image/CopyOnWrite.h
author Henrik Skupin <mail@hskupin.info>
Fri, 30 Nov 2018 13:27:22 +0000
changeset 448968 cd370c35d88d82c719e7b3d70ae1da003c8cb7f6
parent 448947 6f3709b3878117466168c40affa7bca0b60cf75b
permissions -rw-r--r--
Bug 1511311 - [marionette] GeckoInstance classes should not overwrite already set custom preferences. r=automatedtester Custom preferences as passed into both classes via the constructor are getting set in the base GeckoInstance class. But both the DesktopInstance and FennecInstance classes could override those custom preferences if they have the same preference set as required preference. As such the custom preference doesn't have any effect. This patch allows both classes to override required prefs with a different value. Differential Revision: https://phabricator.services.mozilla.com/D13522

/* -*- 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), mReaders(0), mWriter(false) {}
  explicit CopyOnWriteValue(already_AddRefed<T>& aValue)
      : mValue(aValue), mReaders(0), mWriter(false) {}
  explicit CopyOnWriteValue(already_AddRefed<T>&& aValue)
      : mValue(aValue), mReaders(0), mWriter(false) {}
  explicit CopyOnWriteValue(const RefPtr<T>& aValue)
      : mValue(aValue), mReaders(0), mWriter(false) {}
  explicit CopyOnWriteValue(RefPtr<T>&& aValue)
      : mValue(aValue), mReaders(0), mWriter(false) {}

  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