Add HRMP notification handlers to the xcm-executor (#3696)

Currently the xcm-executor returns an `Unimplemented` error if it
receives any HRMP-related instruction.
What I propose here, which is what we are currently doing in our forked
executor at polimec, is to introduce a trait implemented by the executor
which will handle those instructions.

This way, if parachains want to keep the default behavior, they just use
`()` and it will return unimplemented, but they can also implement their
own logic to establish HRMP channels with other chains in an automated
fashion, without requiring to go through governance.

Our implementation is mentioned in the [polkadot HRMP
docs](https://arc.net/l/quote/hduiivbu), and it was suggested to us to
submit a PR to add these changes to polkadot-sdk.

---------

Co-authored-by: Branislav Kontur <bkontur@gmail.com>
Co-authored-by: command-bot <>

polkadot/xcm/xcm-executor/src/config.rs

@@ -16,7 +16,8 @@
 
 use crate::traits::{
 	AssetExchange, AssetLock, CallDispatcher, ClaimAssets, ConvertOrigin, DropAssets, ExportXcm,
-	FeeManager, OnResponse, ProcessTransaction, ShouldExecute, TransactAsset,
+	FeeManager, HandleHrmpChannelAccepted, HandleHrmpChannelClosing,
+	HandleHrmpNewChannelOpenRequest, OnResponse, ProcessTransaction, ShouldExecute, TransactAsset,
 	VersionChangeNotifier, WeightBounds, WeightTrader,
 };
 use frame_support::{
@@ -114,4 +115,11 @@ pub trait Config {
 
 	/// Transactional processor for XCM instructions.
 	type TransactionalProcessor: ProcessTransaction;
+
+	/// Allows optional logic execution for the `HrmpNewChannelOpenRequest` XCM notification.
+	type HrmpNewChannelOpenRequestHandler: HandleHrmpNewChannelOpenRequest;
+	/// Allows optional logic execution for the `HrmpChannelAccepted` XCM notification.
+	type HrmpChannelAcceptedHandler: HandleHrmpChannelAccepted;
+	/// Allows optional logic execution for the `HrmpChannelClosing` XCM notification.
+	type HrmpChannelClosingHandler: HandleHrmpChannelClosing;
 }

polkadot/xcm/xcm-executor/src/lib.rs

@@ -31,7 +31,8 @@ use xcm::latest::prelude::*;
 pub mod traits;
 use traits::{
 	validate_export, AssetExchange, AssetLock, CallDispatcher, ClaimAssets, ConvertOrigin,
-	DropAssets, Enact, ExportXcm, FeeManager, FeeReason, OnResponse, ProcessTransaction,
+	DropAssets, Enact, ExportXcm, FeeManager, FeeReason, HandleHrmpChannelAccepted,
+	HandleHrmpChannelClosing, HandleHrmpNewChannelOpenRequest, OnResponse, ProcessTransaction,
 	Properties, ShouldExecute, TransactAsset, VersionChangeNotifier, WeightBounds, WeightTrader,
 	XcmAssetTransfers,
 };
@@ -1212,9 +1213,21 @@ impl<Config: config::Config> XcmExecutor<Config> {
 				);
 				Ok(())
 			},
-			HrmpNewChannelOpenRequest { .. } => Err(XcmError::Unimplemented),
-			HrmpChannelAccepted { .. } => Err(XcmError::Unimplemented),
-			HrmpChannelClosing { .. } => Err(XcmError::Unimplemented),
+			HrmpNewChannelOpenRequest { sender, max_message_size, max_capacity } =>
+				Config::TransactionalProcessor::process(|| {
+					Config::HrmpNewChannelOpenRequestHandler::handle(
+						sender,
+						max_message_size,
+						max_capacity,
+					)
+				}),
+			HrmpChannelAccepted { recipient } => Config::TransactionalProcessor::process(|| {
+				Config::HrmpChannelAcceptedHandler::handle(recipient)
+			}),
+			HrmpChannelClosing { initiator, sender, recipient } =>
+				Config::TransactionalProcessor::process(|| {
+					Config::HrmpChannelClosingHandler::handle(initiator, sender, recipient)
+				}),
 		}
 	}
 }

polkadot/xcm/xcm-executor/src/traits/hrmp.rs

@@ -0,0 +1,56 @@
+// 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/>.
+
+use xcm::latest::Result as XcmResult;
+
+/// Executes logic when a `HrmpNewChannelOpenRequest` XCM notification is received.
+pub trait HandleHrmpNewChannelOpenRequest {
+	fn handle(sender: u32, max_message_size: u32, max_capacity: u32) -> XcmResult;
+}
+
+/// Executes optional logic when a `HrmpChannelAccepted` XCM notification is received.
+pub trait HandleHrmpChannelAccepted {
+	fn handle(recipient: u32) -> XcmResult;
+}
+
+/// Executes optional logic when a `HrmpChannelClosing` XCM notification is received.
+pub trait HandleHrmpChannelClosing {
+	fn handle(initiator: u32, sender: u32, recipient: u32) -> XcmResult;
+}
+
+#[impl_trait_for_tuples::impl_for_tuples(30)]
+impl HandleHrmpNewChannelOpenRequest for Tuple {
+	fn handle(sender: u32, max_message_size: u32, max_capacity: u32) -> XcmResult {
+		for_tuples!( #( Tuple::handle(sender, max_message_size, max_capacity)?; )* );
+		Ok(())
+	}
+}
+
+#[impl_trait_for_tuples::impl_for_tuples(30)]
+impl HandleHrmpChannelAccepted for Tuple {
+	fn handle(recipient: u32) -> XcmResult {
+		for_tuples!( #( Tuple::handle(recipient)?; )* );
+		Ok(())
+	}
+}
+
+#[impl_trait_for_tuples::impl_for_tuples(30)]
+impl HandleHrmpChannelClosing for Tuple {
+	fn handle(initiator: u32, sender: u32, recipient: u32) -> XcmResult {
+		for_tuples!( #( Tuple::handle(initiator, sender, recipient)?; )* );
+		Ok(())
+	}
+}

polkadot/xcm/xcm-executor/src/traits/mod.rs

@@ -45,6 +45,10 @@ mod should_execute;
 pub use should_execute::{CheckSuspension, Properties, ShouldExecute};
 mod transact_asset;
 pub use transact_asset::TransactAsset;
+mod hrmp;
+pub use hrmp::{
+	HandleHrmpChannelAccepted, HandleHrmpChannelClosing, HandleHrmpNewChannelOpenRequest,
+};
 mod weight;
 #[deprecated = "Use `sp_runtime::traits::` instead"]
 pub use sp_runtime::traits::{Identity, TryConvertInto as JustTry};