In the US (and other countries) the door is locked but there is a PIN to open it that the cabin crew have access to. Even then, there's a time delay to the door

In the US (and other countries) the door is locked but there is a PIN to open it that the cabin crew have access to. Even then, there's a time delay to the door opening, during which people in the cockpit are notified and can reject the request. That means that no food cart should ever be needed to block the door. But the cockpit should never be totally empty anyway, as surely only one of the two crew should be using the toilet at a time. (Note: I'm not a pilot, just a reader of Admiral Cloudberg's blog.)