getinfo/event to learn what circid you would use if you get a stream for ip:port
Some users want to know what country their connections would appear as if they use Tor right now. They want to display the output in e.g. their browser window status bar, or maybe somewhere in Vidalia.
There are some Firefox extensions that solve this by doing periodic connections to some whats-my-ip service, and put a flag in the corner. Doing that via Tor is bad news for two reasons though -- first because the address you used a while ago may not be the address you'd use next, and second because this query would cause Tor to use circuits even when you're not really using them, so if you query it every 10 minutes you add a lot of new circuit load on the network.
So we should refactor the circuit choosing code so the controller can ask, for a given ip:port, what circuit Tor would pick for it.
We might also want an event that triggers every time we change what our answer would be. Then we could imagine a Torbutton-style controller that changes the flag in the browser corner.