Trace precompiled contracts when the transfer value is not zero (#8486)

* Trace precompiled contracts when the transfer value is not zero

* Add tests for precompiled CALL tracing

* Use byzantium test machine for the new test

* Add notes in comments on why we don't trace all precompileds

* Use is_transferred instead of transferred

ethcore/src/executive.rs

@@ -428,8 +428,14 @@ impl<'a, B: 'a + StateBackend> Executive<'a, B> {
 					self.state.discard_checkpoint();
 					output.write(0, &builtin_out_buffer);
 
-					// trace only top level calls to builtins to avoid DDoS attacks
-					if self.depth == 0 {
+					// Trace only top level calls and calls with balance transfer to builtins. The reason why we don't
+					// trace all internal calls to builtin contracts is that memcpy (IDENTITY) is a heavily used
+					// function.
+					let is_transferred = match params.value {
+						ActionValue::Transfer(value) => value != U256::zero(),
+						ActionValue::Apparent(_) => false,
+					};
+					if self.depth == 0 || is_transferred {
 						let mut trace_output = tracer.prepare_trace_output();
 						if let Some(out) = trace_output.as_mut() {
 							*out = output.to_owned();
@@ -722,6 +728,12 @@ mod tests {
 		machine
 	}
 
+	fn make_byzantium_machine(max_depth: usize) -> EthereumMachine {
+		let mut machine = ::ethereum::new_byzantium_test_machine();
+		machine.set_schedule_creation_rules(Box::new(move |s, _| s.max_depth = max_depth));
+		machine
+	}
+
 	#[test]
 	fn test_contract_address() {
 		let address = Address::from_str("0f572e5295c57f15886f9b263e2f6d2d6c7b5ec6").unwrap();
@@ -813,6 +825,76 @@ mod tests {
 		assert_eq!(substate.contracts_created.len(), 0);
 	}
 
+	#[test]
+	fn test_call_to_precompiled_tracing() {
+		// code:
+		//
+		// 60 00 - push 00 out size
+		// 60 00 - push 00 out offset
+		// 60 00 - push 00 in size
+		// 60 00 - push 00 in offset
+		// 60 01 - push 01 value
+		// 60 03 - push 03 to
+		// 61 ffff - push fff gas
+		// f1 - CALL
+
+		let code = "60006000600060006001600361fffff1".from_hex().unwrap();
+		let sender = Address::from_str("4444444444444444444444444444444444444444").unwrap();
+		let address = Address::from_str("5555555555555555555555555555555555555555").unwrap();
+
+		let mut params = ActionParams::default();
+		params.address = address.clone();
+		params.code_address = address.clone();
+		params.sender = sender.clone();
+		params.origin = sender.clone();
+		params.gas = U256::from(100_000);
+		params.code = Some(Arc::new(code));
+		params.value = ActionValue::Transfer(U256::from(100));
+		params.call_type = CallType::Call;
+		let mut state = get_temp_state();
+		state.add_balance(&sender, &U256::from(100), CleanupMode::NoEmpty).unwrap();
+		let info = EnvInfo::default();
+		let machine = make_byzantium_machine(5);
+		let mut substate = Substate::new();
+		let mut tracer = ExecutiveTracer::default();
+		let mut vm_tracer = ExecutiveVMTracer::toplevel();
+
+		let mut ex = Executive::new(&mut state, &info, &machine);
+		let output = BytesRef::Fixed(&mut[0u8;0]);
+		ex.call(params, &mut substate, output, &mut tracer, &mut vm_tracer).unwrap();
+
+		assert_eq!(tracer.drain(), vec![FlatTrace {
+			action: trace::Action::Call(trace::Call {
+				from: "4444444444444444444444444444444444444444".into(),
+				to: "5555555555555555555555555555555555555555".into(),
+				value: 100.into(),
+				gas: 100_000.into(),
+				input: vec![],
+				call_type: CallType::Call
+			}),
+			result: trace::Res::Call(trace::CallResult {
+				gas_used: 33021.into(),
+				output: vec![]
+			}),
+			subtraces: 1,
+			trace_address: Default::default()
+		}, FlatTrace {
+			action: trace::Action::Call(trace::Call {
+				from: "5555555555555555555555555555555555555555".into(),
+				to: "0000000000000000000000000000000000000003".into(),
+				value: 1.into(),
+				gas: 66560.into(),
+				input: vec![],
+				call_type: CallType::Call
+			}), result: trace::Res::Call(trace::CallResult {
+				gas_used: 600.into(),
+				output: vec![]
+			}),
+			subtraces: 0,
+			trace_address: vec![0].into_iter().collect(),
+		}]);
+	}
+
 	#[test]
 	// Tracing is not suported in JIT
 	fn test_call_to_create() {