Merge pull request #52 from OffchainLabs/guard-tracer-empty-callstack

eth: Guard callTracer and erc7562Tracer OnTxEnd against empty callstack

Author: tsahee

eth/tracers/native/call.go

@@ -239,6 +239,11 @@ func (t *callTracer) OnTxEnd(receipt *types.Receipt, err error) {
 	if err != nil {
 		return
 	}
+	// Tracing may have been interrupted (e.g. a timeout) before the top-level frame was
+	// captured, leaving nothing to finalize.
+	if len(t.callstack) == 0 {
+		return
+	}
 	if receipt != nil {
 		t.callstack[0].GasUsed = receipt.GasUsed
 	}
@@ -273,6 +278,10 @@ func (t *callTracer) OnLog(log *types.Log) {
 // GetResult returns the json-encoded nested list of call traces, and any
 // error arising from the encoding or forceful termination (via `Stop`).
 func (t *callTracer) GetResult() (json.RawMessage, error) {
+	// Tracing was interrupted (e.g. a timeout): report the interruption instead of continuing
+	if t.interrupt.Load() {
+		return nil, t.reason
+	}
 	if len(t.callstack) != 1 {
 		return nil, errors.New("incorrect number of top-level calls")
 	}

eth/tracers/native/call_flat.go

@@ -232,6 +232,10 @@ func (t *flatCallTracer) OnTxEnd(receipt *types.Receipt, err error) {
 
 // GetResult returns an empty json object.
 func (t *flatCallTracer) GetResult() (json.RawMessage, error) {
+	// Tracing was interrupted (e.g. a timeout): report the interruption instead of continuing
+	if t.interrupt.Load() {
+		return nil, t.tracer.reason
+	}
 	if len(t.tracer.callstack) < 1 {
 		return nil, errors.New("invalid number of calls")
 	}

eth/tracers/native/erc7562.go

@@ -288,6 +288,10 @@ func (t *erc7562Tracer) OnTxEnd(receipt *types.Receipt, err error) {
 	if err != nil {
 		return
 	}
+	// An interrupted trace may leave no top-level frame to finalize.
+	if len(t.callstackWithOpcodes) == 0 {
+		return
+	}
 	t.callstackWithOpcodes[0].GasUsed = receipt.GasUsed
 	if t.config.WithLog {
 		// Logs are not emitted when the call fails

eth/tracers/native/tracer_interrupt_test.go

@@ -0,0 +1,70 @@
+// Copyright 2025 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package native_test
+
+import (
+	"errors"
+	"math/big"
+	"testing"
+
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/core/tracing"
+	"github.com/ethereum/go-ethereum/core/types"
+	"github.com/ethereum/go-ethereum/core/vm"
+	"github.com/ethereum/go-ethereum/eth/tracers"
+	"github.com/ethereum/go-ethereum/params"
+	"github.com/stretchr/testify/require"
+)
+
+// TestTracerInterruptBeforeTopFrame guards against the empty-callstack panic when a trace
+// is interrupted before its top-level frame is captured. Calling Stop before OnEnter makes
+// that window deterministic, unlike the scheduler-dependent system-test repro.
+func TestTracerInterruptBeforeTopFrame(t *testing.T) {
+	stopError := errors.New("execution timeout")
+
+	tx := types.NewTx(&types.LegacyTx{
+		Nonce:    0,
+		To:       &common.Address{},
+		Value:    big.NewInt(0),
+		Gas:      0,
+		GasPrice: big.NewInt(0),
+		Data:     nil,
+	})
+
+	for _, name := range []string{"callTracer", "flatCallTracer", "erc7562Tracer"} {
+		t.Run(name, func(t *testing.T) {
+			tracer, err := tracers.DefaultDirectory.New(name, &tracers.Context{}, nil, params.MainnetChainConfig)
+			require.NoError(t, err)
+
+			tracer.OnTxStart(&tracing.VMContext{}, tx, common.Address{})
+
+			// Interrupt before the top-level frame is captured; OnEnter then no-ops on the
+			// interrupt flag, leaving the callstack empty.
+			tracer.Stop(stopError)
+			tracer.OnEnter(0, byte(vm.CALL), common.Address{}, common.Address{}, nil, 0, big.NewInt(0))
+
+			var res []byte
+			require.NotPanics(t, func() {
+				tracer.OnTxEnd(&types.Receipt{GasUsed: 0}, nil)
+				var tracerErr error
+				res, tracerErr = tracer.GetResult()
+				require.Equal(t, stopError, tracerErr)
+			})
+			require.Nil(t, res)
+		})
+	}
+}