If client for some reason does not reply to the callback request for dbcrypt key (for example - segfault in callbak code) server waits for reply packet for 60 sec and may be 60 more sec for one more callback. During this time server can process other users' requests except attach/detach database and can't exit/restart normally.
During this time server *can* process other users' requests but not all - attach and detach are impossible. Existing attachments continue work normally. There is mutex locked that is in use during attach and detach.