rabbit_db: Register projections during virgin db init

This covers a specific case where we need to register projections not
covered by the enable callback of the `khepri_db` feature flag. The
feature flag may be enabled if a node has been part of a cluster which
enabled the flag, but the metadata store might be reset. Upon init the
feature flag will be enabled but the store will be empty and the
projections will not exist, so operations like inserting default data
will fail when asserting that a vhost exists for example.

This fixes the `cluster_management_SUITE:forget_cluster_node_in_khepri/1`
case when running the suite with `RABBITMQ_METADATA_STORE=khepri`, which
fails as mentioned above.

We could run projection registration always when using Khepri (i.e. the
`init_using_khepri/0` function) but once projections are registered the
command is idempotent so there's no need to, and the commands are
somewhat large.

Author: the-mikedavis

deps/rabbit/src/rabbit_db.erl

@@ -67,8 +67,8 @@ init() ->
     end,
 
     Ret = case rabbit_khepri:is_enabled() of
-              true  -> init_using_khepri();
-              false -> init_using_mnesia()
+              true  -> init_using_khepri(IsVirgin);
+              false -> init_using_mnesia(IsVirgin)
           end,
     case Ret of
         ok ->
@@ -91,23 +91,29 @@ pre_init(IsVirgin) ->
     OtherMembers = rabbit_nodes:nodes_excl_me(Members),
     rabbit_db_cluster:ensure_feature_flags_are_in_sync(OtherMembers, IsVirgin).
 
-init_using_mnesia() ->
+init_using_mnesia(_IsVirgin) ->
     ?LOG_DEBUG(
       "DB: initialize Mnesia",
       #{domain => ?RMQLOG_DOMAIN_DB}),
     ok = rabbit_mnesia:init(),
     ?assertEqual(rabbit:data_dir(), mnesia_dir()),
     rabbit_sup:start_child(mnesia_sync).
 
-init_using_khepri() ->
+init_using_khepri(IsVirgin) ->
     case rabbit_khepri:members() of
         [] ->
             timer:sleep(1000),
-            init_using_khepri();
+            init_using_khepri(IsVirgin);
         Members ->
             ?LOG_WARNING(
                "Found the following metadata store members: ~p", [Members],
-               #{domain => ?RMQLOG_DOMAIN_DB})
+               #{domain => ?RMQLOG_DOMAIN_DB}),
+            case IsVirgin of
+                true ->
+                    rabbit_khepri:register_projections();
+                false ->
+                    ok
+            end
     end.
 
 init_finished() ->