Watch existing extrinsics RPC (#3873)

* Transaction pool watch intristics.

* Track extrinsic rpc methods.

* Test for pool watcher.

* Track extrinsic rpc test.

* Fix rpc naming.

* review fixes

* Update jsonrpc and use une subcription.

* Naming and dependencies.

substrate/core/transaction-pool/graph/src/pool.rs

@@ -165,6 +165,17 @@ impl<B: ChainApi> Pool<B> {
 		)
 	}
 
+	/// Watch existing transaction
+	///
+	/// Get notified when some existing transaction is finished verifying or gets finalized
+	/// in a new block.
+	pub fn watch(
+		&self,
+		hash: ExHash<B>,
+	) -> Watcher<ExHash<B>, BlockHash<B>> {
+		self.validated_pool.watch(hash)
+	}
+
 	/// Prunes ready transactions.
 	///
 	/// Used to clear the pool from transactions that were part of recently imported block.
@@ -783,7 +794,6 @@ mod tests {
 			// when
 			pool.validated_pool.remove_invalid(&[*watcher.hash()]);
 
-
 			// then
 			let mut stream = futures::executor::block_on_stream(watcher.into_stream());
 			assert_eq!(stream.next(), Some(watcher::Status::Ready));
@@ -907,5 +917,37 @@ mod tests {
 			assert_eq!(pool.status().ready, 1);
 			assert_eq!(pool.status().future, 0);
 		}
+
+		#[test]
+		fn should_watch_existing() {
+			let limit = Limit {
+				count: 1,
+				total_bytes: 1000,
+			};
+			let pool = Pool::new(Options {
+				ready: limit.clone(),
+				future: limit.clone(),
+			}, TestApi::default());
+
+			let xt = uxt(Transfer {
+				from: AccountId::from_h256(H256::from_low_u64_be(1)),
+				to: AccountId::from_h256(H256::from_low_u64_be(2)),
+				amount: 5,
+				nonce: 0,
+			});
+			let hash = block_on(pool.submit_one(&BlockId::Number(0), xt)).expect("Failed to submit");
+			assert_eq!(pool.status().ready, 1);
+
+			let watcher = pool.watch(hash);
+			block_on(pool.prune_tags(&BlockId::Number(2), vec![], vec![hash]))
+				.expect("Failed to prune tags");
+
+			let mut stream = futures::executor::block_on_stream(
+				watcher.into_stream()
+			);
+
+			assert_eq!(stream.next(), Some(watcher::Status::Finalized(H256::from_low_u64_be(2).into())));
+			assert_eq!(stream.next(), None);
+		}
 	}
 }