It drives me nuts when people are rude when the person wasn't being rude to them. I mean, I get that it's work and they don't want to be there. But still. You're going to be there either way, so why not try to have it go as nice and smooth as possible for everyone, you know? When it happens to me I try to stay calm, and nice, and finish up as fast as I can. I don't complain about them or anything in case something was a misunderstanding or in case it's an unusually bad day for them. If someone is rude to them first though, then I don't blame them for being rude back.

When a worker is rude to me I go the route of being instantly that sort of stereotypical super-sweet motherly figure thing, like "Oh, you poor dear, you must be having a simply awful time of it, if you're reduced to lashing out at customers! How terrible for you," et cetera.

I find it separates them out pretty quick.

The ones who have no real reason to be rude feel patronized and get all defensive and hate it but you've essentially just called them out on it so they have nowhere to go with it. (Hint: don't ever actually use the phrase "bless your heart" on them, because that's a declaration of war.)

The ones who really HAVE had a rough time of it lately are usually really grateful for the understanding -I've had a couple of people burst into tears and cry on my shoulder, and that's actually kind of good because if the manager/shift boss comes over and starts making trouble I can always do the whole routine of "isn't this place so amazing, you all make me feel like family here, I'm so flattered, this is my favorite place to come, it just feels like a real community; if one of your workers feels such loyalty that they come in while dealing with all this, you must be someone amazing to inspire that kind of closeness," et cetera.
Which typically turns them right around from "Must contain potentially problematic worker/situation" into flipping right into "I have a chance to play the hero" mode. Which means that poor fried worker actually gets a chance to decompress a little.

It's worked great so far.