Reward validators for participating in parachains (#2089)

* plumbing for rewarding backers

* give validators reward points for participating

* fix tests

* add bitfield rewarding

* add mocks for backing rewards

* add testing for backing & availability rewards

* implement RewardValidators on top of staking

* add to test-runtime and rococo

* add to test-runtime & rococo

* point to source on rewards values

* fix common tests

* do not reward availability anymore

polkadot/runtime/parachains/src/reward_points.rs

@@ -0,0 +1,55 @@
+// Copyright 2020 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/>.
+
+//! An implementation of the `RewardValidators` trait used by `inclusion` that employs
+//! `pallet-staking` to compute the rewards.
+//!
+//! Based on https://w3f-research.readthedocs.io/en/latest/polkadot/Token%20Economics.html
+//! which doesn't currently mention availability bitfields. As such, we don't reward them
+//! for the time being, although we will build schemes to do so in the future.
+
+use primitives::v1::ValidatorIndex;
+use pallet_staking::SessionInterface;
+
+/// The amount of era points given by backing a candidate that is included.
+pub const BACKING_POINTS: u32 = 20;
+
+/// Rewards validators for participating in parachains with era points in pallet-staking.
+pub struct RewardValidatorsWithEraPoints<C>(sp_std::marker::PhantomData<C>);
+
+fn reward_by_indices<C, I>(points: u32, indices: I) where
+	C: pallet_staking::Config,
+	I: IntoIterator<Item = ValidatorIndex>
+{
+	// Fetch the validators from the _session_ because sessions are offset from eras
+	// and we are rewarding for behavior in current session.
+	let validators = C::SessionInterface::validators();
+	let rewards = indices.into_iter()
+		.filter_map(|i| validators.get(i as usize).map(|v| v.clone()))
+		.map(|v| (v, points));
+
+	<pallet_staking::Module<C>>::reward_by_ids(rewards);
+}
+
+impl<C> crate::inclusion::RewardValidators for RewardValidatorsWithEraPoints<C>
+	where C: pallet_staking::Config
+{
+	fn reward_backing(validators: impl IntoIterator<Item=ValidatorIndex>) {
+		reward_by_indices::<C, _>(BACKING_POINTS, validators);
+	}
+
+	fn reward_bitfields(_validators: impl IntoIterator<Item=ValidatorIndex>) { }
+}