Fix cycle pointer value

encode.go

@@ -22,6 +22,7 @@ type Encoder struct {
 	indent                         int
 	structTypeToCompiledCode       map[uintptr]*compiledCode
 	structTypeToCompiledIndentCode map[uintptr]*compiledCode
+	seenPtr                        map[uintptr]struct{}
 }
 
 type compiledCode struct {
@@ -67,6 +68,7 @@ func init() {
 				buf:                            make([]byte, 0, bufSize),
 				structTypeToCompiledCode:       map[uintptr]*compiledCode{},
 				structTypeToCompiledIndentCode: map[uintptr]*compiledCode{},
+				seenPtr:                        map[uintptr]struct{}{},
 			}
 		},
 	}

encode_vm.go

@@ -80,6 +80,13 @@ func (e *Encoder) run(code *opcode) error {
 				typ: ifaceCode.typ,
 				ptr: unsafe.Pointer(ptr),
 			}))
+			if _, exists := e.seenPtr[ptr]; exists {
+				return &UnsupportedValueError{
+					Value: reflect.ValueOf(v),
+					Str:   fmt.Sprintf("encountered a cycle via %s", code.typ),
+				}
+			}
+			e.seenPtr[ptr] = struct{}{}
 			rv := reflect.ValueOf(v)
 			if rv.IsNil() {
 				e.encodeNull()
@@ -498,6 +505,17 @@ func (e *Encoder) run(code *opcode) error {
 			code = c.next
 		case opStructFieldRecursive:
 			recursive := code.toRecursiveCode()
+			if _, exists := e.seenPtr[recursive.ptr]; exists {
+				v := *(*interface{})(unsafe.Pointer(&interfaceHeader{
+					typ: code.typ,
+					ptr: unsafe.Pointer(recursive.ptr),
+				}))
+				return &UnsupportedValueError{
+					Value: reflect.ValueOf(v),
+					Str:   fmt.Sprintf("encountered a cycle via %s", code.typ),
+				}
+			}
+			e.seenPtr[recursive.ptr] = struct{}{}
 			if err := e.run(newRecursiveCode(recursive)); err != nil {
 				return err
 			}