gecko-dev/js/public/StructuredClone.h

/* -*- 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/. */

#ifndef js_StructuredClone_h
#define js_StructuredClone_h

#include "mozilla/Attributes.h"
#include "mozilla/BufferList.h"
#include "mozilla/MemoryReporting.h"

#include <stdint.h>
#include <utility>

#include "jstypes.h"

#include "js/AllocPolicy.h"
#include "js/RootingAPI.h"
#include "js/TypeDecls.h"
#include "js/Vector.h"

/*
 * API for safe passing of structured data, HTML 2018 Feb 21 section 2.7.
 * <https://html.spec.whatwg.org/multipage/structured-data.html>
 *
 * This is a serialization scheme for JS values, somewhat like JSON. It
 * preserves some aspects of JS objects (strings, numbers, own data properties
 * with string keys, array elements) but not others (methods, getters and
 * setters, prototype chains). Unlike JSON, structured data:
 *
 * -   can contain cyclic references.
 *
 * -   handles Maps, Sets, and some other object types.
 *
 * -   supports *transferring* objects of certain types from one realm to
 *     another, rather than cloning them.
 *
 * -   is specified by a living standard, and continues to evolve.
 *
 * -   is encoded in a nonstandard binary format, and is never exposed to Web
 *     content in its serialized form. It's used internally by the browser to
 *     send data from one thread/realm/domain to another, not across the
 *     network.
 */

struct JSStructuredCloneReader;
struct JSStructuredCloneWriter;

/**
 * The structured-clone serialization format version number.
 *
 * When serialized data is stored as bytes, e.g. in your Firefox profile, later
 * versions of the engine may have to read it. When you upgrade Firefox, we
 * don't crawl through your whole profile converting all saved data from the
 * previous version of the serialization format to the latest version. So it is
 * normal to have data in old formats stored in your profile.
 *
 * The JS engine can *write* data only in the current format version.
 *
 * It can *read* any data written in the current version, and data written for
 * DifferentProcess scope in earlier versions.
 *
 *
 * ## When to bump this version number
 *
 * When making a change so drastic that the JS engine needs to know whether
 * it's reading old or new serialized data in order to handle both correctly,
 * increment this version number. Make sure the engine can still read all
 * old data written with previous versions.
 *
 * If StructuredClone.cpp doesn't contain code that distinguishes between
 * version 8 and version 9, there should not be a version 9.
 *
 * Do not increment for changes that only affect SameProcess encoding.
 *
 * Increment only for changes that would otherwise break old serialized data.
 * Do not increment for new data types. (Rationale: Modulo bugs, older versions
 * of the JS engine can already correctly throw errors when they encounter new,
 * unrecognized features. A version number bump does not actually help them.)
 */
#define JS_STRUCTURED_CLONE_VERSION 8

namespace JS {

/**
 * Indicates the "scope of validity" of serialized data.
 *
 * Writing plain JS data produces an array of bytes that can be copied and
 * read in another process or whatever. The serialized data is Plain Old Data.
 * However, HTML also supports `Transferable` objects, which, when cloned, can
 * be moved from the source object into the clone, like when you take a
 * photograph of someone and it steals their soul.
 * See <https://developer.mozilla.org/en-US/docs/Web/API/Transferable>.
 * We support cloning and transferring objects of many types.
 *
 * For example, when we transfer an ArrayBuffer (within a process), we "detach"
 * the ArrayBuffer, embed the raw buffer pointer in the serialized data, and
 * later install it in a new ArrayBuffer in the destination realm. Ownership
 * of that buffer memory is transferred from the original ArrayBuffer to the
 * serialized data and then to the clone.
 *
 * This only makes sense within a single address space. When we transfer an
 * ArrayBuffer to another process, the contents of the buffer must be copied
 * into the serialized data. (The original ArrayBuffer is still detached,
 * though, for consistency; in some cases the caller shouldn't know or care if
 * the recipient is in the same process.)
 *
 * ArrayBuffers are actually a lucky case; some objects (like MessagePorts)
 * can't reasonably be stored by value in serialized data -- it's pointers or
 * nothing.
 *
 * So there is a tradeoff between scope of validity -- how far away the
 * serialized data may be sent and still make sense -- and efficiency or
 * features. The read and write algorithms therefore take an argument of this
 * type, allowing the user to control those trade-offs.
 */
enum class StructuredCloneScope : uint32_t {
  /**
   * The most restrictive scope, with greatest efficiency and features.
   *
   * When writing, this means: The caller promises that the serialized data
   * will **not** be shipped off to a different process or stored in a
   * database. However, it may be shipped to another thread. It's OK to
   * produce serialized data that contains pointers to data that is safe to
   * send across threads, such as array buffers. In Rust terms, the
   * serialized data will be treated as `Send` but not `Copy`.
   *
   * When reading, this means: Accept transferred objects and buffers
   * (pointers). The caller promises that the serialized data was written
   * using this API (otherwise, the serialized data may contain bogus
   * pointers, leading to undefined behavior).
   *
   * Starts from 1 because there used to be a SameProcessSameThread enum value
   * of 0 and these values are encoded into the structured serialization format
   * as part of the SCTAG_HEADER, and IndexedDB persists the representation to
   * disk.
   */
  SameProcess = 1,

  /**
   * When writing, this means we're writing for an audience in a different
   * process. Produce serialized data that can be sent to other processes,
   * bitwise copied, or even stored as bytes in a database and read by later
   * versions of Firefox years from now. The HTML5 spec refers to this as
   * "ForStorage" as in StructuredSerializeForStorage, though we use
   * DifferentProcess for IPC as well as storage.
   *
   * Transferable objects are limited to ArrayBuffers, whose contents are
   * copied into the serialized data (rather than just writing a pointer).
   *
   * When reading, this means: Do not accept pointers.
   */
  DifferentProcess,

  /**
   * Handle a backwards-compatibility case with IndexedDB (bug 1434308): when
   * reading, this means to treat legacy SameProcess data as if it were
   * DifferentProcess.
   *
   * Do not use this for writing; use DifferentProcess instead.
   */
  DifferentProcessForIndexedDB,

  /**
   * Existing code wants to be able to create an uninitialized
   * JSStructuredCloneData without knowing the scope, then populate it with
   * data (at which point the scope *is* known.)
   */
  Unassigned,

  /**
   * This scope is used when the deserialization context is unknown. When
   * writing, DifferentProcess or SameProcess scope is chosen based on the
   * nature of the object.
   */
  UnknownDestination,
};

/** Values used to describe the ownership individual Transferables.
 *
 * Note that these *can* show up in DifferentProcess clones, since
 * DifferentProcess ArrayBuffers can be Transferred. In that case, this will
 * distinguish the specific ownership mechanism: is it a malloc pointer or a
 * memory mapping? */
enum TransferableOwnership {
  /** Transferable data has not been filled in yet. */
  SCTAG_TMO_UNFILLED = 0,

  /** Structured clone buffer does not yet own the data. */
  SCTAG_TMO_UNOWNED = 1,

  /** All enum values at least this large are owned by the clone buffer. */
  SCTAG_TMO_FIRST_OWNED = 2,

  /** Data is a pointer that can be freed. */
  SCTAG_TMO_ALLOC_DATA = SCTAG_TMO_FIRST_OWNED,

  /** Data is a memory mapped pointer. */
  SCTAG_TMO_MAPPED_DATA = 3,

  /**
   * Data is embedding-specific. The engine can free it by calling the
   * freeTransfer op. */
  SCTAG_TMO_CUSTOM = 4,

  /**
   * Same as SCTAG_TMO_CUSTOM, but the embedding can also use
   * SCTAG_TMO_USER_MIN and greater, up to 2^32-1, to distinguish specific
   * ownership variants.
   */
  SCTAG_TMO_USER_MIN
};

class CloneDataPolicy {
  bool allowIntraClusterClonableSharedObjects_;
  bool allowSharedMemoryObjects_;

 public:
  // The default is to deny all policy-controlled aspects.

  CloneDataPolicy()
      : allowIntraClusterClonableSharedObjects_(false),
        allowSharedMemoryObjects_(false) {}

  // SharedArrayBuffers and WASM modules can only be cloned intra-process
  // because the shared memory areas are allocated in process-private memory or
  // because there are security issues of sharing them cross agent clusters.
  // y default, we don't allow shared-memory and intra-cluster objects. Clients
  // should therefore enable these 2 clone features when needed.

  void allowIntraClusterClonableSharedObjects() {
    allowIntraClusterClonableSharedObjects_ = true;
  }
  bool areIntraClusterClonableSharedObjectsAllowed() const {
    return allowIntraClusterClonableSharedObjects_;
  }

  void allowSharedMemoryObjects() { allowSharedMemoryObjects_ = true; }
  bool areSharedMemoryObjectsAllowed() const {
    return allowSharedMemoryObjects_;
  }
};

} /* namespace JS */

/**
 * Read structured data from the reader r. This hook is used to read a value
 * previously serialized by a call to the WriteStructuredCloneOp hook.
 *
 * tag and data are the pair of uint32_t values from the header. The callback
 * may use the JS_Read* APIs to read any other relevant parts of the object
 * from the reader r. closure is any value passed to the JS_ReadStructuredClone
 * function. Return the new object on success, nullptr on error/exception.
 */
typedef JSObject* (*ReadStructuredCloneOp)(
    JSContext* cx, JSStructuredCloneReader* r,
    const JS::CloneDataPolicy& cloneDataPolicy, uint32_t tag, uint32_t data,
    void* closure);

/**
 * Structured data serialization hook. The engine can write primitive values,
 * Objects, Arrays, Dates, RegExps, TypedArrays, ArrayBuffers, Sets, Maps,
 * and SharedTypedArrays. Any other type of object requires application support.
 * This callback must first use the JS_WriteUint32Pair API to write an object
 * header, passing a value greater than JS_SCTAG_USER to the tag parameter.
 * Then it can use the JS_Write* APIs to write any other relevant parts of
 * the value v to the writer w. closure is any value passed to the
 * JS_WriteStructuredClone function.
 *
 * Return true on success, false on error/exception.
 */
typedef bool (*WriteStructuredCloneOp)(JSContext* cx,
                                       JSStructuredCloneWriter* w,
                                       JS::HandleObject obj,
                                       bool* sameProcessScopeRequired,
                                       void* closure);

/**
 * This is called when JS_WriteStructuredClone is given an invalid transferable.
 * To follow HTML5, the application must throw a DATA_CLONE_ERR DOMException
 * with error set to one of the JS_SCERR_* values.
 */
typedef void (*StructuredCloneErrorOp)(JSContext* cx, uint32_t errorid,
                                       void* closure, const char* errorMessage);

/**
 * This is called when JS_ReadStructuredClone receives a transferable object
 * not known to the engine. If this hook does not exist or returns false, the
 * JS engine calls the reportError op if set, otherwise it throws a
 * DATA_CLONE_ERR DOM Exception. This method is called before any other
 * callback and must return a non-null object in returnObject on success.
 */
typedef bool (*ReadTransferStructuredCloneOp)(
    JSContext* cx, JSStructuredCloneReader* r, uint32_t tag, void* content,
    uint64_t extraData, void* closure, JS::MutableHandleObject returnObject);

/**
 * Called when JS_WriteStructuredClone receives a transferable object not
 * handled by the engine. If this hook does not exist or returns false, the JS
 * engine will call the reportError hook or fall back to throwing a
 * DATA_CLONE_ERR DOM Exception. This method is called before any other
 * callback.
 *
 *  tag: indicates what type of transferable this is. Must be greater than
 *       0xFFFF0201 (value of the internal SCTAG_TRANSFER_MAP_PENDING_ENTRY)
 *
 *  ownership: see TransferableOwnership, above. Used to communicate any needed
 *       ownership info to the FreeTransferStructuredCloneOp.
 *
 *  content, extraData: what the ReadTransferStructuredCloneOp will receive
 */
typedef bool (*TransferStructuredCloneOp)(JSContext* cx,
                                          JS::Handle<JSObject*> obj,
                                          void* closure,
                                          // Output:
                                          uint32_t* tag,
                                          JS::TransferableOwnership* ownership,
                                          void** content, uint64_t* extraData);

/**
 * Called when freeing a transferable handled by the embedding. Note that it
 * should never trigger a garbage collection (and will assert in a
 * debug build if it does.)
 *
 * This callback will be used to release ownership in three situations:
 *
 * 1. During serialization: an object is Transferred from, then an error is
 *    encountered later and the incomplete serialization is discarded.
 *
 * 2. During deserialization: before an object is Transferred to, an error
 *    is encountered and the incompletely deserialized clone is discarded.
 *
 * 3. Serialized data that includes Transferring is never deserialized (eg when
 *    the receiver disappears before reading in the message), and the clone data
 * is destroyed.
 *
 */
typedef void (*FreeTransferStructuredCloneOp)(
    uint32_t tag, JS::TransferableOwnership ownership, void* content,
    uint64_t extraData, void* closure);

/**
 * Called when the transferring objects are checked. If this function returns
 * false, the serialization ends throwing a DataCloneError exception.
 */
typedef bool (*CanTransferStructuredCloneOp)(JSContext* cx,
                                             JS::Handle<JSObject*> obj,
                                             bool* sameProcessScopeRequired,
                                             void* closure);

/**
 * Called when a SharedArrayBuffer (including one owned by a Wasm memory object)
 * has been processed in context `cx` by structured cloning.  If `receiving` is
 * true then the SAB has been received from a channel and a new SAB object has
 * been created; if false then an existing SAB has been serialized onto a
 * channel.
 *
 * If the callback returns false then the clone operation (read or write) will
 * signal a failure.
 */
typedef bool (*SharedArrayBufferClonedOp)(JSContext* cx, bool receiving,
                                          void* closure);

struct JSStructuredCloneCallbacks {
  ReadStructuredCloneOp read;
  WriteStructuredCloneOp write;
  StructuredCloneErrorOp reportError;
  ReadTransferStructuredCloneOp readTransfer;
  TransferStructuredCloneOp writeTransfer;
  FreeTransferStructuredCloneOp freeTransfer;
  CanTransferStructuredCloneOp canTransfer;
  SharedArrayBufferClonedOp sabCloned;
};

enum OwnTransferablePolicy {
  /**
   * The buffer owns any Transferables that it might contain, and should
   * properly release them upon destruction.
   */
  OwnsTransferablesIfAny,

  /**
   * Do not free any Transferables within this buffer when deleting it. This
   * is used to mark a clone buffer as containing data from another process,
   * and so it can't legitimately contain pointers. If the buffer claims to
   * have transferables, it's a bug or an attack. This is also used for
   * abandon(), where a buffer still contains raw data but the ownership has
   * been given over to some other entity.
   */
  IgnoreTransferablesIfAny,

  /**
   * A buffer that cannot contain Transferables at all. This usually means
   * the buffer is empty (not yet filled in, or having been cleared).
   */
  NoTransferables
};

namespace js {
class SharedArrayRawBuffer;

class SharedArrayRawBufferRefs {
 public:
  SharedArrayRawBufferRefs() = default;
  SharedArrayRawBufferRefs(SharedArrayRawBufferRefs&& other) = default;
  SharedArrayRawBufferRefs& operator=(SharedArrayRawBufferRefs&& other);
  ~SharedArrayRawBufferRefs();

  [[nodiscard]] bool acquire(JSContext* cx, SharedArrayRawBuffer* rawbuf);
  [[nodiscard]] bool acquireAll(JSContext* cx,
                                const SharedArrayRawBufferRefs& that);
  void takeOwnership(SharedArrayRawBufferRefs&&);
  void releaseAll();

 private:
  js::Vector<js::SharedArrayRawBuffer*, 0, js::SystemAllocPolicy> refs_;
};

template <typename T, typename AllocPolicy>
struct BufferIterator;
}  // namespace js

/**
 * JSStructuredCloneData represents structured clone data together with the
 * information needed to read/write/transfer/free the records within it, in the
 * form of a set of callbacks.
 */
class MOZ_NON_MEMMOVABLE JS_PUBLIC_API JSStructuredCloneData {
 public:
  using BufferList = mozilla::BufferList<js::SystemAllocPolicy>;
  using Iterator = BufferList::IterImpl;

 private:
  static const size_t kStandardCapacity = 4096;

  BufferList bufList_;

  // The (address space, thread) scope within which this clone is valid. Note
  // that this must be either set during construction, or start out as
  // Unassigned and transition once to something else.
  JS::StructuredCloneScope scope_;

  const JSStructuredCloneCallbacks* callbacks_ = nullptr;
  void* closure_ = nullptr;
  OwnTransferablePolicy ownTransferables_ =
      OwnTransferablePolicy::NoTransferables;
  js::SharedArrayRawBufferRefs refsHeld_;

  friend struct JSStructuredCloneWriter;
  friend class JS_PUBLIC_API JSAutoStructuredCloneBuffer;
  template <typename T, typename AllocPolicy>
  friend struct js::BufferIterator;

 public:
  // The constructor must be infallible but SystemAllocPolicy is not, so both
  // the initial size and initial capacity of the BufferList must be zero.
  explicit JSStructuredCloneData(JS::StructuredCloneScope scope)
      : bufList_(0, 0, kStandardCapacity, js::SystemAllocPolicy()),
        scope_(scope),
        callbacks_(nullptr),
        closure_(nullptr),
        ownTransferables_(OwnTransferablePolicy::NoTransferables) {}

  // Steal the raw data from a BufferList. In this case, we don't know the
  // scope and none of the callback info is assigned yet.
  JSStructuredCloneData(BufferList&& buffers, JS::StructuredCloneScope scope)
      : bufList_(std::move(buffers)),
        scope_(scope),
        callbacks_(nullptr),
        closure_(nullptr),
        ownTransferables_(OwnTransferablePolicy::NoTransferables) {}
  MOZ_IMPLICIT JSStructuredCloneData(BufferList&& buffers)
      : JSStructuredCloneData(std::move(buffers),
                              JS::StructuredCloneScope::Unassigned) {}
  JSStructuredCloneData(JSStructuredCloneData&& other) = default;
  JSStructuredCloneData& operator=(JSStructuredCloneData&& other) = default;
  ~JSStructuredCloneData();

  void setCallbacks(const JSStructuredCloneCallbacks* callbacks, void* closure,
                    OwnTransferablePolicy policy) {
    callbacks_ = callbacks;
    closure_ = closure;
    ownTransferables_ = policy;
  }

  [[nodiscard]] bool Init(size_t initialCapacity = 0) {
    return bufList_.Init(0, initialCapacity);
  }

  JS::StructuredCloneScope scope() const {
    if (scope_ == JS::StructuredCloneScope::UnknownDestination) {
      return JS::StructuredCloneScope::DifferentProcess;
    }
    return scope_;
  }

  void sameProcessScopeRequired() {
    if (scope_ == JS::StructuredCloneScope::UnknownDestination) {
      scope_ = JS::StructuredCloneScope::SameProcess;
    }
  }

  void initScope(JS::StructuredCloneScope newScope) {
    MOZ_ASSERT(Size() == 0, "initScope() of nonempty JSStructuredCloneData");
    if (scope() != JS::StructuredCloneScope::Unassigned) {
      MOZ_ASSERT(scope() == newScope,
                 "Cannot change scope after it has been initialized");
    }
    scope_ = newScope;
  }

  size_t Size() const { return bufList_.Size(); }

  const Iterator Start() const { return bufList_.Iter(); }

  [[nodiscard]] bool Advance(Iterator& iter, size_t distance) const {
    return iter.AdvanceAcrossSegments(bufList_, distance);
  }

  [[nodiscard]] bool ReadBytes(Iterator& iter, char* buffer,
                               size_t size) const {
    return bufList_.ReadBytes(iter, buffer, size);
  }

  // Append new data to the end of the buffer.
  [[nodiscard]] bool AppendBytes(const char* data, size_t size) {
    MOZ_ASSERT(scope() != JS::StructuredCloneScope::Unassigned);
    return bufList_.WriteBytes(data, size);
  }

  // Update data stored within the existing buffer. There must be at least
  // 'size' bytes between the position of 'iter' and the end of the buffer.
  [[nodiscard]] bool UpdateBytes(Iterator& iter, const char* data,
                                 size_t size) const {
    MOZ_ASSERT(scope() != JS::StructuredCloneScope::Unassigned);
    while (size > 0) {
      size_t remaining = iter.RemainingInSegment();
      size_t nbytes = std::min(remaining, size);
      memcpy(iter.Data(), data, nbytes);
      data += nbytes;
      size -= nbytes;
      iter.Advance(bufList_, nbytes);
    }
    return true;
  }

  char* AllocateBytes(size_t maxSize, size_t* size) {
    return bufList_.AllocateBytes(maxSize, size);
  }

  void Clear() {
    discardTransferables();
    bufList_.Clear();
  }

  // Return a new read-only JSStructuredCloneData that "borrows" the contents
  // of |this|. Its lifetime should not exceed the donor's. This is only
  // allowed for DifferentProcess clones, so finalization of the borrowing
  // clone will do nothing.
  JSStructuredCloneData Borrow(Iterator& iter, size_t size,
                               bool* success) const {
    MOZ_ASSERT(scope() == JS::StructuredCloneScope::DifferentProcess);
    return JSStructuredCloneData(
        bufList_.Borrow<js::SystemAllocPolicy>(iter, size, success), scope());
  }

  // Iterate over all contained data, one BufferList segment's worth at a
  // time, and invoke the given FunctionToApply with the data pointer and
  // size. The function should return a bool value, and this loop will exit
  // with false if the function ever returns false.
  template <typename FunctionToApply>
  bool ForEachDataChunk(FunctionToApply&& function) const {
    Iterator iter = bufList_.Iter();
    while (!iter.Done()) {
      if (!function(iter.Data(), iter.RemainingInSegment())) {
        return false;
      }
      iter.Advance(bufList_, iter.RemainingInSegment());
    }
    return true;
  }

  // Append the entire contents of other's bufList_ to our own.
  [[nodiscard]] bool Append(const JSStructuredCloneData& other) {
    MOZ_ASSERT(scope() == other.scope());
    return other.ForEachDataChunk(
        [&](const char* data, size_t size) { return AppendBytes(data, size); });
  }

  size_t SizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf) {
    return bufList_.SizeOfExcludingThis(mallocSizeOf);
  }

  void discardTransferables();

 private:
  // This internal method exposes the real value of scope_. It's meant to be
  // used only when starting the writing.
  JS::StructuredCloneScope scopeForInternalWriting() const { return scope_; }
};

/**
 * Implements StructuredDeserialize and StructuredDeserializeWithTransfer.
 *
 * Note: If `data` contains transferable objects, it can be read only once.
 */
JS_PUBLIC_API bool JS_ReadStructuredClone(
    JSContext* cx, const JSStructuredCloneData& data, uint32_t version,
    JS::StructuredCloneScope scope, JS::MutableHandleValue vp,
    const JS::CloneDataPolicy& cloneDataPolicy,
    const JSStructuredCloneCallbacks* optionalCallbacks, void* closure);

/**
 * Implements StructuredSerialize, StructuredSerializeForStorage, and
 * StructuredSerializeWithTransfer.
 *
 * Note: If the scope is DifferentProcess then the cloneDataPolicy must deny
 * shared-memory objects, or an error will be signaled if a shared memory object
 * is seen.
 */
JS_PUBLIC_API bool JS_WriteStructuredClone(
    JSContext* cx, JS::HandleValue v, JSStructuredCloneData* data,
    JS::StructuredCloneScope scope, const JS::CloneDataPolicy& cloneDataPolicy,
    const JSStructuredCloneCallbacks* optionalCallbacks, void* closure,
    JS::HandleValue transferable);

JS_PUBLIC_API bool JS_StructuredCloneHasTransferables(
    JSStructuredCloneData& data, bool* hasTransferable);

JS_PUBLIC_API bool JS_StructuredClone(
    JSContext* cx, JS::HandleValue v, JS::MutableHandleValue vp,
    const JSStructuredCloneCallbacks* optionalCallbacks, void* closure);

/**
 * The C-style API calls to read and write structured clones are fragile --
 * they rely on the caller to properly handle ownership of the clone data, and
 * the handling of the input data as well as the interpretation of the contents
 * of the clone buffer are dependent on the callbacks passed in. If you
 * serialize and deserialize with different callbacks, the results are
 * questionable.
 *
 * JSAutoStructuredCloneBuffer wraps things up in an RAII class for data
 * management, and uses the same callbacks for both writing and reading
 * (serializing and deserializing).
 */
class JS_PUBLIC_API JSAutoStructuredCloneBuffer {
  JSStructuredCloneData data_;
  uint32_t version_;

 public:
  JSAutoStructuredCloneBuffer(JS::StructuredCloneScope scope,
                              const JSStructuredCloneCallbacks* callbacks,
                              void* closure)
      : data_(scope), version_(JS_STRUCTURED_CLONE_VERSION) {
    data_.setCallbacks(callbacks, closure,
                       OwnTransferablePolicy::NoTransferables);
  }

  JSAutoStructuredCloneBuffer(JSAutoStructuredCloneBuffer&& other);
  JSAutoStructuredCloneBuffer& operator=(JSAutoStructuredCloneBuffer&& other);

  ~JSAutoStructuredCloneBuffer() { clear(); }

  JSStructuredCloneData& data() { return data_; }
  bool empty() const { return !data_.Size(); }

  void clear();

  JS::StructuredCloneScope scope() const { return data_.scope(); }

  /**
   * Adopt some memory. It will be automatically freed by the destructor.
   * data must have been allocated by the JS engine (e.g., extracted via
   * JSAutoStructuredCloneBuffer::steal).
   */
  void adopt(JSStructuredCloneData&& data,
             uint32_t version = JS_STRUCTURED_CLONE_VERSION,
             const JSStructuredCloneCallbacks* callbacks = nullptr,
             void* closure = nullptr);

  /**
   * Release ownership of the buffer and assign it and ownership of it to
   * `data`.
   */
  void giveTo(JSStructuredCloneData* data);

  bool read(JSContext* cx, JS::MutableHandleValue vp,
            const JS::CloneDataPolicy& cloneDataPolicy = JS::CloneDataPolicy(),
            const JSStructuredCloneCallbacks* optionalCallbacks = nullptr,
            void* closure = nullptr);

  bool write(JSContext* cx, JS::HandleValue v,
             const JSStructuredCloneCallbacks* optionalCallbacks = nullptr,
             void* closure = nullptr);

  bool write(JSContext* cx, JS::HandleValue v, JS::HandleValue transferable,
             const JS::CloneDataPolicy& cloneDataPolicy,
             const JSStructuredCloneCallbacks* optionalCallbacks = nullptr,
             void* closure = nullptr);

  size_t sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf) {
    return data_.SizeOfExcludingThis(mallocSizeOf);
  }

  size_t sizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf) {
    return mallocSizeOf(this) + sizeOfExcludingThis(mallocSizeOf);
  }

 private:
  // Copy and assignment are not supported.
  JSAutoStructuredCloneBuffer(const JSAutoStructuredCloneBuffer& other) =
      delete;
  JSAutoStructuredCloneBuffer& operator=(
      const JSAutoStructuredCloneBuffer& other) = delete;
};

// The range of tag values the application may use for its own custom object
// types.
#define JS_SCTAG_USER_MIN ((uint32_t)0xFFFF8000)
#define JS_SCTAG_USER_MAX ((uint32_t)0xFFFFFFFF)

#define JS_SCERR_RECURSION 0
#define JS_SCERR_TRANSFERABLE 1
#define JS_SCERR_DUP_TRANSFERABLE 2
#define JS_SCERR_UNSUPPORTED_TYPE 3
#define JS_SCERR_SHMEM_TRANSFERABLE 4
#define JS_SCERR_TYPED_ARRAY_DETACHED 5
#define JS_SCERR_WASM_NO_TRANSFER 6
#define JS_SCERR_NOT_CLONABLE 7
#define JS_SCERR_NOT_CLONABLE_WITH_COOP_COEP 8

JS_PUBLIC_API bool JS_ReadUint32Pair(JSStructuredCloneReader* r, uint32_t* p1,
                                     uint32_t* p2);

JS_PUBLIC_API bool JS_ReadBytes(JSStructuredCloneReader* r, void* p,
                                size_t len);

JS_PUBLIC_API bool JS_ReadString(JSStructuredCloneReader* r,
                                 JS::MutableHandleString str);

JS_PUBLIC_API bool JS_ReadDouble(JSStructuredCloneReader* r, double* v);

JS_PUBLIC_API bool JS_ReadTypedArray(JSStructuredCloneReader* r,
                                     JS::MutableHandleValue vp);

JS_PUBLIC_API bool JS_WriteUint32Pair(JSStructuredCloneWriter* w, uint32_t tag,
                                      uint32_t data);

JS_PUBLIC_API bool JS_WriteBytes(JSStructuredCloneWriter* w, const void* p,
                                 size_t len);

JS_PUBLIC_API bool JS_WriteString(JSStructuredCloneWriter* w,
                                  JS::HandleString str);

JS_PUBLIC_API bool JS_WriteDouble(JSStructuredCloneWriter* w, double v);

JS_PUBLIC_API bool JS_WriteTypedArray(JSStructuredCloneWriter* w,
                                      JS::HandleValue v);

JS_PUBLIC_API bool JS_ObjectNotWritten(JSStructuredCloneWriter* w,
                                       JS::HandleObject obj);

JS_PUBLIC_API JS::StructuredCloneScope JS_GetStructuredCloneScope(
    JSStructuredCloneWriter* w);

#endif /* js_StructuredClone_h */