value-serializer.cc

// Copyright 2016 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "src/objects/value-serializer.h"

#include <type_traits>

#include "include/v8-maybe.h"
#include "include/v8-value-serializer-version.h"
#include "include/v8-value-serializer.h"
#include "include/v8-wasm.h"
#include "src/api/api-inl.h"
#include "src/base/logging.h"
#include "src/base/platform/memory.h"
#include "src/execution/isolate.h"
#include "src/flags/flags.h"
#include "src/handles/global-handles-inl.h"
#include "src/handles/handles-inl.h"
#include "src/handles/maybe-handles-inl.h"
#include "src/handles/shared-object-conveyor-handles.h"
#include "src/heap/factory.h"
#include "src/numbers/conversions.h"
#include "src/objects/heap-number-inl.h"
#include "src/objects/js-array-buffer-inl.h"
#include "src/objects/js-array-buffer.h"
#include "src/objects/js-array-inl.h"
#include "src/objects/js-collection-inl.h"
#include "src/objects/js-regexp-inl.h"
#include "src/objects/js-shared-array-inl.h"
#include "src/objects/js-struct-inl.h"
#include "src/objects/map-updater.h"
#include "src/objects/objects-inl.h"
#include "src/objects/objects.h"
#include "src/objects/oddball-inl.h"
#include "src/objects/ordered-hash-table-inl.h"
#include "src/objects/property-descriptor.h"
#include "src/objects/property-details.h"
#include "src/objects/smi.h"
#include "src/objects/transitions-inl.h"
#include "src/regexp/regexp.h"
#include "src/snapshot/code-serializer.h"

#if V8_ENABLE_WEBASSEMBLY
#include "src/wasm/wasm-objects-inl.h"
#endif  // V8_ENABLE_WEBASSEMBLY

namespace v8 {
namespace internal {

// WARNING: This serialization format MUST remain backward compatible!
//
// This format is used by APIs to persist values to disk, e.g. IndexedDB.
//
// Backward compatibility means that when the format changes, deserializing
// valid values in the older format must behave identically as before the
// change. To maintain compatibility, either a format change does not affect the
// deserializing behavior of valid values in the older format, or the
// kLatestVersion constant is bumped.
//
// Adding a new tag is backwards compatible because no valid serialized value in
// older formats would contain the new object tag.
//
// On the other hand, changing the format of a particular tag is backwards
// incompatible and the version must be bumped. For example, a JSArrayBufferView
// tag prior to version 14 was followed by the sub-tag, the byte offset, and the
// byte length. Starting with version 14, a JSArrayBufferView tag is followed by
// the sub-tag, the byte offset, the byte length, and flags. Due the addition of
// flags, older valid serialized values for JSArrayBufferViews would be
// misinterpreted by newer deserializers. This requires the version to be bumped
// and the deserializer to handle both the old and new formats depending on the
// version.

// Version 9: (imported from Blink)
// Version 10: one-byte (Latin-1) strings
// Version 11: properly separate undefined from the hole in arrays
// Version 12: regexp and string objects share normal string encoding
// Version 13: host objects have an explicit tag (rather than handling all
//             unknown tags)
// Version 14: flags for JSArrayBufferViews
// Version 15: support for shared objects with an explicit tag
//
// WARNING: Increasing this value is a change which cannot safely be rolled
// back without breaking compatibility with data stored on disk. It is
// strongly recommended that you do not make such changes near a release
// milestone branch point.
//
// Recent changes are routinely reverted in preparation for branch, and this
// has been the cause of at least one bug in the past.
static const uint32_t kLatestVersion = 15;
static_assert(kLatestVersion == v8::CurrentValueSerializerFormatVersion(),
              "Exported format version must match latest version.");

namespace {
// For serializing JSArrayBufferView flags. Instead of serializing /
// deserializing the flags directly, we serialize them bit by bit. This is for
// ensuring backwards compatibility in the case where the representation
// changes. Note that the ValueSerializer data can be stored on disk.
using JSArrayBufferViewIsLengthTracking = base::BitField<bool, 0, 1>;
using JSArrayBufferViewIsBackedByRab =
    JSArrayBufferViewIsLengthTracking::Next<bool, 1>;

}  // namespace

template <typename T>
static size_t BytesNeededForVarint(T value) {
  static_assert(std::is_integral<T>::value && std::is_unsigned<T>::value,
                "Only unsigned integer types can be written as varints.");
  size_t result = 0;
  do {
    result++;
    value >>= 7;
  } while (value);
  return result;
}

enum class SerializationTag : uint8_t {
  // version:uint32_t (if at beginning of data, sets version > 0)
  kVersion = 0xFF,
  // ignore
  kPadding = '\0',
  // refTableSize:uint32_t (previously used for sanity checks; safe to ignore)
  kVerifyObjectCount = '?',
  // Oddballs (no data).
  kTheHole = '-',
  kUndefined = '_',
  kNull = '0',
  kTrue = 'T',
  kFalse = 'F',
  // Number represented as 32-bit integer, ZigZag-encoded
  // (like sint32 in protobuf)
  kInt32 = 'I',
  // Number represented as 32-bit unsigned integer, varint-encoded
  // (like uint32 in protobuf)
  kUint32 = 'U',
  // Number represented as a 64-bit double.
  // Host byte order is used (N.B. this makes the format non-portable).
  kDouble = 'N',
  // BigInt. Bitfield:uint32_t, then raw digits storage.
  kBigInt = 'Z',
  // byteLength:uint32_t, then raw data
  kUtf8String = 'S',
  kOneByteString = '"',
  kTwoByteString = 'c',
  // Reference to a serialized object. objectID:uint32_t
  kObjectReference = '^',
  // Beginning of a JS object.
  kBeginJSObject = 'o',
  // End of a JS object. numProperties:uint32_t
  kEndJSObject = '{',
  // Beginning of a sparse JS array. length:uint32_t
  // Elements and properties are written as key/value pairs, like objects.
  kBeginSparseJSArray = 'a',
  // End of a sparse JS array. numProperties:uint32_t length:uint32_t
  kEndSparseJSArray = '@',
  // Beginning of a dense JS array. length:uint32_t
  // |length| elements, followed by properties as key/value pairs
  kBeginDenseJSArray = 'A',
  // End of a dense JS array. numProperties:uint32_t length:uint32_t
  kEndDenseJSArray = '$',
  // Date. millisSinceEpoch:double
  kDate = 'D',
  // Boolean object. No data.
  kTrueObject = 'y',
  kFalseObject = 'x',
  // Number object. value:double
  kNumberObject = 'n',
  // BigInt object. Bitfield:uint32_t, then raw digits storage.
  kBigIntObject = 'z',
  // String object, UTF-8 encoding. byteLength:uint32_t, then raw data.
  kStringObject = 's',
  // Regular expression, UTF-8 encoding. byteLength:uint32_t, raw data,
  // flags:uint32_t.
  kRegExp = 'R',
  // Beginning of a JS map.
  kBeginJSMap = ';',
  // End of a JS map. length:uint32_t.
  kEndJSMap = ':',
  // Beginning of a JS set.
  kBeginJSSet = '\'',
  // End of a JS set. length:uint32_t.
  kEndJSSet = ',',
  // Array buffer. byteLength:uint32_t, then raw data.
  kArrayBuffer = 'B',
  // Resizable ArrayBuffer.
  kResizableArrayBuffer = '~',
  // Array buffer (transferred). transferID:uint32_t
  kArrayBufferTransfer = 't',
  // View into an array buffer.
  // subtag:ArrayBufferViewTag, byteOffset:uint32_t, byteLength:uint32_t
  // For typed arrays, byteOffset and byteLength must be divisible by the size
  // of the element.
  // Note: kArrayBufferView is special, and should have an ArrayBuffer (or an
  // ObjectReference to one) serialized just before it. This is a quirk arising
  // from the previous stack-based implementation.
  kArrayBufferView = 'V',
  // Shared array buffer. transferID:uint32_t
  kSharedArrayBuffer = 'u',
  // A HeapObject shared across Isolates. sharedValueID:uint32_t
  kSharedObject = 'p',
  // A wasm module object transfer. next value is its index.
  kWasmModuleTransfer = 'w',
  // The delegate is responsible for processing all following data.
  // This "escapes" to whatever wire format the delegate chooses.
  kHostObject = '\\',
  // A transferred WebAssembly.Memory object. maximumPages:int32_t, then by
  // SharedArrayBuffer tag and its data.
  kWasmMemoryTransfer = 'm',
  // A list of (subtag: ErrorTag, [subtag dependent data]). See ErrorTag for
  // details.
  kError = 'r',

  // The following tags are reserved because they were in use in Chromium before
  // the kHostObject tag was introduced in format version 13, at
  //   v8           refs/heads/master@{#43466}
  //   chromium/src refs/heads/master@{#453568}
  //
  // They must not be reused without a version check to prevent old values from
  // starting to deserialize incorrectly. For simplicity, it's recommended to
  // avoid them altogether.
  //
  // This is the set of tags that existed in SerializationTag.h at that time and
  // still exist at the time of this writing (i.e., excluding those that were
  // removed on the Chromium side because there should be no real user data
  // containing them).
  //
  // It might be possible to also free up other tags which were never persisted
  // (e.g. because they were used only for transfer) in the future.
  kLegacyReservedMessagePort = 'M',
  kLegacyReservedBlob = 'b',
  kLegacyReservedBlobIndex = 'i',
  kLegacyReservedFile = 'f',
  kLegacyReservedFileIndex = 'e',
  kLegacyReservedDOMFileSystem = 'd',
  kLegacyReservedFileList = 'l',
  kLegacyReservedFileListIndex = 'L',
  kLegacyReservedImageData = '#',
  kLegacyReservedImageBitmap = 'g',
  kLegacyReservedImageBitmapTransfer = 'G',
  kLegacyReservedOffscreenCanvas = 'H',
  kLegacyReservedCryptoKey = 'K',
  kLegacyReservedRTCCertificate = 'k',
};

namespace {

enum class ArrayBufferViewTag : uint8_t {
  kInt8Array = 'b',
  kUint8Array = 'B',
  kUint8ClampedArray = 'C',
  kInt16Array = 'w',
  kUint16Array = 'W',
  kInt32Array = 'd',
  kUint32Array = 'D',
  kFloat16Array = 'h',
  kFloat32Array = 'f',
  kFloat64Array = 'F',
  kBigInt64Array = 'q',
  kBigUint64Array = 'Q',
  kDataView = '?',
};

// Sub-tags only meaningful for error serialization.
enum class ErrorTag : uint8_t {
  // The error is a EvalError. No accompanying data.
  kEvalErrorPrototype = 'E',
  // The error is a RangeError. No accompanying data.
  kRangeErrorPrototype = 'R',
  // The error is a ReferenceError. No accompanying data.
  kReferenceErrorPrototype = 'F',
  // The error is a SyntaxError. No accompanying data.
  kSyntaxErrorPrototype = 'S',
  // The error is a TypeError. No accompanying data.
  kTypeErrorPrototype = 'T',
  // The error is a URIError. No accompanying data.
  kUriErrorPrototype = 'U',
  // Followed by message: string.
  kMessage = 'm',
  // Followed by a JS object: cause.
  kCause = 'c',
  // Followed by stack: string.
  kStack = 's',
  // The end of this error information.
  kEnd = '.',
};

}  // namespace

ValueSerializer::ValueSerializer(Isolate* isolate,
                                 v8::ValueSerializer::Delegate* delegate)
    : isolate_(isolate),
      delegate_(delegate),
      zone_(isolate->allocator(), ZONE_NAME),
      id_map_(isolate->heap(), ZoneAllocationPolicy(&zone_)),
      array_buffer_transfer_map_(isolate->heap(),
                                 ZoneAllocationPolicy(&zone_)) {
  if (delegate_) {
    v8::Isolate* v8_isolate = reinterpret_cast<v8::Isolate*>(isolate_);
    has_custom_host_objects_ = delegate_->HasCustomHostObject(v8_isolate);
  }
}

ValueSerializer::~ValueSerializer() {
  if (buffer_) {
    if (delegate_) {
      delegate_->FreeBufferMemory(buffer_);
    } else {
      base::Free(buffer_);
    }
  }
}

void ValueSerializer::WriteHeader() {
  WriteTag(SerializationTag::kVersion);
  WriteVarint(kLatestVersion);
}

void ValueSerializer::SetTreatArrayBufferViewsAsHostObjects(bool mode) {
  treat_array_buffer_views_as_host_objects_ = mode;
}

void ValueSerializer::WriteTag(SerializationTag tag) {
  uint8_t raw_tag = static_cast<uint8_t>(tag);
  WriteRawBytes(&raw_tag, sizeof(raw_tag));
}

template <typename T>
void ValueSerializer::WriteVarint(T value) {
  // Writes an unsigned integer as a base-128 varint.
  // The number is written, 7 bits at a time, from the least significant to the
  // most significant 7 bits. Each byte, except the last, has the MSB set.
  // See also https://developers.google.com/protocol-buffers/docs/encoding
  static_assert(std::is_integral<T>::value && std::is_unsigned<T>::value,
                "Only unsigned integer types can be written as varints.");
  uint8_t stack_buffer[sizeof(T) * 8 / 7 + 1];
  uint8_t* next_byte = &stack_buffer[0];
  do {
    *next_byte = (value & 0x7F) | 0x80;
    next_byte++;
    value >>= 7;
  } while (value);
  *(next_byte - 1) &= 0x7F;
  WriteRawBytes(stack_buffer, next_byte - stack_buffer);
}

template <typename T>
void ValueSerializer::WriteZigZag(T value) {
  // Writes a signed integer as a varint using ZigZag encoding (i.e. 0 is
  // encoded as 0, -1 as 1, 1 as 2, -2 as 3, and so on).
  // See also https://developers.google.com/protocol-buffers/docs/encoding
  // Note that this implementation relies on the right shift being arithmetic.
  static_assert(std::is_integral<T>::value && std::is_signed<T>::value,
                "Only signed integer types can be written as zigzag.");
  using UnsignedT = typename std::make_unsigned<T>::type;
  WriteVarint((static_cast<UnsignedT>(value) << 1) ^
              (value >> (8 * sizeof(T) - 1)));
}

template EXPORT_TEMPLATE_DEFINE(
    V8_EXPORT_PRIVATE) void ValueSerializer::WriteZigZag(int32_t value);

void ValueSerializer::WriteDouble(double value) {
  // Warning: this uses host endianness.
  WriteRawBytes(&value, sizeof(value));
}

void ValueSerializer::WriteOneByteString(base::Vector<const uint8_t> chars) {
  WriteVarint<uint32_t>(chars.length());
  WriteRawBytes(chars.begin(), chars.length() * sizeof(uint8_t));
}

void ValueSerializer::WriteTwoByteString(base::Vector<const base::uc16> chars) {
  // Warning: this uses host endianness.
  WriteVarint<uint32_t>(chars.length() * sizeof(base::uc16));
  WriteRawBytes(chars.begin(), chars.length() * sizeof(base::uc16));
}

void ValueSerializer::WriteBigIntContents(Tagged<BigInt> bigint) {
  uint32_t bitfield = bigint->GetBitfieldForSerialization();
  int bytelength = BigInt::DigitsByteLengthForBitfield(bitfield);
  WriteVarint<uint32_t>(bitfield);
  uint8_t* dest;
  if (ReserveRawBytes(bytelength).To(&dest)) {
    bigint->SerializeDigits(dest);
  }
}

void ValueSerializer::WriteRawBytes(const void* source, size_t length) {
  uint8_t* dest;
  if (ReserveRawBytes(length).To(&dest) && length > 0) {
    memcpy(dest, source, length);
  }
}

Maybe<uint8_t*> ValueSerializer::ReserveRawBytes(size_t bytes) {
  size_t old_size = buffer_size_;
  size_t new_size = old_size + bytes;
  if (V8_UNLIKELY(new_size > buffer_capacity_)) {
    bool ok;
    if (!ExpandBuffer(new_size).To(&ok)) {
      return Nothing<uint8_t*>();
    }
  }
  buffer_size_ = new_size;
  return Just(&buffer_[old_size]);
}

Maybe<bool> ValueSerializer::ExpandBuffer(size_t required_capacity) {
  DCHECK_GT(required_capacity, buffer_capacity_);
  size_t requested_capacity =
      std::max(required_capacity, buffer_capacity_ * 2) + 64;
  size_t provided_capacity = 0;
  void* new_buffer = nullptr;
  if (delegate_) {
    new_buffer = delegate_->ReallocateBufferMemory(buffer_, requested_capacity,
                                                   &provided_capacity);
  } else {
    new_buffer = base::Realloc(buffer_, requested_capacity);
    provided_capacity = requested_capacity;
  }
  if (new_buffer) {
    DCHECK(provided_capacity >= requested_capacity);
    buffer_ = reinterpret_cast<uint8_t*>(new_buffer);
    buffer_capacity_ = provided_capacity;
    return Just(true);
  } else {
    out_of_memory_ = true;
    return Nothing<bool>();
  }
}

void ValueSerializer::WriteByte(uint8_t value) {
  uint8_t* dest;
  if (ReserveRawBytes(sizeof(uint8_t)).To(&dest)) {
    *dest = value;
  }
}

void ValueSerializer::WriteUint32(uint32_t value) {
  WriteVarint<uint32_t>(value);
}

void ValueSerializer::WriteUint64(uint64_t value) {
  WriteVarint<uint64_t>(value);
}

std::pair<uint8_t*, size_t> ValueSerializer::Release() {
  auto result = std::make_pair(buffer_, buffer_size_);
  buffer_ = nullptr;
  buffer_size_ = 0;
  buffer_capacity_ = 0;
  return result;
}

void ValueSerializer::TransferArrayBuffer(uint32_t transfer_id,
                                          Handle<JSArrayBuffer> array_buffer) {
  DCHECK(!array_buffer_transfer_map_.Find(array_buffer));
  DCHECK(!array_buffer->is_shared());
  array_buffer_transfer_map_.Insert(array_buffer, transfer_id);
}

Maybe<bool> ValueSerializer::WriteObject(Handle<Object> object) {
  // There is no sense in trying to proceed if we've previously run out of
  // memory. Bail immediately, as this likely implies that some write has
  // previously failed and so the buffer is corrupt.
  if (V8_UNLIKELY(out_of_memory_)) return ThrowIfOutOfMemory();

  if (IsSmi(*object)) {
    WriteSmi(Smi::cast(*object));
    return ThrowIfOutOfMemory();
  }

  DCHECK(IsHeapObject(*object));
  InstanceType instance_type =
      HeapObject::cast(*object)->map(isolate_)->instance_type();
  switch (instance_type) {
    case ODDBALL_TYPE:
      WriteOddball(Oddball::cast(*object));
      return ThrowIfOutOfMemory();
    case HEAP_NUMBER_TYPE:
      WriteHeapNumber(HeapNumber::cast(*object));
      return ThrowIfOutOfMemory();
    case BIGINT_TYPE:
      WriteBigInt(BigInt::cast(*object));
      return ThrowIfOutOfMemory();
    case JS_TYPED_ARRAY_TYPE:
    case JS_DATA_VIEW_TYPE:
    case JS_RAB_GSAB_DATA_VIEW_TYPE: {
      // Despite being JSReceivers, these have their wrapped buffer serialized
      // first. That makes this logic a little quirky, because it needs to
      // happen before we assign object IDs.
      // TODO(jbroman): It may be possible to avoid materializing a typed
      // array's buffer here.
      Handle<JSArrayBufferView> view = Handle<JSArrayBufferView>::cast(object);
      if (!id_map_.Find(view) && !treat_array_buffer_views_as_host_objects_) {
        Handle<JSArrayBuffer> buffer(
            InstanceTypeChecker::IsJSTypedArray(instance_type)
                ? Handle<JSTypedArray>::cast(view)->GetBuffer()
                : handle(JSArrayBuffer::cast(view->buffer()), isolate_));
        if (!WriteJSReceiver(buffer).FromMaybe(false)) return Nothing<bool>();
      }
      return WriteJSReceiver(view);
    }
    default:
      if (InstanceTypeChecker::IsString(instance_type)) {
        WriteString(Handle<String>::cast(object));
        return ThrowIfOutOfMemory();
      } else if (InstanceTypeChecker::IsJSReceiver(instance_type)) {
        return WriteJSReceiver(Handle<JSReceiver>::cast(object));
      } else {
        return ThrowDataCloneError(MessageTemplate::kDataCloneError, object);
      }
  }
}

void ValueSerializer::WriteOddball(Tagged<Oddball> oddball) {
  SerializationTag tag = SerializationTag::kUndefined;
  switch (oddball->kind()) {
    case Oddball::kUndefined:
      tag = SerializationTag::kUndefined;
      break;
    case Oddball::kFalse:
      tag = SerializationTag::kFalse;
      break;
    case Oddball::kTrue:
      tag = SerializationTag::kTrue;
      break;
    case Oddball::kNull:
      tag = SerializationTag::kNull;
      break;
    default:
      UNREACHABLE();
  }
  WriteTag(tag);
}

void ValueSerializer::WriteSmi(Tagged<Smi> smi) {
  static_assert(kSmiValueSize <= 32, "Expected SMI <= 32 bits.");
  WriteTag(SerializationTag::kInt32);
  WriteZigZag<int32_t>(smi.value());
}

void ValueSerializer::WriteHeapNumber(Tagged<HeapNumber> number) {
  WriteTag(SerializationTag::kDouble);
  WriteDouble(number->value());
}

void ValueSerializer::WriteBigInt(Tagged<BigInt> bigint) {
  WriteTag(SerializationTag::kBigInt);
  WriteBigIntContents(bigint);
}

void ValueSerializer::WriteString(Handle<String> string) {
  string = String::Flatten(isolate_, string);
  DisallowGarbageCollection no_gc;
  String::FlatContent flat = string->GetFlatContent(no_gc);
  DCHECK(flat.IsFlat());
  if (flat.IsOneByte()) {
    base::Vector<const uint8_t> chars = flat.ToOneByteVector();
    WriteTag(SerializationTag::kOneByteString);
    WriteOneByteString(chars);
  } else if (flat.IsTwoByte()) {
    base::Vector<const base::uc16> chars = flat.ToUC16Vector();
    uint32_t byte_length = chars.length() * sizeof(base::uc16);
    // The existing reading code expects 16-byte strings to be aligned.
    if ((buffer_size_ + 1 + BytesNeededForVarint(byte_length)) & 1)
      WriteTag(SerializationTag::kPadding);
    WriteTag(SerializationTag::kTwoByteString);
    WriteTwoByteString(chars);
  } else {
    UNREACHABLE();
  }
}

Maybe<bool> ValueSerializer::WriteJSReceiver(Handle<JSReceiver> receiver) {
  // If the object has already been serialized, just write its ID.
  auto find_result = id_map_.FindOrInsert(receiver);
  if (find_result.already_exists) {
    WriteTag(SerializationTag::kObjectReference);
    WriteVarint(*find_result.entry - 1);
    return ThrowIfOutOfMemory();
  }

  // Otherwise, allocate an ID for it.
  uint32_t id = next_id_++;
  *find_result.entry = id + 1;

  // Eliminate callable and exotic objects, which should not be serialized.
  InstanceType instance_type = receiver->map()->instance_type();
  if (IsCallable(*receiver) || (IsSpecialReceiverInstanceType(instance_type) &&
                                instance_type != JS_SPECIAL_API_OBJECT_TYPE)) {
    return ThrowDataCloneError(MessageTemplate::kDataCloneError, receiver);
  }

  // If we are at the end of the stack, abort. This function may recurse.
  STACK_CHECK(isolate_, Nothing<bool>());

  HandleScope scope(isolate_);
  switch (instance_type) {
    case JS_ARRAY_TYPE:
      return WriteJSArray(Handle<JSArray>::cast(receiver));
    case JS_ARRAY_ITERATOR_PROTOTYPE_TYPE:
    case JS_ITERATOR_PROTOTYPE_TYPE:
    case JS_MAP_ITERATOR_PROTOTYPE_TYPE:
    case JS_OBJECT_PROTOTYPE_TYPE:
    case JS_OBJECT_TYPE:
    case JS_PROMISE_PROTOTYPE_TYPE:
    case JS_REG_EXP_PROTOTYPE_TYPE:
    case JS_SET_ITERATOR_PROTOTYPE_TYPE:
    case JS_SET_PROTOTYPE_TYPE:
    case JS_STRING_ITERATOR_PROTOTYPE_TYPE:
    case JS_TYPED_ARRAY_PROTOTYPE_TYPE:
    case JS_API_OBJECT_TYPE: {
      Handle<JSObject> js_object = Handle<JSObject>::cast(receiver);
      Maybe<bool> is_host_object = IsHostObject(js_object);
      if (is_host_object.IsNothing()) {
        return is_host_object;
      }
      if (is_host_object.FromJust()) {
        return WriteHostObject(js_object);
      } else {
        return WriteJSObject(js_object);
      }
    }
    case JS_SPECIAL_API_OBJECT_TYPE:
      return WriteHostObject(Handle<JSObject>::cast(receiver));
    case JS_DATE_TYPE:
      WriteJSDate(JSDate::cast(*receiver));
      return ThrowIfOutOfMemory();
    case JS_PRIMITIVE_WRAPPER_TYPE:
      return WriteJSPrimitiveWrapper(
          Handle<JSPrimitiveWrapper>::cast(receiver));
    case JS_REG_EXP_TYPE:
      WriteJSRegExp(Handle<JSRegExp>::cast(receiver));
      return ThrowIfOutOfMemory();
    case JS_MAP_TYPE:
      return WriteJSMap(Handle<JSMap>::cast(receiver));
    case JS_SET_TYPE:
      return WriteJSSet(Handle<JSSet>::cast(receiver));
    case JS_ARRAY_BUFFER_TYPE:
      return WriteJSArrayBuffer(Handle<JSArrayBuffer>::cast(receiver));
    case JS_TYPED_ARRAY_TYPE:
    case JS_DATA_VIEW_TYPE:
    case JS_RAB_GSAB_DATA_VIEW_TYPE:
      return WriteJSArrayBufferView(JSArrayBufferView::cast(*receiver));
    case JS_ERROR_TYPE: {
      Handle<JSObject> js_error = Handle<JSObject>::cast(receiver);
      Maybe<bool> is_host_object = IsHostObject(js_error);
      if (is_host_object.IsNothing()) {
        return is_host_object;
      }
      if (is_host_object.FromJust()) {
        return WriteHostObject(js_error);
      }
      return WriteJSError(js_error);
    }
    case JS_SHARED_ARRAY_TYPE:
      return WriteJSSharedArray(Handle<JSSharedArray>::cast(receiver));
    case JS_SHARED_STRUCT_TYPE:
      return WriteJSSharedStruct(Handle<JSSharedStruct>::cast(receiver));
    case JS_ATOMICS_MUTEX_TYPE:
    case JS_ATOMICS_CONDITION_TYPE:
      return WriteSharedObject(receiver);
#if V8_ENABLE_WEBASSEMBLY
    case WASM_MODULE_OBJECT_TYPE:
      return WriteWasmModule(Handle<WasmModuleObject>::cast(receiver));
    case WASM_MEMORY_OBJECT_TYPE:
      return WriteWasmMemory(Handle<WasmMemoryObject>::cast(receiver));
#endif  // V8_ENABLE_WEBASSEMBLY
    default:
      break;
  }

  return ThrowDataCloneError(MessageTemplate::kDataCloneError, receiver);
}

Maybe<bool> ValueSerializer::WriteJSObject(Handle<JSObject> object) {
  DCHECK(!IsCustomElementsReceiverMap(object->map()));
  const bool can_serialize_fast =
      object->HasFastProperties(isolate_) && object->elements()->length() == 0;
  if (!can_serialize_fast) return WriteJSObjectSlow(object);

  Handle<Map> map(object->map(), isolate_);
  WriteTag(SerializationTag::kBeginJSObject);

  // Write out fast properties as long as they are only data properties and the
  // map doesn't change.
  uint32_t properties_written = 0;
  bool map_changed = false;
  for (InternalIndex i : map->IterateOwnDescriptors()) {
    Handle<Name> key(map->instance_descriptors(isolate_)->GetKey(i), isolate_);
    if (!IsString(*key, isolate_)) continue;
    PropertyDetails details =
        map->instance_descriptors(isolate_)->GetDetails(i);
    if (details.IsDontEnum()) continue;

    Handle<Object> value;
    if (V8_LIKELY(!map_changed)) map_changed = *map != object->map();
    if (V8_LIKELY(!map_changed &&
                  details.location() == PropertyLocation::kField)) {
      DCHECK_EQ(PropertyKind::kData, details.kind());
      FieldIndex field_index = FieldIndex::ForDetails(*map, details);
      value = JSObject::FastPropertyAt(isolate_, object,
                                       details.representation(), field_index);
    } else {
      // This logic should essentially match WriteJSObjectPropertiesSlow.
      // If the property is no longer found, do not serialize it.
      // This could happen if a getter deleted the property.
      LookupIterator it(isolate_, object, key, LookupIterator::OWN);
      if (!it.IsFound()) continue;
      if (!Object::GetProperty(&it).ToHandle(&value)) return Nothing<bool>();
    }

    if (!WriteObject(key).FromMaybe(false) ||
        !WriteObject(value).FromMaybe(false)) {
      return Nothing<bool>();
    }
    properties_written++;
  }

  WriteTag(SerializationTag::kEndJSObject);
  WriteVarint<uint32_t>(properties_written);
  return ThrowIfOutOfMemory();
}

Maybe<bool> ValueSerializer::WriteJSObjectSlow(Handle<JSObject> object) {
  WriteTag(SerializationTag::kBeginJSObject);
  Handle<FixedArray> keys;
  uint32_t properties_written = 0;
  if (!KeyAccumulator::GetKeys(isolate_, object, KeyCollectionMode::kOwnOnly,
                               ENUMERABLE_STRINGS)
           .ToHandle(&keys) ||
      !WriteJSObjectPropertiesSlow(object, keys).To(&properties_written)) {
    return Nothing<bool>();
  }
  WriteTag(SerializationTag::kEndJSObject);
  WriteVarint<uint32_t>(properties_written);
  return ThrowIfOutOfMemory();
}

Maybe<bool> ValueSerializer::WriteJSArray(Handle<JSArray> array) {
  PtrComprCageBase cage_base(isolate_);
  uint32_t length = 0;
  bool valid_length = Object::ToArrayLength(array->length(), &length);
  DCHECK(valid_length);
  USE(valid_length);

  // To keep things simple, for now we decide between dense and sparse
  // serialization based on elements kind. A more principled heuristic could
  // count the elements, but would need to take care to note which indices
  // existed (as only indices which were enumerable own properties at this point
  // should be serialized).
  const bool should_serialize_densely =
      array->HasFastElements(cage_base) && !array->HasHoleyElements(cage_base);

  if (should_serialize_densely) {
    DCHECK_LE(length, static_cast<uint32_t>(FixedArray::kMaxLength));
    WriteTag(SerializationTag::kBeginDenseJSArray);
    WriteVarint<uint32_t>(length);
    uint32_t i = 0;

    // Fast paths. Note that PACKED_ELEMENTS in particular can bail due to the
    // structure of the elements changing.
    switch (array->GetElementsKind(cage_base)) {
      case PACKED_SMI_ELEMENTS: {
        DisallowGarbageCollection no_gc;
        Tagged<FixedArray> elements = FixedArray::cast(array->elements());
        for (i = 0; i < length; i++) {
          WriteSmi(Smi::cast(elements->get(i)));
        }
        break;
      }
      case PACKED_DOUBLE_ELEMENTS: {
        // Elements are empty_fixed_array, not a FixedDoubleArray, if the array
        // is empty. No elements to encode in this case anyhow.
        if (length == 0) break;
        DisallowGarbageCollection no_gc;
        Tagged<FixedDoubleArray> elements =
            FixedDoubleArray::cast(array->elements());
        for (i = 0; i < length; i++) {
          WriteTag(SerializationTag::kDouble);
          WriteDouble(elements->get_scalar(i));
        }
        break;
      }
      case PACKED_ELEMENTS: {
        Handle<Object> old_length(array->length(cage_base), isolate_);
        for (; i < length; i++) {
          if (array->length(cage_base) != *old_length ||
              array->GetElementsKind(cage_base) != PACKED_ELEMENTS) {
            // Fall back to slow path.
            break;
          }
          Handle<Object> element(FixedArray::cast(array->elements())->get(i),
                                 isolate_);
          if (!WriteObject(element).FromMaybe(false)) return Nothing<bool>();
        }
        break;
      }
      default:
        break;
    }

    // If there are elements remaining, serialize them slowly.
    for (; i < length; i++) {
      // Serializing the array's elements can have arbitrary side effects, so we
      // cannot rely on still having fast elements, even if it did to begin
      // with.
      Handle<Object> element;
      LookupIterator it(isolate_, array, i, array, LookupIterator::OWN);
      if (!it.IsFound()) {
        // This can happen in the case where an array that was originally dense
        // became sparse during serialization. It's too late to switch to the
        // sparse format, but we can mark the elements as absent.
        WriteTag(SerializationTag::kTheHole);
        continue;
      }
      if (!Object::GetProperty(&it).ToHandle(&element) ||
          !WriteObject(element).FromMaybe(false)) {
        return Nothing<bool>();
      }
    }

    Handle<FixedArray> keys;
    if (!KeyAccumulator::GetKeys(isolate_, array, KeyCollectionMode::kOwnOnly,
                                 ENUMERABLE_STRINGS,
                                 GetKeysConversion::kKeepNumbers, false, true)
             .ToHandle(&keys)) {
      return Nothing<bool>();
    }

    uint32_t properties_written;
    if (!WriteJSObjectPropertiesSlow(array, keys).To(&properties_written)) {
      return Nothing<bool>();
    }
    WriteTag(SerializationTag::kEndDenseJSArray);
    WriteVarint<uint32_t>(properties_written);
    WriteVarint<uint32_t>(length);
  } else {
    WriteTag(SerializationTag::kBeginSparseJSArray);
    WriteVarint<uint32_t>(length);
    Handle<FixedArray> keys;
    uint32_t properties_written = 0;
    if (!KeyAccumulator::GetKeys(isolate_, array, KeyCollectionMode::kOwnOnly,
                                 ENUMERABLE_STRINGS)
             .ToHandle(&keys) ||
        !WriteJSObjectPropertiesSlow(array, keys).To(&properties_written)) {
      return Nothing<bool>();
    }
    WriteTag(SerializationTag::kEndSparseJSArray);
    WriteVarint<uint32_t>(properties_written);
    WriteVarint<uint32_t>(length);
  }
  return ThrowIfOutOfMemory();
}

void ValueSerializer::WriteJSDate(Tagged<JSDate> date) {
  WriteTag(SerializationTag::kDate);
  WriteDouble(Object::Number(date->value()));
}

Maybe<bool> ValueSerializer::WriteJSPrimitiveWrapper(
    Handle<JSPrimitiveWrapper> value) {
  PtrComprCageBase cage_base(isolate_);
  {
    DisallowGarbageCollection no_gc;
    Tagged<Object> inner_value = value->value();
    if (IsTrue(inner_value, isolate_)) {
      WriteTag(SerializationTag::kTrueObject);
    } else if (IsFalse(inner_value, isolate_)) {
      WriteTag(SerializationTag::kFalseObject);
    } else if (IsNumber(inner_value, cage_base)) {
      WriteTag(SerializationTag::kNumberObject);
      WriteDouble(Object::Number(inner_value));
    } else if (IsBigInt(inner_value, cage_base)) {
      WriteTag(SerializationTag::kBigIntObject);
      WriteBigIntContents(BigInt::cast(inner_value));
    } else if (IsString(inner_value, cage_base)) {
      WriteTag(SerializationTag::kStringObject);
      WriteString(handle(String::cast(inner_value), isolate_));
    } else {
      AllowGarbageCollection allow_gc;
      DCHECK(IsSymbol(inner_value));
      return ThrowDataCloneError(MessageTemplate::kDataCloneError, value);
    }
  }
  return ThrowIfOutOfMemory();
}

void ValueSerializer::WriteJSRegExp(Handle<JSRegExp> regexp) {
  WriteTag(SerializationTag::kRegExp);
  WriteString(handle(regexp->source(), isolate_));
  WriteVarint(static_cast<uint32_t>(regexp->flags()));
}

Maybe<bool> ValueSerializer::WriteJSMap(Handle<JSMap> js_map) {
  // First copy the key-value pairs, since getters could mutate them.
  Handle<OrderedHashMap> table(OrderedHashMap::cast(js_map->table()), isolate_);
  int length = table->NumberOfElements() * 2;
  Handle<FixedArray> entries = isolate_->factory()->NewFixedArray(length);
  {
    DisallowGarbageCollection no_gc;
    Tagged<OrderedHashMap> raw_table = *table;
    Tagged<FixedArray> raw_entries = *entries;
    Tagged<Hole> hash_table_hole =
        ReadOnlyRoots(isolate_).hash_table_hole_value();
    int result_index = 0;
    for (InternalIndex entry : raw_table->IterateEntries()) {
      Tagged<Object> key = raw_table->KeyAt(entry);
      if (key == hash_table_hole) continue;
      raw_entries->set(result_index++, key);
      raw_entries->set(result_index++, raw_table->ValueAt(entry));
    }
    DCHECK_EQ(result_index, length);
  }

  // Then write it out.
  WriteTag(SerializationTag::kBeginJSMap);
  for (int i = 0; i < length; i++) {
    if (!WriteObject(handle(entries->get(i), isolate_)).FromMaybe(false)) {
      return Nothing<bool>();
    }
  }
  WriteTag(SerializationTag::kEndJSMap);
  WriteVarint<uint32_t>(length);
  return ThrowIfOutOfMemory();
}

Maybe<bool> ValueSerializer::WriteJSSet(Handle<JSSet> js_set) {
  // First copy the element pointers, since getters could mutate them.
  Handle<OrderedHashSet> table(OrderedHashSet::cast(js_set->table()), isolate_);
  int length = table->NumberOfElements();
  Handle<FixedArray> entries = isolate_->factory()->NewFixedArray(length);
  {
    DisallowGarbageCollection no_gc;
    Tagged<OrderedHashSet> raw_table = *table;
    Tagged<FixedArray> raw_entries = *entries;
    Tagged<Hole> hash_table_hole =
        ReadOnlyRoots(isolate_).hash_table_hole_value();
    int result_index = 0;
    for (InternalIndex entry : raw_table->IterateEntries()) {
      Tagged<Object> key = raw_table->KeyAt(entry);
      if (key == hash_table_hole) continue;
      raw_entries->set(result_index++, key);
    }
    DCHECK_EQ(result_index, length);
  }

  // Then write it out.
  WriteTag(SerializationTag::kBeginJSSet);
  for (int i = 0; i < length; i++) {
    if (!WriteObject(handle(entries->get(i), isolate_)).FromMaybe(false)) {
      return Nothing<bool>();
    }
  }
  WriteTag(SerializationTag::kEndJSSet);
  WriteVarint<uint32_t>(length);
  return ThrowIfOutOfMemory();
}

Maybe<bool> ValueSerializer::WriteJSArrayBuffer(
    Handle<JSArrayBuffer> array_buffer) {
  if (array_buffer->is_shared()) {
    if (!delegate_) {
      return ThrowDataCloneError(MessageTemplate::kDataCloneError,
                                 array_buffer);
    }

    v8::Isolate* v8_isolate = reinterpret_cast<v8::Isolate*>(isolate_);
    Maybe<uint32_t> index = delegate_->GetSharedArrayBufferId(
        v8_isolate, Utils::ToLocalShared(array_buffer));
    RETURN_VALUE_IF_EXCEPTION(isolate_, Nothing<bool>());

    WriteTag(SerializationTag::kSharedArrayBuffer);
    WriteVarint(index.FromJust());
    return ThrowIfOutOfMemory();
  }

  uint32_t* transfer_entry = array_buffer_transfer_map_.Find(array_buffer);
  if (transfer_entry) {
    WriteTag(SerializationTag::kArrayBufferTransfer);
    WriteVarint(*transfer_entry);
    return ThrowIfOutOfMemory();
  }
  if (array_buffer->was_detached()) {
    return ThrowDataCloneError(
        MessageTemplate::kDataCloneErrorDetachedArrayBuffer);
  }
  size_t byte_length = array_buffer->byte_length();
  if (byte_length > std::numeric_limits<uint32_t>::max()) {
    return ThrowDataCloneError(MessageTemplate::kDataCloneError, array_buffer);
  }
  if (array_buffer->is_resizable_by_js()) {
    size_t max_byte_length = array_buffer->max_byte_length();
    if (max_byte_length > std::numeric_limits<uint32_t>::max()) {
      return ThrowDataCloneError(MessageTemplate::kDataCloneError,
                                 array_buffer);
    }

    WriteTag(SerializationTag::kResizableArrayBuffer);
    WriteVarint<uint32_t>(static_cast<uint32_t>(byte_length));
    WriteVarint<uint32_t>(static_cast<uint32_t>(max_byte_length));
    WriteRawBytes(array_buffer->backing_store(), byte_length);
    return ThrowIfOutOfMemory();
  }
  WriteTag(SerializationTag::kArrayBuffer);
  WriteVarint<uint32_t>(static_cast<uint32_t>(byte_length));
  WriteRawBytes(array_buffer->backing_store(), byte_length);
  return ThrowIfOutOfMemory();
}

Maybe<bool> ValueSerializer::WriteJSArrayBufferView(
    Tagged<JSArrayBufferView> view) {
  if (treat_array_buffer_views_as_host_objects_) {
    return WriteHostObject(handle(view, isolate_));
  }
  WriteTag(SerializationTag::kArrayBufferView);
  ArrayBufferViewTag tag = ArrayBufferViewTag::kInt8Array;
  if (IsJSTypedArray(view)) {
    if (JSTypedArray::cast(view)->IsOutOfBounds()) {
      return ThrowDataCloneError(MessageTemplate::kDataCloneError,
                                 handle(view, isolate_));
    }
    switch (JSTypedArray::cast(view)->type()) {
#define TYPED_ARRAY_CASE(Type, type, TYPE, ctype) \
  case kExternal##Type##Array:                    \
    tag = ArrayBufferViewTag::k##Type##Array;     \
    break;
      TYPED_ARRAYS(TYPED_ARRAY_CASE)
#undef TYPED_ARRAY_CASE
    }
  } else {
    DCHECK(IsJSDataViewOrRabGsabDataView(view));
    if (IsJSRabGsabDataView(view) &&
        JSRabGsabDataView::cast(view)->IsOutOfBounds()) {
      return ThrowDataCloneError(MessageTemplate::kDataCloneError,
                                 handle(view, isolate_));
    }

    tag = ArrayBufferViewTag::kDataView;
  }
  WriteVarint(static_cast<uint8_t>(tag));
  WriteVarint(static_cast<uint32_t>(view->byte_offset()));
  WriteVarint(static_cast<uint32_t>(view->byte_length()));
  uint32_t flags =
      JSArrayBufferViewIsLengthTracking::encode(view->is_length_tracking()) |
      JSArrayBufferViewIsBackedByRab::encode(view->is_backed_by_rab());
  WriteVarint(flags);
  return ThrowIfOutOfMemory();
}

Maybe<bool> ValueSerializer::WriteJSError(Handle<JSObject> error) {
  Handle<Object> stack;
  PropertyDescriptor message_desc;
  Maybe<bool> message_found = JSReceiver::GetOwnPropertyDescriptor(
      isolate_, error, isolate_->factory()->message_string(), &message_desc);
  MAYBE_RETURN(message_found, Nothing<bool>());
  PropertyDescriptor cause_desc;
  Maybe<bool> cause_found = JSReceiver::GetOwnPropertyDescriptor(
      isolate_, error, isolate_->factory()->cause_string(), &cause_desc);

  WriteTag(SerializationTag::kError);

  Handle<Object> name_object;
  if (!JSObject::GetProperty(isolate_, error, "name").ToHandle(&name_object)) {
    return Nothing<bool>();
  }
  Handle<String> name;
  if (!Object::ToString(isolate_, name_object).ToHandle(&name)) {
    return Nothing<bool>();
  }

  if (name->IsOneByteEqualTo(base::CStrVector("EvalError"))) {
    WriteVarint(static_cast<uint8_t>(ErrorTag::kEvalErrorPrototype));
  } else if (name->IsOneByteEqualTo(base::CStrVector("RangeError"))) {
    WriteVarint(static_cast<uint8_t>(ErrorTag::kRangeErrorPrototype));
  } else if (name->IsOneByteEqualTo(base::CStrVector("ReferenceError"))) {
    WriteVarint(static_cast<uint8_t>(ErrorTag::kReferenceErrorPrototype));
  } else if (name->IsOneByteEqualTo(base::CStrVector("SyntaxError"))) {
    WriteVarint(static_cast<uint8_t>(ErrorTag::kSyntaxErrorPrototype));
  } else if (name->IsOneByteEqualTo(base::CStrVector("TypeError"))) {
    WriteVarint(static_cast<uint8_t>(ErrorTag::kTypeErrorPrototype));
  } else if (name->IsOneByteEqualTo(base::CStrVector("URIError"))) {
    WriteVarint(static_cast<uint8_t>(ErrorTag::kUriErrorPrototype));
  } else {
    // The default prototype in the deserialization side is Error.prototype, so
    // we don't have to do anything here.
  }

  if (message_found.FromJust() &&
      PropertyDescriptor::IsDataDescriptor(&message_desc)) {
    Handle<String> message;
    if (!Object::ToString(isolate_, message_desc.value()).ToHandle(&message)) {
      return Nothing<bool>();
    }
    WriteVarint(static_cast<uint8_t>(ErrorTag::kMessage));
    WriteString(message);
  }

  if (!Object::GetProperty(isolate_, error, isolate_->factory()->stack_string())
           .ToHandle(&stack)) {
    return Nothing<bool>();
  }
  if (IsString(*stack)) {
    WriteVarint(static_cast<uint8_t>(ErrorTag::kStack));
    WriteString(Handle<String>::cast(stack));
  }

  // The {cause} can self-reference the error. We add at the end, so that we can
  // create the Error first when deserializing.
  if (cause_found.FromJust() &&
      PropertyDescriptor::IsDataDescriptor(&cause_desc)) {
    Handle<Object> cause = cause_desc.value();
    WriteVarint(static_cast<uint8_t>(ErrorTag::kCause));
    if (!WriteObject(cause).FromMaybe(false)) {
      return Nothing<bool>();
    }
  }

  WriteVarint(static_cast<uint8_t>(ErrorTag::kEnd));
  return ThrowIfOutOfMemory();
}

Maybe<bool> ValueSerializer::WriteJSSharedStruct(
    Handle<JSSharedStruct> shared_struct) {
  // TODO(v8:12547): Support copying serialization for shared structs as well.
  return WriteSharedObject(shared_struct);
}

Maybe<bool> ValueSerializer::WriteJSSharedArray(
    Handle<JSSharedArray> shared_array) {
  return WriteSharedObject(shared_array);
}

#if V8_ENABLE_WEBASSEMBLY
Maybe<bool> ValueSerializer::WriteWasmModule(Handle<WasmModuleObject> object) {
  if (delegate_ == nullptr) {
    return ThrowDataCloneError(MessageTemplate::kDataCloneError, object);
  }

  // TODO(titzer): introduce a Utils::ToLocal for WasmModuleObject.
  Maybe<uint32_t> transfer_id = delegate_->GetWasmModuleTransferId(
      reinterpret_cast<v8::Isolate*>(isolate_),
      v8::Local<v8::WasmModuleObject>::Cast(
          Utils::ToLocal(Handle<JSObject>::cast(object))));
  RETURN_VALUE_IF_EXCEPTION(isolate_, Nothing<bool>());
  uint32_t id = 0;
  if (transfer_id.To(&id)) {
    WriteTag(SerializationTag::kWasmModuleTransfer);
    WriteVarint<uint32_t>(id);
    return Just(true);
  }
  return ThrowIfOutOfMemory();
}

Maybe<bool> ValueSerializer::WriteWasmMemory(Handle<WasmMemoryObject> object) {
  if (!object->array_buffer()->is_shared()) {
    return ThrowDataCloneError(MessageTemplate::kDataCloneError, object);
  }

  GlobalBackingStoreRegistry::Register(
      object->array_buffer()->GetBackingStore());

  WriteTag(SerializationTag::kWasmMemoryTransfer);
  WriteZigZag<int32_t>(object->maximum_pages());
  WriteByte(object->is_memory64() ? 1 : 0);
  return WriteJSReceiver(Handle<JSReceiver>(object->array_buffer(), isolate_));
}
#endif  // V8_ENABLE_WEBASSEMBLY

Maybe<bool> ValueSerializer::WriteSharedObject(Handle<HeapObject> object) {
  if (!delegate_ || !isolate_->has_shared_space()) {
    return ThrowDataCloneError(MessageTemplate::kDataCloneError, object);
  }

  DCHECK(IsShared(*object));

  // The first time a shared object is serialized, a new conveyor is made. This
  // conveyor is used for every shared object in this serialization and
  // subsequent deserialization sessions. The embedder owns the lifetime of the
  // conveyor.
  if (!shared_object_conveyor_) {
    v8::Isolate* v8_isolate = reinterpret_cast<v8::Isolate*>(isolate_);
    v8::SharedValueConveyor v8_conveyor(v8_isolate);
    shared_object_conveyor_ = v8_conveyor.private_.get();
    if (!delegate_->AdoptSharedValueConveyor(v8_isolate,
                                             std::move(v8_conveyor))) {
      shared_object_conveyor_ = nullptr;
      RETURN_VALUE_IF_EXCEPTION(isolate_, Nothing<bool>());
      return Nothing<bool>();
    }
  }

  WriteTag(SerializationTag::kSharedObject);
  WriteVarint(shared_object_conveyor_->Persist(*object));

  return ThrowIfOutOfMemory();
}

Maybe<bool> ValueSerializer::WriteHostObject(Handle<JSObject> object) {
  WriteTag(SerializationTag::kHostObject);
  if (!delegate_) {
    isolate_->Throw(*isolate_->factory()->NewError(
        isolate_->error_function(), MessageTemplate::kDataCloneError, object));
    return Nothing<bool>();
  }
  v8::Isolate* v8_isolate = reinterpret_cast<v8::Isolate*>(isolate_);
  Maybe<bool> result =
      delegate_->WriteHostObject(v8_isolate, Utils::ToLocal(object));
  RETURN_VALUE_IF_EXCEPTION(isolate_, Nothing<bool>());
  USE(result);
  DCHECK(!result.IsNothing());
  DCHECK(result.ToChecked());
  return ThrowIfOutOfMemory();
}

Maybe<uint32_t> ValueSerializer::WriteJSObjectPropertiesSlow(
    Handle<JSObject> object, Handle<FixedArray> keys) {
  uint32_t properties_written = 0;
  int length = keys->length();
  for (int i = 0; i < length; i++) {
    Handle<Object> key(keys->get(i), isolate_);

    PropertyKey lookup_key(isolate_, key);
    LookupIterator it(isolate_, object, lookup_key, LookupIterator::OWN);
    Handle<Object> value;
    if (!Object::GetProperty(&it).ToHandle(&value)) return Nothing<uint32_t>();

    // If the property is no longer found, do not serialize it.
    // This could happen if a getter deleted the property.
    if (!it.IsFound()) continue;

    if (!WriteObject(key).FromMaybe(false) ||
        !WriteObject(value).FromMaybe(false)) {
      return Nothing<uint32_t>();
    }

    properties_written++;
  }
  return Just(properties_written);
}

Maybe<bool> ValueSerializer::IsHostObject(Handle<JSObject> js_object) {
  if (!has_custom_host_objects_) {
    return Just<bool>(
        JSObject::GetEmbedderFieldCount(js_object->map(isolate_)));
  }
  DCHECK_NOT_NULL(delegate_);

  v8::Isolate* v8_isolate = reinterpret_cast<v8::Isolate*>(isolate_);
  Maybe<bool> result =
      delegate_->IsHostObject(v8_isolate, Utils::ToLocal(js_object));
  RETURN_VALUE_IF_EXCEPTION(isolate_, Nothing<bool>());
  DCHECK(!result.IsNothing());

  if (V8_UNLIKELY(out_of_memory_)) return ThrowIfOutOfMemory();
  return result;
}

Maybe<bool> ValueSerializer::ThrowIfOutOfMemory() {
  if (out_of_memory_) {
    return ThrowDataCloneError(MessageTemplate::kDataCloneErrorOutOfMemory);
  }
  return Just(true);
}

Maybe<bool> ValueSerializer::ThrowDataCloneError(
    MessageTemplate template_index) {
  return ThrowDataCloneError(template_index,
                             isolate_->factory()->empty_string());
}

Maybe<bool> ValueSerializer::ThrowDataCloneError(MessageTemplate index,
                                                 Handle<Object> arg0) {
  Handle<String> message =
      MessageFormatter::Format(isolate_, index, base::VectorOf({arg0}));
  if (delegate_) {
    delegate_->ThrowDataCloneError(Utils::ToLocal(message));
  } else {
    isolate_->Throw(
        *isolate_->factory()->NewError(isolate_->error_function(), message));
  }
  return Nothing<bool>();
}

ValueDeserializer::ValueDeserializer(Isolate* isolate,
                                     base::Vector<const uint8_t> data,
                                     v8::ValueDeserializer::Delegate* delegate)
    : isolate_(isolate),
      delegate_(delegate),
      position_(data.begin()),
      end_(data.end()),
      id_map_(isolate->global_handles()->Create(
          ReadOnlyRoots(isolate_).empty_fixed_array())) {}

ValueDeserializer::ValueDeserializer(Isolate* isolate, const uint8_t* data,
                                     size_t size)
    : isolate_(isolate),
      delegate_(nullptr),
      position_(data),
      end_(data + size),
      id_map_(isolate->global_handles()->Create(
          ReadOnlyRoots(isolate_).empty_fixed_array())) {}

ValueDeserializer::~ValueDeserializer() {
  DCHECK_LE(position_, end_);
  GlobalHandles::Destroy(id_map_.location());

  Handle<Object> transfer_map_handle;
  if (array_buffer_transfer_map_.ToHandle(&transfer_map_handle)) {
    GlobalHandles::Destroy(transfer_map_handle.location());
  }
}

Maybe<bool> ValueDeserializer::ReadHeader() {
  if (position_ < end_ &&
      *position_ == static_cast<uint8_t>(SerializationTag::kVersion)) {
    ReadTag().ToChecked();
    if (!ReadVarintLoop<uint32_t>().To(&version_) ||
        version_ > kLatestVersion) {
      isolate_->Throw(*isolate_->factory()->NewError(
          MessageTemplate::kDataCloneDeserializationVersionError));
      return Nothing<bool>();
    }
  }
  return Just(true);
}

Maybe<SerializationTag> ValueDeserializer::PeekTag() const {
  const uint8_t* peek_position = position_;
  SerializationTag tag;
  do {
    if (peek_position >= end_) return Nothing<SerializationTag>();
    tag = static_cast<SerializationTag>(*peek_position);
    peek_position++;
  } while (tag == SerializationTag::kPadding);
  return Just(tag);
}

void ValueDeserializer::ConsumeTag(SerializationTag peeked_tag) {
  SerializationTag actual_tag = ReadTag().ToChecked();
  DCHECK(actual_tag == peeked_tag);
  USE(actual_tag);
}

Maybe<SerializationTag> ValueDeserializer::ReadTag() {
  SerializationTag tag;
  do {
    if (position_ >= end_) return Nothing<SerializationTag>();
    tag = static_cast<SerializationTag>(*position_);
    position_++;
  } while (tag == SerializationTag::kPadding);
  return Just(tag);
}

template <typename T>
Maybe<T> ValueDeserializer::ReadVarint() {
  // Reads an unsigned integer as a base-128 varint.
  // The number is written, 7 bits at a time, from the least significant to the
  // most significant 7 bits. Each byte, except the last, has the MSB set.
  // If the varint is larger than T, any more significant bits are discarded.
  // See also https://developers.google.com/protocol-buffers/docs/encoding
  static_assert(std::is_integral<T>::value && std::is_unsigned<T>::value,
                "Only unsigned integer types can be read as varints.");
  if (sizeof(T) > 4) return ReadVarintLoop<T>();
  auto max_read_position = position_ + sizeof(T) + 1;
  if (V8_UNLIKELY(max_read_position >= end_)) return ReadVarintLoop<T>();
#ifdef DEBUG
  // DCHECK code to make sure the manually unrolled loop yields the exact
  // same end state and result.
  auto previous_position = position_;
  Maybe<T> maybe_expected_value = ReadVarintLoop<T>();
  // ReadVarintLoop can't return Nothing here; all such conditions have been
  // checked above.
  T expected_value = maybe_expected_value.ToChecked();
  auto expected_position = position_;
  position_ = previous_position;
#endif  // DEBUG
#define EXIT_DCHECK()                      \
  DCHECK_LE(position_, end_);              \
  DCHECK_EQ(position_, expected_position); \
  DCHECK_EQ(value, expected_value)

  T value = 0;
#define ITERATION_SHIFTED(shift)                     \
  if (shift < sizeof(T) * 8) {                       \
    uint8_t byte = *position_;                       \
    position_++;                                     \
    if (byte < 0x80) {                               \
      value |= static_cast<T>(byte) << shift;        \
      EXIT_DCHECK();                                 \
      return Just(value);                            \
    } else {                                         \
      value |= static_cast<T>(byte & 0x7F) << shift; \
    }                                                \
  }
  // Manually unroll the loop to achieve the best measured peformance.
  // This is ~15% faster than ReadVarintLoop.
  ITERATION_SHIFTED(0);
  ITERATION_SHIFTED(7);
  ITERATION_SHIFTED(14);
  ITERATION_SHIFTED(21);
  ITERATION_SHIFTED(28);

  EXIT_DCHECK();
  return Just(value);
#undef ITERATION_SHIFTED
#undef EXIT_DCHECK
}

template <typename T>
Maybe<T> ValueDeserializer::ReadVarintLoop() {
  static_assert(std::is_integral<T>::value && std::is_unsigned<T>::value,
                "Only unsigned integer types can be read as varints.");
  T value = 0;
  unsigned shift = 0;
  bool has_another_byte;
  do {
    if (position_ >= end_) return Nothing<T>();
    uint8_t byte = *position_;
    has_another_byte = byte & 0x80;
    if (V8_LIKELY(shift < sizeof(T) * 8)) {
      value |= static_cast<T>(byte & 0x7F) << shift;
      shift += 7;
    } else {
      // For consistency with the fast unrolled loop in ReadVarint we return
      // after we have read size(T) + 1 bytes.
#ifdef V8_VALUE_DESERIALIZER_HARD_FAIL
      CHECK(!has_another_byte);
#endif  // V8_VALUE_DESERIALIZER_HARD_FAIL
      return Just(value);
    }
    position_++;
  } while (has_another_byte);
  return Just(value);
}

template <typename T>
Maybe<T> ValueDeserializer::ReadZigZag() {
  // Writes a signed integer as a varint using ZigZag encoding (i.e. 0 is
  // encoded as 0, -1 as 1, 1 as 2, -2 as 3, and so on).
  // See also https://developers.google.com/protocol-buffers/docs/encoding
  static_assert(std::is_integral<T>::value && std::is_signed<T>::value,
                "Only signed integer types can be read as zigzag.");
  using UnsignedT = typename std::make_unsigned<T>::type;
  UnsignedT unsigned_value;
  if (!ReadVarint<UnsignedT>().To(&unsigned_value)) return Nothing<T>();
  return Just(static_cast<T>((unsigned_value >> 1) ^
                             -static_cast<T>(unsigned_value & 1)));
}

template EXPORT_TEMPLATE_DEFINE(
    V8_EXPORT_PRIVATE) Maybe<int32_t> ValueDeserializer::ReadZigZag();

Maybe<double> ValueDeserializer::ReadDouble() {
  // Warning: this uses host endianness.
  if (sizeof(double) > static_cast<unsigned>(end_ - position_)) {
    return Nothing<double>();
  }
  double value;
  memcpy(&value, position_, sizeof(double));
  position_ += sizeof(double);
  if (std::isnan(value)) value = std::numeric_limits<double>::quiet_NaN();
  return Just(value);
}

Maybe<base::Vector<const uint8_t>> ValueDeserializer::ReadRawBytes(
    size_t size) {
  if (size > static_cast<size_t>(end_ - position_)) {
    return Nothing<base::Vector<const uint8_t>>();
  }
  const uint8_t* start = position_;
  position_ += size;
  return Just(base::Vector<const uint8_t>(start, size));
}

bool ValueDeserializer::ReadByte(uint8_t* value) {
  if (static_cast<size_t>(end_ - position_) < sizeof(uint8_t)) return false;
  *value = *position_;
  position_++;
  return true;
}

bool ValueDeserializer::ReadUint32(uint32_t* value) {
  return ReadVarint<uint32_t>().To(value);
}

bool ValueDeserializer::ReadUint64(uint64_t* value) {
  return ReadVarint<uint64_t>().To(value);
}

bool ValueDeserializer::ReadDouble(double* value) {
  return ReadDouble().To(value);
}

bool ValueDeserializer::ReadRawBytes(size_t length, const void** data) {
  if (length > static_cast<size_t>(end_ - position_)) return false;
  *data = position_;
  position_ += length;
  return true;
}

void ValueDeserializer::TransferArrayBuffer(
    uint32_t transfer_id, Handle<JSArrayBuffer> array_buffer) {
  if (array_buffer_transfer_map_.is_null()) {
    array_buffer_transfer_map_ = isolate_->global_handles()->Create(
        *SimpleNumberDictionary::New(isolate_, 0));
  }
  Handle<SimpleNumberDictionary> dictionary =
      array_buffer_transfer_map_.ToHandleChecked();
  Handle<SimpleNumberDictionary> new_dictionary = SimpleNumberDictionary::Set(
      isolate_, dictionary, transfer_id, array_buffer);
  if (!new_dictionary.is_identical_to(dictionary)) {
    GlobalHandles::Destroy(dictionary.location());
    array_buffer_transfer_map_ =
        isolate_->global_handles()->Create(*new_dictionary);
  }
}

MaybeHandle<Object> ValueDeserializer::ReadObjectWrapper() {
  // We had a bug which produced invalid version 13 data (see
  // crbug.com/1284506). This compatibility mode tries to first read the data
  // normally, and if it fails, and the version is 13, tries to read the broken
  // format.
  const uint8_t* original_position = position_;
  suppress_deserialization_errors_ = true;
  MaybeHandle<Object> result = ReadObject();

  // The deserialization code doesn't throw errors for invalid data. It throws
  // errors for stack overflows, though, and in that case we won't retry.
  if (result.is_null() && version_ == 13 && !isolate_->has_exception()) {
    version_13_broken_data_mode_ = true;
    position_ = original_position;
    result = ReadObject();
  }

  if (result.is_null() && !isolate_->has_exception()) {
    isolate_->Throw(*isolate_->factory()->NewError(
        MessageTemplate::kDataCloneDeserializationError));
  }

  return result;
}

MaybeHandle<Object> ValueDeserializer::ReadObject() {
  DisallowJavascriptExecution no_js(isolate_);
  // If we are at the end of the stack, abort. This function may recurse.
  STACK_CHECK(isolate_, MaybeHandle<Object>());

  MaybeHandle<Object> result = ReadObjectInternal();

  // ArrayBufferView is special in that it consumes the value before it, even
  // after format version 0.
  Handle<Object> object;
  SerializationTag tag;
  if (result.ToHandle(&object) && V8_UNLIKELY(IsJSArrayBuffer(*object)) &&
      PeekTag().To(&tag) && tag == SerializationTag::kArrayBufferView) {
    ConsumeTag(SerializationTag::kArrayBufferView);
    result = ReadJSArrayBufferView(Handle<JSArrayBuffer>::cast(object));
  }

  if (result.is_null() && !suppress_deserialization_errors_ &&
      !isolate_->has_exception()) {
    isolate_->Throw(*isolate_->factory()->NewError(
        MessageTemplate::kDataCloneDeserializationError));
  }
#if defined(DEBUG) && defined(VERIFY_HEAP)
  if (!result.is_null() && v8_flags.enable_slow_asserts &&
      v8_flags.verify_heap) {
    Object::ObjectVerify(*object, isolate_);
  }
#endif

  return result;
}

MaybeHandle<Object> ValueDeserializer::ReadObjectInternal() {
  SerializationTag tag;
  if (!ReadTag().To(&tag)) return MaybeHandle<Object>();
  switch (tag) {
    case SerializationTag::kVerifyObjectCount:
      // Read the count and ignore it.
      if (ReadVarint<uint32_t>().IsNothing()) return MaybeHandle<Object>();
      return ReadObject();
    case SerializationTag::kUndefined:
      return isolate_->factory()->undefined_value();
    case SerializationTag::kNull:
      return isolate_->factory()->null_value();
    case SerializationTag::kTrue:
      return isolate_->factory()->true_value();
    case SerializationTag::kFalse:
      return isolate_->factory()->false_value();
    case SerializationTag::kInt32: {
      Maybe<int32_t> number = ReadZigZag<int32_t>();
      if (number.IsNothing()) return MaybeHandle<Object>();
      return isolate_->factory()->NewNumberFromInt(number.FromJust());
    }
    case SerializationTag::kUint32: {
      Maybe<uint32_t> number = ReadVarint<uint32_t>();
      if (number.IsNothing()) return MaybeHandle<Object>();
      return isolate_->factory()->NewNumberFromUint(number.FromJust());
    }
    case SerializationTag::kDouble: {
      Maybe<double> number = ReadDouble();
      if (number.IsNothing()) return MaybeHandle<Object>();
      return isolate_->factory()->NewNumber(number.FromJust());
    }
    case SerializationTag::kBigInt:
      return ReadBigInt();
    case SerializationTag::kUtf8String:
      return ReadUtf8String();
    case SerializationTag::kOneByteString:
      return ReadOneByteString();
    case SerializationTag::kTwoByteString:
      return ReadTwoByteString();
    case SerializationTag::kObjectReference: {
      uint32_t id;
      if (!ReadVarint<uint32_t>().To(&id)) return MaybeHandle<Object>();
      return GetObjectWithID(id);
    }
    case SerializationTag::kBeginJSObject:
      return ReadJSObject();
    case SerializationTag::kBeginSparseJSArray:
      return ReadSparseJSArray();
    case SerializationTag::kBeginDenseJSArray:
      return ReadDenseJSArray();
    case SerializationTag::kDate:
      return ReadJSDate();
    case SerializationTag::kTrueObject:
    case SerializationTag::kFalseObject:
    case SerializationTag::kNumberObject:
    case SerializationTag::kBigIntObject:
    case SerializationTag::kStringObject:
      return ReadJSPrimitiveWrapper(tag);
    case SerializationTag::kRegExp:
      return ReadJSRegExp();
    case SerializationTag::kBeginJSMap:
      return ReadJSMap();
    case SerializationTag::kBeginJSSet:
      return ReadJSSet();
    case SerializationTag::kArrayBuffer: {
      constexpr bool is_shared = false;
      constexpr bool is_resizable = false;
      return ReadJSArrayBuffer(is_shared, is_resizable);
    }
    case SerializationTag::kResizableArrayBuffer: {
      constexpr bool is_shared = false;
      constexpr bool is_resizable = true;
      return ReadJSArrayBuffer(is_shared, is_resizable);
    }
    case SerializationTag::kArrayBufferTransfer: {
      return ReadTransferredJSArrayBuffer();
    }
    case SerializationTag::kSharedArrayBuffer: {
      constexpr bool is_shared = true;
      constexpr bool is_resizable = false;
      return ReadJSArrayBuffer(is_shared, is_resizable);
    }
    case SerializationTag::kError:
      return ReadJSError();
#if V8_ENABLE_WEBASSEMBLY
    case SerializationTag::kWasmModuleTransfer:
      return ReadWasmModuleTransfer();
    case SerializationTag::kWasmMemoryTransfer:
      return ReadWasmMemory();
#endif  // V8_ENABLE_WEBASSEMBLY
    case SerializationTag::kHostObject:
      return ReadHostObject();
    case SerializationTag::kSharedObject:
      if (version_ >= 15) return ReadSharedObject();
      // If the data doesn't support shared values because it is from an older
      // version, treat the tag as unknown.
      [[fallthrough]];
    default:
      // Before there was an explicit tag for host objects, all unknown tags
      // were delegated to the host.
      if (version_ < 13) {
        position_--;
        return ReadHostObject();
      }
      return MaybeHandle<Object>();
  }
}

MaybeHandle<String> ValueDeserializer::ReadString() {
  if (version_ < 12) return ReadUtf8String();
  Handle<Object> object;
  if (!ReadObject().ToHandle(&object) || !IsString(*object, isolate_)) {
    return MaybeHandle<String>();
  }
  return Handle<String>::cast(object);
}

MaybeHandle<BigInt> ValueDeserializer::ReadBigInt() {
  uint32_t bitfield;
  if (!ReadVarint<uint32_t>().To(&bitfield)) return MaybeHandle<BigInt>();
  int bytelength = BigInt::DigitsByteLengthForBitfield(bitfield);
  base::Vector<const uint8_t> digits_storage;
  if (!ReadRawBytes(bytelength).To(&digits_storage)) {
    return MaybeHandle<BigInt>();
  }
  return BigInt::FromSerializedDigits(isolate_, bitfield, digits_storage);
}

MaybeHandle<String> ValueDeserializer::ReadUtf8String(
    AllocationType allocation) {
  uint32_t utf8_length;
  if (!ReadVarint<uint32_t>().To(&utf8_length)) return {};
  // utf8_length is checked in ReadRawBytes.
  base::Vector<const uint8_t> utf8_bytes;
  if (!ReadRawBytes(utf8_length).To(&utf8_bytes)) return {};
  return isolate_->factory()->NewStringFromUtf8(
      base::Vector<const char>::cast(utf8_bytes), allocation);
}

MaybeHandle<String> ValueDeserializer::ReadOneByteString(
    AllocationType allocation) {
  uint32_t byte_length;
  base::Vector<const uint8_t> bytes;
  if (!ReadVarint<uint32_t>().To(&byte_length)) return {};
  // byte_length is checked in ReadRawBytes.
  if (!ReadRawBytes(byte_length).To(&bytes)) return {};
  return isolate_->factory()->NewStringFromOneByte(bytes, allocation);
}

MaybeHandle<String> ValueDeserializer::ReadTwoByteString(
    AllocationType allocation) {
  uint32_t byte_length;
  base::Vector<const uint8_t> bytes;
  if (!ReadVarint<uint32_t>().To(&byte_length)) return {};
  // byte_length is checked in ReadRawBytes.
  if (byte_length % sizeof(base::uc16) != 0 ||
      !ReadRawBytes(byte_length).To(&bytes)) {
    return MaybeHandle<String>();
  }

  // Allocate an uninitialized string so that we can do a raw memcpy into the
  // string on the heap (regardless of alignment).
  if (byte_length == 0) return isolate_->factory()->empty_string();
  Handle<SeqTwoByteString> string;
  if (!isolate_->factory()
           ->NewRawTwoByteString(byte_length / sizeof(base::uc16), allocation)
           .ToHandle(&string)) {
    return MaybeHandle<String>();
  }

  // Copy the bytes directly into the new string.
  // Warning: this uses host endianness.
  DisallowGarbageCollection no_gc;
  memcpy(string->GetChars(no_gc), bytes.begin(), bytes.length());
  return string;
}

bool ValueDeserializer::ReadExpectedString(Handle<String> expected) {
  DisallowGarbageCollection no_gc;
  // In the case of failure, the position in the stream is reset.
  const uint8_t* original_position = position_;

  SerializationTag tag;
  uint32_t byte_length;
  base::Vector<const uint8_t> bytes;
  if (!ReadTag().To(&tag) || !ReadVarint<uint32_t>().To(&byte_length)) {
    return {};
  }
  // Length is also checked in ReadRawBytes.
#ifdef V8_VALUE_DESERIALIZER_HARD_FAIL
  CHECK_LE(byte_length,
           static_cast<uint32_t>(std::numeric_limits<int32_t>::max()));
#endif  // V8_VALUE_DESERIALIZER_HARD_FAIL
  if (!ReadRawBytes(byte_length).To(&bytes)) {
    position_ = original_position;
    return false;
  }

  String::FlatContent flat = expected->GetFlatContent(no_gc);

  // If the bytes are verbatim what is in the flattened string, then the string
  // is successfully consumed.
  if (tag == SerializationTag::kOneByteString && flat.IsOneByte()) {
    base::Vector<const uint8_t> chars = flat.ToOneByteVector();
    if (byte_length == static_cast<size_t>(chars.length()) &&
        memcmp(bytes.begin(), chars.begin(), byte_length) == 0) {
      return true;
    }
  } else if (tag == SerializationTag::kTwoByteString && flat.IsTwoByte()) {
    base::Vector<const base::uc16> chars = flat.ToUC16Vector();
    if (byte_length ==
            static_cast<unsigned>(chars.length()) * sizeof(base::uc16) &&
        memcmp(bytes.begin(), chars.begin(), byte_length) == 0) {
      return true;
    }
  } else if (tag == SerializationTag::kUtf8String && flat.IsOneByte()) {
    base::Vector<const uint8_t> chars = flat.ToOneByteVector();
    if (byte_length == static_cast<size_t>(chars.length()) &&
        String::IsAscii(chars.begin(), chars.length()) &&
        memcmp(bytes.begin(), chars.begin(), byte_length) == 0) {
      return true;
    }
  }

  position_ = original_position;
  return false;
}

MaybeHandle<JSObject> ValueDeserializer::ReadJSObject() {
  // If we are at the end of the stack, abort. This function may recurse.
  STACK_CHECK(isolate_, MaybeHandle<JSObject>());

  uint32_t id = next_id_++;
  HandleScope scope(isolate_);
  Handle<JSObject> object =
      isolate_->factory()->NewJSObject(isolate_->object_function());
  AddObjectWithID(id, object);

  uint32_t num_properties;
  uint32_t expected_num_properties;
  if (!ReadJSObjectProperties(object, SerializationTag::kEndJSObject, true)
           .To(&num_properties) ||
      !ReadVarint<uint32_t>().To(&expected_num_properties) ||
      num_properties != expected_num_properties) {
    return MaybeHandle<JSObject>();
  }

  DCHECK(HasObjectWithID(id));
  return scope.CloseAndEscape(object);
}

MaybeHandle<JSArray> ValueDeserializer::ReadSparseJSArray() {
  // If we are at the end of the stack, abort. This function may recurse.
  STACK_CHECK(isolate_, MaybeHandle<JSArray>());

  uint32_t length;
  if (!ReadVarint<uint32_t>().To(&length)) return MaybeHandle<JSArray>();

  uint32_t id = next_id_++;
  HandleScope scope(isolate_);
  Handle<JSArray> array =
      isolate_->factory()->NewJSArray(0, TERMINAL_FAST_ELEMENTS_KIND);
  MAYBE_RETURN(JSArray::SetLength(array, length), MaybeHandle<JSArray>());
  AddObjectWithID(id, array);

  uint32_t num_properties;
  uint32_t expected_num_properties;
  uint32_t expected_length;
  if (!ReadJSObjectProperties(array, SerializationTag::kEndSparseJSArray, false)
           .To(&num_properties) ||
      !ReadVarint<uint32_t>().To(&expected_num_properties) ||
      !ReadVarint<uint32_t>().To(&expected_length) ||
      num_properties != expected_num_properties || length != expected_length) {
    return MaybeHandle<JSArray>();
  }

  DCHECK(HasObjectWithID(id));
  return scope.CloseAndEscape(array);
}

MaybeHandle<JSArray> ValueDeserializer::ReadDenseJSArray() {
  // If we are at the end of the stack, abort. This function may recurse.
  STACK_CHECK(isolate_, MaybeHandle<JSArray>());

  // We shouldn't permit an array larger than the biggest we can request from
  // V8. As an additional sanity check, since each entry will take at least one
  // byte to encode, if there are fewer bytes than that we can also fail fast.
  uint32_t length;
  if (!ReadVarint<uint32_t>().To(&length) ||
      length > static_cast<uint32_t>(FixedArray::kMaxLength) ||
      length > static_cast<size_t>(end_ - position_)) {
    return MaybeHandle<JSArray>();
  }

  uint32_t id = next_id_++;
  HandleScope scope(isolate_);
  Handle<JSArray> array = isolate_->factory()->NewJSArray(
      HOLEY_ELEMENTS, length, length,
      ArrayStorageAllocationMode::INITIALIZE_ARRAY_ELEMENTS_WITH_HOLE);
  AddObjectWithID(id, array);

  Handle<FixedArray> elements(FixedArray::cast(array->elements()), isolate_);
  auto elements_length = static_cast<uint32_t>(elements->length());
  for (uint32_t i = 0; i < length; i++) {
    SerializationTag tag;
    if (PeekTag().To(&tag) && tag == SerializationTag::kTheHole) {
      ConsumeTag(SerializationTag::kTheHole);
      continue;
    }

    Handle<Object> element;
    if (!ReadObject().ToHandle(&element)) return MaybeHandle<JSArray>();

    // Serialization versions less than 11 encode the hole the same as
    // undefined. For consistency with previous behavior, store these as the
    // hole. Past version 11, undefined means undefined.
    if (version_ < 11 && IsUndefined(*element, isolate_)) continue;

    // Safety check.
    if (i >= elements_length) return MaybeHandle<JSArray>();

    elements->set(i, *element);
  }

  uint32_t num_properties;
  uint32_t expected_num_properties;
  uint32_t expected_length;
  if (!ReadJSObjectProperties(array, SerializationTag::kEndDenseJSArray, false)
           .To(&num_properties) ||
      !ReadVarint<uint32_t>().To(&expected_num_properties) ||
      !ReadVarint<uint32_t>().To(&expected_length) ||
      num_properties != expected_num_properties || length != expected_length) {
    return MaybeHandle<JSArray>();
  }

  DCHECK(HasObjectWithID(id));
  return scope.CloseAndEscape(array);
}

MaybeHandle<JSDate> ValueDeserializer::ReadJSDate() {
  double value;
  if (!ReadDouble().To(&value)) return MaybeHandle<JSDate>();
  uint32_t id = next_id_++;
  Handle<JSDate> date;
  if (!JSDate::New(isolate_->date_function(), isolate_->date_function(), value)
           .ToHandle(&date)) {
    return MaybeHandle<JSDate>();
  }
  AddObjectWithID(id, date);
  return date;
}

MaybeHandle<JSPrimitiveWrapper> ValueDeserializer::ReadJSPrimitiveWrapper(
    SerializationTag tag) {
  uint32_t id = next_id_++;
  Handle<JSPrimitiveWrapper> value;
  switch (tag) {
    case SerializationTag::kTrueObject:
      value = Handle<JSPrimitiveWrapper>::cast(
          isolate_->factory()->NewJSObject(isolate_->boolean_function()));
      value->set_value(ReadOnlyRoots(isolate_).true_value());
      break;
    case SerializationTag::kFalseObject:
      value = Handle<JSPrimitiveWrapper>::cast(
          isolate_->factory()->NewJSObject(isolate_->boolean_function()));
      value->set_value(ReadOnlyRoots(isolate_).false_value());
      break;
    case SerializationTag::kNumberObject: {
      double number;
      if (!ReadDouble().To(&number)) return MaybeHandle<JSPrimitiveWrapper>();
      value = Handle<JSPrimitiveWrapper>::cast(
          isolate_->factory()->NewJSObject(isolate_->number_function()));
      Handle<Object> number_object = isolate_->factory()->NewNumber(number);
      value->set_value(*number_object);
      break;
    }
    case SerializationTag::kBigIntObject: {
      Handle<BigInt> bigint;
      if (!ReadBigInt().ToHandle(&bigint))
        return MaybeHandle<JSPrimitiveWrapper>();
      value = Handle<JSPrimitiveWrapper>::cast(
          isolate_->factory()->NewJSObject(isolate_->bigint_function()));
      value->set_value(*bigint);
      break;
    }
    case SerializationTag::kStringObject: {
      Handle<String> string;
      if (!ReadString().ToHandle(&string))
        return MaybeHandle<JSPrimitiveWrapper>();
      value = Handle<JSPrimitiveWrapper>::cast(
          isolate_->factory()->NewJSObject(isolate_->string_function()));
      value->set_value(*string);
      break;
    }
    default:
      UNREACHABLE();
  }
  AddObjectWithID(id, value);
  return value;
}

MaybeHandle<JSRegExp> ValueDeserializer::ReadJSRegExp() {
  uint32_t id = next_id_++;
  Handle<String> pattern;
  uint32_t raw_flags;
  Handle<JSRegExp> regexp;
  if (!ReadString().ToHandle(&pattern) ||
      !ReadVarint<uint32_t>().To(&raw_flags)) {
    return MaybeHandle<JSRegExp>();
  }

  // Ensure the deserialized flags are valid.
  uint32_t bad_flags_mask = static_cast<uint32_t>(-1) << JSRegExp::kFlagCount;
  // kLinear is accepted only with the appropriate flag.
  if (!v8_flags.enable_experimental_regexp_engine) {
    bad_flags_mask |= JSRegExp::kLinear;
  }
  if ((raw_flags & bad_flags_mask) ||
      !RegExp::VerifyFlags(static_cast<RegExpFlags>(raw_flags)) ||
      !JSRegExp::New(isolate_, pattern, static_cast<JSRegExp::Flags>(raw_flags))
           .ToHandle(&regexp)) {
    return MaybeHandle<JSRegExp>();
  }

  AddObjectWithID(id, regexp);
  return regexp;
}

MaybeHandle<JSMap> ValueDeserializer::ReadJSMap() {
  // If we are at the end of the stack, abort. This function may recurse.
  STACK_CHECK(isolate_, MaybeHandle<JSMap>());

  HandleScope scope(isolate_);
  uint32_t id = next_id_++;
  Handle<JSMap> map = isolate_->factory()->NewJSMap();
  AddObjectWithID(id, map);

  Handle<JSFunction> map_set = isolate_->map_set();
  uint32_t length = 0;
  while (true) {
    SerializationTag tag;
    if (!PeekTag().To(&tag)) return MaybeHandle<JSMap>();
    if (tag == SerializationTag::kEndJSMap) {
      ConsumeTag(SerializationTag::kEndJSMap);
      break;
    }

    Handle<Object> argv[2];
    if (!ReadObject().ToHandle(&argv[0]) || !ReadObject().ToHandle(&argv[1])) {
      return MaybeHandle<JSMap>();
    }

    AllowJavascriptExecution allow_js(isolate_);
    if (Execution::Call(isolate_, map_set, map, arraysize(argv), argv)
            .is_null()) {
      return MaybeHandle<JSMap>();
    }
    length += 2;
  }

  uint32_t expected_length;
  if (!ReadVarint<uint32_t>().To(&expected_length) ||
      length != expected_length) {
    return MaybeHandle<JSMap>();
  }
  DCHECK(HasObjectWithID(id));
  return scope.CloseAndEscape(map);
}

MaybeHandle<JSSet> ValueDeserializer::ReadJSSet() {
  // If we are at the end of the stack, abort. This function may recurse.
  STACK_CHECK(isolate_, MaybeHandle<JSSet>());

  HandleScope scope(isolate_);
  uint32_t id = next_id_++;
  Handle<JSSet> set = isolate_->factory()->NewJSSet();
  AddObjectWithID(id, set);
  Handle<JSFunction> set_add = isolate_->set_add();
  uint32_t length = 0;
  while (true) {
    SerializationTag tag;
    if (!PeekTag().To(&tag)) return MaybeHandle<JSSet>();
    if (tag == SerializationTag::kEndJSSet) {
      ConsumeTag(SerializationTag::kEndJSSet);
      break;
    }

    Handle<Object> argv[1];
    if (!ReadObject().ToHandle(&argv[0])) return MaybeHandle<JSSet>();

    AllowJavascriptExecution allow_js(isolate_);
    if (Execution::Call(isolate_, set_add, set, arraysize(argv), argv)
            .is_null()) {
      return MaybeHandle<JSSet>();
    }
    length++;
  }

  uint32_t expected_length;
  if (!ReadVarint<uint32_t>().To(&expected_length) ||
      length != expected_length) {
    return MaybeHandle<JSSet>();
  }
  DCHECK(HasObjectWithID(id));
  return scope.CloseAndEscape(set);
}

MaybeHandle<JSArrayBuffer> ValueDeserializer::ReadJSArrayBuffer(
    bool is_shared, bool is_resizable) {
  uint32_t id = next_id_++;
  if (is_shared) {
    uint32_t clone_id;
    Local<SharedArrayBuffer> sab_value;
    if (!ReadVarint<uint32_t>().To(&clone_id) || delegate_ == nullptr ||
        !delegate_
             ->GetSharedArrayBufferFromId(
                 reinterpret_cast<v8::Isolate*>(isolate_), clone_id)
             .ToLocal(&sab_value)) {
      RETURN_EXCEPTION_IF_EXCEPTION(isolate_, JSArrayBuffer);
      return MaybeHandle<JSArrayBuffer>();
    }
    Handle<JSArrayBuffer> array_buffer = Utils::OpenHandle(*sab_value);
    DCHECK_EQ(is_shared, array_buffer->is_shared());
    AddObjectWithID(id, array_buffer);
    return array_buffer;
  }
  uint32_t byte_length;
  if (!ReadVarint<uint32_t>().To(&byte_length)) {
    return MaybeHandle<JSArrayBuffer>();
  }
  uint32_t max_byte_length = byte_length;
  if (is_resizable) {
    if (!ReadVarint<uint32_t>().To(&max_byte_length)) {
      return MaybeHandle<JSArrayBuffer>();
    }
    if (byte_length > max_byte_length) {
      return MaybeHandle<JSArrayBuffer>();
    }
  }
  if (byte_length > static_cast<size_t>(end_ - position_)) {
    return MaybeHandle<JSArrayBuffer>();
  }
  MaybeHandle<JSArrayBuffer> result =
      isolate_->factory()->NewJSArrayBufferAndBackingStore(
          byte_length, max_byte_length, InitializedFlag::kUninitialized,
          is_resizable ? ResizableFlag::kResizable
                       : ResizableFlag::kNotResizable);

  Handle<JSArrayBuffer> array_buffer;
  if (!result.ToHandle(&array_buffer)) return result;

  if (byte_length > 0) {
    memcpy(array_buffer->backing_store(), position_, byte_length);
  }
  position_ += byte_length;
  AddObjectWithID(id, array_buffer);
  return array_buffer;
}

MaybeHandle<JSArrayBuffer> ValueDeserializer::ReadTransferredJSArrayBuffer() {
  uint32_t id = next_id_++;
  uint32_t transfer_id;
  Handle<SimpleNumberDictionary> transfer_map;
  if (!ReadVarint<uint32_t>().To(&transfer_id) ||
      !array_buffer_transfer_map_.ToHandle(&transfer_map)) {
    return MaybeHandle<JSArrayBuffer>();
  }
  InternalIndex index = transfer_map->FindEntry(isolate_, transfer_id);
  if (index.is_not_found()) {
    return MaybeHandle<JSArrayBuffer>();
  }
  Handle<JSArrayBuffer> array_buffer(
      JSArrayBuffer::cast(transfer_map->ValueAt(index)), isolate_);
  AddObjectWithID(id, array_buffer);
  return array_buffer;
}

MaybeHandle<JSArrayBufferView> ValueDeserializer::ReadJSArrayBufferView(
    Handle<JSArrayBuffer> buffer) {
  uint32_t buffer_byte_length = static_cast<uint32_t>(buffer->GetByteLength());
  uint8_t tag = 0;
  uint32_t byte_offset = 0;
  uint32_t byte_length = 0;
  uint32_t flags = 0;
  if (!ReadVarint<uint8_t>().To(&tag) ||
      !ReadVarint<uint32_t>().To(&byte_offset) ||
      !ReadVarint<uint32_t>().To(&byte_length) ||
      byte_offset > buffer_byte_length ||
      byte_length > buffer_byte_length - byte_offset) {
    return MaybeHandle<JSArrayBufferView>();
  }
  const bool should_read_flags = version_ >= 14 || version_13_broken_data_mode_;
  if (should_read_flags && !ReadVarint<uint32_t>().To(&flags)) {
    return MaybeHandle<JSArrayBufferView>();
  }
  uint32_t id = next_id_++;
  ExternalArrayType external_array_type = kExternalInt8Array;
  unsigned element_size = 0;

  switch (static_cast<ArrayBufferViewTag>(tag)) {
    case ArrayBufferViewTag::kDataView: {
      bool is_length_tracking = false;
      bool is_backed_by_rab = false;
      if (!ValidateJSArrayBufferViewFlags(*buffer, flags, is_length_tracking,
                                          is_backed_by_rab)) {
        return MaybeHandle<JSArrayBufferView>();
      }
      Handle<JSDataViewOrRabGsabDataView> data_view =
          isolate_->factory()->NewJSDataViewOrRabGsabDataView(
              buffer, byte_offset, byte_length, is_length_tracking);
      CHECK_EQ(is_backed_by_rab, data_view->is_backed_by_rab());
      CHECK_EQ(is_length_tracking, data_view->is_length_tracking());
      AddObjectWithID(id, data_view);
      return data_view;
    }
#define TYPED_ARRAY_CASE(Type, type, TYPE, ctype) \
  case ArrayBufferViewTag::k##Type##Array:        \
    external_array_type = kExternal##Type##Array; \
    element_size = sizeof(ctype);                 \
    break;
      TYPED_ARRAYS_BASE(TYPED_ARRAY_CASE)
#undef TYPED_ARRAY_CASE
    case ArrayBufferViewTag::kFloat16Array: {
      if (i::v8_flags.js_float16array) {
        external_array_type = kExternalFloat16Array;
        element_size = sizeof(uint16_t);
      }
      break;
    }
  }
  if (element_size == 0 || byte_offset % element_size != 0 ||
      byte_length % element_size != 0) {
    return MaybeHandle<JSArrayBufferView>();
  }
  bool is_length_tracking = false;
  bool is_backed_by_rab = false;
  if (!ValidateJSArrayBufferViewFlags(*buffer, flags, is_length_tracking,
                                      is_backed_by_rab)) {
    return MaybeHandle<JSArrayBufferView>();
  }
  Handle<JSTypedArray> typed_array = isolate_->factory()->NewJSTypedArray(
      external_array_type, buffer, byte_offset, byte_length / element_size,
      is_length_tracking);
  CHECK_EQ(is_length_tracking, typed_array->is_length_tracking());
  CHECK_EQ(is_backed_by_rab, typed_array->is_backed_by_rab());
  AddObjectWithID(id, typed_array);
  return typed_array;
}

bool ValueDeserializer::ValidateJSArrayBufferViewFlags(
    Tagged<JSArrayBuffer> buffer, uint32_t serialized_flags,
    bool& is_length_tracking, bool& is_backed_by_rab) {
  is_length_tracking =
      JSArrayBufferViewIsLengthTracking::decode(serialized_flags);
  is_backed_by_rab = JSArrayBufferViewIsBackedByRab::decode(serialized_flags);

  // TODO(marja): When the version number is bumped the next time, check that
  // serialized_flags doesn't contain spurious 1-bits.

  if (is_backed_by_rab || is_length_tracking) {
    if (!buffer->is_resizable_by_js()) {
      return false;
    }
    if (is_backed_by_rab && buffer->is_shared()) {
      return false;
    }
  }
  // The RAB-ness of the buffer and the TA's "is_backed_by_rab" need to be in
  // sync.
  if (buffer->is_resizable_by_js() && !buffer->is_shared() &&
      !is_backed_by_rab) {
    return false;
  }
  return true;
}

MaybeHandle<Object> ValueDeserializer::ReadJSError() {
  uint32_t id = next_id_++;

#define READ_NEXT_ERROR_TAG()              \
  do {                                     \
    if (!ReadVarint<uint8_t>().To(&tag)) { \
      return MaybeHandle<JSObject>();      \
    }                                      \
  } while (false)

  uint8_t tag;
  READ_NEXT_ERROR_TAG();

  // Read error type constructor.
  Handle<JSFunction> constructor;
  switch (static_cast<ErrorTag>(tag)) {
    case ErrorTag::kEvalErrorPrototype:
      constructor = isolate_->eval_error_function();
      READ_NEXT_ERROR_TAG();
      break;
    case ErrorTag::kRangeErrorPrototype:
      constructor = isolate_->range_error_function();
      READ_NEXT_ERROR_TAG();
      break;
    case ErrorTag::kReferenceErrorPrototype:
      constructor = isolate_->reference_error_function();
      READ_NEXT_ERROR_TAG();
      break;
    case ErrorTag::kSyntaxErrorPrototype:
      constructor = isolate_->syntax_error_function();
      READ_NEXT_ERROR_TAG();
      break;
    case ErrorTag::kTypeErrorPrototype:
      constructor = isolate_->type_error_function();
      READ_NEXT_ERROR_TAG();
      break;
    case ErrorTag::kUriErrorPrototype:
      constructor = isolate_->uri_error_function();
      READ_NEXT_ERROR_TAG();
      break;
    default:
      // The default prototype in the deserialization side is Error.prototype,
      // so we don't have to do anything here.
      constructor = isolate_->error_function();
      break;
  }

  // Check for message property.
  Handle<Object> message = isolate_->factory()->undefined_value();
  if (static_cast<ErrorTag>(tag) == ErrorTag::kMessage) {
    Handle<String> message_string;
    if (!ReadString().ToHandle(&message_string)) {
      return MaybeHandle<JSObject>();
    }
    message = message_string;
    READ_NEXT_ERROR_TAG();
  }

  // Check for stack property.
  Handle<Object> stack = isolate_->factory()->undefined_value();
  if (static_cast<ErrorTag>(tag) == ErrorTag::kStack) {
    Handle<String> stack_string;
    if (!ReadString().ToHandle(&stack_string)) {
      return MaybeHandle<JSObject>();
    }
    stack = stack_string;
    READ_NEXT_ERROR_TAG();
  }

  // Create error object before adding the cause property.
  Handle<JSObject> error;
  Handle<Object> no_caller;
  Handle<Object> undefined_options = isolate_->factory()->undefined_value();
  if (!ErrorUtils::Construct(isolate_, constructor, constructor, message,
                             undefined_options, SKIP_NONE, no_caller,
                             ErrorUtils::StackTraceCollection::kDisabled)
           .ToHandle(&error)) {
    return MaybeHandle<Object>();
  }
  ErrorUtils::SetFormattedStack(isolate_, error, stack);
  AddObjectWithID(id, error);

  // Add cause property if needed.
  if (static_cast<ErrorTag>(tag) == ErrorTag::kCause) {
    Handle<Object> cause;
    if (!ReadObject().ToHandle(&cause)) {
      return MaybeHandle<JSObject>();
    }
    Handle<Name> cause_string = isolate_->factory()->cause_string();
    if (JSObject::SetOwnPropertyIgnoreAttributes(error, cause_string, cause,
                                                 DONT_ENUM)
            .is_null()) {
      return MaybeHandle<JSObject>();
    }
    READ_NEXT_ERROR_TAG();
  }

#undef READ_NEXT_ERROR_TAG

  if (static_cast<ErrorTag>(tag) != ErrorTag::kEnd) {
    return MaybeHandle<Object>();
  }
  return error;
}

#if V8_ENABLE_WEBASSEMBLY
MaybeHandle<JSObject> ValueDeserializer::ReadWasmModuleTransfer() {
  uint32_t transfer_id = 0;
  Local<Value> module_value;
  if (!ReadVarint<uint32_t>().To(&transfer_id) || delegate_ == nullptr ||
      !delegate_
           ->GetWasmModuleFromId(reinterpret_cast<v8::Isolate*>(isolate_),
                                 transfer_id)
           .ToLocal(&module_value)) {
    RETURN_EXCEPTION_IF_EXCEPTION(isolate_, JSObject);
    return MaybeHandle<JSObject>();
  }
  uint32_t id = next_id_++;
  Handle<JSObject> module =
      Handle<JSObject>::cast(Utils::OpenHandle(*module_value));
  AddObjectWithID(id, module);
  return module;
}

MaybeHandle<WasmMemoryObject> ValueDeserializer::ReadWasmMemory() {
  uint32_t id = next_id_++;

  int32_t maximum_pages;
  if (!ReadZigZag<int32_t>().To(&maximum_pages)) return {};
  uint8_t memory64_byte;
  if (!ReadByte(&memory64_byte)) return {};
  if (memory64_byte > 1) return {};
  bool is_memory64 = memory64_byte;

  Handle<Object> buffer_object;
  if (!ReadObject().ToHandle(&buffer_object)) return {};
  if (!IsJSArrayBuffer(*buffer_object)) return {};

  Handle<JSArrayBuffer> buffer = Handle<JSArrayBuffer>::cast(buffer_object);
  if (!buffer->is_shared()) return {};

  Handle<WasmMemoryObject> result =
      WasmMemoryObject::New(isolate_, buffer, maximum_pages,
                            is_memory64 ? WasmMemoryFlag::kWasmMemory64
                                        : WasmMemoryFlag::kWasmMemory32);

  AddObjectWithID(id, result);
  return result;
}
#endif  // V8_ENABLE_WEBASSEMBLY

namespace {

// Throws a generic "deserialization failed" exception by default, unless a more
// specific exception has already been thrown.
void ThrowDeserializationExceptionIfNonePending(Isolate* isolate) {
  if (!isolate->has_exception()) {
    isolate->Throw(*isolate->factory()->NewError(
        MessageTemplate::kDataCloneDeserializationError));
  }
  DCHECK(isolate->has_exception());
}

}  // namespace

MaybeHandle<HeapObject> ValueDeserializer::ReadSharedObject() {
  STACK_CHECK(isolate_, MaybeHandle<HeapObject>());
  DCHECK_GE(version_, 15);

  uint32_t shared_object_id;
  if (!ReadVarint<uint32_t>().To(&shared_object_id)) {
    RETURN_EXCEPTION_IF_EXCEPTION(isolate_, HeapObject);
    return MaybeHandle<HeapObject>();
  }

  if (!delegate_) {
    ThrowDeserializationExceptionIfNonePending(isolate_);
    return MaybeHandle<HeapObject>();
  }

  if (shared_object_conveyor_ == nullptr) {
    const v8::SharedValueConveyor* conveyor = delegate_->GetSharedValueConveyor(
        reinterpret_cast<v8::Isolate*>(isolate_));
    if (!conveyor) {
      RETURN_EXCEPTION_IF_EXCEPTION(isolate_, HeapObject);
      return MaybeHandle<HeapObject>();
    }
    shared_object_conveyor_ = conveyor->private_.get();
  }

  Handle<HeapObject> shared_object(
      shared_object_conveyor_->GetPersisted(shared_object_id), isolate_);
  DCHECK(IsShared(*shared_object));
  return shared_object;
}

MaybeHandle<JSObject> ValueDeserializer::ReadHostObject() {
  if (!delegate_) return MaybeHandle<JSObject>();
  STACK_CHECK(isolate_, MaybeHandle<JSObject>());
  uint32_t id = next_id_++;
  v8::Isolate* v8_isolate = reinterpret_cast<v8::Isolate*>(isolate_);
  v8::Local<v8::Object> object;
  if (!delegate_->ReadHostObject(v8_isolate).ToLocal(&object)) {
    RETURN_EXCEPTION_IF_EXCEPTION(isolate_, JSObject);
    return MaybeHandle<JSObject>();
  }
  Handle<JSObject> js_object =
      Handle<JSObject>::cast(Utils::OpenHandle(*object));
  AddObjectWithID(id, js_object);
  return js_object;
}

// Copies a vector of property values into an object, given the map that should
// be used.
static void CommitProperties(Handle<JSObject> object, Handle<Map> map,
                             const std::vector<Handle<Object>>& properties) {
  JSObject::AllocateStorageForMap(object, map);
  DCHECK(!object->map()->is_dictionary_map());

  DisallowGarbageCollection no_gc;
  Tagged<DescriptorArray> descriptors = object->map()->instance_descriptors();
  for (InternalIndex i : InternalIndex::Range(properties.size())) {
    // Initializing store.
    object->WriteToField(i, descriptors->GetDetails(i),
                         *properties[i.raw_value()]);
  }
}

static bool IsValidObjectKey(Tagged<Object> value, Isolate* isolate) {
  if (IsSmi(value)) return true;
  auto instance_type = HeapObject::cast(value)->map(isolate)->instance_type();
  return InstanceTypeChecker::IsName(instance_type) ||
         InstanceTypeChecker::IsHeapNumber(instance_type);
}

Maybe<uint32_t> ValueDeserializer::ReadJSObjectProperties(
    Handle<JSObject> object, SerializationTag end_tag,
    bool can_use_transitions) {
  uint32_t num_properties = 0;

  // Fast path (following map transitions).
  if (can_use_transitions) {
    bool transitioning = true;
    Handle<Map> map(object->map(), isolate_);
    DCHECK(!map->is_dictionary_map());
    DCHECK_EQ(0, map->instance_descriptors(isolate_)->number_of_descriptors());
    std::vector<Handle<Object>> properties;
    properties.reserve(8);

    while (transitioning) {
      // If there are no more properties, finish.
      SerializationTag tag;
      if (!PeekTag().To(&tag)) return Nothing<uint32_t>();
      if (tag == end_tag) {
        ConsumeTag(end_tag);
        CommitProperties(object, map, properties);
        CHECK_LT(properties.size(), std::numeric_limits<uint32_t>::max());
        return Just(static_cast<uint32_t>(properties.size()));
      }

      // Determine the key to be used and the target map to transition to, if
      // possible. Transitioning may abort if the key is not a string, or if no
      // transition was found.
      Handle<Object> key;
      Handle<Map> target;
      Handle<String> expected_key;
      {
        TransitionsAccessor transitions(isolate_, *map);
        expected_key = transitions.ExpectedTransitionKey();
        if (!expected_key.is_null()) {
          target = transitions.ExpectedTransitionTarget();
        }
      }
      if (!expected_key.is_null() && ReadExpectedString(expected_key)) {
        key = expected_key;
      } else {
        if (!ReadObject().ToHandle(&key) || !IsValidObjectKey(*key, isolate_)) {
          return Nothing<uint32_t>();
        }
        if (IsString(*key, isolate_)) {
          key =
              isolate_->factory()->InternalizeString(Handle<String>::cast(key));
          // Don't reuse |transitions| because it could be stale.
          transitioning = TransitionsAccessor(isolate_, *map)
                              .FindTransitionToField(Handle<String>::cast(key))
                              .ToHandle(&target);
        } else {
          transitioning = false;
        }
      }

      // Read the value that corresponds to it.
      Handle<Object> value;
      if (!ReadObject().ToHandle(&value)) return Nothing<uint32_t>();

      // If still transitioning and the value fits the field representation
      // (though generalization may be required), store the property value so
      // that we can copy them all at once. Otherwise, stop transitioning.
      if (transitioning) {
        // Deserializaton of |value| might have deprecated current |target|,
        // ensure we are working with the up-to-date version.
        target = Map::Update(isolate_, target);
        if (!target->is_dictionary_map()) {
          InternalIndex descriptor(properties.size());
          PropertyDetails details =
              target->instance_descriptors(isolate_)->GetDetails(descriptor);
          Representation expected_representation = details.representation();
          if (Object::FitsRepresentation(*value, expected_representation)) {
            if (expected_representation.IsHeapObject() &&
                !FieldType::NowContains(
                    target->instance_descriptors(isolate_)->GetFieldType(
                        descriptor),
                    value)) {
              Handle<FieldType> value_type = Object::OptimalType(
                  *value, isolate_, expected_representation);
              MapUpdater::GeneralizeField(isolate_, target, descriptor,
                                          details.constness(),
                                          expected_representation, value_type);
            }
            DCHECK(FieldType::NowContains(
                target->instance_descriptors(isolate_)->GetFieldType(
                    descriptor),
                value));
            properties.push_back(value);
            map = target;
            continue;
          }
        }
        transitioning = false;
      }

      // Fell out of transitioning fast path. Commit the properties gathered so
      // far, and then start setting properties slowly instead.
      DCHECK(!transitioning);
      CHECK_LT(properties.size(), std::numeric_limits<uint32_t>::max());
      CHECK(!map->is_dictionary_map());
      CommitProperties(object, map, properties);
      num_properties = static_cast<uint32_t>(properties.size());

      // We checked earlier that IsValidObjectKey(key).
      PropertyKey lookup_key(isolate_, key);
      LookupIterator it(isolate_, object, lookup_key, LookupIterator::OWN);
      if (it.state() != LookupIterator::NOT_FOUND ||
          JSObject::DefineOwnPropertyIgnoreAttributes(&it, value, NONE)
              .is_null()) {
        return Nothing<uint32_t>();
      }
      num_properties++;
    }

    // At this point, transitioning should be done, but at least one property
    // should have been written (in the zero-property case, there is an early
    // return).
    DCHECK(!transitioning);
    DCHECK_GE(num_properties, 1u);
  }

  // Slow path.
  for (;; num_properties++) {
    SerializationTag tag;
    if (!PeekTag().To(&tag)) return Nothing<uint32_t>();
    if (tag == end_tag) {
      ConsumeTag(end_tag);
      return Just(num_properties);
    }

    Handle<Object> key;
    if (!ReadObject().ToHandle(&key) || !IsValidObjectKey(*key, isolate_)) {
      return Nothing<uint32_t>();
    }
    Handle<Object> value;
    if (!ReadObject().ToHandle(&value)) return Nothing<uint32_t>();

    // We checked earlier that IsValidObjectKey(key).
    PropertyKey lookup_key(isolate_, key);
    LookupIterator it(isolate_, object, lookup_key, LookupIterator::OWN);
    if (it.state() != LookupIterator::NOT_FOUND ||
        JSObject::DefineOwnPropertyIgnoreAttributes(&it, value, NONE)
            .is_null()) {
      return Nothing<uint32_t>();
    }
  }
}

bool ValueDeserializer::HasObjectWithID(uint32_t id) {
  return id < static_cast<unsigned>(id_map_->length()) &&
         !IsTheHole(id_map_->get(id), isolate_);
}

MaybeHandle<JSReceiver> ValueDeserializer::GetObjectWithID(uint32_t id) {
  if (id >= static_cast<unsigned>(id_map_->length())) {
    return MaybeHandle<JSReceiver>();
  }
  Tagged<Object> value = id_map_->get(id);
  if (IsTheHole(value, isolate_)) return MaybeHandle<JSReceiver>();
  DCHECK(IsJSReceiver(value));
  return Handle<JSReceiver>(JSReceiver::cast(value), isolate_);
}

void ValueDeserializer::AddObjectWithID(uint32_t id,
                                        Handle<JSReceiver> object) {
  DCHECK(!HasObjectWithID(id));
  Handle<FixedArray> new_array =
      FixedArray::SetAndGrow(isolate_, id_map_, id, object);

  // If the dictionary was reallocated, update the global handle.
  if (!new_array.is_identical_to(id_map_)) {
    GlobalHandles::Destroy(id_map_.location());
    id_map_ = isolate_->global_handles()->Create(*new_array);
  }
}

static Maybe<bool> SetPropertiesFromKeyValuePairs(Isolate* isolate,
                                                  Handle<JSObject> object,
                                                  Handle<Object>* data,
                                                  uint32_t num_properties) {
  for (unsigned i = 0; i < 2 * num_properties; i += 2) {
    Handle<Object> key = data[i];
    if (!IsValidObjectKey(*key, isolate)) return Nothing<bool>();
    Handle<Object> value = data[i + 1];
    PropertyKey lookup_key(isolate, key);
    LookupIterator it(isolate, object, lookup_key, LookupIterator::OWN);
    if (it.state() != LookupIterator::NOT_FOUND ||
        JSObject::DefineOwnPropertyIgnoreAttributes(&it, value, NONE)
            .is_null()) {
      return Nothing<bool>();
    }
  }
  return Just(true);
}

MaybeHandle<Object>
ValueDeserializer::ReadObjectUsingEntireBufferForLegacyFormat() {
  DCHECK_EQ(version_, 0u);
  HandleScope scope(isolate_);
  std::vector<Handle<Object>> stack;
  while (position_ < end_) {
    SerializationTag tag;
    if (!PeekTag().To(&tag)) break;

    Handle<Object> new_object;
    switch (tag) {
      case SerializationTag::kEndJSObject: {
        ConsumeTag(SerializationTag::kEndJSObject);

        // JS Object: Read the last 2*n values from the stack and use them as
        // key-value pairs.
        uint32_t num_properties;
        if (!ReadVarint<uint32_t>().To(&num_properties) ||
            stack.size() / 2 < num_properties) {
          isolate_->Throw(*isolate_->factory()->NewError(
              MessageTemplate::kDataCloneDeserializationError));
          return MaybeHandle<Object>();
        }

        size_t begin_properties =
            stack.size() - 2 * static_cast<size_t>(num_properties);
        Handle<JSObject> js_object =
            isolate_->factory()->NewJSObject(isolate_->object_function());
        if (num_properties &&
            !SetPropertiesFromKeyValuePairs(
                 isolate_, js_object, &stack[begin_properties], num_properties)
                 .FromMaybe(false)) {
          ThrowDeserializationExceptionIfNonePending(isolate_);
          return MaybeHandle<Object>();
        }

        stack.resize(begin_properties);
        new_object = js_object;
        break;
      }
      case SerializationTag::kEndSparseJSArray: {
        ConsumeTag(SerializationTag::kEndSparseJSArray);

        // Sparse JS Array: Read the last 2*|num_properties| from the stack.
        uint32_t num_properties;
        uint32_t length;
        if (!ReadVarint<uint32_t>().To(&num_properties) ||
            !ReadVarint<uint32_t>().To(&length) ||
            stack.size() / 2 < num_properties) {
          isolate_->Throw(*isolate_->factory()->NewError(
              MessageTemplate::kDataCloneDeserializationError));
          return MaybeHandle<Object>();
        }

        Handle<JSArray> js_array =
            isolate_->factory()->NewJSArray(0, TERMINAL_FAST_ELEMENTS_KIND);
        MAYBE_RETURN_NULL(JSArray::SetLength(js_array, length));
        size_t begin_properties =
            stack.size() - 2 * static_cast<size_t>(num_properties);
        if (num_properties &&
            !SetPropertiesFromKeyValuePairs(
                 isolate_, js_array, &stack[begin_properties], num_properties)
                 .FromMaybe(false)) {
          ThrowDeserializationExceptionIfNonePending(isolate_);
          return MaybeHandle<Object>();
        }

        stack.resize(begin_properties);
        new_object = js_array;
        break;
      }
      case SerializationTag::kEndDenseJSArray: {
        // This was already broken in Chromium, and apparently wasn't missed.
        isolate_->Throw(*isolate_->factory()->NewError(
            MessageTemplate::kDataCloneDeserializationError));
        return MaybeHandle<Object>();
      }
      default:
        if (!ReadObject().ToHandle(&new_object)) return MaybeHandle<Object>();
        break;
    }
    stack.push_back(new_object);
  }

// Nothing remains but padding.
#ifdef DEBUG
  while (position_ < end_) {
    DCHECK(*position_++ == static_cast<uint8_t>(SerializationTag::kPadding));
  }
#endif
  position_ = end_;

  if (stack.size() != 1) {
    isolate_->Throw(*isolate_->factory()->NewError(
        MessageTemplate::kDataCloneDeserializationError));
    return MaybeHandle<Object>();
  }
  return scope.CloseAndEscape(stack[0]);
}

}  // namespace internal
}  // namespace v8