rabbit_feature_flags: Sync enabled feature flags differently on virgin node

If a virgin node starts as clustered (thanks to peer discovery), we need
to mark feature flags already enabled remotely as enabled locally too.

We can't do a regular cluster sync because remote nodes may have
required feature flags which are disabled locally on the virgin node.
Therefore, those nodes don't have the migration code for these feature
flags anymore and the feature flags' state can't be changed.

By doing this special sync, we allow a clustered virgin node to join a
cluster with remote nodes having required feature flags.

Author: dumbbell

deps/rabbit/src/rabbit_feature_flags.erl

@@ -1872,7 +1872,44 @@ sync_cluster_v1([], NodeIsVirgin, _) ->
               "current state"),
             ok
     end;
-sync_cluster_v1(Nodes, _, Timeout) ->
+sync_cluster_v1(Nodes, true = _NodeIsVirgin, Timeout) ->
+    [Node | _] = Nodes,
+    rabbit_log_feature_flags:debug(
+      "Feature flags: marking feature flags enabled remotely as enabled "
+      "locally because this node is virgin; using ~s as the remote node",
+     [Node]),
+    ?assertNotEqual(node(), Node),
+    Ret1 = rpc:call(Node, ?MODULE, list, [enabled], Timeout),
+    case Ret1 of
+        FeatureFlags when is_map(FeatureFlags) ->
+            FeatureNames = lists:sort(maps:keys(FeatureFlags)),
+            rabbit_log_feature_flags:error(
+              "Feature flags: list of feature flags to automatically mark as "
+              "enabled on this virgin node: ~0p",
+              [FeatureNames]),
+            Ret2 =
+            lists:foldl(
+              fun
+                  (FeatureName, ok) ->
+                      case is_supported_locally(FeatureName) of
+                          true  -> mark_as_enabled_locally(FeatureName, true);
+                          false -> ok
+                      end;
+                  (_, Acc) ->
+                      Acc
+              end, ok, FeatureNames),
+            case Ret2 of
+                ok -> sync_cluster_v1(Nodes, false, Timeout);
+                _  -> Ret2
+            end;
+        {badrpc, _} = Reason ->
+            rabbit_log_feature_flags:error(
+              "Feature flags: failed to query remotely enabled feature flags "
+              "on node ~s: ~p",
+              [Node, Reason]),
+            {error, Reason}
+    end;
+sync_cluster_v1(Nodes, _NodeIsVirgin, Timeout) ->
     case sync_feature_flags_v2_first(Nodes, Timeout) of
         true ->
             ?LOG_DEBUG(