gecko-dev/ipc/glue/IPCMessageUtils.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 __IPC_GLUE_IPCMESSAGEUTILS_H__
#define __IPC_GLUE_IPCMESSAGEUTILS_H__

#include "base/process_util.h"
#include "chrome/common/ipc_message_utils.h"

#include "mozilla/ArrayUtils.h"
#include "mozilla/Attributes.h"
#include "mozilla/DebugOnly.h"
#include "mozilla/dom/ipc/StructuredCloneData.h"
#include "mozilla/EnumSet.h"
#include "mozilla/Maybe.h"
#include "mozilla/net/WebSocketFrame.h"
#include "mozilla/TimeStamp.h"
#ifdef XP_WIN
#include "mozilla/TimeStamp_windows.h"
#endif
#include "mozilla/TypeTraits.h"
#include "mozilla/IntegerTypeTraits.h"

#include <stdint.h>

#include "nsExceptionHandler.h"
#include "nsID.h"
#include "nsIWidget.h"
#include "nsMemory.h"
#include "nsString.h"
#include "nsTArray.h"
#include "js/StructuredClone.h"
#include "nsCSSPropertyID.h"

#ifdef _MSC_VER
#pragma warning( disable : 4800 )
#endif

#if !defined(OS_POSIX)
// This condition must be kept in sync with the one in
// ipc_message_utils.h, but this dummy definition of
// base::FileDescriptor acts as a static assert that we only get one
// def or the other (or neither, in which case code using
// FileDescriptor fails to build)
namespace base { struct FileDescriptor { }; }
#endif

namespace mozilla {

// This is a cross-platform approximation to HANDLE, which we expect
// to be typedef'd to void* or thereabouts.
typedef uintptr_t WindowsHandle;

// XXX there are out of place and might be generally useful.  Could
// move to nscore.h or something.
struct void_t {
  bool operator==(const void_t&) const { return true; }
};
struct null_t {
  bool operator==(const null_t&) const { return true; }
};

struct SerializedStructuredCloneBuffer final
{
  SerializedStructuredCloneBuffer&
  operator=(const SerializedStructuredCloneBuffer& aOther)
  {
    data.Clear();
    auto iter = aOther.data.Iter();
    while (!iter.Done()) {
      data.WriteBytes(iter.Data(), iter.RemainingInSegment());
      iter.Advance(aOther.data, iter.RemainingInSegment());
    }
    return *this;
  }

  bool
  operator==(const SerializedStructuredCloneBuffer& aOther) const
  {
    // The copy assignment operator and the equality operator are
    // needed by the IPDL generated code. We relied on the copy
    // assignment operator at some places but we never use the
    // equality operator.
    return false;
  }

  JSStructuredCloneData data;
};

} // namespace mozilla

namespace IPC {

/**
 * Maximum size, in bytes, of a single IPC message.
 */
static const uint32_t MAX_MESSAGE_SIZE = 65536;

/**
 * Generic enum serializer.
 *
 * Consider using the specializations below, such as ContiguousEnumSerializer.
 *
 * This is a generic serializer for any enum type used in IPDL.
 * Programmers can define ParamTraits<E> for enum type E by deriving
 * EnumSerializer<E, MyEnumValidator> where MyEnumValidator is a struct
 * that has to define a static IsLegalValue function returning whether
 * a given value is a legal value of the enum type at hand.
 *
 * \sa https://developer.mozilla.org/en/IPDL/Type_Serialization
 */
template <typename E, typename EnumValidator>
struct EnumSerializer {
  typedef E paramType;
  typedef typename mozilla::UnsignedStdintTypeForSize<sizeof(paramType)>::Type
          uintParamType;

  static void Write(Message* aMsg, const paramType& aValue) {
    MOZ_RELEASE_ASSERT(EnumValidator::IsLegalValue(aValue));
    WriteParam(aMsg, uintParamType(aValue));
  }

  static bool Read(const Message* aMsg, PickleIterator* aIter, paramType* aResult) {
    uintParamType value;
    if (!ReadParam(aMsg, aIter, &value)) {
      CrashReporter::AnnotateCrashReport(NS_LITERAL_CSTRING("IPCReadErrorReason"),
                                         NS_LITERAL_CSTRING("Bad iter"));
      return false;
    } else if (!EnumValidator::IsLegalValue(paramType(value))) {
      CrashReporter::AnnotateCrashReport(NS_LITERAL_CSTRING("IPCReadErrorReason"),
                                         NS_LITERAL_CSTRING("Illegal value"));
      return false;
    }
    *aResult = paramType(value);
    return true;
  }
};

template <typename E,
          E MinLegal,
          E HighBound>
class ContiguousEnumValidator
{
  // Silence overzealous -Wtype-limits bug in GCC fixed in GCC 4.8:
  // "comparison of unsigned expression >= 0 is always true"
  // http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11856
  template <typename T>
  static bool IsLessThanOrEqual(T a, T b) { return a <= b; }

public:
  static bool IsLegalValue(E e)
  {
    return IsLessThanOrEqual(MinLegal, e) && e < HighBound;
  }
};

template <typename E,
          E MinLegal,
          E MaxLegal>
class ContiguousEnumValidatorInclusive
{
  // Silence overzealous -Wtype-limits bug in GCC fixed in GCC 4.8:
  // "comparison of unsigned expression >= 0 is always true"
  // http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11856
  template <typename T>
  static bool IsLessThanOrEqual(T a, T b) { return a <= b; }

public:
  static bool IsLegalValue(E e)
  {
    return IsLessThanOrEqual(MinLegal, e) && e <= MaxLegal;
  }
};

template <typename E,
          E AllBits>
struct BitFlagsEnumValidator
{
  static bool IsLegalValue(E e)
  {
    return (e & AllBits) == e;
  }
};

/**
 * Specialization of EnumSerializer for enums with contiguous enum values.
 *
 * Provide two values: MinLegal, HighBound. An enum value x will be
 * considered legal if MinLegal <= x < HighBound.
 *
 * For example, following is definition of serializer for enum type FOO.
 * \code
 * enum FOO { FOO_FIRST, FOO_SECOND, FOO_LAST, NUM_FOO };
 *
 * template <>
 * struct ParamTraits<FOO>:
 *     public ContiguousEnumSerializer<FOO, FOO_FIRST, NUM_FOO> {};
 * \endcode
 * FOO_FIRST, FOO_SECOND, and FOO_LAST are valid value.
 */
template <typename E,
          E MinLegal,
          E HighBound>
struct ContiguousEnumSerializer
  : EnumSerializer<E,
                   ContiguousEnumValidator<E, MinLegal, HighBound>>
{};

/**
 * This is similar to ContiguousEnumSerializer, but the last template
 * parameter is expected to be the highest legal value, rather than a
 * sentinel value. This is intended to support enumerations that don't
 * have sentinel values.
 */
template <typename E,
          E MinLegal,
          E MaxLegal>
struct ContiguousEnumSerializerInclusive
  : EnumSerializer<E,
                   ContiguousEnumValidatorInclusive<E, MinLegal, MaxLegal>>
{};

/**
 * Specialization of EnumSerializer for enums representing bit flags.
 *
 * Provide one value: AllBits. An enum value x will be
 * considered legal if (x & AllBits) == x;
 *
 * Example:
 * \code
 * enum FOO {
 *   FOO_FIRST =  1 << 0,
 *   FOO_SECOND = 1 << 1,
 *   FOO_LAST =   1 << 2,
 *   ALL_BITS =   (1 << 3) - 1
 * };
 *
 * template <>
 * struct ParamTraits<FOO>:
 *     public BitFlagsEnumSerializer<FOO, FOO::ALL_BITS> {};
 * \endcode
 */
template <typename E,
          E AllBits>
struct BitFlagsEnumSerializer
  : EnumSerializer<E,
                   BitFlagsEnumValidator<E, AllBits>>
{};

/**
 * A helper class for serializing plain-old data (POD) structures.
 * The memory representation of the structure is written to and read from
 * the serialized stream directly, without individual processing of the
 * structure's members.
 *
 * Derive ParamTraits<T> from PlainOldDataSerializer<T> if T is POD.
 *
 * Note: For POD structures with enumeration fields, this will not do
 *   validation of the enum values the way serializing the fields
 *   individually would. Prefer serializing the fields individually
 *   in such cases.
 */
template <typename T>
struct PlainOldDataSerializer
{
  // TODO: Once the mozilla::IsPod trait is in good enough shape (bug 900042),
  //       static_assert that mozilla::IsPod<T>::value is true.
  typedef T paramType;

  static void Write(Message* aMsg, const paramType& aParam) {
    aMsg->WriteBytes(&aParam, sizeof(aParam));
  }

  static bool Read(const Message* aMsg, PickleIterator* aIter, paramType* aResult) {
    return aMsg->ReadBytesInto(aIter, aResult, sizeof(paramType));
  }
};

/**
 * A helper class for serializing empty structs. Since the struct is empty there
 * is nothing to write, and a priori we know the result of the read.
 */
template <typename T>
struct EmptyStructSerializer
{
  typedef T paramType;

  static void Write(Message* aMsg, const paramType& aParam) {}

  static bool Read(const Message* aMsg, PickleIterator* aIter, paramType* aResult) {
    *aResult = {};
    return true;
  }
};

template<>
struct ParamTraits<int8_t>
{
  typedef int8_t paramType;

  static void Write(Message* aMsg, const paramType& aParam)
  {
    aMsg->WriteBytes(&aParam, sizeof(aParam));
  }

  static bool Read(const Message* aMsg, PickleIterator* aIter, paramType* aResult)
  {
    return aMsg->ReadBytesInto(aIter, aResult, sizeof(*aResult));
  }
};

template<>
struct ParamTraits<uint8_t>
{
  typedef uint8_t paramType;

  static void Write(Message* aMsg, const paramType& aParam)
  {
    aMsg->WriteBytes(&aParam, sizeof(aParam));
  }

  static bool Read(const Message* aMsg, PickleIterator* aIter, paramType* aResult)
  {
    return aMsg->ReadBytesInto(aIter, aResult, sizeof(*aResult));
  }
};

#if !defined(OS_POSIX)
// See above re: keeping definitions in sync
template<>
struct ParamTraits<base::FileDescriptor>
{
  typedef base::FileDescriptor paramType;
  static void Write(Message* aMsg, const paramType& aParam) {
    MOZ_CRASH("FileDescriptor isn't meaningful on this platform");
  }
  static bool Read(const Message* aMsg, PickleIterator* aIter, paramType* aResult) {
    MOZ_CRASH("FileDescriptor isn't meaningful on this platform");
    return false;
  }
};
#endif  // !defined(OS_POSIX)

template <>
struct ParamTraits<nsACString>
{
  typedef nsACString paramType;

  static void Write(Message* aMsg, const paramType& aParam)
  {
    bool isVoid = aParam.IsVoid();
    aMsg->WriteBool(isVoid);

    if (isVoid)
      // represents a nullptr pointer
      return;

    uint32_t length = aParam.Length();
    WriteParam(aMsg, length);
    aMsg->WriteBytes(aParam.BeginReading(), length);
  }

  static bool Read(const Message* aMsg, PickleIterator* aIter, paramType* aResult)
  {
    bool isVoid;
    if (!aMsg->ReadBool(aIter, &isVoid))
      return false;

    if (isVoid) {
      aResult->SetIsVoid(true);
      return true;
    }

    uint32_t length;
    if (!ReadParam(aMsg, aIter, &length)) {
      return false;
    }
    aResult->SetLength(length);

    return aMsg->ReadBytesInto(aIter, aResult->BeginWriting(), length);
  }

  static void Log(const paramType& aParam, std::wstring* aLog)
  {
    if (aParam.IsVoid())
      aLog->append(L"(NULL)");
    else
      aLog->append(UTF8ToWide(aParam.BeginReading()));
  }
};

template <>
struct ParamTraits<nsAString>
{
  typedef nsAString paramType;

  static void Write(Message* aMsg, const paramType& aParam)
  {
    bool isVoid = aParam.IsVoid();
    aMsg->WriteBool(isVoid);

    if (isVoid)
      // represents a nullptr pointer
      return;

    uint32_t length = aParam.Length();
    WriteParam(aMsg, length);
    aMsg->WriteBytes(aParam.BeginReading(), length * sizeof(char16_t));
  }

  static bool Read(const Message* aMsg, PickleIterator* aIter, paramType* aResult)
  {
    bool isVoid;
    if (!aMsg->ReadBool(aIter, &isVoid))
      return false;

    if (isVoid) {
      aResult->SetIsVoid(true);
      return true;
    }

    uint32_t length;
    if (!ReadParam(aMsg, aIter, &length)) {
      return false;
    }

    aResult->SetLength(length);

    mozilla::CheckedInt<uint32_t> byteLength = mozilla::CheckedInt<uint32_t>(length) * sizeof(char16_t);
    if (!byteLength.isValid()) {
      return false;
    }

    return aMsg->ReadBytesInto(aIter, aResult->BeginWriting(), byteLength.value());
  }

  static void Log(const paramType& aParam, std::wstring* aLog)
  {
    if (aParam.IsVoid())
      aLog->append(L"(NULL)");
    else {
#ifdef WCHAR_T_IS_UTF16
      aLog->append(reinterpret_cast<const wchar_t*>(aParam.BeginReading()));
#else
      uint32_t length = aParam.Length();
      for (uint32_t index = 0; index < length; index++) {
        aLog->push_back(std::wstring::value_type(aParam[index]));
      }
#endif
    }
  }
};

template <>
struct ParamTraits<nsCString> : ParamTraits<nsACString>
{
  typedef nsCString paramType;
};

template <>
struct ParamTraits<nsLiteralCString> : ParamTraits<nsACString>
{
  typedef nsLiteralCString paramType;
};

#ifdef MOZILLA_INTERNAL_API

template<>
struct ParamTraits<nsAutoCString> : ParamTraits<nsCString>
{
  typedef nsAutoCString paramType;
};

#endif  // MOZILLA_INTERNAL_API

template <>
struct ParamTraits<nsString> : ParamTraits<nsAString>
{
  typedef nsString paramType;
};

template <>
struct ParamTraits<nsLiteralString> : ParamTraits<nsAString>
{
  typedef nsLiteralString paramType;
};

template <>
struct ParamTraits<nsDependentSubstring> : ParamTraits<nsAString>
{
  typedef nsDependentSubstring paramType;
};

template <>
struct ParamTraits<nsDependentCSubstring> : ParamTraits<nsACString>
{
  typedef nsDependentCSubstring paramType;
};

#ifdef MOZILLA_INTERNAL_API

template<>
struct ParamTraits<nsAutoString> : ParamTraits<nsString>
{
  typedef nsAutoString paramType;
};

#endif  // MOZILLA_INTERNAL_API

// Pickle::ReadBytes and ::WriteBytes take the length in ints, so we must
// ensure there is no overflow. This returns |false| if it would overflow.
// Otherwise, it returns |true| and places the byte length in |aByteLength|.
bool ByteLengthIsValid(uint32_t aNumElements, size_t aElementSize, int* aByteLength);

// Note: IPDL will sometimes codegen specialized implementations of
// nsTArray serialization and deserialization code in
// implementSpecialArrayPickling(). This is needed when ParamTraits<E>
// is not defined.
template <typename E>
struct ParamTraits<nsTArray<E>>
{
  typedef nsTArray<E> paramType;

  // We write arrays of integer or floating-point data using a single pickling
  // call, rather than writing each element individually.  We deliberately do
  // not use mozilla::IsPod here because it is perfectly reasonable to have
  // a data structure T for which IsPod<T>::value is true, yet also have a
  // ParamTraits<T> specialization.
  static const bool sUseWriteBytes = (mozilla::IsIntegral<E>::value ||
                                      mozilla::IsFloatingPoint<E>::value);

  static void Write(Message* aMsg, const paramType& aParam)
  {
    uint32_t length = aParam.Length();
    WriteParam(aMsg, length);

    if (sUseWriteBytes) {
      int pickledLength = 0;
      MOZ_RELEASE_ASSERT(ByteLengthIsValid(length, sizeof(E), &pickledLength));
      aMsg->WriteBytes(aParam.Elements(), pickledLength);
    } else {
      const E* elems = aParam.Elements();
      for (uint32_t index = 0; index < length; index++) {
        WriteParam(aMsg, elems[index]);
      }
    }
  }

  // This method uses infallible allocation so that an OOM failure will
  // show up as an OOM crash rather than an IPC FatalError.
  static bool Read(const Message* aMsg, PickleIterator* aIter, paramType* aResult)
  {
    uint32_t length;
    if (!ReadParam(aMsg, aIter, &length)) {
      return false;
    }

    if (sUseWriteBytes) {
      int pickledLength = 0;
      if (!ByteLengthIsValid(length, sizeof(E), &pickledLength)) {
        return false;
      }

      E* elements = aResult->AppendElements(length);
      return aMsg->ReadBytesInto(aIter, elements, pickledLength);
    } else {
      aResult->SetCapacity(length);

      for (uint32_t index = 0; index < length; index++) {
        E* element = aResult->AppendElement();
        if (!ReadParam(aMsg, aIter, element)) {
          return false;
        }
      }
      return true;
    }
  }

  static void Log(const paramType& aParam, std::wstring* aLog)
  {
    for (uint32_t index = 0; index < aParam.Length(); index++) {
      if (index) {
        aLog->append(L" ");
      }
      LogParam(aParam[index], aLog);
    }
  }
};

template<typename E>
struct ParamTraits<FallibleTArray<E>>
{
  typedef FallibleTArray<E> paramType;

  static void Write(Message* aMsg, const paramType& aParam)
  {
    WriteParam(aMsg, static_cast<const nsTArray<E>&>(aParam));
  }

  // Deserialize the array infallibly, but return a FallibleTArray.
  static bool Read(const Message* aMsg, PickleIterator* aIter, paramType* aResult)
  {
    nsTArray<E> temp;
    if (!ReadParam(aMsg, aIter, &temp))
      return false;

    aResult->SwapElements(temp);
    return true;
  }

  static void Log(const paramType& aParam, std::wstring* aLog)
  {
    LogParam(static_cast<const nsTArray<E>&>(aParam), aLog);
  }
};

template<typename E, size_t N>
struct ParamTraits<AutoTArray<E, N>> : ParamTraits<nsTArray<E>>
{
  typedef AutoTArray<E, N> paramType;
};

template<>
struct ParamTraits<float>
{
  typedef float paramType;

  static void Write(Message* aMsg, const paramType& aParam)
  {
    aMsg->WriteBytes(&aParam, sizeof(paramType));
  }

  static bool Read(const Message* aMsg, PickleIterator* aIter, paramType* aResult)
  {
    return aMsg->ReadBytesInto(aIter, aResult, sizeof(*aResult));
  }

  static void Log(const paramType& aParam, std::wstring* aLog)
  {
    aLog->append(StringPrintf(L"%g", aParam));
  }
};

template <>
struct ParamTraits<nsCSSPropertyID>
  : public ContiguousEnumSerializer<nsCSSPropertyID,
                                    eCSSProperty_UNKNOWN,
                                    eCSSProperty_COUNT>
{};

template<>
struct ParamTraits<mozilla::void_t>
{
  typedef mozilla::void_t paramType;
  static void Write(Message* aMsg, const paramType& aParam) { }
  static bool
  Read(const Message* aMsg, PickleIterator* aIter, paramType* aResult)
  {
    *aResult = paramType();
    return true;
  }
};

template<>
struct ParamTraits<mozilla::null_t>
{
  typedef mozilla::null_t paramType;
  static void Write(Message* aMsg, const paramType& aParam) { }
  static bool
  Read(const Message* aMsg, PickleIterator* aIter, paramType* aResult)
  {
    *aResult = paramType();
    return true;
  }
};

template<>
struct ParamTraits<nsID>
{
  typedef nsID paramType;

  static void Write(Message* aMsg, const paramType& aParam)
  {
    WriteParam(aMsg, aParam.m0);
    WriteParam(aMsg, aParam.m1);
    WriteParam(aMsg, aParam.m2);
    for (unsigned int i = 0; i < mozilla::ArrayLength(aParam.m3); i++) {
      WriteParam(aMsg, aParam.m3[i]);
    }
  }

  static bool Read(const Message* aMsg, PickleIterator* aIter, paramType* aResult)
  {
    if(!ReadParam(aMsg, aIter, &(aResult->m0)) ||
       !ReadParam(aMsg, aIter, &(aResult->m1)) ||
       !ReadParam(aMsg, aIter, &(aResult->m2)))
      return false;

    for (unsigned int i = 0; i < mozilla::ArrayLength(aResult->m3); i++)
      if (!ReadParam(aMsg, aIter, &(aResult->m3[i])))
        return false;

    return true;
  }

  static void Log(const paramType& aParam, std::wstring* aLog)
  {
    aLog->append(L"{");
    aLog->append(StringPrintf(L"%8.8X-%4.4X-%4.4X-",
                              aParam.m0,
                              aParam.m1,
                              aParam.m2));
    for (unsigned int i = 0; i < mozilla::ArrayLength(aParam.m3); i++)
      aLog->append(StringPrintf(L"%2.2X", aParam.m3[i]));
    aLog->append(L"}");
  }
};

template<>
struct ParamTraits<mozilla::TimeDuration>
{
  typedef mozilla::TimeDuration paramType;
  static void Write(Message* aMsg, const paramType& aParam)
  {
    WriteParam(aMsg, aParam.mValue);
  }
  static bool Read(const Message* aMsg, PickleIterator* aIter, paramType* aResult)
  {
    return ReadParam(aMsg, aIter, &aResult->mValue);
  };
};

template<>
struct ParamTraits<mozilla::TimeStamp>
{
  typedef mozilla::TimeStamp paramType;
  static void Write(Message* aMsg, const paramType& aParam)
  {
    WriteParam(aMsg, aParam.mValue);
  }
  static bool Read(const Message* aMsg, PickleIterator* aIter, paramType* aResult)
  {
    return ReadParam(aMsg, aIter, &aResult->mValue);
  };
};

#ifdef XP_WIN

template<>
struct ParamTraits<mozilla::TimeStampValue>
{
  typedef mozilla::TimeStampValue paramType;
  static void Write(Message* aMsg, const paramType& aParam)
  {
    WriteParam(aMsg, aParam.mGTC);
    WriteParam(aMsg, aParam.mQPC);
    WriteParam(aMsg, aParam.mHasQPC);
    WriteParam(aMsg, aParam.mIsNull);
  }
  static bool Read(const Message* aMsg, PickleIterator* aIter, paramType* aResult)
  {
    return (ReadParam(aMsg, aIter, &aResult->mGTC) &&
            ReadParam(aMsg, aIter, &aResult->mQPC) &&
            ReadParam(aMsg, aIter, &aResult->mHasQPC) &&
            ReadParam(aMsg, aIter, &aResult->mIsNull));
  }
};

#endif

template <>
struct ParamTraits<mozilla::dom::ipc::StructuredCloneData>
{
  typedef mozilla::dom::ipc::StructuredCloneData paramType;

  static void Write(Message* aMsg, const paramType& aParam)
  {
    aParam.WriteIPCParams(aMsg);
  }

  static bool Read(const Message* aMsg, PickleIterator* aIter, paramType* aResult)
  {
    return aResult->ReadIPCParams(aMsg, aIter);
  }

  static void Log(const paramType& aParam, std::wstring* aLog)
  {
    LogParam(aParam.DataLength(), aLog);
  }
};

template <>
struct ParamTraits<mozilla::net::WebSocketFrameData>
{
  typedef mozilla::net::WebSocketFrameData paramType;

  static void Write(Message* aMsg, const paramType& aParam)
  {
    aParam.WriteIPCParams(aMsg);
  }

  static bool Read(const Message* aMsg, PickleIterator* aIter, paramType* aResult)
  {
    return aResult->ReadIPCParams(aMsg, aIter);
  }
};

template <>
struct ParamTraits<JSStructuredCloneData>
{
  typedef JSStructuredCloneData paramType;

  static void Write(Message* aMsg, const paramType& aParam)
  {
    MOZ_ASSERT(!(aParam.Size() % sizeof(uint64_t)));
    WriteParam(aMsg, aParam.Size());
    auto iter = aParam.Iter();
    while (!iter.Done()) {
      aMsg->WriteBytes(iter.Data(), iter.RemainingInSegment(), sizeof(uint64_t));
      iter.Advance(aParam, iter.RemainingInSegment());
    }
  }

  static bool Read(const Message* aMsg, PickleIterator* aIter, paramType* aResult)
  {
    size_t length = 0;
    if (!ReadParam(aMsg, aIter, &length)) {
      return false;
    }
    MOZ_ASSERT(!(length % sizeof(uint64_t)));

    mozilla::BufferList<InfallibleAllocPolicy> buffers(0, 0, 4096);

    // Borrowing is not suitable to use for IPC to hand out data
    // because we often want to store the data somewhere for
    // processing after IPC has released the underlying buffers. One
    // case is PContentChild::SendGetXPCOMProcessAttributes. We can't
    // return a borrowed buffer because the out param outlives the
    // IPDL callback.
    if (length && !aMsg->ExtractBuffers(aIter, length, &buffers, sizeof(uint64_t))) {
      return false;
    }

    bool success;
    mozilla::BufferList<js::SystemAllocPolicy> out =
      buffers.MoveFallible<js::SystemAllocPolicy>(&success);
    if (!success) {
      return false;
    }

    *aResult = JSStructuredCloneData(Move(out));

    return true;
  }
};

template <>
struct ParamTraits<mozilla::SerializedStructuredCloneBuffer>
{
  typedef mozilla::SerializedStructuredCloneBuffer paramType;

  static void Write(Message* aMsg, const paramType& aParam)
  {
    WriteParam(aMsg, aParam.data);
  }

  static bool Read(const Message* aMsg, PickleIterator* aIter, paramType* aResult)
  {
    return ReadParam(aMsg, aIter, &aResult->data);
  }

  static void Log(const paramType& aParam, std::wstring* aLog)
  {
    LogParam(aParam.data.Size(), aLog);
  }
};

template <>
struct ParamTraits<nsIWidget::TouchPointerState>
  : public BitFlagsEnumSerializer<nsIWidget::TouchPointerState,
                                  nsIWidget::TouchPointerState::ALL_BITS>
{
};

template<class T>
struct ParamTraits<mozilla::Maybe<T>>
{
  typedef mozilla::Maybe<T> paramType;

  static void Write(Message* msg, const paramType& param)
  {
    if (param.isSome()) {
      WriteParam(msg, true);
      WriteParam(msg, param.value());
    } else {
      WriteParam(msg, false);
    }
  }

  static bool Read(const Message* msg, PickleIterator* iter, paramType* result)
  {
    bool isSome;
    if (!ReadParam(msg, iter, &isSome)) {
      return false;
    }
    if (isSome) {
      T tmp;
      if (!ReadParam(msg, iter, &tmp)) {
        return false;
      }
      *result = mozilla::Some(mozilla::Move(tmp));
    } else {
      *result = mozilla::Nothing();
    }
    return true;
  }
};

template<typename T>
struct ParamTraits<mozilla::EnumSet<T>>
{
  typedef mozilla::EnumSet<T> paramType;

  static void Write(Message* msg, const paramType& param)
  {
    WriteParam(msg, param.serialize());
  }

  static bool Read(const Message* msg, PickleIterator* iter, paramType* result)
  {
    decltype(result->serialize()) tmp;
    if (ReadParam(msg, iter, &tmp)) {
      result->deserialize(tmp);
      return true;
    }
    return false;
  }
};

template<class... Ts>
struct ParamTraits<mozilla::Variant<Ts...>>
{
  typedef mozilla::Variant<Ts...> paramType;
  using Tag = typename mozilla::detail::VariantTag<Ts...>::Type;

  struct VariantWriter
  {
    Message* msg;

    template<class T>
    void match(const T& t) {
      WriteParam(msg, t);
    }
  };

  static void Write(Message* msg, const paramType& param)
  {
    WriteParam(msg, param.tag);
    param.match(VariantWriter{msg});
  }

  // Because VariantReader is a nested struct, we need the dummy template
  // parameter to avoid making VariantReader<0> an explicit specialization,
  // which is not allowed for a nested class template
  template<size_t N, typename dummy = void>
  struct VariantReader
  {
    using Next = VariantReader<N-1>;

    static bool Read(const Message* msg, PickleIterator* iter,
        Tag tag, paramType* result)
    {
      // Since the VariantReader specializations start at N , we need to
      // subtract one to look at N - 1, the first valid tag.  This means our
      // comparisons are off by 1.  If we get to N = 0 then we have failed to
      // find a match to the tag.
      if (tag == N - 1) {
        // Recall, even though the template parameter is N, we are
        // actually interested in the N - 1 tag.
        typename mozilla::detail::Nth<N - 1, Ts...>::Type val;
        if (ReadParam(msg, iter, &val)) {
          *result = mozilla::AsVariant(val);
          return true;
        }
        return false;
      } else {
        return Next::Read(msg, iter, tag, result);
      }
    }

  }; // VariantReader<N>

  // Since we are conditioning on tag = N - 1 in the preceding specialization,
  // if we get to `VariantReader<0, dummy>` we have failed to find
  // a matching tag.
  template<typename dummy>
  struct VariantReader<0, dummy>
  {
    static bool Read(const Message* msg, PickleIterator* iter,
        Tag tag, paramType* result)
    {
      return false;
    }
  };

  static bool Read(const Message* msg, PickleIterator* iter, paramType* result)
  {
    Tag tag;
    if (ReadParam(msg, iter, &tag)) {
      return VariantReader<sizeof...(Ts)>::Read(msg, iter, tag, result);
    }
    return false;
  }
};

template<typename T>
struct ParamTraits<mozilla::dom::Optional<T>>
{
  typedef mozilla::dom::Optional<T> paramType;

  static void Write(Message* aMsg, const paramType& aParam)
  {
    if (aParam.WasPassed()) {
      WriteParam(aMsg, true);
      WriteParam(aMsg, aParam.Value());
      return;
    }

    WriteParam(aMsg, false);
  }

  static bool Read(const Message* aMsg, PickleIterator* aIter, paramType* aResult)
  {
    bool wasPassed = false;

    if (!ReadParam(aMsg, aIter, &wasPassed)) {
      return false;
    }

    aResult->Reset();

    if (wasPassed) {
      if (!ReadParam(aMsg, aIter, &aResult->Construct())) {
        return false;
      }
    }

    return true;
  }
};

} /* namespace IPC */

#endif /* __IPC_GLUE_IPCMESSAGEUTILS_H__ */