Introduce partial bp-xcm-bridge-hub - just LocalXcmChannelManager

Cargo.toml

@@ -37,6 +37,7 @@ members = [
 	"primitives/relayers",
 	"primitives/runtime",
 	"primitives/test-utils",
+	"primitives/xcm-bridge-hub",
 	"relays/bin-substrate",
 	"relays/client-bridge-hub-kusama",
 	"relays/client-bridge-hub-polkadot",

primitives/xcm-bridge-hub/Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+name = "bp-xcm-bridge-hub"
+description = "Primitives of the xcm-bridge-hub pallet."
+version = "0.1.0"
+authors = ["Parity Technologies <admin@parity.io>"]
+edition = "2018"
+license = "GPL-3.0-or-later WITH Classpath-exception-2.0"
+
+[dependencies]
+# Polkadot Dependencies
+
+xcm = { git = "https://github.com/paritytech/polkadot", branch = "master", default-features = false }
+
+[features]
+default = ["std"]
+std = [
+	"xcm/std",
+]

primitives/xcm-bridge-hub/src/lib.rs

@@ -0,0 +1,75 @@
+// Copyright 2019-2021 Parity Technologies (UK) Ltd.
+// This file is part of Parity Bridges Common.
+
+// Parity Bridges Common 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.
+
+// Parity Bridges Common 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 Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Primitives of the xcm-bridge-hub pallet.
+
+#![cfg_attr(not(feature = "std"), no_std)]
+
+use xcm::latest::prelude::*;
+
+/// A manager of XCM communication channels between the bridge hub and parent/sibling chains
+/// that have opened bridges at this bridge hub.
+///
+/// We use this interface to suspend and resume channels programmatically to implement backpressure
+/// mechanism for bridge queues.
+#[allow(clippy::result_unit_err)] // XCM uses `Result<(), ()>` everywhere
+pub trait LocalXcmChannelManager {
+	// TODO: https://github.com/paritytech/parity-bridges-common/issues/2255
+	// check following assumptions. They are important at least for following cases:
+	// 1) we now close the associated outbound lane when misbehavior is reported. If we'll keep
+	//    handling inbound XCM messages after the `suspend_inbound_channel`, they will be dropped
+	// 2) the sender will be able to enqueue message to othe lanes if we won't stop handling inbound
+	//    XCM immediately. He even may open additional bridges
+
+	/// Stop handling new incoming XCM messages from given bridge `owner` (parent/sibling chain).
+	///
+	/// We assume that the channel will be suspended immediately, but we don't mind if inbound
+	/// messages will keep piling up here for some time. Once this is communicated to the
+	/// `owner` chain (in any form), we expect it to stop sending messages to us and queue
+	/// messages at that `owner` chain instead.
+	///
+	/// This method will be called if we detect a misbehavior in one of bridges, owned by
+	/// the `owner`. We expect that:
+	///
+	/// - no more incoming XCM messages from the `owner` will be processed until further
+	///  `resume_inbound_channel` call;
+	///
+	/// - soon after the call, the channel will switch to the state when incoming messages are
+	///   piling up at the sending chain, not at the bridge hub.
+	///
+	/// This method shall not fail if the channel is already suspended.
+	fn suspend_inbound_channel(owner: MultiLocation) -> Result<(), ()>;
+
+	/// Start handling incoming messages from from given bridge `owner` (parent/sibling chain)
+	/// again.
+	///
+	/// This method is called when the `owner` tries to resume bridge operations after
+	/// resolving "misbehavior" issues. The channel is assumed to be suspended by the previous
+	/// `suspend_inbound_channel` call, however we don't check it anywhere.
+	///
+	/// This method shall not fail if the channel is already resumed.
+	fn resume_inbound_channel(owner: MultiLocation) -> Result<(), ()>;
+}
+
+impl LocalXcmChannelManager for () {
+	fn suspend_inbound_channel(_owner: MultiLocation) -> Result<(), ()> {
+		Ok(())
+	}
+
+	fn resume_inbound_channel(_owner: MultiLocation) -> Result<(), ()> {
+		Err(())
+	}
+}