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/Cargo.lock

@@ -3548,6 +3548,7 @@ dependencies = [
  "futures 0.3.5",
  "hex-literal",
  "jsonrpc-core",
+ "jsonrpc-pubsub",
  "log",
  "nix",
  "node-executor",
@@ -3680,6 +3681,7 @@ name = "node-rpc"
 version = "2.0.0-rc5"
 dependencies = [
  "jsonrpc-core",
+ "jsonrpc-pubsub",
  "node-primitives",
  "node-runtime",
  "pallet-contracts-rpc",
@@ -3698,7 +3700,6 @@ dependencies = [
  "sp-blockchain",
  "sp-consensus",
  "sp-consensus-babe",
- "sp-runtime",
  "sp-transaction-pool",
  "substrate-frame-rpc-system",
 ]
@@ -6622,11 +6623,23 @@ dependencies = [
  "jsonrpc-core",
  "jsonrpc-core-client",
  "jsonrpc-derive",
+ "jsonrpc-pubsub",
+ "lazy_static",
  "log",
+ "parity-scale-codec",
+ "sc-block-builder",
  "sc-finality-grandpa",
+ "sc-network-test",
+ "sc-rpc",
  "serde",
  "serde_json",
+ "sp-blockchain",
+ "sp-consensus",
  "sp-core",
+ "sp-finality-grandpa",
+ "sp-keyring",
+ "sp-runtime",
+ "substrate-test-runtime-client",
 ]
 
 [[package]]

substrate/bin/node-template/node/src/service.rs

@@ -113,7 +113,7 @@ pub fn new_full(config: Configuration) -> Result<TaskManager, ServiceError> {
 		let client = client.clone();
 		let pool = transaction_pool.clone();
 
-		Box::new(move |deny_unsafe| {
+		Box::new(move |deny_unsafe, _| {
 			let deps = crate::rpc::FullDeps {
 				client: client.clone(),
 				pool: pool.clone(),
@@ -278,7 +278,7 @@ pub fn new_light(config: Configuration) -> Result<TaskManager, ServiceError> {
 		transaction_pool,
 		task_manager: &mut task_manager,
 		on_demand: Some(on_demand),
-		rpc_extensions_builder: Box::new(|_| ()),
+		rpc_extensions_builder: Box::new(|_, _| ()),
 		telemetry_connection_sinks: sc_service::TelemetryConnectionSinks::default(),
 		config,
 		client,

substrate/bin/node/cli/Cargo.toml

@@ -39,6 +39,7 @@ serde = { version = "1.0.102", features = ["derive"] }
 futures = { version = "0.3.1", features = ["compat"] }
 hex-literal = "0.2.1"
 jsonrpc-core = "14.2.0"
+jsonrpc-pubsub = "14.2.0"
 log = "0.4.8"
 rand = "0.7.2"
 structopt = { version = "0.3.8", optional = true }

substrate/bin/node/cli/src/service.rs

@@ -49,7 +49,10 @@ pub fn new_partial(config: &Configuration) -> Result<sc_service::PartialComponen
 	sp_consensus::DefaultImportQueue<Block, FullClient>,
 	sc_transaction_pool::FullPool<Block, FullClient>,
 	(
-		impl Fn(node_rpc::DenyUnsafe) -> node_rpc::IoHandler,
+		impl Fn(
+			node_rpc::DenyUnsafe,
+			jsonrpc_pubsub::manager::SubscriptionManager
+		) -> node_rpc::IoHandler,
 		(
 			sc_consensus_babe::BabeBlockImport<Block, FullClient, FullGrandpaBlockImport>,
 			grandpa::LinkHalf<Block, FullClient, FullSelectChain>,
@@ -101,6 +104,7 @@ pub fn new_partial(config: &Configuration) -> Result<sc_service::PartialComponen
 	let (rpc_extensions_builder, rpc_setup) = {
 		let (_, grandpa_link, babe_link) = &import_setup;
 
+		let justification_stream = grandpa_link.justification_stream();
 		let shared_authority_set = grandpa_link.shared_authority_set().clone();
 		let shared_voter_state = grandpa::SharedVoterState::empty();
 
@@ -114,7 +118,7 @@ pub fn new_partial(config: &Configuration) -> Result<sc_service::PartialComponen
 		let select_chain = select_chain.clone();
 		let keystore = keystore.clone();
 
-		let rpc_extensions_builder = move |deny_unsafe| {
+		let rpc_extensions_builder = move |deny_unsafe, subscriptions| {
 			let deps = node_rpc::FullDeps {
 				client: client.clone(),
 				pool: pool.clone(),
@@ -128,6 +132,8 @@ pub fn new_partial(config: &Configuration) -> Result<sc_service::PartialComponen
 				grandpa: node_rpc::GrandpaDeps {
 					shared_voter_state: shared_voter_state.clone(),
 					shared_authority_set: shared_authority_set.clone(),
+					justification_stream: justification_stream.clone(),
+					subscriptions,
 				},
 			};
 

substrate/bin/node/rpc/Cargo.toml

@@ -11,25 +11,25 @@ repository = "https://github.com/paritytech/substrate/"
 targets = ["x86_64-unknown-linux-gnu"]
 
 [dependencies]
-sc-client-api = { version = "2.0.0-rc5", path = "../../../client/api" }
-sc-rpc = { version = "2.0.0-rc5", path = "../../../client/rpc" }
 jsonrpc-core = "14.2.0"
+jsonrpc-pubsub = "14.2.0"
 node-primitives = { version = "2.0.0-rc5", path = "../primitives" }
 node-runtime = { version = "2.0.0-rc5", path = "../runtime" }
-sp-runtime = { version = "2.0.0-rc5", path = "../../../primitives/runtime" }
-sp-api = { version = "2.0.0-rc5", path = "../../../primitives/api" }
 pallet-contracts-rpc = { version = "0.8.0-rc5", path = "../../../frame/contracts/rpc/" }
 pallet-transaction-payment-rpc = { version = "2.0.0-rc5", path = "../../../frame/transaction-payment/rpc/" }
-substrate-frame-rpc-system = { version = "2.0.0-rc5", path = "../../../utils/frame/rpc/system" }
-sp-transaction-pool = { version = "2.0.0-rc5", path = "../../../primitives/transaction-pool" }
+sc-client-api = { version = "2.0.0-rc5", path = "../../../client/api" }
 sc-consensus-babe = { version = "0.8.0-rc5", path = "../../../client/consensus/babe" }
 sc-consensus-babe-rpc = { version = "0.8.0-rc5", path = "../../../client/consensus/babe/rpc" }
-sp-consensus-babe = { version = "0.8.0-rc5", path = "../../../primitives/consensus/babe" }
-sc-keystore = { version = "2.0.0-rc5", path = "../../../client/keystore" }
 sc-consensus-epochs = { version = "0.8.0-rc5", path = "../../../client/consensus/epochs" }
-sp-consensus = { version = "0.8.0-rc5", path = "../../../primitives/consensus/common" }
-sp-blockchain = { version = "2.0.0-rc5", path = "../../../primitives/blockchain" }
 sc-finality-grandpa = { version = "0.8.0-rc5", path = "../../../client/finality-grandpa" }
 sc-finality-grandpa-rpc = { version = "0.8.0-rc5", path = "../../../client/finality-grandpa/rpc" }
+sc-keystore = { version = "2.0.0-rc5", path = "../../../client/keystore" }
 sc-rpc-api = { version = "0.8.0-rc5", path = "../../../client/rpc-api" }
+sc-rpc = { version = "2.0.0-rc5", path = "../../../client/rpc" }
+sp-api = { version = "2.0.0-rc5", path = "../../../primitives/api" }
 sp-block-builder = { version = "2.0.0-rc5", path = "../../../primitives/block-builder" }
+sp-blockchain = { version = "2.0.0-rc5", path = "../../../primitives/blockchain" }
+sp-consensus = { version = "0.8.0-rc5", path = "../../../primitives/consensus/common" }
+sp-consensus-babe = { version = "0.8.0-rc5", path = "../../../primitives/consensus/babe" }
+sp-transaction-pool = { version = "2.0.0-rc5", path = "../../../primitives/transaction-pool" }
+substrate-frame-rpc-system = { version = "2.0.0-rc5", path = "../../../utils/frame/rpc/system" }

substrate/bin/node/rpc/src/lib.rs

@@ -32,20 +32,21 @@
 
 use std::sync::Arc;
 
+use jsonrpc_pubsub::manager::SubscriptionManager;
 use node_primitives::{Block, BlockNumber, AccountId, Index, Balance, Hash};
-use sp_api::ProvideRuntimeApi;
-use sp_transaction_pool::TransactionPool;
-use sp_blockchain::{Error as BlockChainError, HeaderMetadata, HeaderBackend};
-use sp_consensus::SelectChain;
-use sc_keystore::KeyStorePtr;
-use sp_consensus_babe::BabeApi;
-use sc_consensus_epochs::SharedEpochChanges;
 use sc_consensus_babe::{Config, Epoch};
 use sc_consensus_babe_rpc::BabeRpcHandler;
-use sc_finality_grandpa::{SharedVoterState, SharedAuthoritySet};
+use sc_consensus_epochs::SharedEpochChanges;
+use sc_finality_grandpa::{SharedVoterState, SharedAuthoritySet, GrandpaJustificationStream};
 use sc_finality_grandpa_rpc::GrandpaRpcHandler;
-use sp_block_builder::BlockBuilder;
+use sc_keystore::KeyStorePtr;
 pub use sc_rpc_api::DenyUnsafe;
+use sp_api::ProvideRuntimeApi;
+use sp_block_builder::BlockBuilder;
+use sp_blockchain::{Error as BlockChainError, HeaderMetadata, HeaderBackend};
+use sp_consensus::SelectChain;
+use sp_consensus_babe::BabeApi;
+use sp_transaction_pool::TransactionPool;
 
 /// Light client extra dependencies.
 pub struct LightDeps<C, F, P> {
@@ -75,6 +76,10 @@ pub struct GrandpaDeps {
 	pub shared_voter_state: SharedVoterState,
 	/// Authority set info.
 	pub shared_authority_set: SharedAuthoritySet<Hash, BlockNumber>,
+	/// Receives notifications about justification events from Grandpa.
+	pub justification_stream: GrandpaJustificationStream<Block>,
+	/// Subscription manager to keep track of pubsub subscribers.
+	pub subscriptions: SubscriptionManager,
 }
 
 /// Full client dependencies.
@@ -97,9 +102,9 @@ pub struct FullDeps<C, P, SC> {
 pub type IoHandler = jsonrpc_core::IoHandler<sc_rpc::Metadata>;
 
 /// Instantiate all Full RPC extensions.
-pub fn create_full<C, P, M, SC>(
+pub fn create_full<C, P, SC>(
 	deps: FullDeps<C, P, SC>,
-) -> jsonrpc_core::IoHandler<M> where
+) -> jsonrpc_core::IoHandler<sc_rpc_api::Metadata> where
 	C: ProvideRuntimeApi<Block>,
 	C: HeaderBackend<Block> + HeaderMetadata<Block, Error=BlockChainError> + 'static,
 	C: Send + Sync + 'static,
@@ -109,7 +114,6 @@ pub fn create_full<C, P, M, SC>(
 	C::Api: BabeApi<Block>,
 	C::Api: BlockBuilder<Block>,
 	P: TransactionPool + 'static,
-	M: jsonrpc_core::Metadata + Default,
 	SC: SelectChain<Block> +'static,
 {
 	use substrate_frame_rpc_system::{FullSystem, SystemApi};
@@ -125,6 +129,7 @@ pub fn create_full<C, P, M, SC>(
 		babe,
 		grandpa,
 	} = deps;
+
 	let BabeDeps {
 		keystore,
 		babe_config,
@@ -133,6 +138,8 @@ pub fn create_full<C, P, M, SC>(
 	let GrandpaDeps {
 		shared_voter_state,
 		shared_authority_set,
+		justification_stream,
+		subscriptions,
 	} = grandpa;
 
 	io.extend_with(
@@ -161,7 +168,12 @@ pub fn create_full<C, P, M, SC>(
 	);
 	io.extend_with(
 		sc_finality_grandpa_rpc::GrandpaApi::to_delegate(
-			GrandpaRpcHandler::new(shared_authority_set, shared_voter_state)
+			GrandpaRpcHandler::new(
+				shared_authority_set,
+				shared_voter_state,
+				justification_stream,
+				subscriptions,
+			)
 		)
 	);
 

substrate/client/finality-grandpa/rpc/Cargo.toml

@@ -8,16 +8,29 @@ edition = "2018"
 license = "GPL-3.0-or-later WITH Classpath-exception-2.0"
 
 [dependencies]
+sc-rpc = { version = "2.0.0-rc5", path = "../../rpc" }
+sp-runtime = { version = "2.0.0-rc5", path = "../../../primitives/runtime" }
 sc-finality-grandpa = { version = "0.8.0-rc5", path = "../" }
 finality-grandpa = { version = "0.12.3", features = ["derive-codec"] }
 jsonrpc-core = "14.2.0"
 jsonrpc-core-client = "14.2.0"
 jsonrpc-derive = "14.2.1"
+jsonrpc-pubsub = "14.2.0"
 futures = { version = "0.3.4", features = ["compat"] }
 serde = { version = "1.0.105", features = ["derive"] }
 serde_json = "1.0.50"
 log = "0.4.8"
 derive_more = "0.99.2"
+parity-scale-codec = { version = "1.3.0", features = ["derive"] }
 
 [dev-dependencies]
+sc-block-builder = { version = "0.8.0-rc5", path = "../../block-builder" }
+sc-network-test = { version = "0.8.0-rc5", path = "../../network/test" }
+sc-rpc = { version = "2.0.0-rc5", path = "../../rpc", features = ["test-helpers"] }
+sp-blockchain = { version = "2.0.0-rc5", path = "../../../primitives/blockchain" }
+sp-consensus = { version = "0.8.0-rc5", path = "../../../primitives/consensus/common" }
 sp-core = { version = "2.0.0-rc5", path = "../../../primitives/core" }
+sp-finality-grandpa = { version = "2.0.0-rc5", path = "../../../primitives/finality-grandpa" }
+sp-keyring = { version = "2.0.0-rc5", path = "../../../primitives/keyring" }
+substrate-test-runtime-client = { version = "2.0.0-rc5",  path = "../../../test-utils/runtime/client" }
+lazy_static = "1.4"

substrate/client/finality-grandpa/rpc/src/lib.rs

@@ -19,13 +19,25 @@
 //! RPC API for GRANDPA.
 #![warn(missing_docs)]
 
-use futures::{FutureExt, TryFutureExt};
+use futures::{FutureExt, TryFutureExt, TryStreamExt, StreamExt};
+use log::warn;
 use jsonrpc_derive::rpc;
+use jsonrpc_pubsub::{typed::Subscriber, SubscriptionId, manager::SubscriptionManager};
+use jsonrpc_core::futures::{
+	sink::Sink as Sink01,
+	stream::Stream as Stream01,
+	future::Future as Future01,
+};
 
 mod error;
+mod notification;
 mod report;
 
+use sc_finality_grandpa::GrandpaJustificationStream;
+use sp_runtime::traits::Block as BlockT;
+
 use report::{ReportAuthoritySet, ReportVoterState, ReportedRoundStates};
+use notification::JustificationNotification;
 
 /// Returned when Grandpa RPC endpoint is not ready.
 pub const NOT_READY_ERROR_CODE: i64 = 1;
@@ -35,48 +47,128 @@ type FutureResult<T> =
 
 /// Provides RPC methods for interacting with GRANDPA.
 #[rpc]
-pub trait GrandpaApi {
+pub trait GrandpaApi<Notification> {
+	/// RPC Metadata
+	type Metadata;
+
 	/// Returns the state of the current best round state as well as the
 	/// ongoing background rounds.
 	#[rpc(name = "grandpa_roundState")]
 	fn round_state(&self) -> FutureResult<ReportedRoundStates>;
+
+	/// Returns the block most recently finalized by Grandpa, alongside
+	/// side its justification.
+	#[pubsub(
+		subscription = "grandpa_justifications",
+		subscribe,
+		name = "grandpa_subscribeJustifications"
+	)]
+	fn subscribe_justifications(
+		&self,
+		metadata: Self::Metadata,
+		subscriber: Subscriber<Notification>
+	);
+
+	/// Unsubscribe from receiving notifications about recently finalized blocks.
+	#[pubsub(
+		subscription = "grandpa_justifications",
+		unsubscribe,
+		name = "grandpa_unsubscribeJustifications"
+	)]
+	fn unsubscribe_justifications(
+		&self,
+		metadata: Option<Self::Metadata>,
+		id: SubscriptionId
+	) -> jsonrpc_core::Result<bool>;
 }
 
 /// Implements the GrandpaApi RPC trait for interacting with GRANDPA.
-pub struct GrandpaRpcHandler<AuthoritySet, VoterState> {
+pub struct GrandpaRpcHandler<AuthoritySet, VoterState, Block: BlockT> {
 	authority_set: AuthoritySet,
 	voter_state: VoterState,
+	justification_stream: GrandpaJustificationStream<Block>,
+	manager: SubscriptionManager,
 }
 
-impl<AuthoritySet, VoterState> GrandpaRpcHandler<AuthoritySet, VoterState> {
-	/// Creates a new GrandpaRpcHander instance.
-	pub fn new(authority_set: AuthoritySet, voter_state: VoterState) -> Self {
+impl<AuthoritySet, VoterState, Block: BlockT> GrandpaRpcHandler<AuthoritySet, VoterState, Block> {
+	/// Creates a new GrandpaRpcHandler instance.
+	pub fn new(
+		authority_set: AuthoritySet,
+		voter_state: VoterState,
+		justification_stream: GrandpaJustificationStream<Block>,
+		manager: SubscriptionManager,
+	) -> Self {
 		Self {
 			authority_set,
 			voter_state,
+			justification_stream,
+			manager,
 		}
 	}
 }
 
-impl<AuthoritySet, VoterState> GrandpaApi for GrandpaRpcHandler<AuthoritySet, VoterState>
+impl<AuthoritySet, VoterState, Block> GrandpaApi<JustificationNotification>
+	for GrandpaRpcHandler<AuthoritySet, VoterState, Block>
 where
 	VoterState: ReportVoterState + Send + Sync + 'static,
 	AuthoritySet: ReportAuthoritySet + Send + Sync + 'static,
+	Block: BlockT,
 {
+	type Metadata = sc_rpc::Metadata;
+
 	fn round_state(&self) -> FutureResult<ReportedRoundStates> {
 		let round_states = ReportedRoundStates::from(&self.authority_set, &self.voter_state);
 		let future = async move { round_states }.boxed();
 		Box::new(future.map_err(jsonrpc_core::Error::from).compat())
 	}
+
+	fn subscribe_justifications(
+		&self,
+		_metadata: Self::Metadata,
+		subscriber: Subscriber<JustificationNotification>
+	) {
+		let stream = self.justification_stream.subscribe()
+			.map(|x| Ok::<_,()>(JustificationNotification::from(x)))
+			.map_err(|e| warn!("Notification stream error: {:?}", e))
+			.compat();
+
+		self.manager.add(subscriber, |sink| {
+			let stream = stream.map(|res| Ok(res));
+			sink.sink_map_err(|e| warn!("Error sending notifications: {:?}", e))
+				.send_all(stream)
+				.map(|_| ())
+		});
+	}
+
+	fn unsubscribe_justifications(
+		&self,
+		_metadata: Option<Self::Metadata>,
+		id: SubscriptionId
+	) -> jsonrpc_core::Result<bool> {
+		Ok(self.manager.cancel(id))
+	}
 }
 
 #[cfg(test)]
 mod tests {
 	use super::*;
-	use jsonrpc_core::IoHandler;
-	use sc_finality_grandpa::{report, AuthorityId};
+	use std::{collections::HashSet, convert::TryInto, sync::Arc};
+	use jsonrpc_core::{Notification, Output, types::Params};
+
+	use parity_scale_codec::Decode;
+	use sc_block_builder::BlockBuilder;
+	use sc_finality_grandpa::{report, AuthorityId, GrandpaJustificationSender, GrandpaJustification};
+	use sp_blockchain::HeaderBackend;
+	use sp_consensus::RecordProof;
 	use sp_core::crypto::Public;
-	use std::{collections::HashSet, convert::TryInto};
+	use sp_keyring::Ed25519Keyring;
+	use sp_runtime::traits::Header as HeaderT;
+	use substrate_test_runtime_client::{
+		runtime::Block,
+		DefaultTestClientBuilderExt,
+		TestClientBuilderExt,
+		TestClientBuilder,
+	};
 
 	struct TestAuthoritySet;
 	struct TestVoterState;
@@ -106,7 +198,7 @@ mod tests {
 			let voter_id_1 = AuthorityId::from_slice(&[1; 32]);
 			let voters_best: HashSet<_> = vec![voter_id_1].into_iter().collect();
 
-			let best_round_state = report::RoundState {
+			let best_round_state = sc_finality_grandpa::report::RoundState {
 				total_weight: 100_u64.try_into().unwrap(),
 				threshold_weight: 67_u64.try_into().unwrap(),
 				prevote_current_weight: 50.into(),
@@ -115,7 +207,7 @@ mod tests {
 				precommit_ids: HashSet::new(),
 			};
 
-			let past_round_state = report::RoundState {
+			let past_round_state = sc_finality_grandpa::report::RoundState {
 				total_weight: 100_u64.try_into().unwrap(),
 				threshold_weight: 67_u64.try_into().unwrap(),
 				prevote_current_weight: 100.into(),
@@ -133,23 +225,42 @@ mod tests {
 		}
 	}
 
+	fn setup_io_handler<VoterState>(voter_state: VoterState) -> (
+		jsonrpc_core::MetaIoHandler<sc_rpc::Metadata>,
+		GrandpaJustificationSender<Block>,
+	) where
+		VoterState: ReportVoterState + Send + Sync + 'static,
+	{
+		let (justification_sender, justification_stream) = GrandpaJustificationStream::channel();
+		let manager = SubscriptionManager::new(Arc::new(sc_rpc::testing::TaskExecutor));
+
+		let handler = GrandpaRpcHandler::new(
+			TestAuthoritySet,
+			voter_state,
+			justification_stream,
+			manager,
+		);
+
+		let mut io = jsonrpc_core::MetaIoHandler::default();
+		io.extend_with(GrandpaApi::to_delegate(handler));
+
+		(io, justification_sender)
+	}
+
 	#[test]
 	fn uninitialized_rpc_handler() {
-		let handler = GrandpaRpcHandler::new(TestAuthoritySet, EmptyVoterState);
-		let mut io = IoHandler::new();
-		io.extend_with(GrandpaApi::to_delegate(handler));
+		let (io, _) = setup_io_handler(EmptyVoterState);
 
 		let request = r#"{"jsonrpc":"2.0","method":"grandpa_roundState","params":[],"id":1}"#;
 		let response = r#"{"jsonrpc":"2.0","error":{"code":1,"message":"GRANDPA RPC endpoint not ready"},"id":1}"#;
 
-		assert_eq!(Some(response.into()), io.handle_request_sync(request));
+		let meta = sc_rpc::Metadata::default();
+		assert_eq!(Some(response.into()), io.handle_request_sync(request, meta));
 	}
 
 	#[test]
 	fn working_rpc_handler() {
-		let handler = GrandpaRpcHandler::new(TestAuthoritySet, TestVoterState);
-		let mut io = IoHandler::new();
-		io.extend_with(GrandpaApi::to_delegate(handler));
+		let (io,  _) = setup_io_handler(TestVoterState);
 
 		let request = r#"{"jsonrpc":"2.0","method":"grandpa_roundState","params":[],"id":1}"#;
 		let response = "{\"jsonrpc\":\"2.0\",\"result\":{\
@@ -166,6 +277,154 @@ mod tests {
 			\"setId\":1\
 		},\"id\":1}";
 
-		assert_eq!(io.handle_request_sync(request), Some(response.into()));
+		let meta = sc_rpc::Metadata::default();
+		assert_eq!(io.handle_request_sync(request, meta), Some(response.into()));
+	}
+
+	fn setup_session() -> (sc_rpc::Metadata, jsonrpc_core::futures::sync::mpsc::Receiver<String>) {
+		let (tx, rx) = jsonrpc_core::futures::sync::mpsc::channel(1);
+		let meta = sc_rpc::Metadata::new(tx);
+		(meta, rx)
+	}
+
+	#[test]
+	fn subscribe_and_unsubscribe_to_justifications() {
+		let (io, _) = setup_io_handler(TestVoterState);
+		let (meta, _) = setup_session();
+
+		// Subscribe
+		let sub_request = r#"{"jsonrpc":"2.0","method":"grandpa_subscribeJustifications","params":[],"id":1}"#;
+		let resp = io.handle_request_sync(sub_request, meta.clone());
+		let resp: Output = serde_json::from_str(&resp.unwrap()).unwrap();
+
+		let sub_id = match resp {
+			Output::Success(success) => success.result,
+			_ => panic!(),
+		};
+
+		// Unsubscribe
+		let unsub_req = format!(
+			"{{\"jsonrpc\":\"2.0\",\"method\":\"grandpa_unsubscribeJustifications\",\"params\":[{}],\"id\":1}}",
+			sub_id
+		);
+		assert_eq!(
+			io.handle_request_sync(&unsub_req, meta.clone()),
+			Some(r#"{"jsonrpc":"2.0","result":true,"id":1}"#.into()),
+		);
+
+		// Unsubscribe again and fail
+		assert_eq!(
+			io.handle_request_sync(&unsub_req, meta),
+			Some(r#"{"jsonrpc":"2.0","result":false,"id":1}"#.into()),
+		);
+	}
+
+	#[test]
+	fn subscribe_and_unsubscribe_with_wrong_id() {
+		let (io, _) = setup_io_handler(TestVoterState);
+		let (meta, _) = setup_session();
+
+		// Subscribe
+		let sub_request = r#"{"jsonrpc":"2.0","method":"grandpa_subscribeJustifications","params":[],"id":1}"#;
+		let resp = io.handle_request_sync(sub_request, meta.clone());
+		let resp: Output = serde_json::from_str(&resp.unwrap()).unwrap();
+		assert!(matches!(resp, Output::Success(_)));
+
+		// Unsubscribe with wrong ID
+		assert_eq!(
+			io.handle_request_sync(
+				r#"{"jsonrpc":"2.0","method":"grandpa_unsubscribeJustifications","params":["FOO"],"id":1}"#,
+				meta.clone()
+			),
+			Some(r#"{"jsonrpc":"2.0","result":false,"id":1}"#.into())
+		);
+	}
+
+	fn create_justification() -> GrandpaJustification<Block> {
+		let peers = &[Ed25519Keyring::Alice];
+
+		let builder = TestClientBuilder::new();
+		let backend = builder.backend();
+		let client = builder.build();
+		let client = Arc::new(client);
+
+		let built_block = BlockBuilder::new(
+			&*client,
+			client.info().best_hash,
+			client.info().best_number,
+			RecordProof::Yes,
+			Default::default(),
+			&*backend,
+		).unwrap().build().unwrap();
+
+		let block = built_block.block;
+		let block_hash = block.hash();
+
+		let justification = {
+			let round = 1;
+			let set_id = 0;
+
+			let precommit = finality_grandpa::Precommit {
+				target_hash: block_hash,
+				target_number: *block.header.number(),
+			};
+
+			let msg = finality_grandpa::Message::Precommit(precommit.clone());
+			let encoded = sp_finality_grandpa::localized_payload(round, set_id, &msg);
+			let signature = peers[0].sign(&encoded[..]).into();
+
+			let precommit = finality_grandpa::SignedPrecommit {
+				precommit,
+				signature,
+				id: peers[0].public().into(),
+			};
+
+			let commit = finality_grandpa::Commit {
+				target_hash: block_hash,
+				target_number: *block.header.number(),
+				precommits: vec![precommit],
+			};
+
+			GrandpaJustification::from_commit(&client, round, commit).unwrap()
+		};
+
+		justification
+	}
+
+	#[test]
+	fn subscribe_and_listen_to_one_justification() {
+		let (io, justification_sender) = setup_io_handler(TestVoterState);
+		let (meta, receiver) = setup_session();
+
+		// Subscribe
+		let sub_request =
+			r#"{"jsonrpc":"2.0","method":"grandpa_subscribeJustifications","params":[],"id":1}"#;
+
+		let resp = io.handle_request_sync(sub_request, meta.clone());
+		let mut resp: serde_json::Value = serde_json::from_str(&resp.unwrap()).unwrap();
+		let sub_id: String = serde_json::from_value(resp["result"].take()).unwrap();
+
+		// Notify with a header and justification
+		let justification = create_justification();
+		let _ = justification_sender.notify(justification.clone()).unwrap();
+
+		// Inspect what we received
+		let recv = receiver.take(1).wait().flatten().collect::<Vec<_>>();
+		let recv: Notification = serde_json::from_str(&recv[0]).unwrap();
+		let mut json_map = match recv.params {
+			Params::Map(json_map) => json_map,
+			_ => panic!(),
+		};
+
+		let recv_sub_id: String =
+			serde_json::from_value(json_map["subscription"].take()).unwrap();
+		let recv_justification: Vec<u8> =
+			serde_json::from_value(json_map["result"].take()).unwrap();
+		let recv_justification: GrandpaJustification<Block> =
+			Decode::decode(&mut &recv_justification[..]).unwrap();
+
+		assert_eq!(recv.method, "grandpa_justifications");
+		assert_eq!(recv_sub_id, sub_id);
+		assert_eq!(recv_justification, justification);
 	}
 }

substrate/client/finality-grandpa/rpc/src/notification.rs

@@ -0,0 +1,32 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2020 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: GPL-3.0-or-later WITH Classpath-exception-2.0
+
+// This program 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.
+
+// This program 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 this program. If not, see <https://www.gnu.org/licenses/>.
+
+use serde::{Serialize, Deserialize};
+use parity_scale_codec::Encode;
+use sp_runtime::traits::Block as BlockT;
+use sc_finality_grandpa::GrandpaJustification;
+
+/// An encoded justification proving that the given header has been finalized
+#[derive(Clone, Serialize, Deserialize)]
+pub struct JustificationNotification(Vec<u8>);
+
+impl<Block: BlockT> From<GrandpaJustification<Block>> for JustificationNotification {
+	fn from(notification: GrandpaJustification<Block>) -> Self {
+		JustificationNotification(notification.encode())
+	}
+}

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

@@ -51,6 +51,7 @@ use sp_consensus::SelectChain;
 use crate::authorities::{AuthoritySet, SharedAuthoritySet};
 use crate::communication::Network as NetworkT;
 use crate::consensus_changes::SharedConsensusChanges;
+use crate::notification::GrandpaJustificationSender;
 use crate::justification::GrandpaJustification;
 use crate::until_imported::UntilVoteTargetImported;
 use crate::voting_rule::VotingRule;
@@ -390,7 +391,6 @@ impl Metrics {
 	}
 }
 
-
 /// The environment we run GRANDPA in.
 pub(crate) struct Environment<Backend, Block: BlockT, C, N: NetworkT<Block>, SC, VR> {
 	pub(crate) client: Arc<C>,
@@ -404,6 +404,7 @@ pub(crate) struct Environment<Backend, Block: BlockT, C, N: NetworkT<Block>, SC,
 	pub(crate) voter_set_state: SharedVoterSetState<Block>,
 	pub(crate) voting_rule: VR,
 	pub(crate) metrics: Option<Metrics>,
+	pub(crate) justification_sender: Option<GrandpaJustificationSender<Block>>,
 	pub(crate) _phantom: PhantomData<Backend>,
 }
 
@@ -1022,6 +1023,7 @@ where
 			number,
 			(round, commit).into(),
 			false,
+			&self.justification_sender,
 		)
 	}
 
@@ -1086,6 +1088,7 @@ pub(crate) fn finalize_block<BE, Block, Client>(
 	number: NumberFor<Block>,
 	justification_or_commit: JustificationOrCommit<Block>,
 	initial_sync: bool,
+	justification_sender: &Option<GrandpaJustificationSender<Block>>,
 ) -> Result<(), CommandOrError<Block::Hash, NumberFor<Block>>> where
 	Block:  BlockT,
 	BE: Backend<Block>,
@@ -1097,6 +1100,7 @@ pub(crate) fn finalize_block<BE, Block, Client>(
 	let mut authority_set = authority_set.inner().write();
 
 	let status = client.info();
+
 	if number <= status.finalized_number && client.hash(number)? == Some(hash) {
 		// This can happen after a forced change (triggered by the finality tracker when finality is stalled), since
 		// the voter will be restarted at the median last finalized block, which can be lower than the local best
@@ -1157,7 +1161,7 @@ pub(crate) fn finalize_block<BE, Block, Client>(
 		// justifications for transition blocks which will be requested by
 		// syncing clients.
 		let justification = match justification_or_commit {
-			JustificationOrCommit::Justification(justification) => Some(justification.encode()),
+			JustificationOrCommit::Justification(justification) => Some(justification),
 			JustificationOrCommit::Commit((round_number, commit)) => {
 				let mut justification_required =
 					// justification is always required when block that enacts new authorities
@@ -1184,13 +1188,22 @@ pub(crate) fn finalize_block<BE, Block, Client>(
 						commit,
 					)?;
 
-					Some(justification.encode())
+					Some(justification)
 				} else {
 					None
 				}
 			},
 		};
 
+		// Notify any registered listeners in case we have a justification
+		if let Some(sender) = justification_sender {
+			if let Some(ref justification) = justification {
+				let _ = sender.notify(justification.clone());
+			}
+		}
+
+		let justification = justification.map(|j| j.encode());
+
 		debug!(target: "afg", "Finalizing blocks up to ({:?}, {})", number, hash);
 
 		// ideally some handle to a synchronization oracle would be used

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

@@ -149,7 +149,7 @@ impl<Block: BlockT> AuthoritySetForFinalityChecker<Block> for Arc<dyn FetchCheck
 }
 
 /// Finality proof provider for serving network requests.
-pub struct FinalityProofProvider<B,  Block: BlockT> {
+pub struct FinalityProofProvider<B, Block: BlockT> {
 	backend: Arc<B>,
 	authority_provider: Arc<dyn AuthoritySetForFinalityProver<Block>>,
 }

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 {

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

@@ -37,7 +37,7 @@ use crate::{Commit, Error};
 ///
 /// This is meant to be stored in the db and passed around the network to other
 /// nodes, and are used by syncing nodes to prove authority set handoffs.
-#[derive(Encode, Decode)]
+#[derive(Clone, Encode, Decode, PartialEq, Eq, Debug)]
 pub struct GrandpaJustification<Block: BlockT> {
 	round: u64,
 	pub(crate) commit: Commit<Block>,
@@ -47,7 +47,7 @@ pub struct GrandpaJustification<Block: BlockT> {
 impl<Block: BlockT> GrandpaJustification<Block> {
 	/// Create a GRANDPA justification from the given commit. This method
 	/// assumes the commit is valid and well-formed.
-	pub(crate) fn from_commit<C>(
+	pub fn from_commit<C>(
 		client: &Arc<C>,
 		round: u64,
 		commit: Commit<Block>,

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

@@ -119,12 +119,14 @@ mod finality_proof;
 mod import;
 mod justification;
 mod light_import;
+mod notification;
 mod observer;
 mod until_imported;
 mod voting_rule;
 
 pub use authorities::SharedAuthoritySet;
 pub use finality_proof::{FinalityProofProvider, StorageAndProofProvider};
+pub use notification::{GrandpaJustificationSender, GrandpaJustificationStream};
 pub use import::GrandpaBlockImport;
 pub use justification::GrandpaJustification;
 pub use light_import::{light_block_import, GrandpaLightBlockImport};
@@ -448,6 +450,8 @@ pub struct LinkHalf<Block: BlockT, C, SC> {
 	select_chain: SC,
 	persistent_data: PersistentData<Block>,
 	voter_commands_rx: TracingUnboundedReceiver<VoterCommand<Block::Hash, NumberFor<Block>>>,
+	justification_sender: GrandpaJustificationSender<Block>,
+	justification_stream: GrandpaJustificationStream<Block>,
 }
 
 impl<Block: BlockT, C, SC> LinkHalf<Block, C, SC> {
@@ -455,6 +459,11 @@ impl<Block: BlockT, C, SC> LinkHalf<Block, C, SC> {
 	pub fn shared_authority_set(&self) -> &SharedAuthoritySet<Block::Hash, NumberFor<Block>> {
 		&self.persistent_data.authority_set
 	}
+
+	/// Get the receiving end of justification notifications.
+	pub fn justification_stream(&self) -> GrandpaJustificationStream<Block> {
+		self.justification_stream.clone()
+	}
 }
 
 /// Provider for the Grandpa authority set configured on the genesis block.
@@ -553,6 +562,9 @@ where
 
 	let (voter_commands_tx, voter_commands_rx) = tracing_unbounded("mpsc_grandpa_voter_command");
 
+	let (justification_sender, justification_stream) =
+		GrandpaJustificationStream::channel();
+
 	// create pending change objects with 0 delay and enacted on finality
 	// (i.e. standard changes) for each authority set hard fork.
 	let authority_set_hard_forks = authority_set_hard_forks
@@ -579,12 +591,15 @@ where
 			voter_commands_tx,
 			persistent_data.consensus_changes.clone(),
 			authority_set_hard_forks,
+			justification_sender.clone(),
 		),
 		LinkHalf {
 			client,
 			select_chain,
 			persistent_data,
 			voter_commands_rx,
+			justification_sender,
+			justification_stream,
 		},
 	))
 }
@@ -719,6 +734,8 @@ pub fn run_grandpa_voter<Block: BlockT, BE: 'static, C, N, SC, VR>(
 		select_chain,
 		persistent_data,
 		voter_commands_rx,
+		justification_sender,
+		justification_stream: _,
 	} = link;
 
 	let network = NetworkBridge::new(
@@ -767,6 +784,7 @@ pub fn run_grandpa_voter<Block: BlockT, BE: 'static, C, N, SC, VR>(
 		voter_commands_rx,
 		prometheus_registry,
 		shared_voter_state,
+		justification_sender,
 	);
 
 	let voter_work = voter_work
@@ -827,6 +845,7 @@ where
 		voter_commands_rx: TracingUnboundedReceiver<VoterCommand<Block::Hash, NumberFor<Block>>>,
 		prometheus_registry: Option<prometheus_endpoint::Registry>,
 		shared_voter_state: SharedVoterState,
+		justification_sender: GrandpaJustificationSender<Block>,
 	) -> Self {
 		let metrics = match prometheus_registry.as_ref().map(Metrics::register) {
 			Some(Ok(metrics)) => Some(metrics),
@@ -850,6 +869,7 @@ where
 			consensus_changes: persistent_data.consensus_changes.clone(),
 			voter_set_state: persistent_data.set_state,
 			metrics: metrics.as_ref().map(|m| m.environment.clone()),
+			justification_sender: Some(justification_sender),
 			_phantom: PhantomData,
 		});
 
@@ -988,6 +1008,7 @@ where
 					network: self.env.network.clone(),
 					voting_rule: self.env.voting_rule.clone(),
 					metrics: self.env.metrics.clone(),
+					justification_sender: self.env.justification_sender.clone(),
 					_phantom: PhantomData,
 				});
 

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

@@ -0,0 +1,102 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2020 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: GPL-3.0-or-later WITH Classpath-exception-2.0
+
+// This program 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.
+
+// This program 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 this program. If not, see <https://www.gnu.org/licenses/>.
+
+use std::sync::Arc;
+use parking_lot::Mutex;
+
+use sp_runtime::traits::Block as BlockT;
+use sp_utils::mpsc::{tracing_unbounded, TracingUnboundedSender, TracingUnboundedReceiver};
+
+use crate::justification::GrandpaJustification;
+
+// Stream of justifications returned when subscribing.
+type JustificationStream<Block> = TracingUnboundedReceiver<GrandpaJustification<Block>>;
+
+// Sending endpoint for notifying about justifications.
+type JustificationSender<Block> = TracingUnboundedSender<GrandpaJustification<Block>>;
+
+// Collection of channel sending endpoints shared with the receiver side so they can register
+// themselves.
+type SharedJustificationSenders<Block> = Arc<Mutex<Vec<JustificationSender<Block>>>>;
+
+/// The sending half of the Grandpa justification channel(s).
+///
+/// Used to send notifications about justifications generated
+/// at the end of a Grandpa round.
+#[derive(Clone)]
+pub struct GrandpaJustificationSender<Block: BlockT> {
+	subscribers: SharedJustificationSenders<Block>
+}
+
+impl<Block: BlockT> GrandpaJustificationSender<Block> {
+	/// The `subscribers` should be shared with a corresponding
+	/// `GrandpaJustificationStream`.
+	fn new(subscribers: SharedJustificationSenders<Block>) -> Self {
+		Self {
+			subscribers,
+		}
+	}
+
+	/// Send out a notification to all subscribers that a new justification
+	/// is available for a block.
+	pub fn notify(&self, notification: GrandpaJustification<Block>) -> Result<(), ()> {
+		self.subscribers.lock().retain(|n| {
+			!n.is_closed() && n.unbounded_send(notification.clone()).is_ok()
+		});
+		Ok(())
+	}
+}
+
+/// The receiving half of the Grandpa justification channel.
+///
+/// Used to receive notifications about justifications generated
+/// at the end of a Grandpa round.
+/// The `GrandpaJustificationStream` entity stores the `SharedJustificationSenders`
+/// so it can be used to add more subscriptions.
+#[derive(Clone)]
+pub struct GrandpaJustificationStream<Block: BlockT> {
+	subscribers: SharedJustificationSenders<Block>
+}
+
+impl<Block: BlockT> GrandpaJustificationStream<Block> {
+	/// Creates a new pair of receiver and sender of justification notifications.
+	pub fn channel() -> (GrandpaJustificationSender<Block>, Self) {
+		let subscribers = Arc::new(Mutex::new(vec![]));
+		let receiver = GrandpaJustificationStream::new(subscribers.clone());
+		let sender = GrandpaJustificationSender::new(subscribers.clone());
+		(sender, receiver)
+	}
+
+	/// Create a new receiver of justification notifications.
+	///
+	/// The `subscribers` should be shared with a corresponding
+	/// `GrandpaJustificationSender`.
+	fn new(subscribers: SharedJustificationSenders<Block>) -> Self {
+		Self {
+			subscribers,
+		}
+	}
+
+	/// Subscribe to a channel through which justifications are sent
+	/// at the end of each Grandpa voting round.
+	pub fn subscribe(&self) -> JustificationStream<Block> {
+		let (sender, receiver) = tracing_unbounded("mpsc_justification_notification_stream");
+		self.subscribers.lock().push(sender);
+		receiver
+	}
+}

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

@@ -40,6 +40,7 @@ use crate::{
 use crate::authorities::SharedAuthoritySet;
 use crate::communication::{Network as NetworkT, NetworkBridge};
 use crate::consensus_changes::SharedConsensusChanges;
+use crate::notification::GrandpaJustificationSender;
 use sp_finality_grandpa::AuthorityId;
 use std::marker::{PhantomData, Unpin};
 
@@ -69,6 +70,7 @@ fn grandpa_observer<BE, Block: BlockT, Client, S, F>(
 	authority_set: &SharedAuthoritySet<Block::Hash, NumberFor<Block>>,
 	consensus_changes: &SharedConsensusChanges<Block::Hash, NumberFor<Block>>,
 	voters: &Arc<VoterSet<AuthorityId>>,
+	justification_sender: &Option<GrandpaJustificationSender<Block>>,
 	last_finalized_number: NumberFor<Block>,
 	commits: S,
 	note_round: F,
@@ -85,6 +87,7 @@ fn grandpa_observer<BE, Block: BlockT, Client, S, F>(
 	let consensus_changes = consensus_changes.clone();
 	let client = client.clone();
 	let voters = voters.clone();
+	let justification_sender = justification_sender.clone();
 
 	let observer = commits.try_fold(last_finalized_number, move |last_finalized_number, global| {
 		let (round, commit, callback) = match global {
@@ -127,6 +130,7 @@ fn grandpa_observer<BE, Block: BlockT, Client, S, F>(
 				finalized_number,
 				(round, commit).into(),
 				false,
+				&justification_sender,
 			) {
 				Ok(_) => {},
 				Err(e) => return future::err(e),
@@ -177,6 +181,7 @@ where
 		select_chain: _,
 		persistent_data,
 		voter_commands_rx,
+		justification_sender,
 		..
 	} = link;
 
@@ -192,7 +197,8 @@ where
 		network,
 		persistent_data,
 		config.keystore,
-		voter_commands_rx
+		voter_commands_rx,
+		Some(justification_sender),
 	);
 
 	let observer_work = observer_work
@@ -213,6 +219,7 @@ struct ObserverWork<B: BlockT, BE, Client, N: NetworkT<B>> {
 	persistent_data: PersistentData<B>,
 	keystore: Option<BareCryptoStorePtr>,
 	voter_commands_rx: TracingUnboundedReceiver<VoterCommand<B::Hash, NumberFor<B>>>,
+	justification_sender: Option<GrandpaJustificationSender<B>>,
 	_phantom: PhantomData<BE>,
 }
 
@@ -230,6 +237,7 @@ where
 		persistent_data: PersistentData<B>,
 		keystore: Option<BareCryptoStorePtr>,
 		voter_commands_rx: TracingUnboundedReceiver<VoterCommand<B::Hash, NumberFor<B>>>,
+		justification_sender: Option<GrandpaJustificationSender<B>>,
 	) -> Self {
 
 		let mut work = ObserverWork {
@@ -241,6 +249,7 @@ where
 			persistent_data,
 			keystore: keystore.clone(),
 			voter_commands_rx,
+			justification_sender,
 			_phantom: PhantomData,
 		};
 		work.rebuild_observer();
@@ -287,6 +296,7 @@ where
 			&self.persistent_data.authority_set,
 			&self.persistent_data.consensus_changes,
 			&voters,
+			&self.justification_sender,
 			last_finalized_number,
 			global_in,
 			note_round,
@@ -422,12 +432,14 @@ mod tests {
 		).unwrap();
 
 		let (_tx, voter_command_rx) = tracing_unbounded("");
+
 		let observer = ObserverWork::new(
 			client,
 			tester.net_handle.clone(),
 			persistent_data,
 			None,
 			voter_command_rx,
+			None,
 		);
 
 		// Trigger a reputation change through the gossip validator.

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

@@ -1567,6 +1567,7 @@ where
 		network,
 		voting_rule,
 		metrics: None,
+		justification_sender: None,
 		_phantom: PhantomData,
 	}
 }

substrate/client/rpc-api/src/lib.rs

@@ -22,10 +22,12 @@
 
 mod errors;
 mod helpers;
+mod metadata;
 mod policy;
 
-pub use jsonrpc_core::IoHandlerExtension as RpcExtension;
 pub use helpers::Receiver;
+pub use jsonrpc_core::IoHandlerExtension as RpcExtension;
+pub use metadata::Metadata;
 pub use policy::DenyUnsafe;
 
 pub mod author;

substrate/client/rpc-api/src/metadata.rs

@@ -19,8 +19,8 @@
 //! RPC Metadata
 use std::sync::Arc;
 
+use jsonrpc_core::futures::sync::mpsc;
 use jsonrpc_pubsub::{Session, PubSubMetadata};
-use rpc::futures::sync::mpsc;
 
 /// RPC Metadata.
 ///
@@ -32,7 +32,7 @@ pub struct Metadata {
 	session: Option<Arc<Session>>,
 }
 
-impl rpc::Metadata for Metadata {}
+impl jsonrpc_core::Metadata for Metadata {}
 impl PubSubMetadata for Metadata {
 	fn session(&self) -> Option<Arc<Session>> {
 		self.session.clone()

substrate/client/rpc/Cargo.toml

@@ -37,6 +37,7 @@ sp-transaction-pool = { version = "2.0.0-rc5", path = "../../primitives/transact
 sp-blockchain = { version = "2.0.0-rc5", path = "../../primitives/blockchain" }
 hash-db = { version = "0.15.2", default-features = false }
 parking_lot = "0.10.0"
+lazy_static = { version = "1.4.0", optional = true }
 
 [dev-dependencies]
 assert_matches = "1.3.0"
@@ -46,4 +47,6 @@ sp-io = { version = "2.0.0-rc5", path = "../../primitives/io" }
 substrate-test-runtime-client = { version = "2.0.0-rc5", path = "../../test-utils/runtime/client" }
 tokio = "0.1.22"
 sc-transaction-pool = { version = "2.0.0-rc5", path = "../transaction-pool" }
-lazy_static = "1.4.0"
+
+[features]
+test-helpers = ["lazy_static"]

substrate/client/rpc/src/author/mod.rs

@@ -94,7 +94,7 @@ impl<P, Client> AuthorApi<TxHash<P>, BlockHash<P>> for Author<P, Client>
 		Client: HeaderBackend<P::Block> + ProvideRuntimeApi<P::Block> + Send + Sync + 'static,
 		Client::Api: SessionKeys<P::Block, Error = ClientError>,
 {
-	type Metadata = crate::metadata::Metadata;
+	type Metadata = crate::Metadata;
 
 	fn insert_key(
 		&self,

substrate/client/rpc/src/chain/mod.rs

@@ -106,7 +106,7 @@ trait ChainBackend<Client, Block: BlockT>: Send + Sync + 'static
 	/// All new head subscription
 	fn subscribe_all_heads(
 		&self,
-		_metadata: crate::metadata::Metadata,
+		_metadata: crate::Metadata,
 		subscriber: Subscriber<Block::Header>,
 	) {
 		subscribe_headers(
@@ -123,7 +123,7 @@ trait ChainBackend<Client, Block: BlockT>: Send + Sync + 'static
 	/// Unsubscribe from all head subscription.
 	fn unsubscribe_all_heads(
 		&self,
-		_metadata: Option<crate::metadata::Metadata>,
+		_metadata: Option<crate::Metadata>,
 		id: SubscriptionId,
 	) -> RpcResult<bool> {
 		Ok(self.subscriptions().cancel(id))
@@ -132,7 +132,7 @@ trait ChainBackend<Client, Block: BlockT>: Send + Sync + 'static
 	/// New best head subscription
 	fn subscribe_new_heads(
 		&self,
-		_metadata: crate::metadata::Metadata,
+		_metadata: crate::Metadata,
 		subscriber: Subscriber<Block::Header>,
 	) {
 		subscribe_headers(
@@ -150,7 +150,7 @@ trait ChainBackend<Client, Block: BlockT>: Send + Sync + 'static
 	/// Unsubscribe from new best head subscription.
 	fn unsubscribe_new_heads(
 		&self,
-		_metadata: Option<crate::metadata::Metadata>,
+		_metadata: Option<crate::Metadata>,
 		id: SubscriptionId,
 	) -> RpcResult<bool> {
 		Ok(self.subscriptions().cancel(id))
@@ -159,7 +159,7 @@ trait ChainBackend<Client, Block: BlockT>: Send + Sync + 'static
 	/// Finalized head subscription
 	fn subscribe_finalized_heads(
 		&self,
-		_metadata: crate::metadata::Metadata,
+		_metadata: crate::Metadata,
 		subscriber: Subscriber<Block::Header>,
 	) {
 		subscribe_headers(
@@ -176,7 +176,7 @@ trait ChainBackend<Client, Block: BlockT>: Send + Sync + 'static
 	/// Unsubscribe from finalized head subscription.
 	fn unsubscribe_finalized_heads(
 		&self,
-		_metadata: Option<crate::metadata::Metadata>,
+		_metadata: Option<crate::Metadata>,
 		id: SubscriptionId,
 	) -> RpcResult<bool> {
 		Ok(self.subscriptions().cancel(id))
@@ -230,7 +230,7 @@ impl<Block, Client> ChainApi<NumberFor<Block>, Block::Hash, Block::Header, Signe
 			Block: BlockT + 'static,
 			Client: HeaderBackend<Block> + BlockchainEvents<Block> + 'static,
 {
-	type Metadata = crate::metadata::Metadata;
+	type Metadata = crate::Metadata;
 
 	fn header(&self, hash: Option<Block::Hash>) -> FutureResult<Option<Block::Header>> {
 		self.backend.header(hash)

substrate/client/rpc/src/lib.rs

@@ -27,10 +27,7 @@ use rpc::futures::future::{Executor, ExecuteError, Future};
 use sp_core::traits::SpawnNamed;
 use std::sync::Arc;
 
-mod metadata;
-
-pub use sc_rpc_api::DenyUnsafe;
-pub use self::metadata::Metadata;
+pub use sc_rpc_api::{DenyUnsafe, Metadata};
 pub use rpc::IoHandlerExtension as RpcExtension;
 
 pub mod author;
@@ -38,8 +35,9 @@ pub mod chain;
 pub mod offchain;
 pub mod state;
 pub mod system;
-#[cfg(test)]
-mod testing;
+
+#[cfg(any(test, feature = "test-helpers"))]
+pub mod testing;
 
 /// Task executor that is being used by RPC subscriptions.
 #[derive(Clone)]

substrate/client/rpc/src/state/mod.rs

@@ -140,21 +140,21 @@ pub trait StateBackend<Block: BlockT, Client>: Send + Sync + 'static
 	/// New runtime version subscription
 	fn subscribe_runtime_version(
 		&self,
-		_meta: crate::metadata::Metadata,
+		_meta: crate::Metadata,
 		subscriber: Subscriber<RuntimeVersion>,
 	);
 
 	/// Unsubscribe from runtime version subscription
 	fn unsubscribe_runtime_version(
 		&self,
-		_meta: Option<crate::metadata::Metadata>,
+		_meta: Option<crate::Metadata>,
 		id: SubscriptionId,
 	) -> RpcResult<bool>;
 
 	/// New storage subscription
 	fn subscribe_storage(
 		&self,
-		_meta: crate::metadata::Metadata,
+		_meta: crate::Metadata,
 		subscriber: Subscriber<StorageChangeSet<Block::Hash>>,
 		keys: Option<Vec<StorageKey>>,
 	);
@@ -162,7 +162,7 @@ pub trait StateBackend<Block: BlockT, Client>: Send + Sync + 'static
 	/// Unsubscribe from storage subscription
 	fn unsubscribe_storage(
 		&self,
-		_meta: Option<crate::metadata::Metadata>,
+		_meta: Option<crate::Metadata>,
 		id: SubscriptionId,
 	) -> RpcResult<bool>;
 }
@@ -230,7 +230,7 @@ impl<Block, Client> StateApi<Block::Hash> for State<Block, Client>
 		Block: BlockT + 'static,
 		Client: Send + Sync + 'static,
 {
-	type Metadata = crate::metadata::Metadata;
+	type Metadata = crate::Metadata;
 
 	fn call(&self, method: String, data: Bytes, block: Option<Block::Hash>) -> FutureResult<Bytes> {
 		self.backend.call(block, method, data)
@@ -390,7 +390,7 @@ impl<Block, Client> ChildStateApi<Block::Hash> for ChildState<Block, Client>
 		Block: BlockT + 'static,
 		Client: Send + Sync + 'static,
 {
-	type Metadata = crate::metadata::Metadata;
+	type Metadata = crate::Metadata;
 
 	fn storage(
 		&self,

substrate/client/rpc/src/state/state_full.rs

@@ -373,7 +373,7 @@ impl<BE, Block, Client> StateBackend<Block, Client> for FullState<BE, Block, Cli
 
 	fn subscribe_runtime_version(
 		&self,
-		_meta: crate::metadata::Metadata,
+		_meta: crate::Metadata,
 		subscriber: Subscriber<RuntimeVersion>,
 	) {
 		let stream = match self.client.storage_changes_notification_stream(
@@ -424,7 +424,7 @@ impl<BE, Block, Client> StateBackend<Block, Client> for FullState<BE, Block, Cli
 
 	fn unsubscribe_runtime_version(
 		&self,
-		_meta: Option<crate::metadata::Metadata>,
+		_meta: Option<crate::Metadata>,
 		id: SubscriptionId,
 	) -> RpcResult<bool> {
 		Ok(self.subscriptions.cancel(id))
@@ -432,7 +432,7 @@ impl<BE, Block, Client> StateBackend<Block, Client> for FullState<BE, Block, Cli
 
 	fn subscribe_storage(
 		&self,
-		_meta: crate::metadata::Metadata,
+		_meta: crate::Metadata,
 		subscriber: Subscriber<StorageChangeSet<Block::Hash>>,
 		keys: Option<Vec<StorageKey>>,
 	) {
@@ -484,7 +484,7 @@ impl<BE, Block, Client> StateBackend<Block, Client> for FullState<BE, Block, Cli
 
 	fn unsubscribe_storage(
 		&self,
-		_meta: Option<crate::metadata::Metadata>,
+		_meta: Option<crate::Metadata>,
 		id: SubscriptionId,
 	) -> RpcResult<bool> {
 		Ok(self.subscriptions.cancel(id))

substrate/client/rpc/src/state/state_light.rs

@@ -289,7 +289,7 @@ impl<Block, F, Client> StateBackend<Block, Client> for LightState<Block, F, Clie
 
 	fn subscribe_storage(
 		&self,
-		_meta: crate::metadata::Metadata,
+		_meta: crate::Metadata,
 		subscriber: Subscriber<StorageChangeSet<Block::Hash>>,
 		keys: Option<Vec<StorageKey>>
 	) {
@@ -384,7 +384,7 @@ impl<Block, F, Client> StateBackend<Block, Client> for LightState<Block, F, Clie
 
 	fn unsubscribe_storage(
 		&self,
-		_meta: Option<crate::metadata::Metadata>,
+		_meta: Option<crate::Metadata>,
 		id: SubscriptionId,
 	) -> RpcResult<bool> {
 		if !self.subscriptions.cancel(id.clone()) {
@@ -412,7 +412,7 @@ impl<Block, F, Client> StateBackend<Block, Client> for LightState<Block, F, Clie
 
 	fn subscribe_runtime_version(
 		&self,
-		_meta: crate::metadata::Metadata,
+		_meta: crate::Metadata,
 		subscriber: Subscriber<RuntimeVersion>,
 	) {
 		self.subscriptions.add(subscriber, move |sink| {
@@ -459,7 +459,7 @@ impl<Block, F, Client> StateBackend<Block, Client> for LightState<Block, F, Clie
 
 	fn unsubscribe_runtime_version(
 		&self,
-		_meta: Option<crate::metadata::Metadata>,
+		_meta: Option<crate::Metadata>,
 		id: SubscriptionId,
 	) -> RpcResult<bool> {
 		Ok(self.subscriptions.cancel(id))

substrate/client/rpc/src/testing.rs

@@ -32,6 +32,7 @@ lazy_static::lazy_static! {
 
 type Boxed01Future01 = Box<dyn future01::Future<Item = (), Error = ()> + Send + 'static>;
 
+/// Executor for use in testing
 pub struct TaskExecutor;
 impl future01::Executor<Boxed01Future01> for TaskExecutor {
 	fn execute(

substrate/client/service/src/builder.rs

@@ -73,17 +73,17 @@ pub trait RpcExtensionBuilder {
 
 	/// Returns an instance of the RPC extension for a particular `DenyUnsafe`
 	/// value, e.g. the RPC extension might not expose some unsafe methods.
-	fn build(&self, deny: sc_rpc::DenyUnsafe) -> Self::Output;
+	fn build(&self, deny: sc_rpc::DenyUnsafe, subscriptions: SubscriptionManager) -> Self::Output;
 }
 
 impl<F, R> RpcExtensionBuilder for F where
-	F: Fn(sc_rpc::DenyUnsafe) -> R,
+	F: Fn(sc_rpc::DenyUnsafe, SubscriptionManager) -> R,
 	R: sc_rpc::RpcExtension<sc_rpc::Metadata>,
 {
 	type Output = R;
 
-	fn build(&self, deny: sc_rpc::DenyUnsafe) -> Self::Output {
-		(*self)(deny)
+	fn build(&self, deny: sc_rpc::DenyUnsafe, subscriptions: SubscriptionManager) -> Self::Output {
+		(*self)(deny, subscriptions)
 	}
 }
 
@@ -97,7 +97,7 @@ impl<R> RpcExtensionBuilder for NoopRpcExtensionBuilder<R> where
 {
 	type Output = R;
 
-	fn build(&self, _deny: sc_rpc::DenyUnsafe) -> Self::Output {
+	fn build(&self, _deny: sc_rpc::DenyUnsafe, _subscriptions: SubscriptionManager) -> Self::Output {
 		self.0.clone()
 	}
 }
@@ -764,7 +764,7 @@ fn gen_handler<TBl, TBackend, TExPool, TRpc, TCl>(
 	let author = sc_rpc::author::Author::new(
 		client,
 		transaction_pool,
-		subscriptions,
+		subscriptions.clone(),
 		keystore,
 		deny_unsafe,
 	);
@@ -786,7 +786,7 @@ fn gen_handler<TBl, TBackend, TExPool, TRpc, TCl>(
 		maybe_offchain_rpc,
 		author::AuthorApi::to_delegate(author),
 		system::SystemApi::to_delegate(system),
-		rpc_extensions_builder.build(deny_unsafe),
+		rpc_extensions_builder.build(deny_unsafe, subscriptions),
 	))
 }