cmd/evm: fix tracing

This commit fixes several missing interface-checks, where the code did not properly
verify a Hook before calling it.
It also fixes an error where the tracer was not set in the vmconfig during t8n initialization.

It also adds a testcase which verifies the tracing-output during t8n executio. The
testcase currently only checks the jsonlogger, but can be extended.

This change also removes a spurious 'null' output in the tracing, when
tracing result is null.

cmd/evm/internal/t8ntool/execution.go

@@ -228,7 +228,7 @@ func (pre *Prestate) Apply(vmConfig vm.Config, chainConfig *params.ChainConfig,
 		if err != nil {
 			return nil, nil, nil, err
 		}
-		if vmConfig.Tracer != nil {
+		if tracer != nil {
 			vmConfig.Tracer = tracer.Hooks
 		}
 		statedb.SetTxContext(tx.Hash(), txIndex)
@@ -240,7 +240,7 @@ func (pre *Prestate) Apply(vmConfig vm.Config, chainConfig *params.ChainConfig,
 		)
 		evm := vm.NewEVM(vmContext, txContext, statedb, chainConfig, vmConfig)
 
-		if tracer != nil {
+		if tracer != nil && tracer.OnTxStart != nil {
 			tracer.OnTxStart(evm.GetVMContext(), tx, msg.From)
 		}
 		// (ret []byte, usedGas uint64, failed bool, err error)
@@ -251,7 +251,9 @@ func (pre *Prestate) Apply(vmConfig vm.Config, chainConfig *params.ChainConfig,
 			rejectedTxs = append(rejectedTxs, &rejectedTx{i, err.Error()})
 			gaspool.SetGas(prevGas)
 			if tracer != nil {
-				tracer.OnTxEnd(nil, err)
+				if tracer.OnTxEnd != nil {
+					tracer.OnTxEnd(nil, err)
+				}
 				if err := writeTraceResult(tracer, traceOutput); err != nil {
 					log.Warn("Error writing tracer output", "err", err)
 				}
@@ -298,7 +300,9 @@ func (pre *Prestate) Apply(vmConfig vm.Config, chainConfig *params.ChainConfig,
 			receipt.TransactionIndex = uint(txIndex)
 			receipts = append(receipts, receipt)
 			if tracer != nil {
-				tracer.Hooks.OnTxEnd(receipt, nil)
+				if tracer.Hooks.OnTxEnd != nil {
+					tracer.Hooks.OnTxEnd(receipt, nil)
+				}
 				writeTraceResult(tracer, traceOutput)
 			}
 		}
@@ -418,7 +422,7 @@ func calcDifficulty(config *params.ChainConfig, number, currentTime, parentTime
 func writeTraceResult(tracer *directory.Tracer, f io.WriteCloser) error {
 	defer f.Close()
 	result, err := tracer.GetResult()
-	if err != nil {
+	if err != nil || result == nil {
 		return err
 	}
 	err = json.NewEncoder(f).Encode(result)

cmd/evm/t8n_test.go

@@ -17,9 +17,12 @@
 package main
 
 import (
+	"bufio"
 	"encoding/json"
 	"fmt"
+	"io"
 	"os"
+	"path/filepath"
 	"reflect"
 	"strings"
 	"testing"
@@ -320,6 +323,92 @@ func TestT8n(t *testing.T) {
 	}
 }
 
+func lineIterator(path string) func() (string, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return func() (string, error) { return err.Error(), err }
+	}
+	scanner := bufio.NewScanner(strings.NewReader(string(data)))
+	return func() (string, error) {
+		if scanner.Scan() {
+			return scanner.Text(), nil
+		}
+		if err := scanner.Err(); err != nil {
+			return "", err
+		}
+		return "", io.EOF // scanner gobbles io.EOF, but we want it
+	}
+}
+
+// TestT8nTracing is a test that checks the tracing-output from t8n.
+func TestT8nTracing(t *testing.T) {
+	t.Parallel()
+	tt := new(testT8n)
+	tt.TestCmd = cmdtest.NewTestCmd(t, tt)
+	for i, tc := range []struct {
+		base           string
+		input          t8nInput
+		expExitCode    int
+		expectedTraces []string
+	}{
+		{
+			base: "./testdata/31",
+			input: t8nInput{
+				"alloc.json", "txs.json", "env.json", "Cancun", "",
+			},
+			expectedTraces: []string{"trace-0-0x88f5fbd1524731a81e49f637aa847543268a5aaf2a6b32a69d2c6d978c45dcfb.jsonl"},
+		},
+	} {
+		args := []string{"t8n"}
+		args = append(args, tc.input.get(tc.base)...)
+		// Add tracing-args
+		args = append(args, "--trace")
+		// Place the output somewhere we can find it
+		outdir := t.TempDir()
+
+		args = append(args, "--output.basedir", outdir)
+
+		var qArgs []string // quoted args for debugging purposes
+		for _, arg := range args {
+			if len(arg) == 0 {
+				qArgs = append(qArgs, `""`)
+			} else {
+				qArgs = append(qArgs, arg)
+			}
+		}
+		tt.Logf("args: %v\n", strings.Join(qArgs, " "))
+		tt.Run("evm-test", args...)
+		tt.WaitExit()
+		// Compare the expected traces
+		for _, traceFile := range tc.expectedTraces {
+			haveFn := lineIterator(filepath.Join(outdir, traceFile))
+			wantFn := lineIterator(filepath.Join(tc.base, traceFile))
+
+			for line := 0; ; line++ {
+				want, wErr := wantFn()
+				have, hErr := haveFn()
+				if want != have {
+					t.Fatalf("test %d, trace %v, line %d\nwant: %v\nhave: %v\n",
+						i, traceFile, line, want, have)
+				}
+				if wErr != nil && hErr != nil {
+					break
+				}
+				if wErr != nil {
+					t.Fatal(wErr)
+				}
+				if hErr != nil {
+					t.Fatal(hErr)
+				}
+				t.Logf("%v\n", want)
+			}
+		}
+		if have, want := tt.ExitStatus(), tc.expExitCode; have != want {
+			t.Fatalf("test %d: wrong exit code, have %d, want %d", i, have, want)
+		}
+	}
+}
+
 type t9nInput struct {
 	inTxs  string
 	stFork string

cmd/evm/testdata/31/README.md

@@ -0,0 +1 @@
+This test does some EVM execution, and can be used to test the tracers and trace-outputs.

cmd/evm/testdata/31/alloc.json

@@ -0,0 +1,16 @@
+{
+  "0xa94f5374fce5edbc8e2a8697c15331677e6ebf0b" : {
+    "balance" : "0x016345785d8a0000",
+    "code" : "0x",
+    "nonce" : "0x00",
+    "storage" : {
+    }
+  },
+  "0x1111111111111111111111111111111111111111" : {
+    "balance" : "0x1",
+    "code" : "0x604060406040604000",
+    "nonce" : "0x00",
+    "storage" : {
+    }
+  }
+}

cmd/evm/testdata/31/env.json

@@ -0,0 +1,20 @@
+{
+  "currentCoinbase" : "0x2adc25665018aa1fe0e6bc666dac8fc2697ff9ba",
+  "currentNumber" : "0x01",
+  "currentTimestamp" : "0x03e8",
+  "currentGasLimit" : "0x1000000000",
+  "previousHash" : "0xe4e2a30b340bec696242b67584264f878600dce98354ae0b6328740fd4ff18da",
+  "currentDataGasUsed" : "0x2000",
+  "parentTimestamp" : "0x00",
+  "parentDifficulty" : "0x00",
+  "parentUncleHash" : "0x1dcc4de8dec75d7aab85b567b6ccd41ad312451b948a7413f0a142fd40d49347",
+  "parentBeaconBlockRoot" : "0x1dcc4de8dec75d7aab85b567b6ccd41ad312451b948a7413f0a142fd40d49347",
+  "currentRandom" : "0x0000000000000000000000000000000000000000000000000000000000020000",
+  "withdrawals" : [
+  ],
+  "parentBaseFee" : "0x08",
+  "parentGasUsed" : "0x00",
+  "parentGasLimit" : "0x1000000000",
+  "parentExcessBlobGas" : "0x1000",
+  "parentBlobGasUsed" : "0x2000"
+}

cmd/evm/testdata/31/trace-0-0x88f5fbd1524731a81e49f637aa847543268a5aaf2a6b32a69d2c6d978c45dcfb.jsonl

@@ -0,0 +1,6 @@
+{"pc":0,"op":96,"gas":"0x13498","gasCost":"0x3","memSize":0,"stack":[],"depth":1,"refund":0,"opName":"PUSH1"}
+{"pc":2,"op":96,"gas":"0x13495","gasCost":"0x3","memSize":0,"stack":["0x40"],"depth":1,"refund":0,"opName":"PUSH1"}
+{"pc":4,"op":96,"gas":"0x13492","gasCost":"0x3","memSize":0,"stack":["0x40","0x40"],"depth":1,"refund":0,"opName":"PUSH1"}
+{"pc":6,"op":96,"gas":"0x1348f","gasCost":"0x3","memSize":0,"stack":["0x40","0x40","0x40"],"depth":1,"refund":0,"opName":"PUSH1"}
+{"pc":8,"op":0,"gas":"0x1348c","gasCost":"0x0","memSize":0,"stack":["0x40","0x40","0x40","0x40"],"depth":1,"refund":0,"opName":"STOP"}
+{"output":"","gasUsed":"0xc"}

cmd/evm/testdata/31/txs.json

@@ -0,0 +1,14 @@
+[
+  {
+    "gas": "0x186a0",
+    "gasPrice": "0x600",
+    "input": "0x",
+    "nonce": "0x0",
+    "to": "0x1111111111111111111111111111111111111111",
+    "value": "0x1",
+    "v" : "0x0",
+    "r" : "0x0",
+    "s" : "0x0",
+    "secretKey" : "0x45a915e4d060149eb4365960e6a7a45f334393093061116b197e3240065ff2d8"
+  }
+]