Use previous block hash to compute tx effectiveGasPrice (#1280)

* Use previous block hash to compute tx effectiveGasPrice

* Fall back to current block for base_fee when tx is on genesis

client/rpc/src/eth/transaction.rs

@@ -287,14 +287,31 @@ where
 				let effective_gas_price = match transaction {
 					EthereumTransaction::Legacy(t) => t.gas_price,
 					EthereumTransaction::EIP2930(t) => t.gas_price,
-					EthereumTransaction::EIP1559(t) => self
-						.client
-						.runtime_api()
-						.gas_price(substrate_hash)
-						.unwrap_or_default()
-						.checked_add(t.max_priority_fee_per_gas)
-						.unwrap_or_else(U256::max_value)
-						.min(t.max_fee_per_gas),
+					EthereumTransaction::EIP1559(t) => {
+						let parent_eth_hash = block.header.parent_hash;
+						let base_fee_block_substrate_hash = if parent_eth_hash.is_zero() {
+							substrate_hash
+						} else {
+							frontier_backend_client::load_hash::<B, C>(
+								self.client.as_ref(),
+								self.backend.as_ref(),
+								parent_eth_hash,
+							)
+							.await
+							.map_err(|err| internal_err(format!("{:?}", err)))?
+							.ok_or(internal_err(
+								"Failed to retrieve substrate parent block hash",
+							))?
+						};
+
+						self.client
+							.runtime_api()
+							.gas_price(base_fee_block_substrate_hash)
+							.unwrap_or_default()
+							.checked_add(t.max_priority_fee_per_gas)
+							.unwrap_or_else(U256::max_value)
+							.min(t.max_fee_per_gas)
+					}
 				};
 
 				return Ok(Some(Receipt {