internal/ethapi: add eth_chainId method (#17617)

This implements EIP-695.

eth/api.go

@@ -67,6 +67,15 @@ func (api *PublicEthereumAPI) Hashrate() hexutil.Uint64 {
 	return hexutil.Uint64(api.e.Miner().HashRate())
 }
 
+// ChainId is the EIP-155 replay-protection chain id for the current ethereum chain config.
+func (api *PublicEthereumAPI) ChainId() hexutil.Uint64 {
+	chainID := new(big.Int)
+	if config := api.e.chainConfig; config.IsEIP155(api.e.blockchain.CurrentBlock().Number()) {
+		chainID = config.ChainID
+	}
+	return (hexutil.Uint64)(chainID.Uint64())
+}
+
 // PublicMinerAPI provides an API to control the miner.
 // It offers only methods that operate on data that pose no security risk when it is publicly accessible.
 type PublicMinerAPI struct {