eth/tracers/logger: use omitempty to reduce log bloat (ethereum#24547)

Makes the evm json output less verbose: omitting output of `memory` and `returndata` in case they are empty.

eth/tracers/logger/logger.go

@@ -66,10 +66,10 @@ type StructLog struct {
 	Op            vm.OpCode                   `json:"op"`
 	Gas           uint64                      `json:"gas"`
 	GasCost       uint64                      `json:"gasCost"`
-	Memory        []byte                      `json:"memory"`
+	Memory        []byte                      `json:"memory,omitempty"`
 	MemorySize    int                         `json:"memSize"`
 	Stack         []uint256.Int               `json:"stack"`
-	ReturnData    []byte                      `json:"returnData"`
+	ReturnData    []byte                      `json:"returnData,omitempty"`
 	Storage       map[common.Hash]common.Hash `json:"-"`
 	Depth         int                         `json:"depth"`
 	RefundCounter uint64                      `json:"refund"`
@@ -82,8 +82,8 @@ type structLogMarshaling struct {
 	GasCost     math.HexOrDecimal64
 	Memory      hexutil.Bytes
 	ReturnData  hexutil.Bytes
-	OpName      string `json:"opName"` // adds call to OpName() in MarshalJSON
-	ErrorString string `json:"error"`  // adds call to ErrorString() in MarshalJSON
+	OpName      string `json:"opName"`          // adds call to OpName() in MarshalJSON
+	ErrorString string `json:"error,omitempty"` // adds call to ErrorString() in MarshalJSON
 }
 
 // OpName formats the operand name in a human-readable format.

eth/tracers/logger/logger_test.go

@@ -17,6 +17,8 @@
 package logger
 
 import (
+	"encoding/json"
+	"fmt"
 	"math/big"
 	"testing"
 
@@ -72,3 +74,34 @@ func TestStoreCapture(t *testing.T) {
 		t.Errorf("expected %x, got %x", exp, logger.storage[contract.Address()][index])
 	}
 }
+
+// Tests that blank fields don't appear in logs when JSON marshalled, to reduce
+// logs bloat and confusion. See https://github.com/ethereum/go-ethereum/issues/24487
+func TestStructLogMarshalingOmitEmpty(t *testing.T) {
+	tests := []struct {
+		name string
+		log  *StructLog
+		want string
+	}{
+		{"empty err and no fields", &StructLog{},
+			`{"pc":0,"op":0,"gas":"0x0","gasCost":"0x0","memSize":0,"stack":null,"depth":0,"refund":0,"opName":"STOP"}`},
+		{"with err", &StructLog{Err: fmt.Errorf("this failed")},
+			`{"pc":0,"op":0,"gas":"0x0","gasCost":"0x0","memSize":0,"stack":null,"depth":0,"refund":0,"opName":"STOP","error":"this failed"}`},
+		{"with mem", &StructLog{Memory: make([]byte, 2), MemorySize: 2},
+			`{"pc":0,"op":0,"gas":"0x0","gasCost":"0x0","memory":"0x0000","memSize":2,"stack":null,"depth":0,"refund":0,"opName":"STOP"}`},
+		{"with 0-size mem", &StructLog{Memory: make([]byte, 0)},
+			`{"pc":0,"op":0,"gas":"0x0","gasCost":"0x0","memSize":0,"stack":null,"depth":0,"refund":0,"opName":"STOP"}`},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			blob, err := json.Marshal(tt.log)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if have, want := string(blob), tt.want; have != want {
+				t.Fatalf("mismatched results\n\thave: %v\n\twant: %v", have, want)
+			}
+		})
+	}
+}