pay dispatch fee at target chain (#911)

* pay dispatch fee at target chain

* refund unspent dispatch weight to messages relayer

* test that transfer actually happens

* pay-at-target-cchain benchmarks + fix previous benchmarks (invalid signature)

* include/exclude pay-dispatch-fee weight from delivery_and_dispatch_fee/delivery tx cost

* remvoe some redundant traces

* enum DispatchFeePayment {}

* typo

* update docs

* (revert removal of valid check)

* Update modules/messages/src/benchmarking.rs

Co-authored-by: Tomasz Drwięga <tomusdrw@users.noreply.github.com>

* Update modules/messages/src/benchmarking.rs

Co-authored-by: Tomasz Drwięga <tomusdrw@users.noreply.github.com>

* Update modules/messages/src/benchmarking.rs

Co-authored-by: Tomasz Drwięga <tomusdrw@users.noreply.github.com>

* Update modules/messages/src/benchmarking.rs

Co-authored-by: Tomasz Drwięga <tomusdrw@users.noreply.github.com>

Co-authored-by: Tomasz Drwięga <tomusdrw@users.noreply.github.com>
Co-authored-by: Tomasz Drwięga <tomasz@parity.io>

bridges/primitives/runtime/src/lib.rs

@@ -29,6 +29,8 @@ pub use storage_proof::{Error as StorageProofError, StorageProofChecker};
 #[cfg(feature = "std")]
 pub use storage_proof::craft_valid_storage_proof;
 
+pub mod messages;
+
 mod chain;
 mod storage_proof;
 

bridges/primitives/runtime/src/messages.rs

@@ -0,0 +1,50 @@
+// 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 that may be used by different message delivery and dispatch mechanisms.
+
+use codec::{Decode, Encode};
+use frame_support::{weights::Weight, RuntimeDebug};
+
+/// Where message dispatch fee is paid?
+#[derive(Encode, Decode, RuntimeDebug, Clone, PartialEq, Eq)]
+pub enum DispatchFeePayment {
+	/// The dispacth fee is paid at the source chain.
+	AtSourceChain,
+	/// The dispatch fee is paid at the target chain.
+	///
+	/// The fee will be paid right before the message is dispatched. So in case of any other
+	/// issues (like invalid call encoding, invalid signature, ...) the dispatch module won't
+	/// do any direct transfers. Instead, it'll return fee related to this message dispatch to the
+	/// relayer.
+	AtTargetChain,
+}
+
+/// Message dispatch result.
+#[derive(Encode, Decode, RuntimeDebug, Clone, PartialEq, Eq)]
+pub struct MessageDispatchResult {
+	/// Unspent dispatch weight. This weight that will be deducted from total delivery transaction
+	/// weight, thus reducing the transaction cost. This shall not be zero in (at least) two cases:
+	///
+	/// 1) if message has been dispatched successfully, but post-dispatch weight is less than
+	///    the weight, declared by the message sender;
+	/// 2) if message has not been dispatched at all.
+	pub unspent_weight: Weight,
+	/// Whether the message dispatch fee has been paid during dispatch. This will be true if your
+	/// configuration supports pay-dispatch-fee-at-target-chain option and message sender has enabled
+	/// this option.
+	pub dispatch_fee_paid_during_dispatch: bool,
+}