Mnesia backup and fallback is not working with mnesia

[Mikaka27]

Hi,

We're got this issue ticket: erlang/otp#8045
You can see that trying to run mnesia_rocksdb_fallback:run() produces error.

crasher:
initial call: mnesia_rocksdb:init/1
pid: <0.262.0>
registered_name: []
exception exit: {noproc,
{gen_server,call,
[mnesia_rocksdb_admin,
{rdb,{get_ref,t}},
infinity]}}
in function gen_server:call/3 (gen_server.erl, line 419)
in call from mnesia_rocksdb_admin:call/3 (src/mnesia_rocksdb_admin.erl, line 338)
in call from mnesia_rocksdb:init/1 (src/mnesia_rocksdb.erl, line 786)
in call from gen_server:init_it/2 (gen_server.erl, line 980)
in call from gen_server:init_it/6 (gen_server.erl, line 935)
ancestors: [mnesia_ext_sup,mnesia_sup,<0.243.0>]

Trying to fix that resulted in this PR:
erlang/otp#9152
What it does is it calls init_backend of all registered external backends during backup traversal (when we're restoring the tables from backup).
mnesia_rocksdb doesn't seem to like it, somehow it causes it to fail in a way that many of the backing files are missing (the main "CURRENT" file also).

I see the following ways forward:

1. Mnesia does not call init_backend during backup traversal if there are no tables from this backend
   I've actually tried to do that, and it seems to make mnesia_rocksdb happy, but doesn't make mnesia_rocksdb_fallback:run() successful.
   It fails on ?m([], mnesia:dirty_match_object(t, {t,'_','_'})), on line 48. The cause is that during backup traversal after mnesia calls init_backend, it tries to delete external tables, and only later recreates them.
   This fails on mnesia_rocksdb because delete table is as follows:

   delete_table(Alias, Tab) ->
   case whereis_proc(Alias, Tab) of
       undefined ->
           ok;
       Pid when is_pid(Pid) ->
           call(Alias, Tab, delete_table)
   end.
   

   and we end up in a case where table holding process is undefined (because we didn't call load_table yet).
   The workaround would be to call load_table first and then clear it of all objects before restoring from backup, but I'm not happy with such workaround since mnesia_eleveldb allows to call delete_table without loading it first.

2. mnesia_rocksdb must allow init_backend to be called during mnesia startup procedure and work fine after that

Either of those 2 would also require delete_table to work without loading it first.

Could you assist in moving this forward?