Add Basic XCM (#1934)

* basic setup

* fix compile

* add back xcmsink

* Update primitives.rs

* add downward xcm for sudo

* Update paras_sudo_wrapper.rs

* Update Cargo.lock

* some cleanups

* Add error log

Co-authored-by: Sergei Shulepov <sergei@parity.io>

polkadot/runtime/parachains/src/ump.rs

@@ -52,6 +52,42 @@ impl UmpSink for () {
 	}
 }
 
+/// A specific implementation of a UmpSink where messages are in the XCM format
+/// and will be forwarded to the XCM Executor.
+pub struct XcmSink<Config>(sp_std::marker::PhantomData<Config>);
+
+impl<Config: xcm_executor::Config> UmpSink for XcmSink<Config> {
+	fn process_upward_message(origin: ParaId, msg: Vec<u8>) -> Weight {
+		use parity_scale_codec::Decode;
+		use xcm::VersionedXcm;
+		use xcm::v0::{Junction, MultiLocation, ExecuteXcm};
+		use xcm_executor::XcmExecutor;
+
+		let weight: Weight = 0;
+
+		if let Ok(versioned_xcm_message) = VersionedXcm::decode(&mut &msg[..]) {
+			match versioned_xcm_message {
+				VersionedXcm::V0(xcm_message) => {
+					let xcm_junction: Junction = Junction::Parachain { id: origin.into() };
+					let xcm_location: MultiLocation = xcm_junction.into();
+					// TODO: Do something with result.
+					let _result = XcmExecutor::<Config>::execute_xcm(xcm_location, xcm_message);
+				}
+			}
+		} else {
+			frame_support::debug::error!(
+				target: "xcm",
+				"Failed to decode versioned XCM from upward message.",
+			);
+		}
+
+		// TODO: to be sound, this implementation must ensure that returned (and thus consumed)
+		// weight is limited to some small portion of the total block weight (as a ballpark, 1/4, 1/8
+		// or lower).
+		weight
+	}
+}
+
 /// An error returned by [`check_upward_messages`] that indicates a violation of one of acceptance
 /// criteria rules.
 pub enum AcceptanceCheckErr {