Merge pull request #384 from OffchainLabs/guard-tracer-empty-callstack

Guard callTracer and erc7562Tracer OnTxEnd against empty callstack

Author: tsahee

changelog/bragaigor-jco-nit-5062.md

@@ -0,0 +1,2 @@
+### Fixed
+- `callTracer.OnTxEnd` now returns early if the top-level frame was never captured (e.g. on timeouts)

go-ethereum

@@ -0,0 +1,54 @@
+// Copyright 2025, Offchain Labs, Inc.
+// For license information, see https://github.com/OffchainLabs/nitro/blob/master/LICENSE.md
+
+package arbtest
+
+import (
+	"context"
+	"encoding/json"
+	"math/big"
+	"strings"
+	"testing"
+
+	"github.com/ethereum/go-ethereum/rpc"
+)
+
+// TestReproTraceTimeoutEmptyCallstackPanic ensures a trace that times out before its
+// top-level frame is captured returns an error instead of panicking the RPC handler. A
+// 1ns timeout makes the interrupt win the race against OnEnter on otherwise normal txs.
+func TestReproTraceTimeoutEmptyCallstackPanic(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	builder := NewNodeBuilder(ctx).DefaultConfig(t, false)
+	cleanup := builder.Build(t)
+	defer cleanup()
+
+	builder.L2Info.GenerateAccount("User2")
+	var lastBlock uint64
+	for i := 0; i < 5; i++ {
+		tx := builder.L2Info.PrepareTx("Owner", "User2", builder.L2Info.TransferGas, big.NewInt(1e12), nil)
+		err := builder.L2.Client.SendTransaction(ctx, tx)
+		Require(t, err)
+		receipt, err := builder.L2.EnsureTxSucceeded(tx)
+		Require(t, err)
+		lastBlock = receipt.BlockNumber.Uint64()
+	}
+
+	l2rpc := builder.L2.Stack.Attach()
+	for _, tracer := range []string{"callTracer", "flatCallTracer", "erc7562Tracer"} {
+		for attempt := 0; attempt < 50; attempt++ {
+			for bn := uint64(1); bn <= lastBlock; bn++ {
+				var blockTrace json.RawMessage
+				err := l2rpc.CallContext(ctx, &blockTrace, "debug_traceBlockByNumber",
+					rpc.BlockNumber(bn), // #nosec G115
+					map[string]interface{}{"tracer": tracer, "timeout": "1ns"})
+				// A timed-out trace must either complete (nil) or report the timeout. It must
+				// never crash the handler ("method handler crashed") or return the misleading
+				// "incorrect number of top-level calls" when no top-level frame was captured.
+				if err != nil && !strings.Contains(err.Error(), "execution timeout") {
+					t.Fatalf("tracer %s: timed-out trace returned unexpected error (want nil or \"execution timeout\"): %v", tracer, err)
+				}
+			}
+		}
+	}
+}