xcm_sender.rs

// Copyright (C) Parity Technologies (UK) Ltd.
// This file is part of Polkadot.

// Polkadot is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.

// Polkadot is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.

//! XCM sender for relay chain.

use frame_support::traits::Get;
use parity_scale_codec::Encode;
use primitives::Id as ParaId;
use runtime_parachains::{
	configuration::{self, HostConfiguration},
	dmp,
};
use sp_std::{marker::PhantomData, prelude::*};
use xcm::prelude::*;
use SendError::*;

/// Simple value-bearing trait for determining/expressing the assets required to be paid for a
/// messages to be delivered to a parachain.
pub trait PriceForParachainDelivery {
	/// Return the assets required to deliver `message` to the given `para` destination.
	fn price_for_parachain_delivery(para: ParaId, message: &Xcm<()>) -> MultiAssets;
}
impl PriceForParachainDelivery for () {
	fn price_for_parachain_delivery(_: ParaId, _: &Xcm<()>) -> MultiAssets {
		MultiAssets::new()
	}
}

/// Implementation of `PriceForParachainDelivery` which returns a fixed price.
pub struct ConstantPrice<T>(sp_std::marker::PhantomData<T>);
impl<T: Get<MultiAssets>> PriceForParachainDelivery for ConstantPrice<T> {
	fn price_for_parachain_delivery(_: ParaId, _: &Xcm<()>) -> MultiAssets {
		T::get()
	}
}

/// XCM sender for relay chain. It only sends downward message.
pub struct ChildParachainRouter<T, W, P>(PhantomData<(T, W, P)>);

impl<T: configuration::Config + dmp::Config, W: xcm::WrapVersion, P: PriceForParachainDelivery>
	SendXcm for ChildParachainRouter<T, W, P>
{
	type Ticket = (HostConfiguration<T::BlockNumber>, ParaId, Vec<u8>);

	fn validate(
		dest: &mut Option<MultiLocation>,
		msg: &mut Option<Xcm<()>>,
	) -> SendResult<(HostConfiguration<T::BlockNumber>, ParaId, Vec<u8>)> {
		let d = dest.take().ok_or(MissingArgument)?;
		let id = if let MultiLocation { parents: 0, interior: X1(Parachain(id)) } = &d {
			*id
		} else {
			*dest = Some(d);
			return Err(NotApplicable)
		};

		// Downward message passing.
		let xcm = msg.take().ok_or(MissingArgument)?;
		let config = <configuration::Pallet<T>>::config();
		let para = id.into();
		let price = P::price_for_parachain_delivery(para, &xcm);
		let blob = W::wrap_version(&d, xcm).map_err(|()| DestinationUnsupported)?.encode();
		<dmp::Pallet<T>>::can_queue_downward_message(&config, &para, &blob)
			.map_err(Into::<SendError>::into)?;

		Ok(((config, para, blob), price))
	}

	fn deliver(
		(config, para, blob): (HostConfiguration<T::BlockNumber>, ParaId, Vec<u8>),
	) -> Result<XcmHash, SendError> {
		let hash = sp_io::hashing::blake2_256(&blob[..]);
		<dmp::Pallet<T>>::queue_downward_message(&config, para, blob)
			.map(|()| hash)
			.map_err(|_| SendError::Transport(&"Error placing into DMP queue"))
	}
}