eth/tracers/logger: use JSON struct tag ",omitempty" to reduce log bloat from blank fields

eth/tracers/logger/logger.go

@@ -78,12 +78,12 @@ type StructLog struct {
 
 // overrides for gencodec
 type structLogMarshaling struct {
-	Gas         math.HexOrDecimal64
-	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
+	Gas         math.HexOrDecimal64 `json:",omitempty"`
+	GasCost     math.HexOrDecimal64 `json:",omitempty"`
+	Memory      hexutil.Bytes       `json:",omitempty"`
+	ReturnData  hexutil.Bytes       `json:",omitempty"`
+	OpName      string              `json:"opName,omitempty"` // 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 +17,7 @@
 package logger
 
 import (
+	"encoding/json"
 	"math/big"
 	"testing"
 
@@ -72,3 +73,29 @@ 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  *structLogMarshaling
+		want string
+	}{
+		{"empty err and no fields", &structLogMarshaling{ErrorString: ""}, `{}`},
+		{"with Gas cost only", &structLogMarshaling{GasCost: 10}, `{"GasCost":"0xa"}`},
+		{"with err", &structLogMarshaling{ErrorString: "this failed"}, `{"error":"this failed"}`},
+	}
+
+	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 g, w := string(blob), tt.want; g != w {
+				t.Fatalf("Mismatched results\n\tGot:  %q\n\tWant: %q", g, w)
+			}
+		})
+	}
+}