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/messages/src/target_chain.rs

@@ -18,7 +18,7 @@
 
 use crate::{LaneId, Message, MessageData, MessageKey, OutboundLaneData};
 
-use bp_runtime::Size;
+use bp_runtime::{messages::MessageDispatchResult, Size};
 use codec::{Decode, Encode, Error as CodecError};
 use frame_support::{weights::Weight, Parameter, RuntimeDebug};
 use sp_std::{collections::btree_map::BTreeMap, fmt::Debug, prelude::*};
@@ -84,7 +84,7 @@ pub trait SourceHeaderChain<Fee> {
 }
 
 /// Called when inbound message is received.
-pub trait MessageDispatch<Fee> {
+pub trait MessageDispatch<AccountId, Fee> {
 	/// Decoded message payload type. Valid message may contain invalid payload. In this case
 	/// message is delivered, but dispatch fails. Therefore, two separate types of payload
 	/// (opaque `MessagePayload` used in delivery and this `DispatchPayload` used in dispatch).
@@ -100,7 +100,13 @@ pub trait MessageDispatch<Fee> {
 	///
 	/// It is up to the implementers of this trait to determine whether the message
 	/// is invalid (i.e. improperly encoded, has too large weight, ...) or not.
-	fn dispatch(message: DispatchMessage<Self::DispatchPayload, Fee>);
+	///
+	/// If your configuration allows paying dispatch fee at the target chain, then
+	/// it must be paid inside this method to the `relayer_account`.
+	fn dispatch(
+		relayer_account: &AccountId,
+		message: DispatchMessage<Self::DispatchPayload, Fee>,
+	) -> MessageDispatchResult;
 }
 
 impl<Message> Default for ProvedLaneMessages<Message> {
@@ -149,12 +155,17 @@ impl<Fee> SourceHeaderChain<Fee> for ForbidInboundMessages {
 	}
 }
 
-impl<Fee> MessageDispatch<Fee> for ForbidInboundMessages {
+impl<AccountId, Fee> MessageDispatch<AccountId, Fee> for ForbidInboundMessages {
 	type DispatchPayload = ();
 
 	fn dispatch_weight(_message: &DispatchMessage<Self::DispatchPayload, Fee>) -> Weight {
 		Weight::MAX
 	}
 
-	fn dispatch(_message: DispatchMessage<Self::DispatchPayload, Fee>) {}
+	fn dispatch(_: &AccountId, _: DispatchMessage<Self::DispatchPayload, Fee>) -> MessageDispatchResult {
+		MessageDispatchResult {
+			unspent_weight: 0,
+			dispatch_fee_paid_during_dispatch: false,
+		}
+	}
 }