mfbt/MaybeOneOf.h
author Andrew Osmond <aosmond@mozilla.com>
Tue, 24 Apr 2018 13:51:35 -0400
changeset 415325 1173dccd114e1c86047a6f01e341cf1c4ea8fe9c
parent 344349 83c513bf2e8843b445bfc5768bba6190543894bc
child 420757 b54db66223586b4e04f5cb926fccdacf8a176b91
permissions -rw-r--r--
Bug 1444537 - Part 3. Fix how redecode errors could cause animated image state inconsistencies. r=tnikkel We can discard frames from an animated image if the memory footprint exceeds the threshold. This will cause us to redecode frames on demand instead. However decoders can fail to produce the same results on subsequent runs due to differences in memory pressure, etc. If this happens our state can get inconsistent. In particular, if we keep failing on the first frame, we end up in an infinite loop on the decoder thread. Since we don't have the owning image to signal, as we had to release our reference to it after the first pass, we can do little but stop decoding. From the user's perspective, the animation will come to a stop.

/* -*- 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/. */

/*
 * A class storing one of two optional value types that supports in-place lazy
 * construction.
 */

#ifndef mozilla_MaybeOneOf_h
#define mozilla_MaybeOneOf_h

#include "mozilla/Assertions.h"
#include "mozilla/Move.h"
#include "mozilla/OperatorNewExtensions.h"
#include "mozilla/TemplateLib.h"

#include <new> // for placement new
#include <stddef.h> // for size_t

namespace mozilla {

/*
 * MaybeOneOf<T1, T2> is like Maybe, but it supports constructing either T1
 * or T2. When a MaybeOneOf<T1, T2> is constructed, it is |empty()|, i.e.,
 * no value has been constructed and no destructor will be called when the
 * MaybeOneOf<T1, T2> is destroyed. Upon calling |construct<T1>()| or
 * |construct<T2>()|, a T1 or T2 object will be constructed with the given
 * arguments and that object will be destroyed when the owning MaybeOneOf is
 * destroyed.
 *
 * Because MaybeOneOf must be aligned suitable to hold any value stored within
 * it, and because |alignas| requirements don't affect platform ABI with respect
 * to how parameters are laid out in memory, MaybeOneOf can't be used as the
 * type of a function parameter.  Pass MaybeOneOf to functions by pointer or
 * reference instead.
 */
template<class T1, class T2>
class MOZ_NON_PARAM MaybeOneOf
{
  static constexpr size_t StorageAlignment =
    tl::Max<alignof(T1), alignof(T2)>::value;
  static constexpr size_t StorageSize =
    tl::Max<sizeof(T1), sizeof(T2)>::value;

  alignas(StorageAlignment) unsigned char storage[StorageSize];

  // GCC fails due to -Werror=strict-aliasing if |storage| is directly cast to
  // T*.  Indirecting through these functions addresses the problem.
  void* data() { return storage; }
  const void* data() const { return storage; }

  enum State { None, SomeT1, SomeT2 } state;
  template <class T, class Ignored = void> struct Type2State {};

  template <class T>
  T& as()
  {
    MOZ_ASSERT(state == Type2State<T>::result);
    return *static_cast<T*>(data());
  }

  template <class T>
  const T& as() const
  {
    MOZ_ASSERT(state == Type2State<T>::result);
    return *static_cast<const T*>(data());
  }

public:
  MaybeOneOf() : state(None) {}
  ~MaybeOneOf() { destroyIfConstructed(); }

  MaybeOneOf(MaybeOneOf&& rhs)
    : state(None)
  {
    if (!rhs.empty()) {
      if (rhs.constructed<T1>()) {
        construct<T1>(Move(rhs.as<T1>()));
        rhs.as<T1>().~T1();
      } else {
        construct<T2>(Move(rhs.as<T2>()));
        rhs.as<T2>().~T2();
      }
      rhs.state = None;
    }
  }

  MaybeOneOf& operator=(MaybeOneOf&& rhs)
  {
    MOZ_ASSERT(this != &rhs, "Self-move is prohibited");
    this->~MaybeOneOf();
    new(this) MaybeOneOf(Move(rhs));
    return *this;
  }

  bool empty() const { return state == None; }

  template <class T>
  bool constructed() const { return state == Type2State<T>::result; }

  template <class T, class... Args>
  void construct(Args&&... aArgs)
  {
    MOZ_ASSERT(state == None);
    state = Type2State<T>::result;
    ::new (KnownNotNull, data()) T(Forward<Args>(aArgs)...);
  }

  template <class T>
  T& ref()
  {
    return as<T>();
  }

  template <class T>
  const T& ref() const
  {
    return as<T>();
  }

  void destroy()
  {
    MOZ_ASSERT(state == SomeT1 || state == SomeT2);
    if (state == SomeT1) {
      as<T1>().~T1();
    } else if (state == SomeT2) {
      as<T2>().~T2();
    }
    state = None;
  }

  void destroyIfConstructed()
  {
    if (!empty()) {
      destroy();
    }
  }

private:
  MaybeOneOf(const MaybeOneOf& aOther) = delete;
  const MaybeOneOf& operator=(const MaybeOneOf& aOther) = delete;
};

template <class T1, class T2>
template <class Ignored>
struct MaybeOneOf<T1, T2>::Type2State<T1, Ignored>
{
  typedef MaybeOneOf<T1, T2> Enclosing;
  static const typename Enclosing::State result = Enclosing::SomeT1;
};

template <class T1, class T2>
template <class Ignored>
struct MaybeOneOf<T1, T2>::Type2State<T2, Ignored>
{
  typedef MaybeOneOf<T1, T2> Enclosing;
  static const typename Enclosing::State result = Enclosing::SomeT2;
};

} // namespace mozilla

#endif /* mozilla_MaybeOneOf_h */