Support eth_chainId RPC method

 * ethereum/EIPs#695
 * Original PR is #6329

rpc/src/v1/impls/eth.rs

@@ -48,6 +48,7 @@ use v1::types::{
 	RichBlock, Block, BlockTransactions, BlockNumber, Bytes, SyncStatus, SyncInfo,
 	Transaction, CallRequest, Index, Filter, Log, Receipt, Work,
 	H64 as RpcH64, H256 as RpcH256, H160 as RpcH160, U256 as RpcU256, block_number_to_id,
+	U64 as RpcU64,
 };
 use v1::metadata::Metadata;
 
@@ -513,6 +514,10 @@ impl<C, SN: ?Sized, S: ?Sized, M, EM, T: StateInfo + 'static> Eth for EthClient<
 		Ok(self.miner.is_currently_sealing())
 	}
 
+	fn chain_id(&self) -> Result<Option<RpcU64>> {
+		Ok(self.client.signing_chain_id().map(RpcU64::from))
+	}
+
 	fn hashrate(&self) -> Result<RpcU256> {
 		Ok(RpcU256::from(self.external_miner.hashrate()))
 	}

rpc/src/v1/impls/light/eth.rs

@@ -49,6 +49,7 @@ use v1::types::{
 	RichBlock, Block, BlockTransactions, BlockNumber, LightBlockNumber, Bytes, SyncStatus, SyncInfo,
 	Transaction, CallRequest, Index, Filter, Log, Receipt, Work,
 	H64 as RpcH64, H256 as RpcH256, H160 as RpcH160, U256 as RpcU256,
+	U64 as RpcU64,
 };
 use v1::metadata::Metadata;
 
@@ -246,6 +247,10 @@ impl<T: LightChainClient + 'static> Eth for EthClient<T> {
 		Ok(false)
 	}
 
+	fn chain_id(&self) -> Result<Option<RpcU64>> {
+		Ok(self.client.signing_chain_id().map(RpcU64::from))
+	}
+
 	fn hashrate(&self) -> Result<RpcU256> {
 		Ok(Default::default())
 	}

rpc/src/v1/tests/mocked/eth.rs

@@ -178,6 +178,17 @@ fn rpc_eth_syncing() {
 	assert_eq!(tester.io.handle_request_sync(request), Some(false_res.to_owned()));
 }
 
+#[test]
+fn rpc_eth_chain_id() {
+	let deps = Dependencies::new();
+	let io = deps.default_client();
+
+	let request = r#"{"jsonrpc": "2.0", "method": "eth_chainId", "params": [], "id": 1}"#;
+	let response = r#"{"jsonrpc":"2.0","result":null,"id":1}"#;
+
+	assert_eq!(tester.io.handle_request_sync(request), Some(response.to_owned()));
+}
+
 #[test]
 fn rpc_eth_hashrate() {
 	let tester = EthTester::default();

rpc/src/v1/traits/eth.rs

@@ -20,7 +20,7 @@ use jsonrpc_macros::Trailing;
 
 use v1::types::{RichBlock, BlockNumber, Bytes, CallRequest, Filter, FilterChanges, Index};
 use v1::types::{Log, Receipt, SyncStatus, Transaction, Work};
-use v1::types::{H64, H160, H256, U256};
+use v1::types::{H64, H160, H256, U256, U64};
 
 build_rpc_trait! {
 	/// Eth rpc interface.
@@ -47,6 +47,12 @@ build_rpc_trait! {
 		#[rpc(name = "eth_mining")]
 		fn is_mining(&self) -> Result<bool>;
 
+		/// Returns the chain ID used for transaction signing at the
+		/// current best block. None is returned if not
+		/// available.
+		#[rpc(name = "eth_chainId")]
+		fn chain_id(&self) -> Result<Option<U64>>;
+
 		/// Returns current gas_price.
 		#[rpc(name = "eth_gasPrice")]
 		fn gas_price(&self) -> Result<U256>;