Add Subscription RPC for Grandpa Finality (#5732)

* Rough skeleton for what I think the RPC should look like

* Create channel for sending justifications

Sends finalized header and justification from Grandpa to the
client. This lays the groundwork for hooking into the RPC module.

* WIP: Add subscribers for justifications to Grandpa

Adds the Sender end of a channel into Grandpa, through which notifications
about block finality events can be sent.

* WIP: Add a struct for managing subscriptions

Slightly different approach from the last commit, but same
basic idea. Still a rough sketch, very much doesn't compile yet.

* Make naming more clear and lock data in Arc

* Rough idea of what RPC would look like

* Remove code from previous approach

* Missed some things

* Update client/rpc-api/src/chain/mod.rs

Co-Authored-By: Tomasz Drwięga <tomusdrw@users.noreply.github.com>

* Update client/rpc-api/src/chain/mod.rs

Co-Authored-By: Tomasz Drwięga <tomusdrw@users.noreply.github.com>

* Split justification subscription into sender and receiver halves

* Replace RwLock with a Mutex

* Add sample usage from the Service's point of view

* Remove code that referred to "chain_" RPC

* Use the Justification sender/receivers from Grandpa LinkHalf

* Add some PubSub boilerplate

* Add guiding comments

* TMP: comment out to fix compilation

* Return MetaIoHandler from PubSubHandler in create_full

* Uncomment pubsub methods in rpc handler (fails to build)

* node/rpc: make Metadata concrete in create_full to fix compilation

* node: pass in SubscriptionManger to grandpa rpc handler

* grandpa-rpc: use SubscriptionManger to add subscriber

* grandpa-rpc: attempt at setting up the justification stream (fails to build)

* grandpa-rpc: fix compilation of connecting stream to sink

* grandpa-rpc: implement unsubscribe

* grandpa-rpc: update older tests

* grandpa-rpc: add full prefix to avoid confusing rust-analyzer

* grandpa-rpc: add test for pubsub not available

* grandpa-rpc: tidy up leftover code

* grandpa-rpc: add test for sub and unsub of justifications

* grandpa-rpc: minor stylistic changes

* grandpa-rpc: split unit test

* grandpa-rpc: minor stylistic changes in test

* grandpa-rpc: skip returning future when cancelling

* grandpa-rpc: reuse testing executor from sc-rpc

* grandpa-rpc: don't need to use PubSubHandler in tests

* node-rpc: use MetaIoHandler rather than PubSubHandler

* grandpa: log if getting header failed

* grandpa: move justification channel creation into factory function

* grandpa: make the justification sender optional

* grandpa: fix compilation warnings

* grandpa: move justification notification types to new file

* grandpa-rpc: move JustificationNotification to grandpa-rpc

* grandpa-rpc: move JustificationNotification to its own file

* grandpa: rename justification channel pairs

* grandpa: rename notifier types

* grandpa: pass justification as GrandpaJustification to the rpc module

* Move Metadata to sc-rpc-api

* grandpa-rpc: remove unsed error code

* grandpa: fix bug for checking if channel is closed before sendind

* grandpa-rpc: unit test for sending justifications

* grandpa-rpc: update comments for the pubsub test

* grandpa-rpc: update pubsub tests with more steps

* grandpa-rpc: fix pubsub test

* grandpa-rpc: minor indendation

* grandpa-rpc: decode instead of encode in test

* grandpa: fix review comments

* grandpa: remove unused serde dependency

Co-authored-by: Tomasz Drwięga <tomusdrw@users.noreply.github.com>
Co-authored-by: Jon Häggblad <jon.haggblad@gmail.com>
Co-authored-by: Tomasz Drwięga <tomasz@parity.io>

substrate/client/finality-grandpa/src/import.rs

@@ -44,6 +44,7 @@ use crate::authorities::{AuthoritySet, SharedAuthoritySet, DelayKind, PendingCha
 use crate::consensus_changes::SharedConsensusChanges;
 use crate::environment::finalize_block;
 use crate::justification::GrandpaJustification;
+use crate::notification::GrandpaJustificationSender;
 use std::marker::PhantomData;
 
 /// A block-import handler for GRANDPA.
@@ -62,6 +63,7 @@ pub struct GrandpaBlockImport<Backend, Block: BlockT, Client, SC> {
 	send_voter_commands: TracingUnboundedSender<VoterCommand<Block::Hash, NumberFor<Block>>>,
 	consensus_changes: SharedConsensusChanges<Block::Hash, NumberFor<Block>>,
 	authority_set_hard_forks: HashMap<Block::Hash, PendingChange<Block::Hash, NumberFor<Block>>>,
+	justification_sender: GrandpaJustificationSender<Block>,
 	_phantom: PhantomData<Backend>,
 }
 
@@ -76,6 +78,7 @@ impl<Backend, Block: BlockT, Client, SC: Clone> Clone for
 			send_voter_commands: self.send_voter_commands.clone(),
 			consensus_changes: self.consensus_changes.clone(),
 			authority_set_hard_forks: self.authority_set_hard_forks.clone(),
+			justification_sender: self.justification_sender.clone(),
 			_phantom: PhantomData,
 		}
 	}
@@ -560,6 +563,7 @@ impl<Backend, Block: BlockT, Client, SC> GrandpaBlockImport<Backend, Block, Clie
 		send_voter_commands: TracingUnboundedSender<VoterCommand<Block::Hash, NumberFor<Block>>>,
 		consensus_changes: SharedConsensusChanges<Block::Hash, NumberFor<Block>>,
 		authority_set_hard_forks: Vec<(SetId, PendingChange<Block::Hash, NumberFor<Block>>)>,
+		justification_sender: GrandpaJustificationSender<Block>,
 	) -> GrandpaBlockImport<Backend, Block, Client, SC> {
 		// check for and apply any forced authority set hard fork that applies
 		// to the *current* authority set.
@@ -603,6 +607,7 @@ impl<Backend, Block: BlockT, Client, SC> GrandpaBlockImport<Backend, Block, Clie
 			send_voter_commands,
 			consensus_changes,
 			authority_set_hard_forks,
+			justification_sender,
 			_phantom: PhantomData,
 		}
 	}
@@ -648,6 +653,7 @@ where
 			number,
 			justification.into(),
 			initial_sync,
+			&Some(self.justification_sender.clone()),
 		);
 
 		match result {