Turkey is Sunni and vies for influence in the region, Erdogan won't lose any sleep over the U.S. attacking Iran. He'll wax indignant because that's what he does best, but he'll be looking for geopolitical gain out the situation. He's a whore.
China isn't going to go to the mat for Iran either. And they certainly don't want the U.S. providing lethal arms to Taiwan. And there'll be no PLA in Iran either, with or without nukes. China will see what advantage it can make out of an attack, but putting their own people in to get clocked by the Americans isn't something they'd want; it would pop their little bubble of how effective their military is.
That said, the U.S. isn't going to attack Iran any time soon, they have nothing the U.S. wants. And the U.S. realizes attacking the nuke sites will only delay Iran and its love affair with nuclear weapons, not stop it...well, short of nuking Iran which won't happen because the U.S. has friends downwind. It would make the Saudis happy campers though.
My own opinion is that Iran will tiptoe up to a nuclear weapon so they can toss one together at a moment's notice...unless they think they can produce them without being noticed. They also realize Obama will be gone in two years, so the days of the Joe Biden Bunny World Foreign Policy are likely over. That won't stop Saudi Arabia from attempting to get nukes. Sooner or later, something will slip out and either a terrorist cell will get their hands on a nuke and let it off, or a crazy government will do it for them. The only question is where and how long we have until that happens. N. Korea is unstable or at least their leader is. And Iran's mullahs do not have a lock on their government, the right ideolog in Iran could easily think it a bright idea to send one Israel's way thinking that if Iran is the one to knock off Israel, all the Sunnis will switch to being Shi'ite and they'll be in Muslim shit-heaven.