mfbt/Maybe.h
author Andrea Marchesini <amarchesini@mozilla.com>
Wed, 31 Oct 2018 18:30:18 +0100
changeset 500246 544498045a9cfe55968fa6500bffbc3181869fce
parent 499485 99d33ba5e213387846e04e635d993e30364c815e
child 505383 6f3709b3878117466168c40affa7bca0b60cf75b
permissions -rw-r--r--
Bug 1486698 - Update Fetch+Stream implementation to throw when the stream is disturbed or locked, r=bz In this patch, I went through any place in DOM fetch code, where there are ReadableStreams and update the locked, disturbed, readable checks. Because we expose streams more often, we need an extra care in the use of ErrorResult objects. JS streams can now throw exceptions and we need to handle them. This patch also fixes a bug in FileStreamReader::CloseAndRelease() which could be called in case mReader creation fails.

/* -*- 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 for optional values and in-place lazy construction. */

#ifndef mozilla_Maybe_h
#define mozilla_Maybe_h

#include "mozilla/Alignment.h"
#include "mozilla/Assertions.h"
#include "mozilla/Attributes.h"
#include "mozilla/MemoryChecking.h"
#include "mozilla/Move.h"
#include "mozilla/OperatorNewExtensions.h"
#include "mozilla/Poison.h"
#include "mozilla/TypeTraits.h"

#include <new>  // for placement new
#include <ostream>
#include <type_traits>

namespace mozilla {

struct Nothing { };

namespace detail {

// You would think that poisoning Maybe instances could just be a call
// to mozWritePoison.  Unfortunately, using a simple call to
// mozWritePoison generates poor code on MSVC for small structures.  The
// generated code contains (always not-taken) branches and does a bunch
// of setup for `rep stos{l,q}`, even though we know at compile time
// exactly how many words we're poisoning.  Instead, we're going to
// force MSVC to generate the code we want via recursive templates.

// Write the given poisonValue into p at offset*sizeof(uintptr_t).
template<size_t offset>
inline void
WritePoisonAtOffset(void* p, const uintptr_t poisonValue)
{
    memcpy(static_cast<char*>(p) + offset*sizeof(poisonValue),
           &poisonValue, sizeof(poisonValue));
}


template<size_t Offset, size_t NOffsets>
struct InlinePoisoner
{
    static void poison(void* p, const uintptr_t poisonValue)
    {
        WritePoisonAtOffset<Offset>(p, poisonValue);
        InlinePoisoner<Offset+1, NOffsets>::poison(p, poisonValue);
    }
};

template<size_t N>
struct InlinePoisoner<N, N>
{
    static void poison(void*, const uintptr_t)
    {
        // All done!
    }
};

// We can't generate inline code for large structures, though, because we'll
// blow out recursive template instantiation limits, and the code would be
// bloated to boot.  So provide a fallback to the out-of-line poisoner.
template<size_t ObjectSize>
struct OutOfLinePoisoner
{
  static void poison(void* p, const uintptr_t)
  {
    mozWritePoison(p, ObjectSize);
  }
};

template<typename T>
inline void
PoisonObject(T* p)
{
  const uintptr_t POISON = mozPoisonValue();
  Conditional<(sizeof(T) <= 8*sizeof(POISON)),
    InlinePoisoner<0, sizeof(T) / sizeof(POISON)>,
    OutOfLinePoisoner<sizeof(T)>>::Type::poison(p, POISON);
}

template<typename T>
struct MaybePoisoner
{
  static const size_t N = sizeof(T);

  static void poison(void* aPtr)
  {
#ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
    if (N >= sizeof(uintptr_t)) {
      PoisonObject(static_cast<typename RemoveCV<T>::Type*>(aPtr));
    }
#endif
    MOZ_MAKE_MEM_UNDEFINED(aPtr, N);
  }
};

} // namespace detail

/*
 * Maybe is a container class which contains either zero or one elements. It
 * serves two roles. It can represent values which are *semantically* optional,
 * augmenting a type with an explicit 'Nothing' value. In this role, it provides
 * methods that make it easy to work with values that may be missing, along with
 * equality and comparison operators so that Maybe values can be stored in
 * containers. Maybe values can be constructed conveniently in expressions using
 * type inference, as follows:
 *
 *   void doSomething(Maybe<Foo> aFoo) {
 *     if (aFoo)                  // Make sure that aFoo contains a value...
 *       aFoo->takeAction();      // and then use |aFoo->| to access it.
 *   }                            // |*aFoo| also works!
 *
 *   doSomething(Nothing());      // Passes a Maybe<Foo> containing no value.
 *   doSomething(Some(Foo(100))); // Passes a Maybe<Foo> containing |Foo(100)|.
 *
 * You'll note that it's important to check whether a Maybe contains a value
 * before using it, using conversion to bool, |isSome()|, or |isNothing()|. You
 * can avoid these checks, and sometimes write more readable code, using
 * |valueOr()|, |ptrOr()|, and |refOr()|, which allow you to retrieve the value
 * in the Maybe and provide a default for the 'Nothing' case.  You can also use
 * |apply()| to call a function only if the Maybe holds a value, and |map()| to
 * transform the value in the Maybe, returning another Maybe with a possibly
 * different type.
 *
 * Maybe's other role is to support lazily constructing objects without using
 * dynamic storage. A Maybe directly contains storage for a value, but it's
 * empty by default. |emplace()|, as mentioned above, can be used to construct a
 * value in Maybe's storage.  The value a Maybe contains can be destroyed by
 * calling |reset()|; this will happen automatically if a Maybe is destroyed
 * while holding a value.
 *
 * It's a common idiom in C++ to use a pointer as a 'Maybe' type, with a null
 * value meaning 'Nothing' and any other value meaning 'Some'. You can convert
 * from such a pointer to a Maybe value using 'ToMaybe()'.
 *
 * Maybe is inspired by similar types in the standard library of many other
 * languages (e.g. Haskell's Maybe and Rust's Option). In the C++ world it's
 * very similar to std::optional, which was proposed for C++14 and originated in
 * Boost. The most important differences between Maybe and std::optional are:
 *
 *   - std::optional<T> may be compared with T. We deliberately forbid that.
 *   - std::optional allows in-place construction without a separate call to
 *     |emplace()| by using a dummy |in_place_t| value to tag the appropriate
 *     constructor.
 *   - std::optional has |valueOr()|, equivalent to Maybe's |valueOr()|, but
 *     lacks corresponding methods for |refOr()| and |ptrOr()|.
 *   - std::optional lacks |map()| and |apply()|, making it less suitable for
 *     functional-style code.
 *   - std::optional lacks many convenience functions that Maybe has. Most
 *     unfortunately, it lacks equivalents of the type-inferred constructor
 *     functions |Some()| and |Nothing()|.
 */
template<class T>
class MOZ_NON_PARAM MOZ_INHERIT_TYPE_ANNOTATIONS_FROM_TEMPLATE_ARGS Maybe
{
  MOZ_ALIGNAS_IN_STRUCT(T) unsigned char mStorage[sizeof(T)];
  char mIsSome; // not bool -- guarantees minimal space consumption

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

  void poisonData()
  {
    detail::MaybePoisoner<T>::poison(data());
  }

public:
  using ValueType = T;

  MOZ_ALLOW_TEMPORARY Maybe() : mIsSome(false)
  {
  }
  ~Maybe() { reset(); }

  MOZ_ALLOW_TEMPORARY MOZ_IMPLICIT Maybe(Nothing) : mIsSome(false)
  {
  }

  Maybe(const Maybe& aOther)
    : mIsSome(false)
  {
    if (aOther.mIsSome) {
      emplace(*aOther);
    }
  }

  /**
   * Maybe<T> can be copy-constructed from a Maybe<U> if U is convertible to T.
   */
  template<typename U,
           typename =
             typename std::enable_if<std::is_convertible<U, T>::value>::type>
  MOZ_IMPLICIT
  Maybe(const Maybe<U>& aOther)
    : mIsSome(false)
  {
    if (aOther.isSome()) {
      emplace(*aOther);
    }
  }

  Maybe(Maybe&& aOther)
    : mIsSome(false)
  {
    if (aOther.mIsSome) {
      emplace(std::move(*aOther));
      aOther.reset();
    }
  }

  /**
   * Maybe<T> can be move-constructed from a Maybe<U> if U is convertible to T.
   */
  template<typename U,
           typename =
             typename std::enable_if<std::is_convertible<U, T>::value>::type>
  MOZ_IMPLICIT
  Maybe(Maybe<U>&& aOther)
    : mIsSome(false)
  {
    if (aOther.isSome()) {
      emplace(std::move(*aOther));
      aOther.reset();
    }
  }

  Maybe& operator=(const Maybe& aOther)
  {
    if (&aOther != this) {
      if (aOther.mIsSome) {
        if (mIsSome) {
          ref() = aOther.ref();
        } else {
          emplace(*aOther);
        }
      } else {
        reset();
      }
    }
    return *this;
  }

  template<typename U,
           typename =
             typename std::enable_if<std::is_convertible<U, T>::value>::type>
  Maybe& operator=(const Maybe<U>& aOther)
  {
    if (aOther.isSome()) {
      if (mIsSome) {
        ref() = aOther.ref();
      } else {
        emplace(*aOther);
      }
    } else {
      reset();
    }
    return *this;
  }

  Maybe& operator=(Maybe&& aOther)
  {
    MOZ_ASSERT(this != &aOther, "Self-moves are prohibited");

    if (aOther.mIsSome) {
      if (mIsSome) {
        ref() = std::move(aOther.ref());
      } else {
        emplace(std::move(*aOther));
      }
      aOther.reset();
    } else {
      reset();
    }

    return *this;
  }

  template<typename U,
           typename =
             typename std::enable_if<std::is_convertible<U, T>::value>::type>
  Maybe& operator=(Maybe<U>&& aOther)
  {
    if (aOther.isSome()) {
      if (mIsSome) {
        ref() = std::move(aOther.ref());
      } else {
        emplace(std::move(*aOther));
      }
      aOther.reset();
    } else {
      reset();
    }

    return *this;
  }

  /* Methods that check whether this Maybe contains a value */
  explicit operator bool() const { return isSome(); }
  bool isSome() const { return mIsSome; }
  bool isNothing() const { return !mIsSome; }

  /* Returns the contents of this Maybe<T> by value. Unsafe unless |isSome()|. */
  T value() const;

  /*
   * Returns the contents of this Maybe<T> by value. If |isNothing()|, returns
   * the default value provided.
   */
  template<typename V>
  T valueOr(V&& aDefault) const
  {
    if (isSome()) {
      return ref();
    }
    return std::forward<V>(aDefault);
  }

  /*
   * Returns the contents of this Maybe<T> by value. If |isNothing()|, returns
   * the value returned from the function or functor provided.
   */
  template<typename F>
  T valueOrFrom(F&& aFunc) const
  {
    if (isSome()) {
      return ref();
    }
    return aFunc();
  }

  /* Returns the contents of this Maybe<T> by pointer. Unsafe unless |isSome()|. */
  T* ptr();
  const T* ptr() const;

  /*
   * Returns the contents of this Maybe<T> by pointer. If |isNothing()|,
   * returns the default value provided.
   */
  T* ptrOr(T* aDefault)
  {
    if (isSome()) {
      return ptr();
    }
    return aDefault;
  }

  const T* ptrOr(const T* aDefault) const
  {
    if (isSome()) {
      return ptr();
    }
    return aDefault;
  }

  /*
   * Returns the contents of this Maybe<T> by pointer. If |isNothing()|,
   * returns the value returned from the function or functor provided.
   */
  template<typename F>
  T* ptrOrFrom(F&& aFunc)
  {
    if (isSome()) {
      return ptr();
    }
    return aFunc();
  }

  template<typename F>
  const T* ptrOrFrom(F&& aFunc) const
  {
    if (isSome()) {
      return ptr();
    }
    return aFunc();
  }

  T* operator->();
  const T* operator->() const;

  /* Returns the contents of this Maybe<T> by ref. Unsafe unless |isSome()|. */
  T& ref();
  const T& ref() const;

  /*
   * Returns the contents of this Maybe<T> by ref. If |isNothing()|, returns
   * the default value provided.
   */
  T& refOr(T& aDefault)
  {
    if (isSome()) {
      return ref();
    }
    return aDefault;
  }

  const T& refOr(const T& aDefault) const
  {
    if (isSome()) {
      return ref();
    }
    return aDefault;
  }

  /*
   * Returns the contents of this Maybe<T> by ref. If |isNothing()|, returns the
   * value returned from the function or functor provided.
   */
  template<typename F>
  T& refOrFrom(F&& aFunc)
  {
    if (isSome()) {
      return ref();
    }
    return aFunc();
  }

  template<typename F>
  const T& refOrFrom(F&& aFunc) const
  {
    if (isSome()) {
      return ref();
    }
    return aFunc();
  }

  T& operator*();
  const T& operator*() const;

  /* If |isSome()|, runs the provided function or functor on the contents of
   * this Maybe. */
  template<typename Func>
  Maybe& apply(Func&& aFunc)
  {
    if (isSome()) {
      std::forward<Func>(aFunc)(ref());
    }
    return *this;
  }

  template<typename Func>
  const Maybe& apply(Func&& aFunc) const
  {
    if (isSome()) {
      std::forward<Func>(aFunc)(ref());
    }
    return *this;
  }

  /*
   * If |isSome()|, runs the provided function and returns the result wrapped
   * in a Maybe. If |isNothing()|, returns an empty Maybe value with the same
   * value type as what the provided function would have returned.
   */
  template<typename Func>
  auto map(Func&& aFunc)
  {
    Maybe<decltype(std::forward<Func>(aFunc)(ref()))> val;
    if (isSome()) {
      val.emplace(std::forward<Func>(aFunc)(ref()));
    }
    return val;
  }

  template<typename Func>
  auto map(Func&& aFunc) const
  {
    Maybe<decltype(std::forward<Func>(aFunc)(ref()))> val;
    if (isSome()) {
      val.emplace(std::forward<Func>(aFunc)(ref()));
    }
    return val;
  }

  /* If |isSome()|, empties this Maybe and destroys its contents. */
  void reset()
  {
    if (isSome()) {
      ref().T::~T();
      mIsSome = false;
      poisonData();
    }
  }

  /*
   * Constructs a T value in-place in this empty Maybe<T>'s storage. The
   * arguments to |emplace()| are the parameters to T's constructor.
   */
  template<typename... Args>
  void emplace(Args&&... aArgs);

  friend std::ostream&
  operator<<(std::ostream& aStream, const Maybe<T>& aMaybe)
  {
    if (aMaybe) {
      aStream << aMaybe.ref();
    } else {
      aStream << "<Nothing>";
    }
    return aStream;
  }
};

template<typename T>
T
Maybe<T>::value() const
{
  MOZ_DIAGNOSTIC_ASSERT(mIsSome);
  return ref();
}

template<typename T>
T*
Maybe<T>::ptr()
{
  MOZ_DIAGNOSTIC_ASSERT(mIsSome);
  return &ref();
}

template<typename T>
const T*
Maybe<T>::ptr() const
{
  MOZ_DIAGNOSTIC_ASSERT(mIsSome);
  return &ref();
}

template<typename T>
T*
Maybe<T>::operator->()
{
  MOZ_DIAGNOSTIC_ASSERT(mIsSome);
  return ptr();
}

template<typename T>
const T*
Maybe<T>::operator->() const
{
  MOZ_DIAGNOSTIC_ASSERT(mIsSome);
  return ptr();
}

template<typename T>
T&
Maybe<T>::ref()
{
  MOZ_DIAGNOSTIC_ASSERT(mIsSome);
  return *static_cast<T*>(data());
}

template<typename T>
const T&
Maybe<T>::ref() const
{
  MOZ_DIAGNOSTIC_ASSERT(mIsSome);
  return *static_cast<const T*>(data());
}

template<typename T>
T&
Maybe<T>::operator*()
{
  MOZ_DIAGNOSTIC_ASSERT(mIsSome);
  return ref();
}

template<typename T>
const T&
Maybe<T>::operator*() const
{
  MOZ_DIAGNOSTIC_ASSERT(mIsSome);
  return ref();
}

template<typename T>
template<typename... Args>
void
Maybe<T>::emplace(Args&&... aArgs)
{
  MOZ_DIAGNOSTIC_ASSERT(!mIsSome);
  ::new (KnownNotNull, data()) T(std::forward<Args>(aArgs)...);
  mIsSome = true;
}

/*
 * Some() creates a Maybe<T> value containing the provided T value. If T has a
 * move constructor, it's used to make this as efficient as possible.
 *
 * Some() selects the type of Maybe it returns by removing any const, volatile,
 * or reference qualifiers from the type of the value you pass to it. This gives
 * it more intuitive behavior when used in expressions, but it also means that
 * if you need to construct a Maybe value that holds a const, volatile, or
 * reference value, you need to use emplace() instead.
 */
template<typename T,
         typename U = typename std::remove_cv<
           typename std::remove_reference<T>::type>::type>
Maybe<U>
Some(T&& aValue)
{
  Maybe<U> value;
  value.emplace(std::forward<T>(aValue));
  return value;
}

template<typename T>
Maybe<typename RemoveCV<typename RemoveReference<T>::Type>::Type>
ToMaybe(T* aPtr)
{
  if (aPtr) {
    return Some(*aPtr);
  }
  return Nothing();
}

/*
 * Two Maybe<T> values are equal if
 * - both are Nothing, or
 * - both are Some, and the values they contain are equal.
 */
template<typename T> bool
operator==(const Maybe<T>& aLHS, const Maybe<T>& aRHS)
{
  if (aLHS.isNothing() != aRHS.isNothing()) {
    return false;
  }
  return aLHS.isNothing() || *aLHS == *aRHS;
}

template<typename T> bool
operator!=(const Maybe<T>& aLHS, const Maybe<T>& aRHS)
{
  return !(aLHS == aRHS);
}

/*
 * We support comparison to Nothing to allow reasonable expressions like:
 *   if (maybeValue == Nothing()) { ... }
 */
template<typename T> bool
operator==(const Maybe<T>& aLHS, const Nothing& aRHS)
{
  return aLHS.isNothing();
}

template<typename T> bool
operator!=(const Maybe<T>& aLHS, const Nothing& aRHS)
{
  return !(aLHS == aRHS);
}

template<typename T> bool
operator==(const Nothing& aLHS, const Maybe<T>& aRHS)
{
  return aRHS.isNothing();
}

template<typename T> bool
operator!=(const Nothing& aLHS, const Maybe<T>& aRHS)
{
  return !(aLHS == aRHS);
}

/*
 * Maybe<T> values are ordered in the same way T values are ordered, except that
 * Nothing comes before anything else.
 */
template<typename T> bool
operator<(const Maybe<T>& aLHS, const Maybe<T>& aRHS)
{
  if (aLHS.isNothing()) {
    return aRHS.isSome();
  }
  if (aRHS.isNothing()) {
    return false;
  }
  return *aLHS < *aRHS;
}

template<typename T> bool
operator>(const Maybe<T>& aLHS, const Maybe<T>& aRHS)
{
  return !(aLHS < aRHS || aLHS == aRHS);
}

template<typename T> bool
operator<=(const Maybe<T>& aLHS, const Maybe<T>& aRHS)
{
  return aLHS < aRHS || aLHS == aRHS;
}

template<typename T> bool
operator>=(const Maybe<T>& aLHS, const Maybe<T>& aRHS)
{
  return !(aLHS < aRHS);
}

} // namespace mozilla

#endif /* mozilla_Maybe_h */