chore: fix flaky test cm_SUITE open session racecondition

test/emqx_cm_SUITE.erl

@@ -141,11 +141,8 @@ t_open_session_race_condition(_) ->
                      end
                    end,
     N = 1000,
-    [spawn(
-      fun() ->
-              spawn(OpenASession),
-              spawn(OpenASession)
-      end) || _ <- lists:seq(1, N)],
+    Pids = lists:flatten([[spawn_monitor(OpenASession), spawn_monitor(OpenASession)] ||
+                          _ <- lists:seq(1, N)]),
 
     WaitingRecv = fun _Wr(N1, N2, 0) ->
                           {N1, N2};
@@ -158,14 +155,29 @@ t_open_session_race_condition(_) ->
 
     {Succeeded, Failed} = WaitingRecv(0, 0, 2 * N),
     ct:pal("Race condition status: succeeded=~p failed=~p~n", [Succeeded, Failed]),
+    ?assertEqual(2 * N, length(Pids)),
+    WaitForDowns =
+        fun _Wd([{Pid, _Ref}]) -> Pid;
+            _Wd(Pids0) ->
+                receive
+                    {'DOWN', DownRef, process, DownPid, _} ->
+                        ?assert(lists:member({DownPid, DownRef}, Pids0)),
+                        _Wd(lists:delete({DownPid, DownRef}, Pids0))
+                after
+                    10000 ->
+                        exit(timeout)
+                end
+        end,
+    Winner = WaitForDowns(Pids),
 
     ?assertMatch([_], ets:lookup(emqx_channel, ClientId)),
-    [Pid] = emqx_cm:lookup_channels(ClientId),
-    ?assertMatch([_], ets:lookup(emqx_channel_conn, {ClientId, Pid})),
+    ?assertEqual([Winner], emqx_cm:lookup_channels(ClientId)),
+    ?assertMatch([_], ets:lookup(emqx_channel_conn, {ClientId, Winner})),
     ?assertMatch([_], ets:lookup(emqx_channel_registry, ClientId)),
 
-    exit(Pid, kill),
-    timer:sleep(100), %% TODO deterministic
+    exit(Winner, kill),
+    receive {'DOWN', _, process, Winner, _} -> ok end,
+    ignored = gen_server:call(emqx_cm, ignore, infinity), %% sync
     ?assertEqual([], emqx_cm:lookup_channels(ClientId)).
 
 t_discard_session(_) ->