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]]