Implement PoV Distribution Subsystem (#1344)

* introduce candidatedescriptor type

* add PoVDistribution message type

* loosen bound on PoV Distribution to account for equivocations

* re-export some types from the messages module

* begin PoV Distribution subsystem

* remove redundant index from PoV distribution

* define state machine for pov distribution

* handle overseer signals

* set up control flow

* remove `ValidatorStatement` section

* implement PoV fetching

* implement distribution logic

* add missing `

* implement some network bridge event handlers

* stub for message processing, handle our view change

* control flow for handling messages

* handle `awaiting` message

* handle any incoming PoVs and redistribute

* actually provide a subsystem implementation

* remove set-builder notation

* begin testing PoV distribution

* test that we send awaiting messages only to peers with same view

* ensure we distribute awaited PoVs to peers on view changes

* test that peers can complete fetch and are rewarded

* test some reporting logic

* ensure peer is reported for flooding

* test punishing peers diverging from awaited protocol

* test that we eagerly complete peers' awaited PoVs based on what we receive

* test that we prune the awaited set after receiving

* expand pov-distribution in guide to match a change I made

* remove unneeded import

polkadot/node/subsystem/src/messages.rs

@@ -28,11 +28,14 @@ use polkadot_primitives::{BlockNumber, Hash, Signature};
 use polkadot_primitives::parachain::{
 	AbridgedCandidateReceipt, PoVBlock, ErasureChunk, BackedCandidate, Id as ParaId,
 	SignedAvailabilityBitfield, SigningContext, ValidatorId, ValidationCode, ValidatorIndex,
+	CandidateDescriptor,
 };
 use polkadot_node_primitives::{
 	MisbehaviorReport, SignedFullStatement, View, ProtocolId,
 };
 
+use std::sync::Arc;
+
 pub use sc_network::{ObservedRole, ReputationChange, PeerId};
 
 /// A notification of a new backed candidate.
@@ -214,6 +217,21 @@ pub enum ProvisionerMessage {
 	ProvisionableData(ProvisionableData),
 }
 
+/// Message to the PoV Distribution Subsystem.
+#[derive(Debug)]
+pub enum PoVDistributionMessage {
+	/// Fetch a PoV from the network.
+	///
+	/// This `CandidateDescriptor` should correspond to a candidate seconded under the provided
+	/// relay-parent hash.
+    FetchPoV(Hash, CandidateDescriptor, oneshot::Sender<Arc<PoVBlock>>),
+    /// Distribute a PoV for the given relay-parent and CandidateDescriptor.
+    /// The PoV should correctly hash to the PoV hash mentioned in the CandidateDescriptor
+    DistributePoV(Hash, CandidateDescriptor, Arc<PoVBlock>),
+    /// An update from the network bridge.
+    NetworkBridgeUpdate(NetworkBridgeEvent),
+}
+
 /// A message type tying together all message types that are used across Subsystems.
 #[derive(Debug)]
 pub enum AllMessages {
@@ -231,6 +249,8 @@ pub enum AllMessages {
 	BitfieldDistribution(BitfieldDistributionMessage),
 	/// Message for the Provisioner subsystem.
 	Provisioner(ProvisionerMessage),
+	/// Message for the PoV Distribution subsystem.
+	PoVDistribution(PoVDistributionMessage),
 	/// Message for the Runtime API subsystem.
 	RuntimeApi(RuntimeApiMessage),
 	/// Message for the availability store subsystem.