Use cets:insert_new/2

And retry

rebar.config

@@ -80,7 +80,7 @@
   {cache_tab, "1.0.30"},
   {segmented_cache, "0.3.0"},
   {worker_pool, "6.0.1"},
-  {cets, {git, "https://github.com/esl/cets.git", {branch, "main"}}},
+  {cets, {git, "https://github.com/esl/cets.git", {branch, "mu-insert-new"}}},
 
   %%% HTTP tools
   {graphql, {git, "https://github.com/esl/graphql-erlang.git", {branch, "master"}}},

rebar.lock

@@ -8,7 +8,7 @@
  {<<"certifi">>,{pkg,<<"certifi">>,<<"2.9.0">>},1},
  {<<"cets">>,
   {git,"https://github.com/esl/cets.git",
-       {ref,"458e2e1df3fb51896fe334385bb0d2c9c53ef87f"}},
+       {ref,"659e0f666782407db99022a4abf3ea8b4dbb0167"}},
   0},
  {<<"cowboy">>,{pkg,<<"cowboy">>,<<"2.9.0">>},0},
  {<<"cowboy_swagger">>,{pkg,<<"cowboy_swagger">>,<<"2.5.1">>},0},

src/muc/mongoose_muc_online_cets.erl

@@ -45,17 +45,23 @@ handle_conflict(_Rec1, Rec2) ->
         Room :: mod_muc:room(),
         Pid :: pid()) -> ok | {exists, pid()} | {error, term()}.
 register_room(HostType, MucHost, Room, Pid) ->
+    register_room(HostType, MucHost, Room, Pid, 3).
+
+register_room(_HostType, _MucHost, _Room, _Pid, 0) ->
+    {error, failed_to_register};
+register_room(HostType, MucHost, Room, Pid, Retries) ->
     Tab = table_name(HostType),
     Rec = {{MucHost, Room}, Pid},
     case find_room_pid(HostType, MucHost, Room) of
-        %% Race condition is possible
-        %% TODO use cets:insert_new/2 once available
-        %% Otherwise cets:insert could overwrite an existing registration
         {ok, OtherPid} ->
             {exists, OtherPid};
         {error, not_found} ->
-            cets:insert(Tab, Rec),
-            ok
+            case cets:insert_new(Tab, Rec) of
+                true ->
+                    ok;
+                false ->
+                     register_room(HostType, MucHost, Room, Pid, Retries - 1)
+            end
     end.
 
 %% Race condition is possible between register and room_destroyed