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/* -*- 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
/**
* 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