@@ -1243,63 +1241,3 @@ sync_feature_flags_with_cluster(_Nodes, _NodeIsVirgin) ->
 
 refresh_feature_flags_after_app_load() ->
     rabbit_ff_controller:refresh_after_app_load().
-
-on_load() ->
-    %% The goal of this `on_load()` code server hook is to prevent this
-    %% module from being loaded in an already running RabbitMQ node if
-    %% the running version does not have the feature flags subsystem.
-    %%
-    %% This situation happens when an upgrade overwrites RabbitMQ files
-    %% with the node still running. This is the case with many packages:
-    %% files are updated on disk, then a post-install step takes care of
-    %% restarting the service.
-    %%
-    %% The problem is that if many nodes in a cluster are updated at the
-    %% same time, one node running the newer version might query feature
-    %% flags on an old node where this module is already available
-    %% (because files were already overwritten). This causes the query
-    %% to report an unexpected answer and the newer node to refuse to
-    %% start.
-    %%
-    %% However, when the module is executed outside of RabbitMQ (for
-    %% debugging purpose or in the context of EUnit for instance), we
-    %% want to allow the load. That's why we first check if RabbitMQ is
-    %% actually running.
-    case rabbit:is_running() of
-        true ->
-            %% RabbitMQ is running.
-            %%
-            %% Now we want to differentiate a pre-feature-flags node
-            %% from one having the subsystem.
-            %%
-            %% To do that, we verify if the `feature_flags_file`
-            %% application environment variable is defined. With a
-            %% feature-flags-enabled node, this application environment
-            %% variable is defined by rabbitmq-server(8).
-            case application:get_env(rabbit, feature_flags_file) of
-                {ok, _} ->
-                    %% This is a feature-flags-enabled version. Loading
-                    %% the module is permitted.
-                    ok;
-                _ ->
-                    %% This is a pre-feature-flags version. We deny the
-                    %% load and report why, possibly specifying the
-                    %% version of RabbitMQ.
-                    Vsn = case application:get_key(rabbit, vsn) of
-                              {ok, V}   -> V;
-                              undefined -> "unknown version"
-                          end,
-                    "Refusing to load '" ?MODULE_STRING "' on this "
-                    "node. It appears to be running a pre-feature-flags "
-                    "version of RabbitMQ (" ++ Vsn ++ "). This is fine: "
-                    "a newer version of RabbitMQ was deployed on this "
-                    "node, but it was not restarted yet. This warning "
-                    "is probably caused by a remote node querying this "
-                    "node for its feature flags."
-            end;
-        false ->
-            %% RabbitMQ is not running. Loading the module is permitted
-            %% because this Erlang node will never be queried for its
-            %% feature flags.
-            ok
-    end.