DXR is a code search and navigation tool aimed at making sense of large projects. It supports full-text and regex searches as well as structural queries.

Mercurial (fddffdeab170)

VCS Links

Line Code
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250
/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* 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/. */

/**
 * CopyOnWrite<T> allows code to safely read from a data structure without
 * worrying that reentrant code will modify it.
 */

#ifndef mozilla_image_CopyOnWrite_h
#define mozilla_image_CopyOnWrite_h

#include "mozilla/RefPtr.h"
#include "MainThreadUtils.h"
#include "nsISupportsImpl.h"

namespace mozilla {
namespace image {

///////////////////////////////////////////////////////////////////////////////
// Implementation Details
///////////////////////////////////////////////////////////////////////////////

namespace detail {

template <typename T>
class CopyOnWriteValue final
{
public:
  NS_INLINE_DECL_REFCOUNTING(CopyOnWriteValue)

  explicit CopyOnWriteValue(T* aValue) : mValue(aValue) { }
  explicit CopyOnWriteValue(already_AddRefed<T>& aValue) : mValue(aValue) { }
  explicit CopyOnWriteValue(already_AddRefed<T>&& aValue) : mValue(aValue) { }
  explicit CopyOnWriteValue(const RefPtr<T>& aValue) : mValue(aValue) { }
  explicit CopyOnWriteValue(RefPtr<T>&& aValue) : mValue(aValue) { }

  T* get() { return mValue.get(); }
  const T* get() const { return mValue.get(); }

  bool HasReaders() const { return mReaders > 0; }
  bool HasWriter() const { return mWriter; }
  bool HasUsers() const { return HasReaders() || HasWriter(); }

  void LockForReading() { MOZ_ASSERT(!HasWriter()); mReaders++; }
  void UnlockForReading() { MOZ_ASSERT(HasReaders()); mReaders--; }

  struct MOZ_STACK_CLASS AutoReadLock
  {
    explicit AutoReadLock(CopyOnWriteValue* aValue)
      : mValue(aValue)
    {
      mValue->LockForReading();
    }
    ~AutoReadLock() { mValue->UnlockForReading(); }
    CopyOnWriteValue<T>* mValue;
  };

  void LockForWriting() { MOZ_ASSERT(!HasUsers()); mWriter = true; }
  void UnlockForWriting() { MOZ_ASSERT(HasWriter()); mWriter = false; }

  struct MOZ_STACK_CLASS AutoWriteLock
  {
    explicit AutoWriteLock(CopyOnWriteValue* aValue)
      : mValue(aValue)
    {
      mValue->LockForWriting();
    }
    ~AutoWriteLock() { mValue->UnlockForWriting(); }
    CopyOnWriteValue<T>* mValue;
  };

private:
  CopyOnWriteValue(const CopyOnWriteValue&) = delete;
  CopyOnWriteValue(CopyOnWriteValue&&) = delete;

  ~CopyOnWriteValue() { }

  RefPtr<T> mValue;
  uint64_t mReaders = 0;
  bool mWriter = false;
};

} // namespace detail


///////////////////////////////////////////////////////////////////////////////
// Public API
///////////////////////////////////////////////////////////////////////////////

/**
 * CopyOnWrite<T> allows code to safely read from a data structure without
 * worrying that reentrant code will modify it. If reentrant code would modify
 * the data structure while other code is reading from it, a copy is made so
 * that readers can continue to use the old version.
 *
 * Note that it's legal to nest a writer inside any number of readers, but
 * nothing can be nested inside a writer. This is because it's assumed that the
 * state of the contained data structure may not be consistent during the write.
 *
 * This is a main-thread-only data structure.
 *
 * To work with CopyOnWrite<T>, a type T needs to be reference counted and to
 * support copy construction.
 */
template <typename T>
class CopyOnWrite final
{
  typedef detail::CopyOnWriteValue<T> CopyOnWriteValue;

public:
  explicit CopyOnWrite(T* aValue)
  : mValue(new CopyOnWriteValue(aValue))
  { }

  explicit CopyOnWrite(already_AddRefed<T>& aValue)
    : mValue(new CopyOnWriteValue(aValue))
  { }

  explicit CopyOnWrite(already_AddRefed<T>&& aValue)
    : mValue(new CopyOnWriteValue(aValue))
  { }

  explicit CopyOnWrite(const RefPtr<T>& aValue)
    : mValue(new CopyOnWriteValue(aValue))
  { }

  explicit CopyOnWrite(RefPtr<T>&& aValue)
    : mValue(new CopyOnWriteValue(aValue))
  { }

  /// @return true if it's safe to read at this time.
  bool CanRead() const { return !mValue->HasWriter(); }

  /**
   * Read from the contained data structure using the function @aReader.
   * @aReader will be passed a pointer of type |const T*|. It's not legal to
   * call this while a writer is active.
   *
   * @return whatever value @aReader returns, or nothing if @aReader is a void
   *         function.
   */
  template <typename ReadFunc>
  auto Read(ReadFunc aReader) const
    -> decltype(aReader(static_cast<const T*>(nullptr)))
  {
    MOZ_ASSERT(NS_IsMainThread());
    MOZ_ASSERT(CanRead());

    // Run the provided function while holding a read lock.
    RefPtr<CopyOnWriteValue> cowValue = mValue;
    typename CopyOnWriteValue::AutoReadLock lock(cowValue);
    return aReader(cowValue->get());
  }

  /**
   * Read from the contained data structure using the function @aReader.
   * @aReader will be passed a pointer of type |const T*|. If it's currently not
   * possible to read because a writer is currently active, @aOnError will be
   * called instead.
   *
   * @return whatever value @aReader or @aOnError returns (their return types
   *         must be consistent), or nothing if the provided functions are void.
   */
  template <typename ReadFunc, typename ErrorFunc>
  auto Read(ReadFunc aReader, ErrorFunc aOnError) const
    -> decltype(aReader(static_cast<const T*>(nullptr)))
  {
    MOZ_ASSERT(NS_IsMainThread());

    if (!CanRead()) {
      return aOnError();
    }

    return Read(aReader);
  }

  /// @return true if it's safe to write at this time.
  bool CanWrite() const { return !mValue->HasWriter(); }

  /**
   * Write to the contained data structure using the function @aWriter.
   * @aWriter will be passed a pointer of type |T*|. It's not legal to call this
   * while another writer is active.
   *
   * If readers are currently active, they will be able to continue reading from
   * a copy of the old version of the data structure. The copy will be destroyed
   * when all its readers finish.  Later readers and writers will see the
   * version of the data structure produced by the most recent call to Write().
   *
   * @return whatever value @aWriter returns, or nothing if @aWriter is a void
   *         function.
   */
  template <typename WriteFunc>
  auto Write(WriteFunc aWriter)
    -> decltype(aWriter(static_cast<T*>(nullptr)))
  {
    MOZ_ASSERT(NS_IsMainThread());
    MOZ_ASSERT(CanWrite());

    // If there are readers, we need to copy first.
    if (mValue->HasReaders()) {
      mValue = new CopyOnWriteValue(new T(*mValue->get()));
    }

    // Run the provided function while holding a write lock.
    RefPtr<CopyOnWriteValue> cowValue = mValue;
    typename CopyOnWriteValue::AutoWriteLock lock(cowValue);
    return aWriter(cowValue->get());
  }

  /**
   * Write to the contained data structure using the function @aWriter.
   * @aWriter will be passed a pointer of type |T*|. If it's currently not
   * possible to write because a writer is currently active, @aOnError will be
   * called instead.
   *
   * If readers are currently active, they will be able to continue reading from
   * a copy of the old version of the data structure. The copy will be destroyed
   * when all its readers finish.  Later readers and writers will see the
   * version of the data structure produced by the most recent call to Write().
   *
   * @return whatever value @aWriter or @aOnError returns (their return types
   *         must be consistent), or nothing if the provided functions are void.
   */
  template <typename WriteFunc, typename ErrorFunc>
  auto Write(WriteFunc aWriter, ErrorFunc aOnError)
    -> decltype(aWriter(static_cast<T*>(nullptr)))
  {
    MOZ_ASSERT(NS_IsMainThread());

    if (!CanWrite()) {
      return aOnError();
    }

    return Write(aWriter);
  }

private:
  CopyOnWrite(const CopyOnWrite&) = delete;
  CopyOnWrite(CopyOnWrite&&) = delete;

  RefPtr<CopyOnWriteValue> mValue;
};

} // namespace image
} // namespace mozilla

#endif // mozilla_image_CopyOnWrite_h