feat(persistent_sessions): don't use rocksdb when unavailable

CHANGES-5.0.md

@@ -16,6 +16,7 @@
 * Fix bad swagger format. [#8517](https://github.com/emqx/emqx/pull/8517)
 * Fix `chars_limit` is not working when `formatter` is `json`. [#8518](http://github.com/emqx/emqx/pull/8518)
 * Ensuring that exhook dispatches the client events are sequential. [#8530](https://github.com/emqx/emqx/pull/8530)
+* Avoid using RocksDB backend for persistent sessions when such backend is unavailable. [#8528](https://github.com/emqx/emqx/pull/8528)
 
 ## Enhancements
 

apps/emqx/src/persistent_session/emqx_persistent_session_backend_builtin.erl

@@ -131,16 +131,23 @@ storage_properties(_, Backend) when ?IS_ETS(Backend) ->
 storage_properties(_, _) ->
     [].
 
+%% Dialyzer sees the compiled literal in
+%% `mria:rocksdb_backend_available/0' and complains about the
+%% complementar match arm...
+-dialyzer({no_match, table_type/1}).
 -spec table_type(atom()) -> mria_table_type().
 table_type(Table) ->
     DiscPersistence = emqx_config:get([?cfg_root, on_disc]),
     RamCache = get_overlayed(Table, ram_cache),
-    case {DiscPersistence, RamCache} of
-        {true, true} ->
+    RocksDBAvailable = mria:rocksdb_backend_available(),
+    case {DiscPersistence, RamCache, RocksDBAvailable} of
+        {true, true, _} ->
             disc_copies;
-        {true, false} ->
+        {true, false, true} ->
             rocksdb_copies;
-        {false, _} ->
+        {true, false, false} ->
+            disc_copies;
+        {false, _, _} ->
             ram_copies
     end.