Bug #34857411 : regression - slow connections/telnet block many other statements

for connect_timeout seconds, causing pileups

[post-push-fix] Fixing the sporadically failing testcase on PB2

Approved by Harin Vadodaria <harin.vadodaria@oracle.com> over Slack.

Author: Sai Tharun Ambati

sql/auth/sql_authentication.cc

@@ -2196,6 +2196,11 @@ acl_authenticate(THD *thd, enum_server_command command)
   compile_time_assert(MYSQL_USERNAME_LENGTH == USERNAME_LENGTH);
   assert(command == COM_CONNECT || command == COM_CHANGE_USER);
 
+  DBUG_EXECUTE_IF("acl_authenticate_begin", {
+    const char act[] = "now SIGNAL conn2_in_acl_auth WAIT_FOR conn1_reached_kill";
+    assert(!debug_sync_set_action(current_thd, STRING_WITH_LEN(act)));
+  });
+
   server_mpvio_initialize(thd, &mpvio, &charset_adapter);
   /*
     Clear thd->db as it points to something, that will be freed when