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/message-dispatch/src/lib.rs

@@ -19,7 +19,10 @@
 #![cfg_attr(not(feature = "std"), no_std)]
 #![warn(missing_docs)]
 
-use bp_runtime::{ChainId, Size};
+use bp_runtime::{
+	messages::{DispatchFeePayment, MessageDispatchResult},
+	ChainId, Size,
+};
 use codec::{Decode, Encode};
 use frame_support::RuntimeDebug;
 use sp_std::prelude::*;
@@ -31,7 +34,7 @@ pub type Weight = u64;
 pub type SpecVersion = u32;
 
 /// A generic trait to dispatch arbitrary messages delivered over the bridge.
-pub trait MessageDispatch<MessageId> {
+pub trait MessageDispatch<AccountId, MessageId> {
 	/// A type of the message to be dispatched.
 	type Message: codec::Decode;
 
@@ -52,7 +55,15 @@ pub trait MessageDispatch<MessageId> {
 	/// a sign that some other component has rejected the message even before it has
 	/// reached `dispatch` method (right now this may only be caused if we fail to decode
 	/// the whole message).
-	fn dispatch(source_chain: ChainId, target_chain: ChainId, id: MessageId, message: Result<Self::Message, ()>);
+	///
+	/// Returns unspent dispatch weight.
+	fn dispatch<P: FnOnce(&AccountId, Weight) -> Result<(), ()>>(
+		source_chain: ChainId,
+		target_chain: ChainId,
+		id: MessageId,
+		message: Result<Self::Message, ()>,
+		pay_dispatch_fee: P,
+	) -> MessageDispatchResult;
 }
 
 /// Origin of a Call when it is dispatched on the target chain.
@@ -110,6 +121,8 @@ pub struct MessagePayload<SourceChainAccountId, TargetChainAccountPublic, Target
 	pub weight: Weight,
 	/// Call origin to be used during dispatch.
 	pub origin: CallOrigin<SourceChainAccountId, TargetChainAccountPublic, TargetChainSignature>,
+	/// Where message dispatch fee is paid?
+	pub dispatch_fee_payment: DispatchFeePayment,
 	/// The call itself.
 	pub call: Call,
 }