About the use of ensure_xcm() about polkadot HOT 4 CLOSED

CC @KiChjang

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In order to make use of ensure_xcm, let's first take a look at the code:

pub fn ensure_xcm<OuterOrigin>(o: OuterOrigin) -> Result<MultiLocation, BadOrigin>
// ...
{
	match o.into() {
		Ok(Origin::Xcm(location)) => Ok(location),
		// ...
	}
}

Here you can see that it's trying to convert the OuterOrigin (we call it RuntimeOrigin nowadays) into an Origin::Xcm variant. This means that the MultiLocation origin must have been converted to a FRAME origin before by an origin converter. Currently, there is a single origin converter that does this, and it's called XcmPassthrough, located in the XCM pallet.

In short, ensure_xcm only works if you have specifically configured your OriginConverter to be XcmPassthrough, otherwise you will never be able to get an Origin::Xcm to match against.

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@KiChjang Ok, so that's the case, thanks for the answer.

I would like to ask, for example, if I want to write a pallet with a test_call method in it, and another chain calls it via DescendOrigin+Trancact instruction, what operation can I use to get the para_id, caller, and other useful information of the calling chain in the test_call method ?

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The most direct way of doing so is to include them as parameters to test_method. Outside of that, XcmPassthrough has the MultiLocation of the sender, so you can quickly check and see which parachain it came from by matching agains the Parachain junction.

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