Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 4267 insertions, 0 deletions

diff --git a/js/src/vm/StructuredClone.cpp b/js/src/vm/StructuredClone.cpp
new file mode 100644
index 0000000000..8f1e131021
--- /dev/null
+++ b/js/src/vm/StructuredClone.cpp
@@ -0,0 +1,4267 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * This file implements the structured data algorithms of
+ * https://html.spec.whatwg.org/multipage/structured-data.html
+ *
+ * The spec is in two parts:
+ *
+ * -   StructuredSerialize examines a JS value and produces a graph of Records.
+ * -   StructuredDeserialize walks the Records and produces a new JS value.
+ *
+ * The differences between our implementation and the spec are minor:
+ *
+ * -   We call the two phases "write" and "read".
+ * -   Our algorithms use an explicit work stack, rather than recursion.
+ * -   Serialized data is a flat array of bytes, not a (possibly cyclic) graph
+ *     of "Records".
+ * -   As a consequence, we handle non-treelike object graphs differently.
+ *     We serialize objects that appear in multiple places in the input as
+ *     backreferences, using sequential integer indexes.
+ *     See `JSStructuredCloneReader::allObjs`, our take on the "memory" map
+ *     in the spec's StructuredDeserialize.
+ */
+
+#include "js/StructuredClone.h"
+
+#include "mozilla/Casting.h"
+#include "mozilla/CheckedInt.h"
+#include "mozilla/EndianUtils.h"
+#include "mozilla/FloatingPoint.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/ScopeExit.h"
+
+#include <algorithm>
+#include <memory>
+#include <utility>
+
+#include "jsdate.h"
+
+#include "builtin/DataViewObject.h"
+#include "builtin/MapObject.h"
+#include "gc/GC.h"           // AutoSelectGCHeap
+#include "js/Array.h"        // JS::GetArrayLength, JS::IsArrayObject
+#include "js/ArrayBuffer.h"  // JS::{ArrayBufferHasData,DetachArrayBuffer,IsArrayBufferObject,New{,Mapped}ArrayBufferWithContents,ReleaseMappedArrayBufferContents}
+#include "js/ColumnNumber.h"  // JS::ColumnNumberOneOrigin, JS::TaggedColumnNumberOneOrigin
+#include "js/Date.h"
+#include "js/experimental/TypedData.h"  // JS_NewDataView, JS_New{{Ui,I}nt{8,16,32},Float{32,64},Uint8Clamped,Big{Ui,I}nt64}ArrayWithBuffer
+#include "js/friend/ErrorMessages.h"    // js::GetErrorMessage, JSMSG_*
+#include "js/GCAPI.h"
+#include "js/GCHashTable.h"
+#include "js/Object.h"              // JS::GetBuiltinClass
+#include "js/PropertyAndElement.h"  // JS_GetElement
+#include "js/RegExpFlags.h"         // JS::RegExpFlag, JS::RegExpFlags
+#include "js/ScalarType.h"          // js::Scalar::Type
+#include "js/SharedArrayBuffer.h"   // JS::IsSharedArrayBufferObject
+#include "js/Wrapper.h"
+#include "util/DifferentialTesting.h"
+#include "vm/BigIntType.h"
+#include "vm/ErrorObject.h"
+#include "vm/JSContext.h"
+#include "vm/PlainObject.h"  // js::PlainObject
+#include "vm/RegExpObject.h"
+#include "vm/SavedFrame.h"
+#include "vm/SharedArrayObject.h"
+#include "vm/TypedArrayObject.h"
+#include "wasm/WasmJS.h"
+
+#include "vm/ArrayObject-inl.h"
+#include "vm/Compartment-inl.h"
+#include "vm/ErrorObject-inl.h"
+#include "vm/InlineCharBuffer-inl.h"
+#include "vm/JSContext-inl.h"
+#include "vm/JSObject-inl.h"
+#include "vm/NativeObject-inl.h"
+#include "vm/ObjectOperations-inl.h"
+#include "vm/Realm-inl.h"
+
+using namespace js;
+
+using JS::CanonicalizeNaN;
+using JS::GetBuiltinClass;
+using JS::RegExpFlag;
+using JS::RegExpFlags;
+using JS::RootedValueVector;
+using mozilla::AssertedCast;
+using mozilla::BitwiseCast;
+using mozilla::Maybe;
+using mozilla::NativeEndian;
+using mozilla::NumbersAreIdentical;
+
+// When you make updates here, make sure you consider whether you need to bump
+// the value of JS_STRUCTURED_CLONE_VERSION in js/public/StructuredClone.h.  You
+// will likely need to increment the version if anything at all changes in the
+// serialization format.
+//
+// Note that SCTAG_END_OF_KEYS is written into the serialized form and should
+// have a stable ID, it need not be at the end of the list and should not be
+// used for sizing data structures.
+
+enum StructuredDataType : uint32_t {
+  // Structured data types provided by the engine
+  SCTAG_FLOAT_MAX = 0xFFF00000,
+  SCTAG_HEADER = 0xFFF10000,
+  SCTAG_NULL = 0xFFFF0000,
+  SCTAG_UNDEFINED,
+  SCTAG_BOOLEAN,
+  SCTAG_INT32,
+  SCTAG_STRING,
+  SCTAG_DATE_OBJECT,
+  SCTAG_REGEXP_OBJECT,
+  SCTAG_ARRAY_OBJECT,
+  SCTAG_OBJECT_OBJECT,
+  SCTAG_ARRAY_BUFFER_OBJECT_V2,  // Old version, for backwards compatibility.
+  SCTAG_BOOLEAN_OBJECT,
+  SCTAG_STRING_OBJECT,
+  SCTAG_NUMBER_OBJECT,
+  SCTAG_BACK_REFERENCE_OBJECT,
+  SCTAG_DO_NOT_USE_1,           // Required for backwards compatibility
+  SCTAG_DO_NOT_USE_2,           // Required for backwards compatibility
+  SCTAG_TYPED_ARRAY_OBJECT_V2,  // Old version, for backwards compatibility.
+  SCTAG_MAP_OBJECT,
+  SCTAG_SET_OBJECT,
+  SCTAG_END_OF_KEYS,
+  SCTAG_DO_NOT_USE_3,         // Required for backwards compatibility
+  SCTAG_DATA_VIEW_OBJECT_V2,  // Old version, for backwards compatibility.
+  SCTAG_SAVED_FRAME_OBJECT,
+
+  // No new tags before principals.
+  SCTAG_JSPRINCIPALS,
+  SCTAG_NULL_JSPRINCIPALS,
+  SCTAG_RECONSTRUCTED_SAVED_FRAME_PRINCIPALS_IS_SYSTEM,
+  SCTAG_RECONSTRUCTED_SAVED_FRAME_PRINCIPALS_IS_NOT_SYSTEM,
+
+  SCTAG_SHARED_ARRAY_BUFFER_OBJECT,
+  SCTAG_SHARED_WASM_MEMORY_OBJECT,
+
+  SCTAG_BIGINT,
+  SCTAG_BIGINT_OBJECT,
+
+  SCTAG_ARRAY_BUFFER_OBJECT,
+  SCTAG_TYPED_ARRAY_OBJECT,
+  SCTAG_DATA_VIEW_OBJECT,
+
+  SCTAG_ERROR_OBJECT,
+
+  SCTAG_RESIZABLE_ARRAY_BUFFER_OBJECT,
+  SCTAG_GROWABLE_SHARED_ARRAY_BUFFER_OBJECT,
+
+  SCTAG_TYPED_ARRAY_V1_MIN = 0xFFFF0100,
+  SCTAG_TYPED_ARRAY_V1_INT8 = SCTAG_TYPED_ARRAY_V1_MIN + Scalar::Int8,
+  SCTAG_TYPED_ARRAY_V1_UINT8 = SCTAG_TYPED_ARRAY_V1_MIN + Scalar::Uint8,
+  SCTAG_TYPED_ARRAY_V1_INT16 = SCTAG_TYPED_ARRAY_V1_MIN + Scalar::Int16,
+  SCTAG_TYPED_ARRAY_V1_UINT16 = SCTAG_TYPED_ARRAY_V1_MIN + Scalar::Uint16,
+  SCTAG_TYPED_ARRAY_V1_INT32 = SCTAG_TYPED_ARRAY_V1_MIN + Scalar::Int32,
+  SCTAG_TYPED_ARRAY_V1_UINT32 = SCTAG_TYPED_ARRAY_V1_MIN + Scalar::Uint32,
+  SCTAG_TYPED_ARRAY_V1_FLOAT32 = SCTAG_TYPED_ARRAY_V1_MIN + Scalar::Float32,
+  SCTAG_TYPED_ARRAY_V1_FLOAT64 = SCTAG_TYPED_ARRAY_V1_MIN + Scalar::Float64,
+  SCTAG_TYPED_ARRAY_V1_UINT8_CLAMPED =
+      SCTAG_TYPED_ARRAY_V1_MIN + Scalar::Uint8Clamped,
+  // BigInt64 and BigUint64 are not supported in the v1 format.
+  SCTAG_TYPED_ARRAY_V1_MAX = SCTAG_TYPED_ARRAY_V1_UINT8_CLAMPED,
+
+  // Define a separate range of numbers for Transferable-only tags, since
+  // they are not used for persistent clone buffers and therefore do not
+  // require bumping JS_STRUCTURED_CLONE_VERSION.
+  SCTAG_TRANSFER_MAP_HEADER = 0xFFFF0200,
+  SCTAG_TRANSFER_MAP_PENDING_ENTRY,
+  SCTAG_TRANSFER_MAP_ARRAY_BUFFER,
+  SCTAG_TRANSFER_MAP_STORED_ARRAY_BUFFER,
+  SCTAG_TRANSFER_MAP_END_OF_BUILTIN_TYPES,
+
+  SCTAG_END_OF_BUILTIN_TYPES
+};
+
+/*
+ * Format of transfer map:
+ *   <SCTAG_TRANSFER_MAP_HEADER, TransferableMapHeader(UNREAD|TRANSFERRED)>
+ *   numTransferables (64 bits)
+ *   array of:
+ *     <SCTAG_TRANSFER_MAP_*, TransferableOwnership>
+ *     pointer (64 bits)
+ *     extraData (64 bits), eg byte length for ArrayBuffers
+ */
+
+// Data associated with an SCTAG_TRANSFER_MAP_HEADER that tells whether the
+// contents have been read out yet or not.
+enum TransferableMapHeader { SCTAG_TM_UNREAD = 0, SCTAG_TM_TRANSFERRED };
+
+static inline uint64_t PairToUInt64(uint32_t tag, uint32_t data) {
+  return uint64_t(data) | (uint64_t(tag) << 32);
+}
+
+namespace js {
+
+template <typename T, typename AllocPolicy>
+struct BufferIterator {
+  using BufferList = mozilla::BufferList<AllocPolicy>;
+
+  explicit BufferIterator(const BufferList& buffer)
+      : mBuffer(buffer), mIter(buffer.Iter()) {
+    static_assert(8 % sizeof(T) == 0);
+  }
+
+  explicit BufferIterator(const JSStructuredCloneData& data)
+      : mBuffer(data.bufList_), mIter(data.Start()) {}
+
+  BufferIterator& operator=(const BufferIterator& other) {
+    MOZ_ASSERT(&mBuffer == &other.mBuffer);
+    mIter = other.mIter;
+    return *this;
+  }
+
+  [[nodiscard]] bool advance(size_t size = sizeof(T)) {
+    return mIter.AdvanceAcrossSegments(mBuffer, size);
+  }
+
+  BufferIterator operator++(int) {
+    BufferIterator ret = *this;
+    if (!advance(sizeof(T))) {
+      MOZ_ASSERT(false, "Failed to read StructuredCloneData. Data incomplete");
+    }
+    return ret;
+  }
+
+  BufferIterator& operator+=(size_t size) {
+    if (!advance(size)) {
+      MOZ_ASSERT(false, "Failed to read StructuredCloneData. Data incomplete");
+    }
+    return *this;
+  }
+
+  size_t operator-(const BufferIterator& other) const {
+    MOZ_ASSERT(&mBuffer == &other.mBuffer);
+    return mBuffer.RangeLength(other.mIter, mIter);
+  }
+
+  bool operator==(const BufferIterator& other) const {
+    return mBuffer.Start() == other.mBuffer.Start() && mIter == other.mIter;
+  }
+  bool operator!=(const BufferIterator& other) const {
+    return !(*this == other);
+  }
+
+  bool done() const { return mIter.Done(); }
+
+  [[nodiscard]] bool readBytes(char* outData, size_t size) {
+    return mBuffer.ReadBytes(mIter, outData, size);
+  }
+
+  void write(const T& data) {
+    MOZ_ASSERT(mIter.HasRoomFor(sizeof(T)));
+    *reinterpret_cast<T*>(mIter.Data()) = data;
+  }
+
+  T peek() const {
+    MOZ_ASSERT(mIter.HasRoomFor(sizeof(T)));
+    return *reinterpret_cast<T*>(mIter.Data());
+  }
+
+  bool canPeek() const { return mIter.HasRoomFor(sizeof(T)); }
+
+  const BufferList& mBuffer;
+  typename BufferList::IterImpl mIter;
+};
+
+SharedArrayRawBufferRefs& SharedArrayRawBufferRefs::operator=(
+    SharedArrayRawBufferRefs&& other) {
+  takeOwnership(std::move(other));
+  return *this;
+}
+
+SharedArrayRawBufferRefs::~SharedArrayRawBufferRefs() { releaseAll(); }
+
+bool SharedArrayRawBufferRefs::acquire(JSContext* cx,
+                                       SharedArrayRawBuffer* rawbuf) {
+  if (!refs_.append(rawbuf)) {
+    ReportOutOfMemory(cx);
+    return false;
+  }
+
+  if (!rawbuf->addReference()) {
+    refs_.popBack();
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                              JSMSG_SC_SAB_REFCNT_OFLO);
+    return false;
+  }
+
+  return true;
+}
+
+bool SharedArrayRawBufferRefs::acquireAll(
+    JSContext* cx, const SharedArrayRawBufferRefs& that) {
+  if (!refs_.reserve(refs_.length() + that.refs_.length())) {
+    ReportOutOfMemory(cx);
+    return false;
+  }
+
+  for (auto ref : that.refs_) {
+    if (!ref->addReference()) {
+      JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                JSMSG_SC_SAB_REFCNT_OFLO);
+      return false;
+    }
+    MOZ_ALWAYS_TRUE(refs_.append(ref));
+  }
+
+  return true;
+}
+
+void SharedArrayRawBufferRefs::takeOwnership(SharedArrayRawBufferRefs&& other) {
+  MOZ_ASSERT(refs_.empty());
+  refs_ = std::move(other.refs_);
+}
+
+void SharedArrayRawBufferRefs::releaseAll() {
+  for (auto ref : refs_) {
+    ref->dropReference();
+  }
+  refs_.clear();
+}
+
+// SCOutput provides an interface to write raw data -- eg uint64_ts, doubles,
+// arrays of bytes -- into a structured clone data output stream. It also knows
+// how to free any transferable data within that stream.
+//
+// Note that it contains a full JSStructuredCloneData object, which holds the
+// callbacks necessary to read/write/transfer/free the data. For the purpose of
+// this class, only the freeTransfer callback is relevant; the rest of the
+// callbacks are used by the higher-level JSStructuredCloneWriter interface.
+struct SCOutput {
+ public:
+  using Iter = BufferIterator<uint64_t, SystemAllocPolicy>;
+
+  SCOutput(JSContext* cx, JS::StructuredCloneScope scope);
+
+  JSContext* context() const { return cx; }
+  JS::StructuredCloneScope scope() const { return buf.scope(); }
+  void sameProcessScopeRequired() { buf.sameProcessScopeRequired(); }
+
+  [[nodiscard]] bool write(uint64_t u);
+  [[nodiscard]] bool writePair(uint32_t tag, uint32_t data);
+  [[nodiscard]] bool writeDouble(double d);
+  [[nodiscard]] bool writeBytes(const void* p, size_t nbytes);
+  [[nodiscard]] bool writeChars(const Latin1Char* p, size_t nchars);
+  [[nodiscard]] bool writeChars(const char16_t* p, size_t nchars);
+
+  template <class T>
+  [[nodiscard]] bool writeArray(const T* p, size_t nelems);
+
+  void setCallbacks(const JSStructuredCloneCallbacks* callbacks, void* closure,
+                    OwnTransferablePolicy policy) {
+    buf.setCallbacks(callbacks, closure, policy);
+  }
+  void extractBuffer(JSStructuredCloneData* data) { *data = std::move(buf); }
+
+  uint64_t tell() const { return buf.Size(); }
+  uint64_t count() const { return buf.Size() / sizeof(uint64_t); }
+  Iter iter() { return Iter(buf); }
+
+  size_t offset(Iter dest) { return dest - iter(); }
+
+  JSContext* cx;
+  JSStructuredCloneData buf;
+};
+
+class SCInput {
+ public:
+  using BufferIterator = js::BufferIterator<uint64_t, SystemAllocPolicy>;
+
+  SCInput(JSContext* cx, const JSStructuredCloneData& data);
+
+  JSContext* context() const { return cx; }
+
+  static void getPtr(uint64_t data, void** ptr);
+  static void getPair(uint64_t data, uint32_t* tagp, uint32_t* datap);
+
+  [[nodiscard]] bool read(uint64_t* p);
+  [[nodiscard]] bool readPair(uint32_t* tagp, uint32_t* datap);
+  [[nodiscard]] bool readDouble(double* p);
+  [[nodiscard]] bool readBytes(void* p, size_t nbytes);
+  [[nodiscard]] bool readChars(Latin1Char* p, size_t nchars);
+  [[nodiscard]] bool readChars(char16_t* p, size_t nchars);
+  [[nodiscard]] bool readPtr(void**);
+
+  [[nodiscard]] bool get(uint64_t* p);
+  [[nodiscard]] bool getPair(uint32_t* tagp, uint32_t* datap);
+
+  const BufferIterator& tell() const { return point; }
+  void seekTo(const BufferIterator& pos) { point = pos; }
+  [[nodiscard]] bool seekBy(size_t pos) {
+    if (!point.advance(pos)) {
+      reportTruncated();
+      return false;
+    }
+    return true;
+  }
+
+  template <class T>
+  [[nodiscard]] bool readArray(T* p, size_t nelems);
+
+  bool reportTruncated() {
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                              JSMSG_SC_BAD_SERIALIZED_DATA, "truncated");
+    return false;
+  }
+
+ private:
+  void staticAssertions() {
+    static_assert(sizeof(char16_t) == 2);
+    static_assert(sizeof(uint32_t) == 4);
+  }
+
+  JSContext* cx;
+  BufferIterator point;
+};
+
+}  // namespace js
+
+struct JSStructuredCloneReader {
+ public:
+  explicit JSStructuredCloneReader(SCInput& in, JS::StructuredCloneScope scope,
+                                   const JS::CloneDataPolicy& cloneDataPolicy,
+                                   const JSStructuredCloneCallbacks* cb,
+                                   void* cbClosure);
+
+  SCInput& input() { return in; }
+  bool read(MutableHandleValue vp, size_t nbytes);
+
+ private:
+  JSContext* context() { return in.context(); }
+
+  bool readHeader();
+  bool readTransferMap();
+
+  [[nodiscard]] bool readUint32(uint32_t* num);
+
+  enum ShouldAtomizeStrings : bool {
+    DontAtomizeStrings = false,
+    AtomizeStrings = true
+  };
+
+  template <typename CharT>
+  JSString* readStringImpl(uint32_t nchars, ShouldAtomizeStrings atomize);
+  JSString* readString(uint32_t data, ShouldAtomizeStrings atomize);
+
+  BigInt* readBigInt(uint32_t data);
+
+  [[nodiscard]] bool readTypedArray(uint32_t arrayType, uint64_t nelems,
+                                    MutableHandleValue vp, bool v1Read = false);
+
+  [[nodiscard]] bool readDataView(uint64_t byteLength, MutableHandleValue vp);
+
+  [[nodiscard]] bool readArrayBuffer(StructuredDataType type, uint32_t data,
+                                     MutableHandleValue vp);
+  [[nodiscard]] bool readV1ArrayBuffer(uint32_t arrayType, uint32_t nelems,
+                                       MutableHandleValue vp);
+
+  [[nodiscard]] bool readSharedArrayBuffer(StructuredDataType type,
+                                           MutableHandleValue vp);
+
+  [[nodiscard]] bool readSharedWasmMemory(uint32_t nbytes,
+                                          MutableHandleValue vp);
+
+  // A serialized SavedFrame contains primitive values in a header followed by
+  // an optional parent frame that is read recursively.
+  [[nodiscard]] JSObject* readSavedFrameHeader(uint32_t principalsTag);
+  [[nodiscard]] bool readSavedFrameFields(Handle<SavedFrame*> frameObj,
+                                          HandleValue parent, bool* state);
+
+  // A serialized Error contains primitive values in a header followed by
+  // 'cause', 'errors', and 'stack' fields that are read recursively.
+  [[nodiscard]] JSObject* readErrorHeader(uint32_t type);
+  [[nodiscard]] bool readErrorFields(Handle<ErrorObject*> errorObj,
+                                     HandleValue cause, bool* state);
+
+  [[nodiscard]] bool readMapField(Handle<MapObject*> mapObj, HandleValue key);
+
+  [[nodiscard]] bool readObjectField(HandleObject obj, HandleValue key);
+
+  [[nodiscard]] bool startRead(
+      MutableHandleValue vp,
+      ShouldAtomizeStrings atomizeStrings = DontAtomizeStrings);
+
+  SCInput& in;
+
+  // The widest scope that the caller will accept, where
+  // SameProcess is the widest (it can store anything it wants)
+  // and DifferentProcess is the narrowest (it cannot contain pointers and must
+  // be valid cross-process.)
+  JS::StructuredCloneScope allowedScope;
+
+  const JS::CloneDataPolicy cloneDataPolicy;
+
+  // Stack of objects with properties remaining to be read.
+  RootedValueVector objs;
+
+  // Maintain a stack of state values for the `objs` stack. Since this is only
+  // needed for a very small subset of objects (those with a known set of
+  // object children), the state information is stored as a stack of
+  // <object, state> pairs where the object determines which element of the
+  // `objs` stack that it corresponds to. So when reading from the `objs` stack,
+  // the state will be retrieved only if the top object on `objState` matches
+  // the top object of `objs`.
+  //
+  // Currently, the only state needed is a boolean indicating whether the fields
+  // have been read yet.
+  Rooted<GCVector<std::pair<HeapPtr<JSObject*>, bool>, 8>> objState;
+
+  // Array of all objects read during this deserialization, for resolving
+  // backreferences.
+  //
+  // For backreferences to work correctly, objects must be added to this
+  // array in exactly the order expected by the version of the Writer that
+  // created the serialized data, even across years and format versions. This
+  // is usually no problem, since both algorithms do a single linear pass
+  // over the serialized data. There is one hitch; see readTypedArray.
+  //
+  // The values in this vector are objects, except it can temporarily have
+  // one `undefined` placeholder value (the readTypedArray hack).
+  RootedValueVector allObjs;
+
+  size_t numItemsRead;
+
+  // The user defined callbacks that will be used for cloning.
+  const JSStructuredCloneCallbacks* callbacks;
+
+  // Any value passed to JS_ReadStructuredClone.
+  void* closure;
+
+  // The heap to use for allocating common GC things. This starts out as the
+  // nursery (the default) but may switch to the tenured heap if nursery
+  // collection occurs, as nursery allocation is pointless after the
+  // deserialized root object is tenured.
+  //
+  // This is only used for the most common kind, e.g. plain objects, strings
+  // and a couple of others.
+  AutoSelectGCHeap gcHeap;
+
+  friend bool JS_ReadString(JSStructuredCloneReader* r,
+                            JS::MutableHandleString str);
+  friend bool JS_ReadTypedArray(JSStructuredCloneReader* r,
+                                MutableHandleValue vp);
+
+  // Provide a way to detect whether any of the clone data is never used. When
+  // "tail" data (currently, this is only stored data for Transferred
+  // ArrayBuffers in the DifferentProcess scope) is read, record the first and
+  // last positions. At the end of deserialization, make sure there's nothing
+  // between the end of the main data and the beginning of the tail, nor after
+  // the end of the tail.
+  mozilla::Maybe<SCInput::BufferIterator> tailStartPos;
+  mozilla::Maybe<SCInput::BufferIterator> tailEndPos;
+};
+
+struct JSStructuredCloneWriter {
+ public:
+  explicit JSStructuredCloneWriter(JSContext* cx,
+                                   JS::StructuredCloneScope scope,
+                                   const JS::CloneDataPolicy& cloneDataPolicy,
+                                   const JSStructuredCloneCallbacks* cb,
+                                   void* cbClosure, const Value& tVal)
+      : out(cx, scope),
+        callbacks(cb),
+        closure(cbClosure),
+        objs(cx),
+        counts(cx),
+        objectEntries(cx),
+        otherEntries(cx),
+        memory(cx),
+        transferable(cx, tVal),
+        transferableObjects(cx, TransferableObjectsList(cx)),
+        cloneDataPolicy(cloneDataPolicy) {
+    out.setCallbacks(cb, cbClosure,
+                     OwnTransferablePolicy::OwnsTransferablesIfAny);
+  }
+
+  bool init() {
+    return parseTransferable() && writeHeader() && writeTransferMap();
+  }
+
+  bool write(HandleValue v);
+
+  SCOutput& output() { return out; }
+
+  void extractBuffer(JSStructuredCloneData* newData) {
+    out.extractBuffer(newData);
+  }
+
+ private:
+  JSStructuredCloneWriter() = delete;
+  JSStructuredCloneWriter(const JSStructuredCloneWriter&) = delete;
+
+  JSContext* context() { return out.context(); }
+
+  bool writeHeader();
+  bool writeTransferMap();
+
+  bool writeString(uint32_t tag, JSString* str);
+  bool writeBigInt(uint32_t tag, BigInt* bi);
+  bool writeArrayBuffer(HandleObject obj);
+  bool writeTypedArray(HandleObject obj);
+  bool writeDataView(HandleObject obj);
+  bool writeSharedArrayBuffer(HandleObject obj);
+  bool writeSharedWasmMemory(HandleObject obj);
+  bool startObject(HandleObject obj, bool* backref);
+  bool writePrimitive(HandleValue v);
+  bool startWrite(HandleValue v);
+  bool traverseObject(HandleObject obj, ESClass cls);
+  bool traverseMap(HandleObject obj);
+  bool traverseSet(HandleObject obj);
+  bool traverseSavedFrame(HandleObject obj);
+  bool traverseError(HandleObject obj);
+
+  template <typename... Args>
+  bool reportDataCloneError(uint32_t errorId, Args&&... aArgs);
+
+  bool parseTransferable();
+  bool transferOwnership();
+
+  inline void checkStack();
+
+  SCOutput out;
+
+  // The user defined callbacks that will be used to signal cloning, in some
+  // cases.
+  const JSStructuredCloneCallbacks* callbacks;
+
+  // Any value passed to the callbacks.
+  void* closure;
+
+  // Vector of objects with properties remaining to be written.
+  //
+  // NB: These can span multiple compartments, so the compartment must be
+  // entered before any manipulation is performed.
+  RootedValueVector objs;
+
+  // counts[i] is the number of entries of objs[i] remaining to be written.
+  // counts.length() == objs.length() and sum(counts) == entries.length().
+  Vector<size_t> counts;
+
+  // For JSObject: Property IDs as value
+  RootedIdVector objectEntries;
+
+  // For Map: Key followed by value
+  // For Set: Key
+  // For SavedFrame: parent SavedFrame
+  // For Error: cause, errors, stack
+  RootedValueVector otherEntries;
+
+  // The "memory" list described in the HTML5 internal structured cloning
+  // algorithm.  memory is a superset of objs; items are never removed from
+  // Memory until a serialization operation is finished
+  using CloneMemory = GCHashMap<JSObject*, uint32_t,
+                                StableCellHasher<JSObject*>, SystemAllocPolicy>;
+  Rooted<CloneMemory> memory;
+
+  // Set of transferable objects
+  RootedValue transferable;
+  using TransferableObjectsList = GCVector<JSObject*>;
+  Rooted<TransferableObjectsList> transferableObjects;
+
+  const JS::CloneDataPolicy cloneDataPolicy;
+
+  friend bool JS_WriteString(JSStructuredCloneWriter* w, HandleString str);
+  friend bool JS_WriteTypedArray(JSStructuredCloneWriter* w, HandleValue v);
+  friend bool JS_ObjectNotWritten(JSStructuredCloneWriter* w, HandleObject obj);
+};
+
+JS_PUBLIC_API uint64_t js::GetSCOffset(JSStructuredCloneWriter* writer) {
+  MOZ_ASSERT(writer);
+  return writer->output().count() * sizeof(uint64_t);
+}
+
+static_assert(SCTAG_END_OF_BUILTIN_TYPES <= JS_SCTAG_USER_MIN);
+static_assert(JS_SCTAG_USER_MIN <= JS_SCTAG_USER_MAX);
+static_assert(Scalar::Int8 == 0);
+
+template <typename... Args>
+static void ReportDataCloneError(JSContext* cx,
+                                 const JSStructuredCloneCallbacks* callbacks,
+                                 uint32_t errorId, void* closure,
+                                 Args&&... aArgs) {
+  unsigned errorNumber;
+  switch (errorId) {
+    case JS_SCERR_DUP_TRANSFERABLE:
+      errorNumber = JSMSG_SC_DUP_TRANSFERABLE;
+      break;
+
+    case JS_SCERR_TRANSFERABLE:
+      errorNumber = JSMSG_SC_NOT_TRANSFERABLE;
+      break;
+
+    case JS_SCERR_UNSUPPORTED_TYPE:
+      errorNumber = JSMSG_SC_UNSUPPORTED_TYPE;
+      break;
+
+    case JS_SCERR_SHMEM_TRANSFERABLE:
+      errorNumber = JSMSG_SC_SHMEM_TRANSFERABLE;
+      break;
+
+    case JS_SCERR_TYPED_ARRAY_DETACHED:
+      errorNumber = JSMSG_TYPED_ARRAY_DETACHED;
+      break;
+
+    case JS_SCERR_WASM_NO_TRANSFER:
+      errorNumber = JSMSG_WASM_NO_TRANSFER;
+      break;
+
+    case JS_SCERR_NOT_CLONABLE:
+      errorNumber = JSMSG_SC_NOT_CLONABLE;
+      break;
+
+    case JS_SCERR_NOT_CLONABLE_WITH_COOP_COEP:
+      errorNumber = JSMSG_SC_NOT_CLONABLE_WITH_COOP_COEP;
+      break;
+
+    default:
+      MOZ_CRASH("Unkown errorId");
+      break;
+  }
+
+  if (callbacks && callbacks->reportError) {
+    MOZ_RELEASE_ASSERT(!cx->isExceptionPending());
+
+    JSErrorReport report;
+    report.errorNumber = errorNumber;
+    // Get js error message if it's possible and propagate it through callback.
+    if (JS_ExpandErrorArgumentsASCII(cx, GetErrorMessage, errorNumber, &report,
+                                     std::forward<Args>(aArgs)...) &&
+        report.message()) {
+      callbacks->reportError(cx, errorId, closure, report.message().c_str());
+    } else {
+      ReportOutOfMemory(cx);
+
+      callbacks->reportError(cx, errorId, closure, "");
+    }
+
+    return;
+  }
+
+  JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, errorNumber,
+                            std::forward<Args>(aArgs)...);
+}
+
+bool WriteStructuredClone(JSContext* cx, HandleValue v,
+                          JSStructuredCloneData* bufp,
+                          JS::StructuredCloneScope scope,
+                          const JS::CloneDataPolicy& cloneDataPolicy,
+                          const JSStructuredCloneCallbacks* cb, void* cbClosure,
+                          const Value& transferable) {
+  JSStructuredCloneWriter w(cx, scope, cloneDataPolicy, cb, cbClosure,
+                            transferable);
+  if (!w.init()) {
+    return false;
+  }
+  if (!w.write(v)) {
+    return false;
+  }
+  w.extractBuffer(bufp);
+  return true;
+}
+
+bool ReadStructuredClone(JSContext* cx, const JSStructuredCloneData& data,
+                         JS::StructuredCloneScope scope, MutableHandleValue vp,
+                         const JS::CloneDataPolicy& cloneDataPolicy,
+                         const JSStructuredCloneCallbacks* cb,
+                         void* cbClosure) {
+  if (data.Size() % 8) {
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                              JSMSG_SC_BAD_SERIALIZED_DATA, "misaligned");
+    return false;
+  }
+  SCInput in(cx, data);
+  JSStructuredCloneReader r(in, scope, cloneDataPolicy, cb, cbClosure);
+  return r.read(vp, data.Size());
+}
+
+static bool StructuredCloneHasTransferObjects(
+    const JSStructuredCloneData& data) {
+  if (data.Size() < sizeof(uint64_t)) {
+    return false;
+  }
+
+  uint64_t u;
+  BufferIterator<uint64_t, SystemAllocPolicy> iter(data);
+  MOZ_ALWAYS_TRUE(iter.readBytes(reinterpret_cast<char*>(&u), sizeof(u)));
+  uint32_t tag = uint32_t(u >> 32);
+  return (tag == SCTAG_TRANSFER_MAP_HEADER);
+}
+
+namespace js {
+
+SCInput::SCInput(JSContext* cx, const JSStructuredCloneData& data)
+    : cx(cx), point(data) {
+  static_assert(JSStructuredCloneData::BufferList::kSegmentAlignment % 8 == 0,
+                "structured clone buffer reads should be aligned");
+  MOZ_ASSERT(data.Size() % 8 == 0);
+}
+
+bool SCInput::read(uint64_t* p) {
+  if (!point.canPeek()) {
+    *p = 0;  // initialize to shut GCC up
+    return reportTruncated();
+  }
+  *p = NativeEndian::swapFromLittleEndian(point.peek());
+  MOZ_ALWAYS_TRUE(point.advance());
+  return true;
+}
+
+bool SCInput::readPair(uint32_t* tagp, uint32_t* datap) {
+  uint64_t u;
+  bool ok = read(&u);
+  if (ok) {
+    *tagp = uint32_t(u >> 32);
+    *datap = uint32_t(u);
+  }
+  return ok;
+}
+
+bool SCInput::get(uint64_t* p) {
+  if (!point.canPeek()) {
+    return reportTruncated();
+  }
+  *p = NativeEndian::swapFromLittleEndian(point.peek());
+  return true;
+}
+
+bool SCInput::getPair(uint32_t* tagp, uint32_t* datap) {
+  uint64_t u = 0;
+  if (!get(&u)) {
+    return false;
+  }
+
+  *tagp = uint32_t(u >> 32);
+  *datap = uint32_t(u);
+  return true;
+}
+
+void SCInput::getPair(uint64_t data, uint32_t* tagp, uint32_t* datap) {
+  uint64_t u = NativeEndian::swapFromLittleEndian(data);
+  *tagp = uint32_t(u >> 32);
+  *datap = uint32_t(u);
+}
+
+bool SCInput::readDouble(double* p) {
+  uint64_t u;
+  if (!read(&u)) {
+    return false;
+  }
+  *p = CanonicalizeNaN(mozilla::BitwiseCast<double>(u));
+  return true;
+}
+
+template <typename T>
+static void swapFromLittleEndianInPlace(T* ptr, size_t nelems) {
+  if (nelems > 0) {
+    NativeEndian::swapFromLittleEndianInPlace(ptr, nelems);
+  }
+}
+
+template <>
+void swapFromLittleEndianInPlace(uint8_t* ptr, size_t nelems) {}
+
+// Data is packed into an integral number of uint64_t words. Compute the
+// padding required to finish off the final word.
+static size_t ComputePadding(size_t nelems, size_t elemSize) {
+  // We want total length mod 8, where total length is nelems * sizeof(T),
+  // but that might overflow. So reduce nelems to nelems mod 8, since we are
+  // going to be doing a mod 8 later anyway.
+  size_t leftoverLength = (nelems % sizeof(uint64_t)) * elemSize;
+  return (-leftoverLength) & (sizeof(uint64_t) - 1);
+}
+
+template <class T>
+bool SCInput::readArray(T* p, size_t nelems) {
+  if (!nelems) {
+    return true;
+  }
+
+  static_assert(sizeof(uint64_t) % sizeof(T) == 0);
+
+  // Fail if nelems is so huge that computing the full size will overflow.
+  mozilla::CheckedInt<size_t> size =
+      mozilla::CheckedInt<size_t>(nelems) * sizeof(T);
+  if (!size.isValid()) {
+    return reportTruncated();
+  }
+
+  if (!point.readBytes(reinterpret_cast<char*>(p), size.value())) {
+    // To avoid any way in which uninitialized data could escape, zero the array
+    // if filling it failed.
+    std::uninitialized_fill_n(p, nelems, 0);
+    return false;
+  }
+
+  swapFromLittleEndianInPlace(p, nelems);
+
+  point += ComputePadding(nelems, sizeof(T));
+
+  return true;
+}
+
+bool SCInput::readBytes(void* p, size_t nbytes) {
+  return readArray((uint8_t*)p, nbytes);
+}
+
+bool SCInput::readChars(Latin1Char* p, size_t nchars) {
+  static_assert(sizeof(Latin1Char) == sizeof(uint8_t),
+                "Latin1Char must fit in 1 byte");
+  return readBytes(p, nchars);
+}
+
+bool SCInput::readChars(char16_t* p, size_t nchars) {
+  MOZ_ASSERT(sizeof(char16_t) == sizeof(uint16_t));
+  return readArray((uint16_t*)p, nchars);
+}
+
+void SCInput::getPtr(uint64_t data, void** ptr) {
+  *ptr = reinterpret_cast<void*>(NativeEndian::swapFromLittleEndian(data));
+}
+
+bool SCInput::readPtr(void** p) {
+  uint64_t u;
+  if (!read(&u)) {
+    return false;
+  }
+  *p = reinterpret_cast<void*>(u);
+  return true;
+}
+
+SCOutput::SCOutput(JSContext* cx, JS::StructuredCloneScope scope)
+    : cx(cx), buf(scope) {}
+
+bool SCOutput::write(uint64_t u) {
+  uint64_t v = NativeEndian::swapToLittleEndian(u);
+  if (!buf.AppendBytes(reinterpret_cast<char*>(&v), sizeof(u))) {
+    ReportOutOfMemory(context());
+    return false;
+  }
+  return true;
+}
+
+bool SCOutput::writePair(uint32_t tag, uint32_t data) {
+  // As it happens, the tag word appears after the data word in the output.
+  // This is because exponents occupy the last 2 bytes of doubles on the
+  // little-endian platforms we care most about.
+  //
+  // For example, TrueValue() is written using writePair(SCTAG_BOOLEAN, 1).
+  // PairToUInt64 produces the number 0xFFFF000200000001.
+  // That is written out as the bytes 01 00 00 00 02 00 FF FF.
+  return write(PairToUInt64(tag, data));
+}
+
+static inline double ReinterpretPairAsDouble(uint32_t tag, uint32_t data) {
+  return BitwiseCast<double>(PairToUInt64(tag, data));
+}
+
+bool SCOutput::writeDouble(double d) {
+  return write(BitwiseCast<uint64_t>(CanonicalizeNaN(d)));
+}
+
+template <class T>
+bool SCOutput::writeArray(const T* p, size_t nelems) {
+  static_assert(8 % sizeof(T) == 0);
+  static_assert(sizeof(uint64_t) % sizeof(T) == 0);
+
+  if (nelems == 0) {
+    return true;
+  }
+
+  for (size_t i = 0; i < nelems; i++) {
+    T value = NativeEndian::swapToLittleEndian(p[i]);
+    if (!buf.AppendBytes(reinterpret_cast<char*>(&value), sizeof(value))) {
+      return false;
+    }
+  }
+
+  // Zero-pad to 8 bytes boundary.
+  size_t padbytes = ComputePadding(nelems, sizeof(T));
+  char zeroes[sizeof(uint64_t)] = {0};
+  if (!buf.AppendBytes(zeroes, padbytes)) {
+    return false;
+  }
+
+  return true;
+}
+
+template <>
+bool SCOutput::writeArray<uint8_t>(const uint8_t* p, size_t nelems) {
+  if (nelems == 0) {
+    return true;
+  }
+
+  if (!buf.AppendBytes(reinterpret_cast<const char*>(p), nelems)) {
+    return false;
+  }
+
+  // zero-pad to 8 bytes boundary
+  size_t padbytes = ComputePadding(nelems, 1);
+  char zeroes[sizeof(uint64_t)] = {0};
+  if (!buf.AppendBytes(zeroes, padbytes)) {
+    return false;
+  }
+
+  return true;
+}
+
+bool SCOutput::writeBytes(const void* p, size_t nbytes) {
+  return writeArray((const uint8_t*)p, nbytes);
+}
+
+bool SCOutput::writeChars(const char16_t* p, size_t nchars) {
+  static_assert(sizeof(char16_t) == sizeof(uint16_t),
+                "required so that treating char16_t[] memory as uint16_t[] "
+                "memory is permissible");
+  return writeArray((const uint16_t*)p, nchars);
+}
+
+bool SCOutput::writeChars(const Latin1Char* p, size_t nchars) {
+  static_assert(sizeof(Latin1Char) == sizeof(uint8_t),
+                "Latin1Char must fit in 1 byte");
+  return writeBytes(p, nchars);
+}
+
+}  // namespace js
+
+JSStructuredCloneData::~JSStructuredCloneData() { discardTransferables(); }
+
+// If the buffer contains Transferables, free them. Note that custom
+// Transferables will use the JSStructuredCloneCallbacks::freeTransfer() to
+// delete their transferables.
+void JSStructuredCloneData::discardTransferables() {
+  if (!Size()) {
+    return;
+  }
+
+  if (ownTransferables_ != OwnTransferablePolicy::OwnsTransferablesIfAny) {
+    return;
+  }
+
+  // DifferentProcess clones cannot contain pointers, so nothing needs to be
+  // released.
+  if (scope() == JS::StructuredCloneScope::DifferentProcess) {
+    return;
+  }
+
+  FreeTransferStructuredCloneOp freeTransfer = nullptr;
+  if (callbacks_) {
+    freeTransfer = callbacks_->freeTransfer;
+  }
+
+  auto point = BufferIterator<uint64_t, SystemAllocPolicy>(*this);
+  if (point.done()) {
+    return;  // Empty buffer
+  }
+
+  uint32_t tag, data;
+  MOZ_RELEASE_ASSERT(point.canPeek());
+  SCInput::getPair(point.peek(), &tag, &data);
+  MOZ_ALWAYS_TRUE(point.advance());
+
+  if (tag == SCTAG_HEADER) {
+    if (point.done()) {
+      return;
+    }
+
+    MOZ_RELEASE_ASSERT(point.canPeek());
+    SCInput::getPair(point.peek(), &tag, &data);
+    MOZ_ALWAYS_TRUE(point.advance());
+  }
+
+  if (tag != SCTAG_TRANSFER_MAP_HEADER) {
+    return;
+  }
+
+  if (TransferableMapHeader(data) == SCTAG_TM_TRANSFERRED) {
+    return;
+  }
+
+  // freeTransfer should not GC
+  JS::AutoSuppressGCAnalysis nogc;
+
+  if (point.done()) {
+    return;
+  }
+
+  MOZ_RELEASE_ASSERT(point.canPeek());
+  uint64_t numTransferables = NativeEndian::swapFromLittleEndian(point.peek());
+  MOZ_ALWAYS_TRUE(point.advance());
+  while (numTransferables--) {
+    if (!point.canPeek()) {
+      return;
+    }
+
+    uint32_t ownership;
+    SCInput::getPair(point.peek(), &tag, &ownership);
+    MOZ_ALWAYS_TRUE(point.advance());
+    MOZ_ASSERT(tag >= SCTAG_TRANSFER_MAP_PENDING_ENTRY);
+    if (!point.canPeek()) {
+      return;
+    }
+
+    void* content;
+    SCInput::getPtr(point.peek(), &content);
+    MOZ_ALWAYS_TRUE(point.advance());
+    if (!point.canPeek()) {
+      return;
+    }
+
+    uint64_t extraData = NativeEndian::swapFromLittleEndian(point.peek());
+    MOZ_ALWAYS_TRUE(point.advance());
+
+    if (ownership < JS::SCTAG_TMO_FIRST_OWNED) {
+      continue;
+    }
+
+    if (ownership == JS::SCTAG_TMO_ALLOC_DATA) {
+      js_free(content);
+    } else if (ownership == JS::SCTAG_TMO_MAPPED_DATA) {
+      JS::ReleaseMappedArrayBufferContents(content, extraData);
+    } else if (freeTransfer) {
+      freeTransfer(tag, JS::TransferableOwnership(ownership), content,
+                   extraData, closure_);
+    } else {
+      MOZ_ASSERT(false, "unknown ownership");
+    }
+  }
+}
+
+static_assert(JSString::MAX_LENGTH < UINT32_MAX);
+
+bool JSStructuredCloneWriter::parseTransferable() {
+  // NOTE: The transferables set is tested for non-emptiness at various
+  //       junctures in structured cloning, so this set must be initialized
+  //       by this method in all non-error cases.
+  MOZ_ASSERT(transferableObjects.empty(),
+             "parseTransferable called with stale data");
+
+  if (transferable.isNull() || transferable.isUndefined()) {
+    return true;
+  }
+
+  if (!transferable.isObject()) {
+    return reportDataCloneError(JS_SCERR_TRANSFERABLE);
+  }
+
+  JSContext* cx = context();
+  RootedObject array(cx, &transferable.toObject());
+  bool isArray;
+  if (!JS::IsArrayObject(cx, array, &isArray)) {
+    return false;
+  }
+  if (!isArray) {
+    return reportDataCloneError(JS_SCERR_TRANSFERABLE);
+  }
+
+  uint32_t length;
+  if (!JS::GetArrayLength(cx, array, &length)) {
+    return false;
+  }
+
+  // Initialize the set for the provided array's length.
+  if (!transferableObjects.reserve(length)) {
+    return false;
+  }
+
+  if (length == 0) {
+    return true;
+  }
+
+  RootedValue v(context());
+  RootedObject tObj(context());
+
+  for (uint32_t i = 0; i < length; ++i) {
+    if (!CheckForInterrupt(cx)) {
+      return false;
+    }
+
+    if (!JS_GetElement(cx, array, i, &v)) {
+      return false;
+    }
+
+    if (!v.isObject()) {
+      return reportDataCloneError(JS_SCERR_TRANSFERABLE);
+    }
+    tObj = &v.toObject();
+
+    RootedObject unwrappedObj(cx, CheckedUnwrapStatic(tObj));
+    if (!unwrappedObj) {
+      ReportAccessDenied(cx);
+      return false;
+    }
+
+    // Shared memory cannot be transferred because it is not possible (nor
+    // desirable) to detach the memory in agents that already hold a
+    // reference to it.
+
+    if (unwrappedObj->is<SharedArrayBufferObject>()) {
+      return reportDataCloneError(JS_SCERR_SHMEM_TRANSFERABLE);
+    }
+
+    else if (unwrappedObj->is<WasmMemoryObject>()) {
+      if (unwrappedObj->as<WasmMemoryObject>().isShared()) {
+        return reportDataCloneError(JS_SCERR_SHMEM_TRANSFERABLE);
+      }
+    }
+
+    // External array buffers may be able to be transferred in the future,
+    // but that is not currently implemented.
+
+    else if (unwrappedObj->is<ArrayBufferObject>()) {
+      if (unwrappedObj->as<ArrayBufferObject>().isExternal()) {
+        return reportDataCloneError(JS_SCERR_TRANSFERABLE);
+      }
+    }
+
+    else {
+      if (!out.buf.callbacks_ || !out.buf.callbacks_->canTransfer) {
+        return reportDataCloneError(JS_SCERR_TRANSFERABLE);
+      }
+
+      JSAutoRealm ar(cx, unwrappedObj);
+      bool sameProcessScopeRequired = false;
+      if (!out.buf.callbacks_->canTransfer(
+              cx, unwrappedObj, &sameProcessScopeRequired, out.buf.closure_)) {
+        return reportDataCloneError(JS_SCERR_TRANSFERABLE);
+      }
+
+      if (sameProcessScopeRequired) {
+        output().sameProcessScopeRequired();
+      }
+    }
+
+    // No duplicates allowed
+    if (std::find(transferableObjects.begin(), transferableObjects.end(),
+                  tObj) != transferableObjects.end()) {
+      return reportDataCloneError(JS_SCERR_DUP_TRANSFERABLE);
+    }
+
+    if (!transferableObjects.append(tObj)) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+template <typename... Args>
+bool JSStructuredCloneWriter::reportDataCloneError(uint32_t errorId,
+                                                   Args&&... aArgs) {
+  ReportDataCloneError(context(), out.buf.callbacks_, errorId, out.buf.closure_,
+                       std::forward<Args>(aArgs)...);
+  return false;
+}
+
+bool JSStructuredCloneWriter::writeString(uint32_t tag, JSString* str) {
+  JSLinearString* linear = str->ensureLinear(context());
+  if (!linear) {
+    return false;
+  }
+
+#if FUZZING_JS_FUZZILLI
+  if (js::SupportDifferentialTesting()) {
+    // TODO we could always output a twoByteChar string
+    return true;
+  }
+#endif
+
+  static_assert(JSString::MAX_LENGTH <= INT32_MAX,
+                "String length must fit in 31 bits");
+
+  uint32_t length = linear->length();
+  uint32_t lengthAndEncoding =
+      length | (uint32_t(linear->hasLatin1Chars()) << 31);
+  if (!out.writePair(tag, lengthAndEncoding)) {
+    return false;
+  }
+
+  JS::AutoCheckCannotGC nogc;
+  return linear->hasLatin1Chars()
+             ? out.writeChars(linear->latin1Chars(nogc), length)
+             : out.writeChars(linear->twoByteChars(nogc), length);
+}
+
+bool JSStructuredCloneWriter::writeBigInt(uint32_t tag, BigInt* bi) {
+  bool signBit = bi->isNegative();
+  size_t length = bi->digitLength();
+  // The length must fit in 31 bits to leave room for a sign bit.
+  if (length > size_t(INT32_MAX)) {
+    return false;
+  }
+  uint32_t lengthAndSign = length | (static_cast<uint32_t>(signBit) << 31);
+
+  if (!out.writePair(tag, lengthAndSign)) {
+    return false;
+  }
+  return out.writeArray(bi->digits().data(), length);
+}
+
+inline void JSStructuredCloneWriter::checkStack() {
+#ifdef DEBUG
+  // To avoid making serialization O(n^2), limit stack-checking at 10.
+  const size_t MAX = 10;
+
+  size_t limit = std::min(counts.length(), MAX);
+  MOZ_ASSERT(objs.length() == counts.length());
+  size_t total = 0;
+  for (size_t i = 0; i < limit; i++) {
+    MOZ_ASSERT(total + counts[i] >= total);
+    total += counts[i];
+  }
+  if (counts.length() <= MAX) {
+    MOZ_ASSERT(total == objectEntries.length() + otherEntries.length());
+  } else {
+    MOZ_ASSERT(total <= objectEntries.length() + otherEntries.length());
+  }
+
+  size_t j = objs.length();
+  for (size_t i = 0; i < limit; i++) {
+    --j;
+    MOZ_ASSERT(memory.has(&objs[j].toObject()));
+  }
+#endif
+}
+
+/*
+ * Write out a typed array. Note that post-v1 structured clone buffers do not
+ * perform endianness conversion on stored data, so multibyte typed arrays
+ * cannot be deserialized into a different endianness machine. Endianness
+ * conversion would prevent sharing ArrayBuffers: if you have Int8Array and
+ * Int16Array views of the same ArrayBuffer, should the data bytes be
+ * byte-swapped when writing or not? The Int8Array requires them to not be
+ * swapped; the Int16Array requires that they are.
+ */
+bool JSStructuredCloneWriter::writeTypedArray(HandleObject obj) {
+  Rooted<TypedArrayObject*> tarr(context(),
+                                 obj->maybeUnwrapAs<TypedArrayObject>());
+  JSAutoRealm ar(context(), tarr);
+
+#ifdef FUZZING_JS_FUZZILLI
+  if (js::SupportDifferentialTesting() && !tarr->hasBuffer()) {
+    // fake oom because differential testing will fail
+    fprintf(stderr, "[unhandlable oom]");
+    _exit(-1);
+    return false;
+  }
+#endif
+
+  if (!TypedArrayObject::ensureHasBuffer(context(), tarr)) {
+    return false;
+  }
+
+  if (!out.writePair(SCTAG_TYPED_ARRAY_OBJECT, uint32_t(tarr->type()))) {
+    return false;
+  }
+
+  mozilla::Maybe<size_t> nelems = tarr->length();
+  if (!nelems) {
+    return reportDataCloneError(JS_SCERR_TYPED_ARRAY_DETACHED);
+  }
+
+  // Auto-length TypedArrays are tagged by storing `-1` for the length. We still
+  // need to query the length to check for detached or out-of-bounds lengths.
+  bool isAutoLength = tarr->is<ResizableTypedArrayObject>() &&
+                      tarr->as<ResizableTypedArrayObject>().isAutoLength();
+  uint64_t length = isAutoLength ? uint64_t(-1) : uint64_t(*nelems);
+  if (!out.write(length)) {
+    return false;
+  }
+
+  // Write out the ArrayBuffer tag and contents
+  RootedValue val(context(), tarr->bufferValue());
+  if (!startWrite(val)) {
+    return false;
+  }
+
+  uint64_t byteOffset = tarr->byteOffset().valueOr(0);
+  return out.write(byteOffset);
+}
+
+bool JSStructuredCloneWriter::writeDataView(HandleObject obj) {
+  Rooted<DataViewObject*> view(context(), obj->maybeUnwrapAs<DataViewObject>());
+  JSAutoRealm ar(context(), view);
+
+  if (!out.writePair(SCTAG_DATA_VIEW_OBJECT, 0)) {
+    return false;
+  }
+
+  mozilla::Maybe<size_t> byteLength = view->byteLength();
+  if (!byteLength) {
+    return reportDataCloneError(JS_SCERR_TYPED_ARRAY_DETACHED);
+  }
+
+  // Auto-length DataViews are tagged by storing `-1` for the length. We still
+  // need to query the length to check for detached or out-of-bounds lengths.
+  bool isAutoLength = view->is<ResizableDataViewObject>() &&
+                      view->as<ResizableDataViewObject>().isAutoLength();
+  uint64_t length = isAutoLength ? uint64_t(-1) : uint64_t(*byteLength);
+  if (!out.write(length)) {
+    return false;
+  }
+
+  // Write out the ArrayBuffer tag and contents
+  RootedValue val(context(), view->bufferValue());
+  if (!startWrite(val)) {
+    return false;
+  }
+
+  uint64_t byteOffset = view->byteOffset().valueOr(0);
+  return out.write(byteOffset);
+}
+
+bool JSStructuredCloneWriter::writeArrayBuffer(HandleObject obj) {
+  Rooted<ArrayBufferObject*> buffer(context(),
+                                    obj->maybeUnwrapAs<ArrayBufferObject>());
+  JSAutoRealm ar(context(), buffer);
+
+  StructuredDataType type = !buffer->isResizable()
+                                ? SCTAG_ARRAY_BUFFER_OBJECT
+                                : SCTAG_RESIZABLE_ARRAY_BUFFER_OBJECT;
+
+  if (!out.writePair(type, 0)) {
+    return false;
+  }
+
+  uint64_t byteLength = buffer->byteLength();
+  if (!out.write(byteLength)) {
+    return false;
+  }
+
+  if (buffer->isResizable()) {
+    uint64_t maxByteLength =
+        buffer->as<ResizableArrayBufferObject>().maxByteLength();
+    if (!out.write(maxByteLength)) {
+      return false;
+    }
+  }
+
+  return out.writeBytes(buffer->dataPointer(), byteLength);
+}
+
+bool JSStructuredCloneWriter::writeSharedArrayBuffer(HandleObject obj) {
+  MOZ_ASSERT(obj->canUnwrapAs<SharedArrayBufferObject>());
+
+  if (!cloneDataPolicy.areSharedMemoryObjectsAllowed()) {
+    auto error = context()->realm()->creationOptions().getCoopAndCoepEnabled()
+                     ? JS_SCERR_NOT_CLONABLE_WITH_COOP_COEP
+                     : JS_SCERR_NOT_CLONABLE;
+    reportDataCloneError(error, "SharedArrayBuffer");
+    return false;
+  }
+
+  output().sameProcessScopeRequired();
+
+  // We must not transmit SAB pointers (including for WebAssembly.Memory)
+  // cross-process.  The cloneDataPolicy should have guarded against this;
+  // since it did not then throw, with a very explicit message.
+
+  if (output().scope() > JS::StructuredCloneScope::SameProcess) {
+    JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                              JSMSG_SC_SHMEM_POLICY);
+    return false;
+  }
+
+  Rooted<SharedArrayBufferObject*> sharedArrayBuffer(
+      context(), obj->maybeUnwrapAs<SharedArrayBufferObject>());
+  SharedArrayRawBuffer* rawbuf = sharedArrayBuffer->rawBufferObject();
+
+  if (!out.buf.refsHeld_.acquire(context(), rawbuf)) {
+    return false;
+  }
+
+  // We must serialize the length so that the buffer object arrives in the
+  // receiver with the same length, and not with the length read from the
+  // rawbuf - that length can be different, and it can change at any time.
+
+  StructuredDataType type = !sharedArrayBuffer->isGrowable()
+                                ? SCTAG_SHARED_ARRAY_BUFFER_OBJECT
+                                : SCTAG_GROWABLE_SHARED_ARRAY_BUFFER_OBJECT;
+
+  intptr_t p = reinterpret_cast<intptr_t>(rawbuf);
+  uint64_t byteLength = sharedArrayBuffer->byteLengthOrMaxByteLength();
+  if (!(out.writePair(type, /* unused data word */ 0) &&
+        out.writeBytes(&byteLength, sizeof(byteLength)) &&
+        out.writeBytes(&p, sizeof(p)))) {
+    return false;
+  }
+
+  if (callbacks && callbacks->sabCloned &&
+      !callbacks->sabCloned(context(), /*receiving=*/false, closure)) {
+    return false;
+  }
+
+  return true;
+}
+
+bool JSStructuredCloneWriter::writeSharedWasmMemory(HandleObject obj) {
+  MOZ_ASSERT(obj->canUnwrapAs<WasmMemoryObject>());
+
+  // Check the policy here so that we can report a sane error.
+  if (!cloneDataPolicy.areSharedMemoryObjectsAllowed()) {
+    auto error = context()->realm()->creationOptions().getCoopAndCoepEnabled()
+                     ? JS_SCERR_NOT_CLONABLE_WITH_COOP_COEP
+                     : JS_SCERR_NOT_CLONABLE;
+    reportDataCloneError(error, "WebAssembly.Memory");
+    return false;
+  }
+
+  // If this changes, might need to change what we write.
+  MOZ_ASSERT(WasmMemoryObject::RESERVED_SLOTS == 3);
+
+  Rooted<WasmMemoryObject*> memoryObj(context(),
+                                      &obj->unwrapAs<WasmMemoryObject>());
+  Rooted<SharedArrayBufferObject*> sab(
+      context(), &memoryObj->buffer().as<SharedArrayBufferObject>());
+
+  return out.writePair(SCTAG_SHARED_WASM_MEMORY_OBJECT, 0) &&
+         out.writePair(SCTAG_BOOLEAN, memoryObj->isHuge()) &&
+         writeSharedArrayBuffer(sab);
+}
+
+bool JSStructuredCloneWriter::startObject(HandleObject obj, bool* backref) {
+  // Handle cycles in the object graph.
+  CloneMemory::AddPtr p = memory.lookupForAdd(obj);
+  if ((*backref = p.found())) {
+    return out.writePair(SCTAG_BACK_REFERENCE_OBJECT, p->value());
+  }
+  if (!memory.add(p, obj, memory.count())) {
+    ReportOutOfMemory(context());
+    return false;
+  }
+
+  if (memory.count() == UINT32_MAX) {
+    JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                              JSMSG_NEED_DIET, "object graph to serialize");
+    return false;
+  }
+
+  return true;
+}
+
+static bool TryAppendNativeProperties(JSContext* cx, HandleObject obj,
+                                      MutableHandleIdVector entries,
+                                      size_t* properties, bool* optimized) {
+  *optimized = false;
+
+  if (!obj->is<NativeObject>()) {
+    return true;
+  }
+
+  Handle<NativeObject*> nobj = obj.as<NativeObject>();
+  if (nobj->isIndexed() || nobj->is<TypedArrayObject>() ||
+      nobj->getClass()->getNewEnumerate() || nobj->getClass()->getEnumerate()) {
+    return true;
+  }
+
+  *optimized = true;
+
+  size_t count = 0;
+  // We iterate from the last to the first property, so the property names
+  // are already in reverse order.
+  for (ShapePropertyIter<NoGC> iter(nobj->shape()); !iter.done(); iter++) {
+    jsid id = iter->key();
+
+    // Ignore symbols and non-enumerable properties.
+    if (!iter->enumerable() || id.isSymbol()) {
+      continue;
+    }
+
+    MOZ_ASSERT(id.isString());
+    if (!entries.append(id)) {
+      return false;
+    }
+
+    count++;
+  }
+
+  // Add dense element ids in reverse order.
+  for (uint32_t i = nobj->getDenseInitializedLength(); i > 0; --i) {
+    if (nobj->getDenseElement(i - 1).isMagic(JS_ELEMENTS_HOLE)) {
+      continue;
+    }
+
+    if (!entries.append(PropertyKey::Int(i - 1))) {
+      return false;
+    }
+
+    count++;
+  }
+
+  *properties = count;
+  return true;
+}
+
+// Objects are written as a "preorder" traversal of the object graph: object
+// "headers" (the class tag and any data needed for initial construction) are
+// visited first, then the children are recursed through (where children are
+// properties, Set or Map entries, etc.). So for example
+//
+//     obj1 = { key1: { key1.1: val1.1, key1.2: val1.2 }, key2: {} }
+//
+// would be stored as:
+//
+//     <Object tag for obj1>
+//       <key1 data>
+//       <Object tag for key1's value>
+//         <key1.1 data>
+//         <val1.1 data>
+//         <key1.2 data>
+//         <val1.2 data>
+//       <end-of-children marker for key1's value>
+//       <key2 data>
+//       <Object tag for key2's value>
+//       <end-of-children marker for key2's value>
+//     <end-of-children marker for obj1>
+//
+// This nests nicely (ie, an entire recursive value starts with its tag and
+// ends with its end-of-children marker) and so it can be presented indented.
+// But see traverseMap below for how this looks different for Maps.
+bool JSStructuredCloneWriter::traverseObject(HandleObject obj, ESClass cls) {
+  size_t count;
+  bool optimized = false;
+  if (!js::SupportDifferentialTesting()) {
+    if (!TryAppendNativeProperties(context(), obj, &objectEntries, &count,
+                                   &optimized)) {
+      return false;
+    }
+  }
+
+  if (!optimized) {
+    // Get enumerable property ids and put them in reverse order so that they
+    // will come off the stack in forward order.
+    RootedIdVector properties(context());
+    if (!GetPropertyKeys(context(), obj, JSITER_OWNONLY, &properties)) {
+      return false;
+    }
+
+    for (size_t i = properties.length(); i > 0; --i) {
+      jsid id = properties[i - 1];
+
+      MOZ_ASSERT(id.isString() || id.isInt());
+      if (!objectEntries.append(id)) {
+        return false;
+      }
+    }
+
+    count = properties.length();
+  }
+
+  // Push obj and count to the stack.
+  if (!objs.append(ObjectValue(*obj)) || !counts.append(count)) {
+    return false;
+  }
+
+  checkStack();
+
+#if DEBUG
+  ESClass cls2;
+  if (!GetBuiltinClass(context(), obj, &cls2)) {
+    return false;
+  }
+  MOZ_ASSERT(cls2 == cls);
+#endif
+
+  // Write the header for obj.
+  if (cls == ESClass::Array) {
+    uint32_t length = 0;
+    if (!JS::GetArrayLength(context(), obj, &length)) {
+      return false;
+    }
+
+    return out.writePair(SCTAG_ARRAY_OBJECT,
+                         NativeEndian::swapToLittleEndian(length));
+  }
+
+  return out.writePair(SCTAG_OBJECT_OBJECT, 0);
+}
+
+// Use the same basic setup as for traverseObject, but now keys can themselves
+// be complex objects. Keys and values are visited first via startWrite(), then
+// the key's children (if any) are handled, then the value's children.
+//
+//     m = new Map();
+//     m.set(key1 = ..., value1 = ...)
+//
+// where key1 and value2 are both objects would be stored as
+//
+//     <Map tag>
+//     <key1 class tag>
+//     <value1 class tag>
+//     ...key1 fields...
+//     <end-of-children marker for key1>
+//     ...value1 fields...
+//     <end-of-children marker for value1>
+//     <end-of-children marker for Map>
+//
+// Notice how the end-of-children marker for key1 is sandwiched between the
+// value1 beginning and end.
+bool JSStructuredCloneWriter::traverseMap(HandleObject obj) {
+  Rooted<GCVector<Value>> newEntries(context(), GCVector<Value>(context()));
+  {
+    // If there is no wrapper, the compartment munging is a no-op.
+    RootedObject unwrapped(context(), obj->maybeUnwrapAs<MapObject>());
+    MOZ_ASSERT(unwrapped);
+    JSAutoRealm ar(context(), unwrapped);
+    if (!MapObject::getKeysAndValuesInterleaved(unwrapped, &newEntries)) {
+      return false;
+    }
+  }
+  if (!context()->compartment()->wrap(context(), &newEntries)) {
+    return false;
+  }
+
+  for (size_t i = newEntries.length(); i > 0; --i) {
+    if (!otherEntries.append(newEntries[i - 1])) {
+      return false;
+    }
+  }
+
+  // Push obj and count to the stack.
+  if (!objs.append(ObjectValue(*obj)) || !counts.append(newEntries.length())) {
+    return false;
+  }
+
+  checkStack();
+
+  // Write the header for obj.
+  return out.writePair(SCTAG_MAP_OBJECT, 0);
+}
+
+// Similar to traverseMap, only there is a single value instead of a key and
+// value, and thus no interleaving is possible: a value will be fully emitted
+// before the next value is begun.
+bool JSStructuredCloneWriter::traverseSet(HandleObject obj) {
+  Rooted<GCVector<Value>> keys(context(), GCVector<Value>(context()));
+  {
+    // If there is no wrapper, the compartment munging is a no-op.
+    RootedObject unwrapped(context(), obj->maybeUnwrapAs<SetObject>());
+    MOZ_ASSERT(unwrapped);
+    JSAutoRealm ar(context(), unwrapped);
+    if (!SetObject::keys(context(), unwrapped, &keys)) {
+      return false;
+    }
+  }
+  if (!context()->compartment()->wrap(context(), &keys)) {
+    return false;
+  }
+
+  for (size_t i = keys.length(); i > 0; --i) {
+    if (!otherEntries.append(keys[i - 1])) {
+      return false;
+    }
+  }
+
+  // Push obj and count to the stack.
+  if (!objs.append(ObjectValue(*obj)) || !counts.append(keys.length())) {
+    return false;
+  }
+
+  checkStack();
+
+  // Write the header for obj.
+  return out.writePair(SCTAG_SET_OBJECT, 0);
+}
+
+bool JSStructuredCloneWriter::traverseSavedFrame(HandleObject obj) {
+  Rooted<SavedFrame*> savedFrame(context(), obj->maybeUnwrapAs<SavedFrame>());
+  MOZ_ASSERT(savedFrame);
+
+  RootedObject parent(context(), savedFrame->getParent());
+  if (!context()->compartment()->wrap(context(), &parent)) {
+    return false;
+  }
+
+  if (!objs.append(ObjectValue(*obj)) ||
+      !otherEntries.append(parent ? ObjectValue(*parent) : NullValue()) ||
+      !counts.append(1)) {
+    return false;
+  }
+
+  checkStack();
+
+  // Write the SavedFrame tag and the SavedFrame's principals.
+
+  if (savedFrame->getPrincipals() ==
+      &ReconstructedSavedFramePrincipals::IsSystem) {
+    if (!out.writePair(SCTAG_SAVED_FRAME_OBJECT,
+                       SCTAG_RECONSTRUCTED_SAVED_FRAME_PRINCIPALS_IS_SYSTEM)) {
+      return false;
+    };
+  } else if (savedFrame->getPrincipals() ==
+             &ReconstructedSavedFramePrincipals::IsNotSystem) {
+    if (!out.writePair(
+            SCTAG_SAVED_FRAME_OBJECT,
+            SCTAG_RECONSTRUCTED_SAVED_FRAME_PRINCIPALS_IS_NOT_SYSTEM)) {
+      return false;
+    }
+  } else {
+    if (auto principals = savedFrame->getPrincipals()) {
+      if (!out.writePair(SCTAG_SAVED_FRAME_OBJECT, SCTAG_JSPRINCIPALS) ||
+          !principals->write(context(), this)) {
+        return false;
+      }
+    } else {
+      if (!out.writePair(SCTAG_SAVED_FRAME_OBJECT, SCTAG_NULL_JSPRINCIPALS)) {
+        return false;
+      }
+    }
+  }
+
+  // Write the SavedFrame's reserved slots, except for the parent, which is
+  // queued on objs for further traversal.
+
+  RootedValue val(context());
+
+  val = BooleanValue(savedFrame->getMutedErrors());
+  if (!writePrimitive(val)) {
+    return false;
+  }
+
+  context()->markAtom(savedFrame->getSource());
+  val = StringValue(savedFrame->getSource());
+  if (!writePrimitive(val)) {
+    return false;
+  }
+
+  val = NumberValue(savedFrame->getLine());
+  if (!writePrimitive(val)) {
+    return false;
+  }
+
+  val = NumberValue(*savedFrame->getColumn().addressOfValueForTranscode());
+  if (!writePrimitive(val)) {
+    return false;
+  }
+
+  auto name = savedFrame->getFunctionDisplayName();
+  if (name) {
+    context()->markAtom(name);
+  }
+  val = name ? StringValue(name) : NullValue();
+  if (!writePrimitive(val)) {
+    return false;
+  }
+
+  auto cause = savedFrame->getAsyncCause();
+  if (cause) {
+    context()->markAtom(cause);
+  }
+  val = cause ? StringValue(cause) : NullValue();
+  if (!writePrimitive(val)) {
+    return false;
+  }
+
+  return true;
+}
+
+// https://html.spec.whatwg.org/multipage/structured-data.html#structuredserializeinternal
+// 2.7.3 StructuredSerializeInternal ( value, forStorage [ , memory ] )
+//
+// Step 17. Otherwise, if value has an [[ErrorData]] internal slot and
+//          value is not a platform object, then:
+//
+// Note: This contains custom extensions for handling non-standard properties.
+bool JSStructuredCloneWriter::traverseError(HandleObject obj) {
+  JSContext* cx = context();
+
+  // 1. Let name be ? Get(value, "name").
+  RootedValue name(cx);
+  if (!GetProperty(cx, obj, obj, cx->names().name, &name)) {
+    return false;
+  }
+
+  // 2. If name is not one of "Error", "EvalError", "RangeError",
+  // "ReferenceError", "SyntaxError", "TypeError", or "URIError",
+  // (not yet specified: or "AggregateError")
+  // then set name to "Error".
+  JSExnType type = JSEXN_ERR;
+  if (name.isString()) {
+    JSLinearString* linear = name.toString()->ensureLinear(cx);
+    if (!linear) {
+      return false;
+    }
+
+    if (EqualStrings(linear, cx->names().Error)) {
+      type = JSEXN_ERR;
+    } else if (EqualStrings(linear, cx->names().EvalError)) {
+      type = JSEXN_EVALERR;
+    } else if (EqualStrings(linear, cx->names().RangeError)) {
+      type = JSEXN_RANGEERR;
+    } else if (EqualStrings(linear, cx->names().ReferenceError)) {
+      type = JSEXN_REFERENCEERR;
+    } else if (EqualStrings(linear, cx->names().SyntaxError)) {
+      type = JSEXN_SYNTAXERR;
+    } else if (EqualStrings(linear, cx->names().TypeError)) {
+      type = JSEXN_TYPEERR;
+    } else if (EqualStrings(linear, cx->names().URIError)) {
+      type = JSEXN_URIERR;
+    } else if (EqualStrings(linear, cx->names().AggregateError)) {
+      type = JSEXN_AGGREGATEERR;
+    }
+  }
+
+  // 3. Let valueMessageDesc be ? value.[[GetOwnProperty]]("message").
+  RootedId messageId(cx, NameToId(cx->names().message));
+  Rooted<Maybe<PropertyDescriptor>> messageDesc(cx);
+  if (!GetOwnPropertyDescriptor(cx, obj, messageId, &messageDesc)) {
+    return false;
+  }
+
+  // 4. Let message be undefined if IsDataDescriptor(valueMessageDesc) is false,
+  //    and ? ToString(valueMessageDesc.[[Value]]) otherwise.
+  RootedString message(cx);
+  if (messageDesc.isSome() && messageDesc->isDataDescriptor()) {
+    RootedValue messageVal(cx, messageDesc->value());
+    message = ToString<CanGC>(cx, messageVal);
+    if (!message) {
+      return false;
+    }
+  }
+
+  // 5. Set serialized to { [[Type]]: "Error", [[Name]]: name, [[Message]]:
+  // message }.
+
+  if (!objs.append(ObjectValue(*obj))) {
+    return false;
+  }
+
+  Rooted<ErrorObject*> unwrapped(cx, obj->maybeUnwrapAs<ErrorObject>());
+  MOZ_ASSERT(unwrapped);
+
+  // Non-standard: Serialize |stack|.
+  // The Error stack property is saved as SavedFrames.
+  RootedValue stack(cx, NullValue());
+  RootedObject stackObj(cx, unwrapped->stack());
+  if (stackObj) {
+    MOZ_ASSERT(stackObj->canUnwrapAs<SavedFrame>());
+    stack.setObject(*stackObj);
+    if (!cx->compartment()->wrap(cx, &stack)) {
+      return false;
+    }
+  }
+  if (!otherEntries.append(stack)) {
+    return false;
+  }
+
+  // Serialize |errors|
+  if (type == JSEXN_AGGREGATEERR) {
+    RootedValue errors(cx);
+    if (!GetProperty(cx, obj, obj, cx->names().errors, &errors)) {
+      return false;
+    }
+    if (!otherEntries.append(errors)) {
+      return false;
+    }
+  } else {
+    if (!otherEntries.append(NullValue())) {
+      return false;
+    }
+  }
+
+  // Non-standard: Serialize |cause|. Because this property
+  // might be missing we also write "hasCause" later.
+  RootedId causeId(cx, NameToId(cx->names().cause));
+  Rooted<Maybe<PropertyDescriptor>> causeDesc(cx);
+  if (!GetOwnPropertyDescriptor(cx, obj, causeId, &causeDesc)) {
+    return false;
+  }
+
+  Rooted<Maybe<Value>> cause(cx);
+  if (causeDesc.isSome() && causeDesc->isDataDescriptor()) {
+    cause = mozilla::Some(causeDesc->value());
+  }
+  if (!cx->compartment()->wrap(cx, &cause)) {
+    return false;
+  }
+  if (!otherEntries.append(cause.get().valueOr(NullValue()))) {
+    return false;
+  }
+
+  // |cause| + |errors| + |stack|, pushed in reverse order
+  if (!counts.append(3)) {
+    return false;
+  }
+
+  checkStack();
+
+  if (!out.writePair(SCTAG_ERROR_OBJECT, type)) {
+    return false;
+  }
+
+  RootedValue val(cx, message ? StringValue(message) : NullValue());
+  if (!writePrimitive(val)) {
+    return false;
+  }
+
+  // hasCause
+  val = BooleanValue(cause.isSome());
+  if (!writePrimitive(val)) {
+    return false;
+  }
+
+  // Non-standard: Also serialize fileName, lineNumber and columnNumber.
+  {
+    JSAutoRealm ar(cx, unwrapped);
+    val = StringValue(unwrapped->fileName(cx));
+  }
+  if (!cx->compartment()->wrap(cx, &val) || !writePrimitive(val)) {
+    return false;
+  }
+
+  val = Int32Value(unwrapped->lineNumber());
+  if (!writePrimitive(val)) {
+    return false;
+  }
+
+  val = Int32Value(*unwrapped->columnNumber().addressOfValueForTranscode());
+  return writePrimitive(val);
+}
+
+bool JSStructuredCloneWriter::writePrimitive(HandleValue v) {
+  MOZ_ASSERT(v.isPrimitive());
+  context()->check(v);
+
+  if (v.isString()) {
+    return writeString(SCTAG_STRING, v.toString());
+  } else if (v.isInt32()) {
+    if (js::SupportDifferentialTesting()) {
+      return out.writeDouble(v.toInt32());
+    }
+    return out.writePair(SCTAG_INT32, v.toInt32());
+  } else if (v.isDouble()) {
+    return out.writeDouble(v.toDouble());
+  } else if (v.isBoolean()) {
+    return out.writePair(SCTAG_BOOLEAN, v.toBoolean());
+  } else if (v.isNull()) {
+    return out.writePair(SCTAG_NULL, 0);
+  } else if (v.isUndefined()) {
+    return out.writePair(SCTAG_UNDEFINED, 0);
+  } else if (v.isBigInt()) {
+    return writeBigInt(SCTAG_BIGINT, v.toBigInt());
+  }
+
+  return reportDataCloneError(JS_SCERR_UNSUPPORTED_TYPE);
+}
+
+bool JSStructuredCloneWriter::startWrite(HandleValue v) {
+  context()->check(v);
+
+  if (v.isPrimitive()) {
+    return writePrimitive(v);
+  }
+
+  if (!v.isObject()) {
+    return reportDataCloneError(JS_SCERR_UNSUPPORTED_TYPE);
+  }
+
+  RootedObject obj(context(), &v.toObject());
+
+  bool backref;
+  if (!startObject(obj, &backref)) {
+    return false;
+  }
+  if (backref) {
+    return true;
+  }
+
+  ESClass cls;
+  if (!GetBuiltinClass(context(), obj, &cls)) {
+    return false;
+  }
+
+  switch (cls) {
+    case ESClass::Object:
+    case ESClass::Array:
+      return traverseObject(obj, cls);
+    case ESClass::Number: {
+      RootedValue unboxed(context());
+      if (!Unbox(context(), obj, &unboxed)) {
+        return false;
+      }
+      return out.writePair(SCTAG_NUMBER_OBJECT, 0) &&
+             out.writeDouble(unboxed.toNumber());
+    }
+    case ESClass::String: {
+      RootedValue unboxed(context());
+      if (!Unbox(context(), obj, &unboxed)) {
+        return false;
+      }
+      return writeString(SCTAG_STRING_OBJECT, unboxed.toString());
+    }
+    case ESClass::Boolean: {
+      RootedValue unboxed(context());
+      if (!Unbox(context(), obj, &unboxed)) {
+        return false;
+      }
+      return out.writePair(SCTAG_BOOLEAN_OBJECT, unboxed.toBoolean());
+    }
+    case ESClass::RegExp: {
+      RegExpShared* re = RegExpToShared(context(), obj);
+      if (!re) {
+        return false;
+      }
+      return out.writePair(SCTAG_REGEXP_OBJECT, re->getFlags().value()) &&
+             writeString(SCTAG_STRING, re->getSource());
+    }
+    case ESClass::ArrayBuffer: {
+      if (JS::IsArrayBufferObject(obj) && JS::ArrayBufferHasData(obj)) {
+        return writeArrayBuffer(obj);
+      }
+      break;
+    }
+    case ESClass::SharedArrayBuffer:
+      if (JS::IsSharedArrayBufferObject(obj)) {
+        return writeSharedArrayBuffer(obj);
+      }
+      break;
+    case ESClass::Date: {
+      RootedValue unboxed(context());
+      if (!Unbox(context(), obj, &unboxed)) {
+        return false;
+      }
+      return out.writePair(SCTAG_DATE_OBJECT, 0) &&
+             out.writeDouble(unboxed.toNumber());
+    }
+    case ESClass::Set:
+      return traverseSet(obj);
+    case ESClass::Map:
+      return traverseMap(obj);
+    case ESClass::Error:
+      return traverseError(obj);
+    case ESClass::BigInt: {
+      RootedValue unboxed(context());
+      if (!Unbox(context(), obj, &unboxed)) {
+        return false;
+      }
+      return writeBigInt(SCTAG_BIGINT_OBJECT, unboxed.toBigInt());
+    }
+    case ESClass::Promise:
+    case ESClass::MapIterator:
+    case ESClass::SetIterator:
+    case ESClass::Arguments:
+    case ESClass::Function:
+      break;
+
+#ifdef ENABLE_RECORD_TUPLE
+    case ESClass::Record:
+    case ESClass::Tuple:
+      MOZ_CRASH("Record and Tuple are not supported");
+#endif
+
+    case ESClass::Other: {
+      if (obj->canUnwrapAs<TypedArrayObject>()) {
+        return writeTypedArray(obj);
+      }
+      if (obj->canUnwrapAs<DataViewObject>()) {
+        return writeDataView(obj);
+      }
+      if (wasm::IsSharedWasmMemoryObject(obj)) {
+        return writeSharedWasmMemory(obj);
+      }
+      if (obj->canUnwrapAs<SavedFrame>()) {
+        return traverseSavedFrame(obj);
+      }
+      break;
+    }
+  }
+
+  if (out.buf.callbacks_ && out.buf.callbacks_->write) {
+    bool sameProcessScopeRequired = false;
+    if (!out.buf.callbacks_->write(context(), this, obj,
+                                   &sameProcessScopeRequired,
+                                   out.buf.closure_)) {
+      return false;
+    }
+
+    if (sameProcessScopeRequired) {
+      output().sameProcessScopeRequired();
+    }
+
+    return true;
+  }
+
+  return reportDataCloneError(JS_SCERR_UNSUPPORTED_TYPE);
+}
+
+bool JSStructuredCloneWriter::writeHeader() {
+  return out.writePair(SCTAG_HEADER, (uint32_t)output().scope());
+}
+
+bool JSStructuredCloneWriter::writeTransferMap() {
+  if (transferableObjects.empty()) {
+    return true;
+  }
+
+  if (!out.writePair(SCTAG_TRANSFER_MAP_HEADER, (uint32_t)SCTAG_TM_UNREAD)) {
+    return false;
+  }
+
+  if (!out.write(transferableObjects.length())) {
+    return false;
+  }
+
+  RootedObject obj(context());
+  for (auto* o : transferableObjects) {
+    obj = o;
+    if (!memory.put(obj, memory.count())) {
+      ReportOutOfMemory(context());
+      return false;
+    }
+
+    // Emit a placeholder pointer.  We defer stealing the data until later
+    // (and, if necessary, detaching this object if it's an ArrayBuffer).
+    if (!out.writePair(SCTAG_TRANSFER_MAP_PENDING_ENTRY,
+                       JS::SCTAG_TMO_UNFILLED)) {
+      return false;
+    }
+    if (!out.write(0)) {  // Pointer to ArrayBuffer contents.
+      return false;
+    }
+    if (!out.write(0)) {  // extraData
+      return false;
+    }
+  }
+
+  return true;
+}
+
+bool JSStructuredCloneWriter::transferOwnership() {
+  if (transferableObjects.empty()) {
+    return true;
+  }
+
+  // Walk along the transferables and the transfer map at the same time,
+  // grabbing out pointers from the transferables and stuffing them into the
+  // transfer map.
+  auto point = out.iter();
+  MOZ_RELEASE_ASSERT(point.canPeek());
+  MOZ_ASSERT(uint32_t(NativeEndian::swapFromLittleEndian(point.peek()) >> 32) ==
+             SCTAG_HEADER);
+  point++;
+  MOZ_RELEASE_ASSERT(point.canPeek());
+  MOZ_ASSERT(uint32_t(NativeEndian::swapFromLittleEndian(point.peek()) >> 32) ==
+             SCTAG_TRANSFER_MAP_HEADER);
+  point++;
+  MOZ_RELEASE_ASSERT(point.canPeek());
+  MOZ_ASSERT(NativeEndian::swapFromLittleEndian(point.peek()) ==
+             transferableObjects.length());
+  point++;
+
+  JSContext* cx = context();
+  RootedObject obj(cx);
+  JS::StructuredCloneScope scope = output().scope();
+  for (auto* o : transferableObjects) {
+    obj = o;
+
+    uint32_t tag;
+    JS::TransferableOwnership ownership;
+    void* content;
+    uint64_t extraData;
+
+#if DEBUG
+    SCInput::getPair(point.peek(), &tag, (uint32_t*)&ownership);
+    MOZ_ASSERT(tag == SCTAG_TRANSFER_MAP_PENDING_ENTRY);
+    MOZ_ASSERT(ownership == JS::SCTAG_TMO_UNFILLED);
+#endif
+
+    ESClass cls;
+    if (!GetBuiltinClass(cx, obj, &cls)) {
+      return false;
+    }
+
+    if (cls == ESClass::ArrayBuffer) {
+      tag = SCTAG_TRANSFER_MAP_ARRAY_BUFFER;
+
+      // The current setup of the array buffer inheritance hierarchy doesn't
+      // lend itself well to generic manipulation via proxies.
+      Rooted<ArrayBufferObject*> arrayBuffer(
+          cx, obj->maybeUnwrapAs<ArrayBufferObject>());
+      JSAutoRealm ar(cx, arrayBuffer);
+
+      if (arrayBuffer->isDetached()) {
+        reportDataCloneError(JS_SCERR_TYPED_ARRAY_DETACHED);
+        return false;
+      }
+
+      if (arrayBuffer->isPreparedForAsmJS()) {
+        reportDataCloneError(JS_SCERR_WASM_NO_TRANSFER);
+        return false;
+      }
+
+      if (scope == JS::StructuredCloneScope::DifferentProcess ||
+          scope == JS::StructuredCloneScope::DifferentProcessForIndexedDB ||
+          arrayBuffer->isResizable()) {
+        // Write Transferred ArrayBuffers in DifferentProcess scope at
+        // the end of the clone buffer, and store the offset within the
+        // buffer to where the ArrayBuffer was written. Note that this
+        // will invalidate the current position iterator.
+        //
+        // Resizable ArrayBuffers need to store two extra data, the byte length
+        // and the maximum byte length, but the current transferables format
+        // supports only a single additional datum. Therefore resizable buffers
+        // currently go through this slower code path.
+
+        size_t pointOffset = out.offset(point);
+        tag = SCTAG_TRANSFER_MAP_STORED_ARRAY_BUFFER;
+        ownership = JS::SCTAG_TMO_UNOWNED;
+        content = nullptr;
+        extraData = out.tell() -
+                    pointOffset;  // Offset from tag to current end of buffer
+        if (!writeArrayBuffer(arrayBuffer)) {
+          ReportOutOfMemory(cx);
+          return false;
+        }
+
+        // Must refresh the point iterator after its collection has
+        // been modified.
+        point = out.iter();
+        point += pointOffset;
+
+        if (!JS::DetachArrayBuffer(cx, arrayBuffer)) {
+          return false;
+        }
+      } else {
+        size_t nbytes = arrayBuffer->byteLength();
+
+        using BufferContents = ArrayBufferObject::BufferContents;
+
+        BufferContents bufContents =
+            ArrayBufferObject::extractStructuredCloneContents(cx, arrayBuffer);
+        if (!bufContents) {
+          return false;  // out of memory
+        }
+
+        content = bufContents.data();
+        if (bufContents.kind() == ArrayBufferObject::MAPPED) {
+          ownership = JS::SCTAG_TMO_MAPPED_DATA;
+        } else {
+          MOZ_ASSERT(
+              bufContents.kind() ==
+                      ArrayBufferObject::MALLOCED_ARRAYBUFFER_CONTENTS_ARENA ||
+                  bufContents.kind() ==
+                      ArrayBufferObject::MALLOCED_UNKNOWN_ARENA,
+              "failing to handle new ArrayBuffer kind?");
+          ownership = JS::SCTAG_TMO_ALLOC_DATA;
+        }
+        extraData = nbytes;
+      }
+    } else {
+      if (!out.buf.callbacks_ || !out.buf.callbacks_->writeTransfer) {
+        return reportDataCloneError(JS_SCERR_TRANSFERABLE);
+      }
+      if (!out.buf.callbacks_->writeTransfer(cx, obj, out.buf.closure_, &tag,
+                                             &ownership, &content,
+                                             &extraData)) {
+        return false;
+      }
+      MOZ_ASSERT(tag > SCTAG_TRANSFER_MAP_PENDING_ENTRY);
+    }
+
+    point.write(NativeEndian::swapToLittleEndian(PairToUInt64(tag, ownership)));
+    MOZ_ALWAYS_TRUE(point.advance());
+    point.write(
+        NativeEndian::swapToLittleEndian(reinterpret_cast<uint64_t>(content)));
+    MOZ_ALWAYS_TRUE(point.advance());
+    point.write(NativeEndian::swapToLittleEndian(extraData));
+    MOZ_ALWAYS_TRUE(point.advance());
+  }
+
+#if DEBUG
+  // Make sure there aren't any more transfer map entries after the expected
+  // number we read out.
+  if (!point.done()) {
+    uint32_t tag, data;
+    SCInput::getPair(point.peek(), &tag, &data);
+    MOZ_ASSERT(tag < SCTAG_TRANSFER_MAP_HEADER ||
+               tag >= SCTAG_TRANSFER_MAP_END_OF_BUILTIN_TYPES);
+  }
+#endif
+  return true;
+}
+
+bool JSStructuredCloneWriter::write(HandleValue v) {
+  if (!startWrite(v)) {
+    return false;
+  }
+
+  RootedObject obj(context());
+  RootedValue key(context());
+  RootedValue val(context());
+  RootedId id(context());
+
+  RootedValue cause(context());
+  RootedValue errors(context());
+  RootedValue stack(context());
+
+  while (!counts.empty()) {
+    obj = &objs.back().toObject();
+    context()->check(obj);
+    if (counts.back()) {
+      counts.back()--;
+
+      ESClass cls;
+      if (!GetBuiltinClass(context(), obj, &cls)) {
+        return false;
+      }
+
+      if (cls == ESClass::Map) {
+        key = otherEntries.popCopy();
+        checkStack();
+
+        counts.back()--;
+        val = otherEntries.popCopy();
+        checkStack();
+
+        if (!startWrite(key) || !startWrite(val)) {
+          return false;
+        }
+      } else if (cls == ESClass::Set || obj->canUnwrapAs<SavedFrame>()) {
+        key = otherEntries.popCopy();
+        checkStack();
+
+        if (!startWrite(key)) {
+          return false;
+        }
+      } else if (cls == ESClass::Error) {
+        cause = otherEntries.popCopy();
+        checkStack();
+
+        counts.back()--;
+        errors = otherEntries.popCopy();
+        checkStack();
+
+        counts.back()--;
+        stack = otherEntries.popCopy();
+        checkStack();
+
+        if (!startWrite(cause) || !startWrite(errors) || !startWrite(stack)) {
+          return false;
+        }
+      } else {
+        id = objectEntries.popCopy();
+        key = IdToValue(id);
+        checkStack();
+
+        // If obj still has an own property named id, write it out.
+        bool found;
+        if (GetOwnPropertyPure(context(), obj, id, val.address(), &found)) {
+          if (found) {
+            if (!writePrimitive(key) || !startWrite(val)) {
+              return false;
+            }
+          }
+          continue;
+        }
+
+        if (!HasOwnProperty(context(), obj, id, &found)) {
+          return false;
+        }
+
+        if (found) {
+#if FUZZING_JS_FUZZILLI
+          // supress calls into user code
+          if (js::SupportDifferentialTesting()) {
+            fprintf(stderr, "Differential testing: cannot call GetProperty\n");
+            return false;
+          }
+#endif
+
+          if (!writePrimitive(key) ||
+              !GetProperty(context(), obj, obj, id, &val) || !startWrite(val)) {
+            return false;
+          }
+        }
+      }
+    } else {
+      if (!out.writePair(SCTAG_END_OF_KEYS, 0)) {
+        return false;
+      }
+      objs.popBack();
+      counts.popBack();
+    }
+  }
+
+  memory.clear();
+  return transferOwnership();
+}
+
+JSStructuredCloneReader::JSStructuredCloneReader(
+    SCInput& in, JS::StructuredCloneScope scope,
+    const JS::CloneDataPolicy& cloneDataPolicy,
+    const JSStructuredCloneCallbacks* cb, void* cbClosure)
+    : in(in),
+      allowedScope(scope),
+      cloneDataPolicy(cloneDataPolicy),
+      objs(in.context()),
+      objState(in.context(), in.context()),
+      allObjs(in.context()),
+      numItemsRead(0),
+      callbacks(cb),
+      closure(cbClosure),
+      gcHeap(in.context()) {
+  // Avoid the need to bounds check by keeping a never-matching element at the
+  // base of the `objState` stack. This append() will always succeed because
+  // the objState vector has a nonzero MinInlineCapacity.
+  MOZ_ALWAYS_TRUE(objState.append(std::make_pair(nullptr, true)));
+}
+
+template <typename CharT>
+JSString* JSStructuredCloneReader::readStringImpl(
+    uint32_t nchars, ShouldAtomizeStrings atomize) {
+  InlineCharBuffer<CharT> chars;
+  if (!chars.maybeAlloc(context(), nchars) ||
+      !in.readChars(chars.get(), nchars)) {
+    return nullptr;
+  }
+
+  if (atomize) {
+    return chars.toAtom(context(), nchars);
+  }
+
+  return chars.toStringDontDeflate(context(), nchars, gcHeap);
+}
+
+JSString* JSStructuredCloneReader::readString(uint32_t data,
+                                              ShouldAtomizeStrings atomize) {
+  uint32_t nchars = data & BitMask(31);
+  bool latin1 = data & (1 << 31);
+
+  if (nchars > JSString::MAX_LENGTH) {
+    JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                              JSMSG_SC_BAD_SERIALIZED_DATA, "string length");
+    return nullptr;
+  }
+
+  return latin1 ? readStringImpl<Latin1Char>(nchars, atomize)
+                : readStringImpl<char16_t>(nchars, atomize);
+}
+
+[[nodiscard]] bool JSStructuredCloneReader::readUint32(uint32_t* num) {
+  Rooted<Value> lineVal(context());
+  if (!startRead(&lineVal)) {
+    return false;
+  }
+  if (!lineVal.isInt32()) {
+    JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                              JSMSG_SC_BAD_SERIALIZED_DATA, "integer required");
+    return false;
+  }
+  *num = uint32_t(lineVal.toInt32());
+  return true;
+}
+
+BigInt* JSStructuredCloneReader::readBigInt(uint32_t data) {
+  size_t length = data & BitMask(31);
+  bool isNegative = data & (1 << 31);
+  if (length == 0) {
+    return BigInt::zero(context());
+  }
+  RootedBigInt result(context(), BigInt::createUninitialized(
+                                     context(), length, isNegative, gcHeap));
+  if (!result) {
+    return nullptr;
+  }
+  if (!in.readArray(result->digits().data(), length)) {
+    return nullptr;
+  }
+  return result;
+}
+
+static uint32_t TagToV1ArrayType(uint32_t tag) {
+  MOZ_ASSERT(tag >= SCTAG_TYPED_ARRAY_V1_MIN &&
+             tag <= SCTAG_TYPED_ARRAY_V1_MAX);
+  return tag - SCTAG_TYPED_ARRAY_V1_MIN;
+}
+
+bool JSStructuredCloneReader::readTypedArray(uint32_t arrayType,
+                                             uint64_t nelems,
+                                             MutableHandleValue vp,
+                                             bool v1Read) {
+  if (arrayType > (v1Read ? Scalar::Uint8Clamped : Scalar::BigUint64)) {
+    JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                              JSMSG_SC_BAD_SERIALIZED_DATA,
+                              "unhandled typed array element type");
+    return false;
+  }
+
+  // Push a placeholder onto the allObjs list to stand in for the typed array.
+  uint32_t placeholderIndex = allObjs.length();
+  Value dummy = UndefinedValue();
+  if (!allObjs.append(dummy)) {
+    return false;
+  }
+
+  // Auto-length TypedArrays are tagged by using `-1` for their length.
+  bool isAutoLength = nelems == uint64_t(-1);
+
+  // Zero |nelems| if it was only used as a tag.
+  if (isAutoLength) {
+    nelems = 0;
+  }
+
+  // Read the ArrayBuffer object and its contents (but no properties)
+  RootedValue v(context());
+  uint64_t byteOffset;
+  if (v1Read) {
+    MOZ_ASSERT(!isAutoLength, "v1Read can't produce auto-length TypedArrays");
+    if (!readV1ArrayBuffer(arrayType, nelems, &v)) {
+      return false;
+    }
+    byteOffset = 0;
+  } else {
+    if (!startRead(&v)) {
+      return false;
+    }
+    if (!in.read(&byteOffset)) {
+      return false;
+    }
+  }
+
+  // Ensure invalid 64-bit values won't be truncated below.
+  if (nelems > ArrayBufferObject::ByteLengthLimit ||
+      byteOffset > ArrayBufferObject::ByteLengthLimit) {
+    JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                              JSMSG_SC_BAD_SERIALIZED_DATA,
+                              "invalid typed array length or offset");
+    return false;
+  }
+
+  if (!v.isObject() || !v.toObject().is<ArrayBufferObjectMaybeShared>()) {
+    JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                              JSMSG_SC_BAD_SERIALIZED_DATA,
+                              "typed array must be backed by an ArrayBuffer");
+    return false;
+  }
+
+  RootedObject buffer(context(), &v.toObject());
+  RootedObject obj(context(), nullptr);
+
+  // Negative values represent an absent length parameter.
+  int64_t length = isAutoLength ? -1 : int64_t(nelems);
+
+  switch (arrayType) {
+#define CREATE_FROM_BUFFER(ExternalType, NativeType, Name)             \
+  case Scalar::Name:                                                   \
+    obj = JS::TypedArray<Scalar::Name>::fromBuffer(context(), buffer,  \
+                                                   byteOffset, length) \
+              .asObject();                                             \
+    break;
+
+    JS_FOR_EACH_TYPED_ARRAY(CREATE_FROM_BUFFER)
+#undef CREATE_FROM_BUFFER
+
+    default:
+      MOZ_CRASH("Can't happen: arrayType range checked above");
+  }
+
+  if (!obj) {
+    return false;
+  }
+  vp.setObject(*obj);
+
+  allObjs[placeholderIndex].set(vp);
+
+  return true;
+}
+
+bool JSStructuredCloneReader::readDataView(uint64_t byteLength,
+                                           MutableHandleValue vp) {
+  // Push a placeholder onto the allObjs list to stand in for the DataView.
+  uint32_t placeholderIndex = allObjs.length();
+  Value dummy = UndefinedValue();
+  if (!allObjs.append(dummy)) {
+    return false;
+  }
+
+  // Read the ArrayBuffer object and its contents (but no properties).
+  RootedValue v(context());
+  if (!startRead(&v)) {
+    return false;
+  }
+  if (!v.isObject() || !v.toObject().is<ArrayBufferObjectMaybeShared>()) {
+    JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                              JSMSG_SC_BAD_SERIALIZED_DATA,
+                              "DataView must be backed by an ArrayBuffer");
+    return false;
+  }
+
+  // Read byteOffset.
+  uint64_t byteOffset;
+  if (!in.read(&byteOffset)) {
+    return false;
+  }
+
+  // Auto-length DataViews are tagged by using `-1` for their byte length.
+  bool isAutoLength = byteLength == uint64_t(-1);
+
+  // Zero |byteLength| if it was only used as a tag.
+  if (isAutoLength) {
+    byteLength = 0;
+  }
+
+  // Ensure invalid 64-bit values won't be truncated below.
+  if (byteLength > ArrayBufferObject::ByteLengthLimit ||
+      byteOffset > ArrayBufferObject::ByteLengthLimit) {
+    JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                              JSMSG_SC_BAD_SERIALIZED_DATA,
+                              "invalid DataView length or offset");
+    return false;
+  }
+
+  RootedObject buffer(context(), &v.toObject());
+  RootedObject obj(context());
+  if (!isAutoLength) {
+    obj = JS_NewDataView(context(), buffer, byteOffset, byteLength);
+  } else {
+    obj = js::NewDataView(context(), buffer, byteOffset);
+  }
+  if (!obj) {
+    return false;
+  }
+  vp.setObject(*obj);
+
+  allObjs[placeholderIndex].set(vp);
+
+  return true;
+}
+
+bool JSStructuredCloneReader::readArrayBuffer(StructuredDataType type,
+                                              uint32_t data,
+                                              MutableHandleValue vp) {
+  // V2 stores the length in |data|. The current version stores the
+  // length separately to allow larger length values.
+  uint64_t nbytes = 0;
+  uint64_t maxbytes = 0;
+  if (type == SCTAG_ARRAY_BUFFER_OBJECT) {
+    if (!in.read(&nbytes)) {
+      return false;
+    }
+  } else if (type == SCTAG_RESIZABLE_ARRAY_BUFFER_OBJECT) {
+    if (!in.read(&nbytes)) {
+      return false;
+    }
+    if (!in.read(&maxbytes)) {
+      return false;
+    }
+  } else {
+    MOZ_ASSERT(type == SCTAG_ARRAY_BUFFER_OBJECT_V2);
+    nbytes = data;
+  }
+
+  // The maximum ArrayBuffer size depends on the platform, and we cast to size_t
+  // below, so we have to check this here.
+  if (nbytes > ArrayBufferObject::ByteLengthLimit ||
+      maxbytes > ArrayBufferObject::ByteLengthLimit) {
+    JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                              JSMSG_BAD_ARRAY_LENGTH);
+    return false;
+  }
+
+  JSObject* obj;
+  if (type != SCTAG_RESIZABLE_ARRAY_BUFFER_OBJECT) {
+    MOZ_ASSERT(maxbytes == 0);
+    obj = ArrayBufferObject::createZeroed(context(), size_t(nbytes));
+  } else {
+    obj = ResizableArrayBufferObject::createZeroed(context(), size_t(nbytes),
+                                                   size_t(maxbytes));
+  }
+  if (!obj) {
+    return false;
+  }
+  vp.setObject(*obj);
+  ArrayBufferObject& buffer = obj->as<ArrayBufferObject>();
+  MOZ_ASSERT(buffer.byteLength() == nbytes);
+  return in.readArray(buffer.dataPointer(), nbytes);
+}
+
+bool JSStructuredCloneReader::readSharedArrayBuffer(StructuredDataType type,
+                                                    MutableHandleValue vp) {
+  MOZ_ASSERT(type == SCTAG_SHARED_ARRAY_BUFFER_OBJECT ||
+             type == SCTAG_GROWABLE_SHARED_ARRAY_BUFFER_OBJECT);
+
+  if (!cloneDataPolicy.areIntraClusterClonableSharedObjectsAllowed() ||
+      !cloneDataPolicy.areSharedMemoryObjectsAllowed()) {
+    auto error = context()->realm()->creationOptions().getCoopAndCoepEnabled()
+                     ? JS_SCERR_NOT_CLONABLE_WITH_COOP_COEP
+                     : JS_SCERR_NOT_CLONABLE;
+    ReportDataCloneError(context(), callbacks, error, closure,
+                         "SharedArrayBuffer");
+    return false;
+  }
+
+  uint64_t byteLength;
+  if (!in.readBytes(&byteLength, sizeof(byteLength))) {
+    return in.reportTruncated();
+  }
+
+  // The maximum ArrayBuffer size depends on the platform, and we cast to size_t
+  // below, so we have to check this here.
+  if (byteLength > ArrayBufferObject::ByteLengthLimit) {
+    JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                              JSMSG_BAD_ARRAY_LENGTH);
+    return false;
+  }
+
+  intptr_t p;
+  if (!in.readBytes(&p, sizeof(p))) {
+    return in.reportTruncated();
+  }
+
+  bool isGrowable = type == SCTAG_GROWABLE_SHARED_ARRAY_BUFFER_OBJECT;
+
+  SharedArrayRawBuffer* rawbuf = reinterpret_cast<SharedArrayRawBuffer*>(p);
+  MOZ_RELEASE_ASSERT(isGrowable == rawbuf->isGrowable());
+
+  // There's no guarantee that the receiving agent has enabled shared memory
+  // even if the transmitting agent has done so.  Ideally we'd check at the
+  // transmission point, but that's tricky, and it will be a very rare problem
+  // in any case.  Just fail at the receiving end if we can't handle it.
+
+  if (!context()
+           ->realm()
+           ->creationOptions()
+           .getSharedMemoryAndAtomicsEnabled()) {
+    JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                              JSMSG_SC_SAB_DISABLED);
+    return false;
+  }
+
+  // The new object will have a new reference to the rawbuf.
+
+  if (!rawbuf->addReference()) {
+    JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                              JSMSG_SC_SAB_REFCNT_OFLO);
+    return false;
+  }
+
+  RootedObject obj(context());
+  if (!isGrowable) {
+    obj = SharedArrayBufferObject::New(context(), rawbuf, byteLength);
+  } else {
+    obj = SharedArrayBufferObject::NewGrowable(context(), rawbuf, byteLength);
+  }
+  if (!obj) {
+    rawbuf->dropReference();
+    return false;
+  }
+
+  // `rawbuf` is now owned by `obj`.
+
+  if (callbacks && callbacks->sabCloned &&
+      !callbacks->sabCloned(context(), /*receiving=*/true, closure)) {
+    return false;
+  }
+
+  vp.setObject(*obj);
+  return true;
+}
+
+bool JSStructuredCloneReader::readSharedWasmMemory(uint32_t nbytes,
+                                                   MutableHandleValue vp) {
+  JSContext* cx = context();
+  if (nbytes != 0) {
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                              JSMSG_SC_BAD_SERIALIZED_DATA,
+                              "invalid shared wasm memory tag");
+    return false;
+  }
+
+  if (!cloneDataPolicy.areIntraClusterClonableSharedObjectsAllowed() ||
+      !cloneDataPolicy.areSharedMemoryObjectsAllowed()) {
+    auto error = context()->realm()->creationOptions().getCoopAndCoepEnabled()
+                     ? JS_SCERR_NOT_CLONABLE_WITH_COOP_COEP
+                     : JS_SCERR_NOT_CLONABLE;
+    ReportDataCloneError(cx, callbacks, error, closure, "WebAssembly.Memory");
+    return false;
+  }
+
+  // Read the isHuge flag
+  RootedValue isHuge(cx);
+  if (!startRead(&isHuge)) {
+    return false;
+  }
+
+  // Read the SharedArrayBuffer object.
+  RootedValue payload(cx);
+  if (!startRead(&payload)) {
+    return false;
+  }
+  if (!payload.isObject() ||
+      !payload.toObject().is<SharedArrayBufferObject>() ||
+      payload.toObject().as<SharedArrayBufferObject>().isGrowable()) {
+    JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                              JSMSG_SC_BAD_SERIALIZED_DATA,
+                              "shared wasm memory must be backed by a "
+                              "non-growable SharedArrayBuffer");
+    return false;
+  }
+
+  Rooted<ArrayBufferObjectMaybeShared*> sab(
+      cx, &payload.toObject().as<SharedArrayBufferObject>());
+
+  // Construct the memory.
+  RootedObject proto(
+      cx, GlobalObject::getOrCreatePrototype(cx, JSProto_WasmMemory));
+  if (!proto) {
+    return false;
+  }
+  RootedObject memory(
+      cx, WasmMemoryObject::create(cx, sab, isHuge.toBoolean(), proto));
+  if (!memory) {
+    return false;
+  }
+
+  vp.setObject(*memory);
+  return true;
+}
+
+/*
+ * Read in the data for a structured clone version 1 ArrayBuffer, performing
+ * endianness-conversion while reading.
+ */
+bool JSStructuredCloneReader::readV1ArrayBuffer(uint32_t arrayType,
+                                                uint32_t nelems,
+                                                MutableHandleValue vp) {
+  if (arrayType > Scalar::Uint8Clamped) {
+    JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                              JSMSG_SC_BAD_SERIALIZED_DATA,
+                              "invalid TypedArray type");
+    return false;
+  }
+
+  mozilla::CheckedInt<size_t> nbytes =
+      mozilla::CheckedInt<size_t>(nelems) *
+      TypedArrayElemSize(static_cast<Scalar::Type>(arrayType));
+  if (!nbytes.isValid() || nbytes.value() > UINT32_MAX) {
+    JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                              JSMSG_SC_BAD_SERIALIZED_DATA,
+                              "invalid typed array size");
+    return false;
+  }
+
+  JSObject* obj = ArrayBufferObject::createZeroed(context(), nbytes.value());
+  if (!obj) {
+    return false;
+  }
+  vp.setObject(*obj);
+  ArrayBufferObject& buffer = obj->as<ArrayBufferObject>();
+  MOZ_ASSERT(buffer.byteLength() == nbytes);
+
+  switch (arrayType) {
+    case Scalar::Int8:
+    case Scalar::Uint8:
+    case Scalar::Uint8Clamped:
+      return in.readArray((uint8_t*)buffer.dataPointer(), nelems);
+    case Scalar::Int16:
+    case Scalar::Uint16:
+      return in.readArray((uint16_t*)buffer.dataPointer(), nelems);
+    case Scalar::Int32:
+    case Scalar::Uint32:
+    case Scalar::Float32:
+      return in.readArray((uint32_t*)buffer.dataPointer(), nelems);
+    case Scalar::Float64:
+    case Scalar::BigInt64:
+    case Scalar::BigUint64:
+      return in.readArray((uint64_t*)buffer.dataPointer(), nelems);
+    default:
+      MOZ_CRASH("Can't happen: arrayType range checked by caller");
+  }
+}
+
+static bool PrimitiveToObject(JSContext* cx, MutableHandleValue vp) {
+  JSObject* obj = js::PrimitiveToObject(cx, vp);
+  if (!obj) {
+    return false;
+  }
+
+  vp.setObject(*obj);
+  return true;
+}
+
+bool JSStructuredCloneReader::startRead(MutableHandleValue vp,
+                                        ShouldAtomizeStrings atomizeStrings) {
+  uint32_t tag, data;
+  bool alreadAppended = false;
+
+  if (!in.readPair(&tag, &data)) {
+    return false;
+  }
+
+  numItemsRead++;
+
+  switch (tag) {
+    case SCTAG_NULL:
+      vp.setNull();
+      break;
+
+    case SCTAG_UNDEFINED:
+      vp.setUndefined();
+      break;
+
+    case SCTAG_INT32:
+      vp.setInt32(data);
+      break;
+
+    case SCTAG_BOOLEAN:
+    case SCTAG_BOOLEAN_OBJECT:
+      vp.setBoolean(!!data);
+      if (tag == SCTAG_BOOLEAN_OBJECT && !PrimitiveToObject(context(), vp)) {
+        return false;
+      }
+      break;
+
+    case SCTAG_STRING:
+    case SCTAG_STRING_OBJECT: {
+      JSString* str = readString(data, atomizeStrings);
+      if (!str) {
+        return false;
+      }
+      vp.setString(str);
+      if (tag == SCTAG_STRING_OBJECT && !PrimitiveToObject(context(), vp)) {
+        return false;
+      }
+      break;
+    }
+
+    case SCTAG_NUMBER_OBJECT: {
+      double d;
+      if (!in.readDouble(&d)) {
+        return false;
+      }
+      vp.setDouble(CanonicalizeNaN(d));
+      if (!PrimitiveToObject(context(), vp)) {
+        return false;
+      }
+      break;
+    }
+
+    case SCTAG_BIGINT:
+    case SCTAG_BIGINT_OBJECT: {
+      RootedBigInt bi(context(), readBigInt(data));
+      if (!bi) {
+        return false;
+      }
+      vp.setBigInt(bi);
+      if (tag == SCTAG_BIGINT_OBJECT && !PrimitiveToObject(context(), vp)) {
+        return false;
+      }
+      break;
+    }
+
+    case SCTAG_DATE_OBJECT: {
+      double d;
+      if (!in.readDouble(&d)) {
+        return false;
+      }
+      JS::ClippedTime t = JS::TimeClip(d);
+      if (!NumbersAreIdentical(d, t.toDouble())) {
+        JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                                  JSMSG_SC_BAD_SERIALIZED_DATA, "date");
+        return false;
+      }
+      JSObject* obj = NewDateObjectMsec(context(), t);
+      if (!obj) {
+        return false;
+      }
+      vp.setObject(*obj);
+      break;
+    }
+
+    case SCTAG_REGEXP_OBJECT: {
+      if ((data & RegExpFlag::AllFlags) != data) {
+        JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                                  JSMSG_SC_BAD_SERIALIZED_DATA, "regexp");
+        return false;
+      }
+
+      RegExpFlags flags(AssertedCast<uint8_t>(data));
+
+      uint32_t tag2, stringData;
+      if (!in.readPair(&tag2, &stringData)) {
+        return false;
+      }
+      if (tag2 != SCTAG_STRING) {
+        JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                                  JSMSG_SC_BAD_SERIALIZED_DATA, "regexp");
+        return false;
+      }
+
+      JSString* str = readString(stringData, AtomizeStrings);
+      if (!str) {
+        return false;
+      }
+
+      Rooted<JSAtom*> atom(context(), &str->asAtom());
+
+      NewObjectKind kind =
+          gcHeap == gc::Heap::Tenured ? TenuredObject : GenericObject;
+      RegExpObject* reobj = RegExpObject::create(context(), atom, flags, kind);
+      if (!reobj) {
+        return false;
+      }
+      vp.setObject(*reobj);
+      break;
+    }
+
+    case SCTAG_ARRAY_OBJECT:
+    case SCTAG_OBJECT_OBJECT: {
+      NewObjectKind kind =
+          gcHeap == gc::Heap::Tenured ? TenuredObject : GenericObject;
+      JSObject* obj;
+      if (tag == SCTAG_ARRAY_OBJECT) {
+        obj = NewDenseUnallocatedArray(
+            context(), NativeEndian::swapFromLittleEndian(data), kind);
+      } else {
+        obj = NewPlainObject(context(), kind);
+      }
+      if (!obj || !objs.append(ObjectValue(*obj))) {
+        return false;
+      }
+
+      vp.setObject(*obj);
+      break;
+    }
+
+    case SCTAG_BACK_REFERENCE_OBJECT: {
+      if (data >= allObjs.length() || !allObjs[data].isObject()) {
+        JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                                  JSMSG_SC_BAD_SERIALIZED_DATA,
+                                  "invalid back reference in input");
+        return false;
+      }
+      vp.set(allObjs[data]);
+      return true;
+    }
+
+    case SCTAG_TRANSFER_MAP_HEADER:
+    case SCTAG_TRANSFER_MAP_PENDING_ENTRY:
+      // We should be past all the transfer map tags.
+      JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                                JSMSG_SC_BAD_SERIALIZED_DATA, "invalid input");
+      return false;
+
+    case SCTAG_ARRAY_BUFFER_OBJECT_V2:
+    case SCTAG_ARRAY_BUFFER_OBJECT:
+    case SCTAG_RESIZABLE_ARRAY_BUFFER_OBJECT:
+      if (!readArrayBuffer(StructuredDataType(tag), data, vp)) {
+        return false;
+      }
+      break;
+
+    case SCTAG_SHARED_ARRAY_BUFFER_OBJECT:
+    case SCTAG_GROWABLE_SHARED_ARRAY_BUFFER_OBJECT:
+      if (!readSharedArrayBuffer(StructuredDataType(tag), vp)) {
+        return false;
+      }
+      break;
+
+    case SCTAG_SHARED_WASM_MEMORY_OBJECT:
+      if (!readSharedWasmMemory(data, vp)) {
+        return false;
+      }
+      break;
+
+    case SCTAG_TYPED_ARRAY_OBJECT_V2: {
+      // readTypedArray adds the array to allObjs.
+      // V2 stores the length (nelems) in |data| and the arrayType separately.
+      uint64_t arrayType;
+      if (!in.read(&arrayType)) {
+        return false;
+      }
+      uint64_t nelems = data;
+      return readTypedArray(arrayType, nelems, vp);
+    }
+
+    case SCTAG_TYPED_ARRAY_OBJECT: {
+      // readTypedArray adds the array to allObjs.
+      // The current version stores the array type in |data| and the length
+      // (nelems) separately to support large TypedArrays.
+      uint32_t arrayType = data;
+      uint64_t nelems;
+      if (!in.read(&nelems)) {
+        return false;
+      }
+      return readTypedArray(arrayType, nelems, vp);
+    }
+
+    case SCTAG_DATA_VIEW_OBJECT_V2: {
+      // readDataView adds the array to allObjs.
+      uint64_t byteLength = data;
+      return readDataView(byteLength, vp);
+    }
+
+    case SCTAG_DATA_VIEW_OBJECT: {
+      // readDataView adds the array to allObjs.
+      uint64_t byteLength;
+      if (!in.read(&byteLength)) {
+        return false;
+      }
+      return readDataView(byteLength, vp);
+    }
+
+    case SCTAG_MAP_OBJECT: {
+      JSObject* obj = MapObject::create(context());
+      if (!obj || !objs.append(ObjectValue(*obj))) {
+        return false;
+      }
+      vp.setObject(*obj);
+      break;
+    }
+
+    case SCTAG_SET_OBJECT: {
+      JSObject* obj = SetObject::create(context());
+      if (!obj || !objs.append(ObjectValue(*obj))) {
+        return false;
+      }
+      vp.setObject(*obj);
+      break;
+    }
+
+    case SCTAG_SAVED_FRAME_OBJECT: {
+      auto* obj = readSavedFrameHeader(data);
+      if (!obj || !objs.append(ObjectValue(*obj)) ||
+          !objState.append(std::make_pair(obj, false))) {
+        return false;
+      }
+      vp.setObject(*obj);
+      break;
+    }
+
+    case SCTAG_ERROR_OBJECT: {
+      auto* obj = readErrorHeader(data);
+      if (!obj || !objs.append(ObjectValue(*obj)) ||
+          !objState.append(std::make_pair(obj, false))) {
+        return false;
+      }
+      vp.setObject(*obj);
+      break;
+    }
+
+    case SCTAG_END_OF_KEYS:
+      JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                                JSMSG_SC_BAD_SERIALIZED_DATA,
+                                "truncated input");
+      return false;
+      break;
+
+    default: {
+      if (tag <= SCTAG_FLOAT_MAX) {
+        double d = ReinterpretPairAsDouble(tag, data);
+        vp.setNumber(CanonicalizeNaN(d));
+        break;
+      }
+
+      if (SCTAG_TYPED_ARRAY_V1_MIN <= tag && tag <= SCTAG_TYPED_ARRAY_V1_MAX) {
+        // A v1-format typed array
+        // readTypedArray adds the array to allObjs
+        return readTypedArray(TagToV1ArrayType(tag), data, vp, true);
+      }
+
+      if (!callbacks || !callbacks->read) {
+        JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                                  JSMSG_SC_BAD_SERIALIZED_DATA,
+                                  "unsupported type");
+        return false;
+      }
+
+      // callbacks->read() might read other objects from the buffer.
+      // In startWrite we always write the object itself before calling
+      // the custom function. We should do the same here to keep
+      // indexing consistent.
+      uint32_t placeholderIndex = allObjs.length();
+      Value dummy = UndefinedValue();
+      if (!allObjs.append(dummy)) {
+        return false;
+      }
+      JSObject* obj =
+          callbacks->read(context(), this, cloneDataPolicy, tag, data, closure);
+      if (!obj) {
+        return false;
+      }
+      vp.setObject(*obj);
+      allObjs[placeholderIndex].set(vp);
+      alreadAppended = true;
+    }
+  }
+
+  if (!alreadAppended && vp.isObject() && !allObjs.append(vp)) {
+    return false;
+  }
+
+  return true;
+}
+
+bool JSStructuredCloneReader::readHeader() {
+  uint32_t tag, data;
+  if (!in.getPair(&tag, &data)) {
+    return in.reportTruncated();
+  }
+
+  JS::StructuredCloneScope storedScope;
+  if (tag == SCTAG_HEADER) {
+    MOZ_ALWAYS_TRUE(in.readPair(&tag, &data));
+    storedScope = JS::StructuredCloneScope(data);
+  } else {
+    // Old structured clone buffer. We must have read it from disk.
+    storedScope = JS::StructuredCloneScope::DifferentProcessForIndexedDB;
+  }
+
+  // Backward compatibility with old structured clone buffers. Value '0' was
+  // used for SameProcessSameThread scope.
+  if ((int)storedScope == 0) {
+    storedScope = JS::StructuredCloneScope::SameProcess;
+  }
+
+  if (storedScope < JS::StructuredCloneScope::SameProcess ||
+      storedScope > JS::StructuredCloneScope::DifferentProcessForIndexedDB) {
+    JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                              JSMSG_SC_BAD_SERIALIZED_DATA,
+                              "invalid structured clone scope");
+    return false;
+  }
+
+  if (allowedScope == JS::StructuredCloneScope::DifferentProcessForIndexedDB) {
+    // Bug 1434308 and bug 1458320 - the scopes stored in old IndexedDB
+    // clones are incorrect. Treat them as if they were DifferentProcess.
+    allowedScope = JS::StructuredCloneScope::DifferentProcess;
+    return true;
+  }
+
+  if (storedScope < allowedScope) {
+    JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                              JSMSG_SC_BAD_SERIALIZED_DATA,
+                              "incompatible structured clone scope");
+    return false;
+  }
+
+  return true;
+}
+
+bool JSStructuredCloneReader::readTransferMap() {
+  JSContext* cx = context();
+  auto headerPos = in.tell();
+
+  uint32_t tag, data;
+  if (!in.getPair(&tag, &data)) {
+    return in.reportTruncated();
+  }
+
+  if (tag != SCTAG_TRANSFER_MAP_HEADER ||
+      TransferableMapHeader(data) == SCTAG_TM_TRANSFERRED) {
+    return true;
+  }
+
+  uint64_t numTransferables;
+  MOZ_ALWAYS_TRUE(in.readPair(&tag, &data));
+  if (!in.read(&numTransferables)) {
+    return false;
+  }
+
+  for (uint64_t i = 0; i < numTransferables; i++) {
+    auto pos = in.tell();
+
+    if (!in.readPair(&tag, &data)) {
+      return false;
+    }
+
+    if (tag == SCTAG_TRANSFER_MAP_PENDING_ENTRY) {
+      ReportDataCloneError(cx, callbacks, JS_SCERR_TRANSFERABLE, closure);
+      return false;
+    }
+
+    RootedObject obj(cx);
+
+    void* content;
+    if (!in.readPtr(&content)) {
+      return false;
+    }
+
+    uint64_t extraData;
+    if (!in.read(&extraData)) {
+      return false;
+    }
+
+    if (tag == SCTAG_TRANSFER_MAP_ARRAY_BUFFER) {
+      if (allowedScope == JS::StructuredCloneScope::DifferentProcess ||
+          allowedScope ==
+              JS::StructuredCloneScope::DifferentProcessForIndexedDB) {
+        // Transferred ArrayBuffers in a DifferentProcess clone buffer
+        // are treated as if they weren't Transferred at all. We should
+        // only see SCTAG_TRANSFER_MAP_STORED_ARRAY_BUFFER.
+        ReportDataCloneError(cx, callbacks, JS_SCERR_TRANSFERABLE, closure);
+        return false;
+      }
+
+      MOZ_RELEASE_ASSERT(extraData <= ArrayBufferObject::ByteLengthLimit);
+      size_t nbytes = extraData;
+
+      MOZ_ASSERT(data == JS::SCTAG_TMO_ALLOC_DATA ||
+                 data == JS::SCTAG_TMO_MAPPED_DATA);
+      if (data == JS::SCTAG_TMO_ALLOC_DATA) {
+        // When the ArrayBuffer can't be allocated, |content| will be free'ed
+        // in `JSStructuredCloneData::discardTransferables()`.
+        obj = JS::NewArrayBufferWithContents(
+            cx, nbytes, content,
+            JS::NewArrayBufferOutOfMemory::CallerMustFreeMemory);
+      } else if (data == JS::SCTAG_TMO_MAPPED_DATA) {
+        obj = JS::NewMappedArrayBufferWithContents(cx, nbytes, content);
+      }
+    } else if (tag == SCTAG_TRANSFER_MAP_STORED_ARRAY_BUFFER) {
+      auto savedPos = in.tell();
+      auto guard = mozilla::MakeScopeExit([&] { in.seekTo(savedPos); });
+      in.seekTo(pos);
+      if (!in.seekBy(static_cast<size_t>(extraData))) {
+        return false;
+      }
+
+      if (tailStartPos.isNothing()) {
+        tailStartPos = mozilla::Some(in.tell());
+      }
+
+      uint32_t tag, data;
+      if (!in.readPair(&tag, &data)) {
+        return false;
+      }
+      if (tag != SCTAG_ARRAY_BUFFER_OBJECT_V2 &&
+          tag != SCTAG_ARRAY_BUFFER_OBJECT &&
+          tag != SCTAG_RESIZABLE_ARRAY_BUFFER_OBJECT) {
+        ReportDataCloneError(cx, callbacks, JS_SCERR_TRANSFERABLE, closure);
+        return false;
+      }
+      RootedValue val(cx);
+      if (!readArrayBuffer(StructuredDataType(tag), data, &val)) {
+        return false;
+      }
+      obj = &val.toObject();
+      tailEndPos = mozilla::Some(in.tell());
+    } else {
+      if (!callbacks || !callbacks->readTransfer) {
+        ReportDataCloneError(cx, callbacks, JS_SCERR_TRANSFERABLE, closure);
+        return false;
+      }
+      if (!callbacks->readTransfer(cx, this, cloneDataPolicy, tag, content,
+                                   extraData, closure, &obj)) {
+        if (!cx->isExceptionPending()) {
+          ReportDataCloneError(cx, callbacks, JS_SCERR_TRANSFERABLE, closure);
+        }
+        return false;
+      }
+      MOZ_ASSERT(obj);
+      MOZ_ASSERT(!cx->isExceptionPending());
+    }
+
+    // On failure, the buffer will still own the data (since its ownership
+    // will not get set to SCTAG_TMO_UNOWNED), so the data will be freed by
+    // DiscardTransferables.
+    if (!obj) {
+      return false;
+    }
+
+    // Mark the SCTAG_TRANSFER_MAP_* entry as no longer owned by the input
+    // buffer.
+    pos.write(PairToUInt64(tag, JS::SCTAG_TMO_UNOWNED));
+    MOZ_ASSERT(!pos.done());
+
+    if (!allObjs.append(ObjectValue(*obj))) {
+      return false;
+    }
+  }
+
+  // Mark the whole transfer map as consumed.
+#ifdef DEBUG
+  SCInput::getPair(headerPos.peek(), &tag, &data);
+  MOZ_ASSERT(tag == SCTAG_TRANSFER_MAP_HEADER);
+  MOZ_ASSERT(TransferableMapHeader(data) != SCTAG_TM_TRANSFERRED);
+#endif
+  headerPos.write(
+      PairToUInt64(SCTAG_TRANSFER_MAP_HEADER, SCTAG_TM_TRANSFERRED));
+
+  return true;
+}
+
+JSObject* JSStructuredCloneReader::readSavedFrameHeader(
+    uint32_t principalsTag) {
+  Rooted<SavedFrame*> savedFrame(context(), SavedFrame::create(context()));
+  if (!savedFrame) {
+    return nullptr;
+  }
+
+  JSPrincipals* principals;
+  if (principalsTag == SCTAG_JSPRINCIPALS) {
+    if (!context()->runtime()->readPrincipals) {
+      JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                                JSMSG_SC_UNSUPPORTED_TYPE);
+      return nullptr;
+    }
+
+    if (!context()->runtime()->readPrincipals(context(), this, &principals)) {
+      return nullptr;
+    }
+  } else if (principalsTag ==
+             SCTAG_RECONSTRUCTED_SAVED_FRAME_PRINCIPALS_IS_SYSTEM) {
+    principals = &ReconstructedSavedFramePrincipals::IsSystem;
+    principals->refcount++;
+  } else if (principalsTag ==
+             SCTAG_RECONSTRUCTED_SAVED_FRAME_PRINCIPALS_IS_NOT_SYSTEM) {
+    principals = &ReconstructedSavedFramePrincipals::IsNotSystem;
+    principals->refcount++;
+  } else if (principalsTag == SCTAG_NULL_JSPRINCIPALS) {
+    principals = nullptr;
+  } else {
+    JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                              JSMSG_SC_BAD_SERIALIZED_DATA,
+                              "bad SavedFrame principals");
+    return nullptr;
+  }
+
+  RootedValue mutedErrors(context());
+  RootedValue source(context());
+  {
+    // Read a |mutedErrors| boolean followed by a |source| string.
+    // The |mutedErrors| boolean is present in all new structured-clone data,
+    // but in older data it will be absent and only the |source| string will be
+    // found.
+    if (!startRead(&mutedErrors, AtomizeStrings)) {
+      return nullptr;
+    }
+
+    if (mutedErrors.isBoolean()) {
+      if (!startRead(&source, AtomizeStrings) || !source.isString()) {
+        return nullptr;
+      }
+    } else if (mutedErrors.isString()) {
+      // Backwards compatibility: Handle missing |mutedErrors| boolean,
+      // this is actually just a |source| string.
+      source = mutedErrors;
+      mutedErrors.setBoolean(true);  // Safe default value.
+    } else {
+      // Invalid type.
+      return nullptr;
+    }
+  }
+
+  savedFrame->initPrincipalsAlreadyHeldAndMutedErrors(principals,
+                                                      mutedErrors.toBoolean());
+
+  savedFrame->initSource(&source.toString()->asAtom());
+
+  uint32_t line;
+  if (!readUint32(&line)) {
+    return nullptr;
+  }
+  savedFrame->initLine(line);
+
+  JS::TaggedColumnNumberOneOrigin column;
+  if (!readUint32(column.addressOfValueForTranscode())) {
+    return nullptr;
+  }
+  savedFrame->initColumn(column);
+
+  // Don't specify a source ID when reading a cloned saved frame, as these IDs
+  // are only valid within a specific process.
+  savedFrame->initSourceId(0);
+
+  RootedValue name(context());
+  if (!startRead(&name, AtomizeStrings)) {
+    return nullptr;
+  }
+  if (!(name.isString() || name.isNull())) {
+    JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                              JSMSG_SC_BAD_SERIALIZED_DATA,
+                              "invalid saved frame cause");
+    return nullptr;
+  }
+  JSAtom* atomName = nullptr;
+  if (name.isString()) {
+    atomName = &name.toString()->asAtom();
+  }
+
+  savedFrame->initFunctionDisplayName(atomName);
+
+  RootedValue cause(context());
+  if (!startRead(&cause, AtomizeStrings)) {
+    return nullptr;
+  }
+  if (!(cause.isString() || cause.isNull())) {
+    JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                              JSMSG_SC_BAD_SERIALIZED_DATA,
+                              "invalid saved frame cause");
+    return nullptr;
+  }
+  JSAtom* atomCause = nullptr;
+  if (cause.isString()) {
+    atomCause = &cause.toString()->asAtom();
+  }
+  savedFrame->initAsyncCause(atomCause);
+
+  return savedFrame;
+}
+
+// SavedFrame object: there is one child value, the parent SavedFrame,
+// which is either null or another SavedFrame object.
+bool JSStructuredCloneReader::readSavedFrameFields(Handle<SavedFrame*> frameObj,
+                                                   HandleValue parent,
+                                                   bool* state) {
+  if (*state) {
+    JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                              JSMSG_SC_BAD_SERIALIZED_DATA,
+                              "multiple SavedFrame parents");
+    return false;
+  }
+
+  SavedFrame* parentFrame;
+  if (parent.isNull()) {
+    parentFrame = nullptr;
+  } else if (parent.isObject() && parent.toObject().is<SavedFrame>()) {
+    parentFrame = &parent.toObject().as<SavedFrame>();
+  } else {
+    JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                              JSMSG_SC_BAD_SERIALIZED_DATA,
+                              "invalid SavedFrame parent");
+    return false;
+  }
+
+  frameObj->initParent(parentFrame);
+  *state = true;
+  return true;
+}
+
+JSObject* JSStructuredCloneReader::readErrorHeader(uint32_t type) {
+  JSContext* cx = context();
+
+  switch (type) {
+    case JSEXN_ERR:
+    case JSEXN_EVALERR:
+    case JSEXN_RANGEERR:
+    case JSEXN_REFERENCEERR:
+    case JSEXN_SYNTAXERR:
+    case JSEXN_TYPEERR:
+    case JSEXN_URIERR:
+    case JSEXN_AGGREGATEERR:
+      break;
+    default:
+      JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                                JSMSG_SC_BAD_SERIALIZED_DATA,
+                                "invalid error type");
+      return nullptr;
+  }
+
+  RootedString message(cx);
+  {
+    RootedValue messageVal(cx);
+    if (!startRead(&messageVal)) {
+      return nullptr;
+    }
+    if (messageVal.isString()) {
+      message = messageVal.toString();
+    } else if (!messageVal.isNull()) {
+      JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                                JSMSG_SC_BAD_SERIALIZED_DATA,
+                                "invalid 'message' field for Error object");
+      return nullptr;
+    }
+  }
+
+  // We have to set |cause| to something if it exists, otherwise the shape
+  // would be wrong. The actual value will be overwritten later.
+  RootedValue val(cx);
+  if (!startRead(&val)) {
+    return nullptr;
+  }
+  bool hasCause = ToBoolean(val);
+  Rooted<Maybe<Value>> cause(cx, mozilla::Nothing());
+  if (hasCause) {
+    cause = mozilla::Some(BooleanValue(true));
+  }
+
+  if (!startRead(&val)) {
+    return nullptr;
+  }
+  if (!val.isString()) {
+    JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                              JSMSG_SC_BAD_SERIALIZED_DATA,
+                              "invalid 'fileName' field for Error object");
+    return nullptr;
+  }
+  RootedString fileName(cx, val.toString());
+
+  uint32_t lineNumber;
+  JS::ColumnNumberOneOrigin columnNumber;
+  if (!readUint32(&lineNumber) ||
+      !readUint32(columnNumber.addressOfValueForTranscode())) {
+    return nullptr;
+  }
+
+  // The |cause| and |stack| slots of the objects might be overwritten later.
+  // For AggregateErrors the |errors| property will be added.
+  RootedObject errorObj(
+      cx, ErrorObject::create(cx, static_cast<JSExnType>(type), nullptr,
+                              fileName, 0, lineNumber, columnNumber, nullptr,
+                              message, cause));
+  if (!errorObj) {
+    return nullptr;
+  }
+
+  return errorObj;
+}
+
+// Error objects have 3 fields, some or all of them null: cause,
+// errors, and stack.
+bool JSStructuredCloneReader::readErrorFields(Handle<ErrorObject*> errorObj,
+                                              HandleValue cause, bool* state) {
+  JSContext* cx = context();
+  if (*state) {
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                              JSMSG_SC_BAD_SERIALIZED_DATA,
+                              "unexpected child value seen for Error object");
+    return false;
+  }
+
+  RootedValue errors(cx);
+  RootedValue stack(cx);
+  if (!startRead(&errors) || !startRead(&stack)) {
+    return false;
+  }
+
+  bool hasCause = errorObj->getCause().isSome();
+  if (hasCause) {
+    errorObj->setCauseSlot(cause);
+  } else if (!cause.isNull()) {
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                              JSMSG_SC_BAD_SERIALIZED_DATA,
+                              "invalid 'cause' field for Error object");
+    return false;
+  }
+
+  if (errorObj->type() == JSEXN_AGGREGATEERR) {
+    if (!DefineDataProperty(context(), errorObj, cx->names().errors, errors,
+                            0)) {
+      return false;
+    }
+  } else if (!errors.isNull()) {
+    JS_ReportErrorNumberASCII(
+        cx, GetErrorMessage, nullptr, JSMSG_SC_BAD_SERIALIZED_DATA,
+        "unexpected 'errors' field seen for non-AggregateError");
+    return false;
+  }
+
+  if (stack.isObject()) {
+    RootedObject stackObj(cx, &stack.toObject());
+    if (!stackObj->is<SavedFrame>()) {
+      JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                JSMSG_SC_BAD_SERIALIZED_DATA,
+                                "invalid 'stack' field for Error object");
+      return false;
+    }
+    errorObj->setStackSlot(stack);
+  } else if (!stack.isNull()) {
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                              JSMSG_SC_BAD_SERIALIZED_DATA,
+                              "invalid 'stack' field for Error object");
+    return false;
+  }
+
+  *state = true;
+  return true;
+}
+
+// Read a value and treat as a key,value pair.
+bool JSStructuredCloneReader::readMapField(Handle<MapObject*> mapObj,
+                                           HandleValue key) {
+  RootedValue val(context());
+  if (!startRead(&val)) {
+    return false;
+  }
+  return MapObject::set(context(), mapObj, key, val);
+}
+
+// Read a value and treat as a key,value pair. Interpret as a plain property
+// value.
+bool JSStructuredCloneReader::readObjectField(HandleObject obj,
+                                              HandleValue key) {
+  if (!key.isString() && !key.isInt32()) {
+    JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                              JSMSG_SC_BAD_SERIALIZED_DATA,
+                              "property key expected");
+    return false;
+  }
+
+  RootedValue val(context());
+  if (!startRead(&val)) {
+    return false;
+  }
+
+  RootedId id(context());
+  if (!PrimitiveValueToId<CanGC>(context(), key, &id)) {
+    return false;
+  }
+
+  // Fast path for adding a new property to a plain object. The property names
+  // we see here should be unique, but we check for duplicates to guard against
+  // corrupt or malicious data.
+  if (id.isString() && obj->is<PlainObject>() &&
+      MOZ_LIKELY(!obj->as<PlainObject>().contains(context(), id))) {
+    return AddDataPropertyToPlainObject(context(), obj.as<PlainObject>(), id,
+                                        val);
+  }
+
+  // Fast path for adding an array element. The index shouldn't exceed the
+  // array's length, but we check for this in `addDenseElementNoLengthChange` to
+  // guard against corrupt or malicious data.
+  if (id.isInt() && obj->is<ArrayObject>()) {
+    ArrayObject* arr = &obj->as<ArrayObject>();
+    switch (arr->addDenseElementNoLengthChange(context(), id.toInt(), val)) {
+      case DenseElementResult::Failure:
+        return false;
+      case DenseElementResult::Success:
+        return true;
+      case DenseElementResult::Incomplete:
+        // Fall-through to slow path.
+        break;
+    }
+  }
+
+  return DefineDataProperty(context(), obj, id, val);
+}
+
+// Perform the whole recursive reading procedure.
+bool JSStructuredCloneReader::read(MutableHandleValue vp, size_t nbytes) {
+  auto startTime = mozilla::TimeStamp::Now();
+
+  if (!readHeader()) {
+    return false;
+  }
+
+  if (!readTransferMap()) {
+    return false;
+  }
+
+  MOZ_ASSERT(objs.length() == 0);
+  MOZ_ASSERT(objState.length() == 1);
+
+  // Start out by reading in the main object and pushing it onto the 'objs'
+  // stack. The data related to this object and its descendants extends from
+  // here to the SCTAG_END_OF_KEYS at the end of the stream.
+  if (!startRead(vp)) {
+    return false;
+  }
+
+  // Stop when the stack shows that all objects have been read.
+  while (objs.length() != 0) {
+    // What happens depends on the top obj on the objs stack.
+    RootedObject obj(context(), &objs.back().toObject());
+
+    uint32_t tag, data;
+    if (!in.getPair(&tag, &data)) {
+      return false;
+    }
+
+    if (tag == SCTAG_END_OF_KEYS) {
+      // Pop the current obj off the stack, since we are done with it and
+      // its children.
+      MOZ_ALWAYS_TRUE(in.readPair(&tag, &data));
+      objs.popBack();
+      if (objState.back().first == obj) {
+        objState.popBack();
+      }
+      continue;
+    }
+
+    // Remember the index of the current top of the state stack, which will
+    // correspond to the state for `obj` iff `obj` is a type that uses state.
+    // startRead() may push additional entries before the state is accessed and
+    // updated while filling in the object's data.
+    size_t objStateIdx = objState.length() - 1;
+
+    // The input stream contains a sequence of "child" values, whose
+    // interpretation depends on the type of obj. These values can be
+    // anything, and startRead() will push onto 'objs' for any non-leaf
+    // value (i.e., anything that may contain children).
+    //
+    // startRead() will allocate the (empty) object, but note that when
+    // startRead() returns, 'key' is not yet initialized with any of its
+    // properties. Those will be filled in by returning to the head of this
+    // loop, processing the first child obj, and continuing until all
+    // children have been fully created.
+    //
+    // Note that this means the ordering in the stream is a little funky for
+    // things like Map. See the comment above traverseMap() for an example.
+
+    bool expectKeyValuePairs =
+        !(obj->is<MapObject>() || obj->is<SetObject>() ||
+          obj->is<SavedFrame>() || obj->is<ErrorObject>());
+
+    RootedValue key(context());
+    ShouldAtomizeStrings atomize =
+        expectKeyValuePairs ? AtomizeStrings : DontAtomizeStrings;
+    if (!startRead(&key, atomize)) {
+      return false;
+    }
+
+    if (key.isNull() && expectKeyValuePairs) {
+      // Backwards compatibility: Null formerly indicated the end of
+      // object properties.
+
+      // No legacy objects used the state stack.
+      MOZ_ASSERT(objState[objStateIdx].first() != obj);
+
+      objs.popBack();
+      continue;
+    }
+
+    context()->check(key);
+
+    if (obj->is<SetObject>()) {
+      // Set object: the values between obj header (from startRead()) and
+      // SCTAG_END_OF_KEYS are all interpreted as values to add to the set.
+      if (!SetObject::add(context(), obj, key)) {
+        return false;
+      }
+    } else if (obj->is<MapObject>()) {
+      Rooted<MapObject*> mapObj(context(), &obj->as<MapObject>());
+      if (!readMapField(mapObj, key)) {
+        return false;
+      }
+    } else if (obj->is<SavedFrame>()) {
+      Rooted<SavedFrame*> frameObj(context(), &obj->as<SavedFrame>());
+      MOZ_ASSERT(objState[objStateIdx].first() == obj);
+      bool state = objState[objStateIdx].second();
+      if (!readSavedFrameFields(frameObj, key, &state)) {
+        return false;
+      }
+      objState[objStateIdx].second() = state;
+    } else if (obj->is<ErrorObject>()) {
+      Rooted<ErrorObject*> errorObj(context(), &obj->as<ErrorObject>());
+      MOZ_ASSERT(objState[objStateIdx].first() == obj);
+      bool state = objState[objStateIdx].second();
+      if (!readErrorFields(errorObj, key, &state)) {
+        return false;
+      }
+      objState[objStateIdx].second() = state;
+    } else {
+      MOZ_ASSERT(expectKeyValuePairs);
+      // Everything else uses a series of key,value,key,value,... Value
+      // objects.
+      if (!readObjectField(obj, key)) {
+        return false;
+      }
+    }
+  }
+
+  allObjs.clear();
+
+  // For fuzzing, it is convenient to allow extra data at the end
+  // of the input buffer so that more possible inputs are considered
+  // valid.
+#ifndef FUZZING
+  bool extraData;
+  if (tailStartPos.isSome()) {
+    // in.tell() is the end of the main data. If "tail" data was consumed,
+    // then check whether there's any data between the main data and the
+    // beginning of the tail, or after the last read point in the tail.
+    extraData = (in.tell() != *tailStartPos || !tailEndPos->done());
+  } else {
+    extraData = !in.tell().done();
+  }
+  if (extraData) {
+    JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr,
+                              JSMSG_SC_BAD_SERIALIZED_DATA,
+                              "extra data after end");
+    return false;
+  }
+#endif
+
+  JSRuntime* rt = context()->runtime();
+  rt->metrics().DESERIALIZE_BYTES(nbytes);
+  rt->metrics().DESERIALIZE_ITEMS(numItemsRead);
+  mozilla::TimeDuration elapsed = mozilla::TimeStamp::Now() - startTime;
+  rt->metrics().DESERIALIZE_US(elapsed);
+
+  return true;
+}
+
+JS_PUBLIC_API bool JS_ReadStructuredClone(
+    JSContext* cx, const JSStructuredCloneData& buf, uint32_t version,
+    JS::StructuredCloneScope scope, MutableHandleValue vp,
+    const JS::CloneDataPolicy& cloneDataPolicy,
+    const JSStructuredCloneCallbacks* optionalCallbacks, void* closure) {
+  AssertHeapIsIdle();
+  CHECK_THREAD(cx);
+
+  if (version > JS_STRUCTURED_CLONE_VERSION) {
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                              JSMSG_SC_BAD_CLONE_VERSION);
+    return false;
+  }
+  const JSStructuredCloneCallbacks* callbacks = optionalCallbacks;
+  return ReadStructuredClone(cx, buf, scope, vp, cloneDataPolicy, callbacks,
+                             closure);
+}
+
+JS_PUBLIC_API bool JS_WriteStructuredClone(
+    JSContext* cx, HandleValue value, JSStructuredCloneData* bufp,
+    JS::StructuredCloneScope scope, const JS::CloneDataPolicy& cloneDataPolicy,
+    const JSStructuredCloneCallbacks* optionalCallbacks, void* closure,
+    HandleValue transferable) {
+  AssertHeapIsIdle();
+  CHECK_THREAD(cx);
+  cx->check(value);
+
+  const JSStructuredCloneCallbacks* callbacks = optionalCallbacks;
+  return WriteStructuredClone(cx, value, bufp, scope, cloneDataPolicy,
+                              callbacks, closure, transferable);
+}
+
+JS_PUBLIC_API bool JS_StructuredCloneHasTransferables(
+    JSStructuredCloneData& data, bool* hasTransferable) {
+  *hasTransferable = StructuredCloneHasTransferObjects(data);
+  return true;
+}
+
+JS_PUBLIC_API bool JS_StructuredClone(
+    JSContext* cx, HandleValue value, MutableHandleValue vp,
+    const JSStructuredCloneCallbacks* optionalCallbacks, void* closure) {
+  AssertHeapIsIdle();
+  CHECK_THREAD(cx);
+
+  // Strings are associated with zones, not compartments,
+  // so we copy the string by wrapping it.
+  if (value.isString()) {
+    RootedString strValue(cx, value.toString());
+    if (!cx->compartment()->wrap(cx, &strValue)) {
+      return false;
+    }
+    vp.setString(strValue);
+    return true;
+  }
+
+  const JSStructuredCloneCallbacks* callbacks = optionalCallbacks;
+
+  JSAutoStructuredCloneBuffer buf(JS::StructuredCloneScope::SameProcess,
+                                  callbacks, closure);
+  {
+    if (value.isObject()) {
+      RootedObject obj(cx, &value.toObject());
+      obj = CheckedUnwrapStatic(obj);
+      if (!obj) {
+        ReportAccessDenied(cx);
+        return false;
+      }
+      AutoRealm ar(cx, obj);
+      RootedValue unwrappedVal(cx, ObjectValue(*obj));
+      if (!buf.write(cx, unwrappedVal, callbacks, closure)) {
+        return false;
+      }
+    } else {
+      if (!buf.write(cx, value, callbacks, closure)) {
+        return false;
+      }
+    }
+  }
+
+  return buf.read(cx, vp, JS::CloneDataPolicy(), callbacks, closure);
+}
+
+JSAutoStructuredCloneBuffer::JSAutoStructuredCloneBuffer(
+    JSAutoStructuredCloneBuffer&& other)
+    : data_(other.scope()) {
+  version_ = other.version_;
+  other.giveTo(&data_);
+}
+
+JSAutoStructuredCloneBuffer& JSAutoStructuredCloneBuffer::operator=(
+    JSAutoStructuredCloneBuffer&& other) {
+  MOZ_ASSERT(&other != this);
+  MOZ_ASSERT(scope() == other.scope());
+  clear();
+  version_ = other.version_;
+  other.giveTo(&data_);
+  return *this;
+}
+
+void JSAutoStructuredCloneBuffer::clear() {
+  data_.discardTransferables();
+  data_.ownTransferables_ = OwnTransferablePolicy::NoTransferables;
+  data_.refsHeld_.releaseAll();
+  data_.Clear();
+  version_ = 0;
+}
+
+void JSAutoStructuredCloneBuffer::adopt(
+    JSStructuredCloneData&& data, uint32_t version,
+    const JSStructuredCloneCallbacks* callbacks, void* closure) {
+  clear();
+  data_ = std::move(data);
+  version_ = version;
+  data_.setCallbacks(callbacks, closure,
+                     OwnTransferablePolicy::OwnsTransferablesIfAny);
+}
+
+void JSAutoStructuredCloneBuffer::giveTo(JSStructuredCloneData* data) {
+  *data = std::move(data_);
+  version_ = 0;
+  data_.setCallbacks(nullptr, nullptr, OwnTransferablePolicy::NoTransferables);
+  data_.Clear();
+}
+
+bool JSAutoStructuredCloneBuffer::read(
+    JSContext* cx, MutableHandleValue vp,
+    const JS::CloneDataPolicy& cloneDataPolicy,
+    const JSStructuredCloneCallbacks* optionalCallbacks, void* closure) {
+  MOZ_ASSERT(cx);
+  return !!JS_ReadStructuredClone(
+      cx, data_, version_, data_.scope(), vp, cloneDataPolicy,
+      optionalCallbacks ? optionalCallbacks : data_.callbacks_,
+      optionalCallbacks ? closure : data_.closure_);
+}
+
+bool JSAutoStructuredCloneBuffer::write(
+    JSContext* cx, HandleValue value,
+    const JSStructuredCloneCallbacks* optionalCallbacks, void* closure) {
+  HandleValue transferable = UndefinedHandleValue;
+  return write(cx, value, transferable, JS::CloneDataPolicy(),
+               optionalCallbacks ? optionalCallbacks : data_.callbacks_,
+               optionalCallbacks ? closure : data_.closure_);
+}
+
+bool JSAutoStructuredCloneBuffer::write(
+    JSContext* cx, HandleValue value, HandleValue transferable,
+    const JS::CloneDataPolicy& cloneDataPolicy,
+    const JSStructuredCloneCallbacks* optionalCallbacks, void* closure) {
+  clear();
+  bool ok = JS_WriteStructuredClone(
+      cx, value, &data_, data_.scopeForInternalWriting(), cloneDataPolicy,
+      optionalCallbacks ? optionalCallbacks : data_.callbacks_,
+      optionalCallbacks ? closure : data_.closure_, transferable);
+  if (!ok) {
+    version_ = JS_STRUCTURED_CLONE_VERSION;
+  }
+  return ok;
+}
+
+JS_PUBLIC_API bool JS_ReadUint32Pair(JSStructuredCloneReader* r, uint32_t* p1,
+                                     uint32_t* p2) {
+  return r->input().readPair((uint32_t*)p1, (uint32_t*)p2);
+}
+
+JS_PUBLIC_API bool JS_ReadBytes(JSStructuredCloneReader* r, void* p,
+                                size_t len) {
+  return r->input().readBytes(p, len);
+}
+
+JS_PUBLIC_API bool JS_ReadString(JSStructuredCloneReader* r,
+                                 MutableHandleString str) {
+  uint32_t tag, data;
+  if (!r->input().readPair(&tag, &data)) {
+    return false;
+  }
+
+  if (tag == SCTAG_STRING) {
+    if (JSString* s =
+            r->readString(data, JSStructuredCloneReader::DontAtomizeStrings)) {
+      str.set(s);
+      return true;
+    }
+    return false;
+  }
+
+  JS_ReportErrorNumberASCII(r->context(), GetErrorMessage, nullptr,
+                            JSMSG_SC_BAD_SERIALIZED_DATA, "expected string");
+  return false;
+}
+
+JS_PUBLIC_API bool JS_ReadDouble(JSStructuredCloneReader* r, double* v) {
+  return r->input().readDouble(v);
+}
+
+JS_PUBLIC_API bool JS_ReadTypedArray(JSStructuredCloneReader* r,
+                                     MutableHandleValue vp) {
+  uint32_t tag, data;
+  if (!r->input().readPair(&tag, &data)) {
+    return false;
+  }
+
+  if (tag >= SCTAG_TYPED_ARRAY_V1_MIN && tag <= SCTAG_TYPED_ARRAY_V1_MAX) {
+    return r->readTypedArray(TagToV1ArrayType(tag), data, vp, true);
+  }
+
+  if (tag == SCTAG_TYPED_ARRAY_OBJECT_V2) {
+    // V2 stores the length (nelems) in |data| and the arrayType separately.
+    uint64_t arrayType;
+    if (!r->input().read(&arrayType)) {
+      return false;
+    }
+    uint64_t nelems = data;
+    return r->readTypedArray(arrayType, nelems, vp);
+  }
+
+  if (tag == SCTAG_TYPED_ARRAY_OBJECT) {
+    // The current version stores the array type in |data| and the length
+    // (nelems) separately to support large TypedArrays.
+    uint32_t arrayType = data;
+    uint64_t nelems;
+    if (!r->input().read(&nelems)) {
+      return false;
+    }
+    return r->readTypedArray(arrayType, nelems, vp);
+  }
+
+  JS_ReportErrorNumberASCII(r->context(), GetErrorMessage, nullptr,
+                            JSMSG_SC_BAD_SERIALIZED_DATA,
+                            "expected type array");
+  return false;
+}
+
+JS_PUBLIC_API bool JS_WriteUint32Pair(JSStructuredCloneWriter* w, uint32_t tag,
+                                      uint32_t data) {
+  return w->output().writePair(tag, data);
+}
+
+JS_PUBLIC_API bool JS_WriteBytes(JSStructuredCloneWriter* w, const void* p,
+                                 size_t len) {
+  return w->output().writeBytes(p, len);
+}
+
+JS_PUBLIC_API bool JS_WriteString(JSStructuredCloneWriter* w,
+                                  HandleString str) {
+  return w->writeString(SCTAG_STRING, str);
+}
+
+JS_PUBLIC_API bool JS_WriteDouble(JSStructuredCloneWriter* w, double v) {
+  return w->output().writeDouble(v);
+}
+
+JS_PUBLIC_API bool JS_WriteTypedArray(JSStructuredCloneWriter* w,
+                                      HandleValue v) {
+  MOZ_ASSERT(v.isObject());
+  w->context()->check(v);
+  RootedObject obj(w->context(), &v.toObject());
+
+  // startWrite can write everything, thus we should check here
+  // and report error if the user passes a wrong type.
+  if (!obj->canUnwrapAs<TypedArrayObject>()) {
+    ReportAccessDenied(w->context());
+    return false;
+  }
+
+  // We should use startWrite instead of writeTypedArray, because
+  // typed array is an object, we should add it to the |memory|
+  // (allObjs) list. Directly calling writeTypedArray won't add it.
+  return w->startWrite(v);
+}
+
+JS_PUBLIC_API bool JS_ObjectNotWritten(JSStructuredCloneWriter* w,
+                                       HandleObject obj) {
+  w->memory.remove(w->memory.lookup(obj));
+
+  return true;
+}
+
+JS_PUBLIC_API JS::StructuredCloneScope JS_GetStructuredCloneScope(
+    JSStructuredCloneWriter* w) {
+  return w->output().scope();
+}