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/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)