mirror of
https://github.com/classilla/tenfourfox.git
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251 lines
7.9 KiB
C++
251 lines
7.9 KiB
C++
/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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/**
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* CopyOnWrite<T> allows code to safely read from a data structure without
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* worrying that reentrant code will modify it.
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*/
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#ifndef mozilla_image_CopyOnWrite_h
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#define mozilla_image_CopyOnWrite_h
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#include "mozilla/RefPtr.h"
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#include "MainThreadUtils.h"
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#include "nsISupportsImpl.h"
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namespace mozilla {
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namespace image {
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///////////////////////////////////////////////////////////////////////////////
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// Implementation Details
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///////////////////////////////////////////////////////////////////////////////
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namespace detail {
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template <typename T>
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class CopyOnWriteValue final
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{
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public:
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NS_INLINE_DECL_REFCOUNTING(CopyOnWriteValue)
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explicit CopyOnWriteValue(T* aValue) : mValue(aValue) { }
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explicit CopyOnWriteValue(already_AddRefed<T>& aValue) : mValue(aValue) { }
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explicit CopyOnWriteValue(already_AddRefed<T>&& aValue) : mValue(aValue) { }
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explicit CopyOnWriteValue(const RefPtr<T>& aValue) : mValue(aValue) { }
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explicit CopyOnWriteValue(RefPtr<T>&& aValue) : mValue(aValue) { }
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T* get() { return mValue.get(); }
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const T* get() const { return mValue.get(); }
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bool HasReaders() const { return mReaders > 0; }
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bool HasWriter() const { return mWriter; }
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bool HasUsers() const { return HasReaders() || HasWriter(); }
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void LockForReading() { MOZ_ASSERT(!HasWriter()); mReaders++; }
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void UnlockForReading() { MOZ_ASSERT(HasReaders()); mReaders--; }
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struct MOZ_STACK_CLASS AutoReadLock
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{
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explicit AutoReadLock(CopyOnWriteValue* aValue)
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: mValue(aValue)
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{
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mValue->LockForReading();
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}
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~AutoReadLock() { mValue->UnlockForReading(); }
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CopyOnWriteValue<T>* mValue;
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};
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void LockForWriting() { MOZ_ASSERT(!HasUsers()); mWriter = true; }
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void UnlockForWriting() { MOZ_ASSERT(HasWriter()); mWriter = false; }
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struct MOZ_STACK_CLASS AutoWriteLock
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{
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explicit AutoWriteLock(CopyOnWriteValue* aValue)
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: mValue(aValue)
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{
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mValue->LockForWriting();
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}
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~AutoWriteLock() { mValue->UnlockForWriting(); }
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CopyOnWriteValue<T>* mValue;
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};
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private:
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CopyOnWriteValue(const CopyOnWriteValue&) = delete;
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CopyOnWriteValue(CopyOnWriteValue&&) = delete;
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~CopyOnWriteValue() { }
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RefPtr<T> mValue;
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uint64_t mReaders = 0;
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bool mWriter = false;
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};
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} // namespace detail
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///////////////////////////////////////////////////////////////////////////////
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// Public API
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///////////////////////////////////////////////////////////////////////////////
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/**
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* CopyOnWrite<T> allows code to safely read from a data structure without
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* worrying that reentrant code will modify it. If reentrant code would modify
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* the data structure while other code is reading from it, a copy is made so
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* that readers can continue to use the old version.
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*
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* Note that it's legal to nest a writer inside any number of readers, but
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* nothing can be nested inside a writer. This is because it's assumed that the
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* state of the contained data structure may not be consistent during the write.
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*
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* This is a main-thread-only data structure.
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*
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* To work with CopyOnWrite<T>, a type T needs to be reference counted and to
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* support copy construction.
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*/
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template <typename T>
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class CopyOnWrite final
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{
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typedef detail::CopyOnWriteValue<T> CopyOnWriteValue;
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public:
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explicit CopyOnWrite(T* aValue)
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: mValue(new CopyOnWriteValue(aValue))
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{ }
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explicit CopyOnWrite(already_AddRefed<T>& aValue)
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: mValue(new CopyOnWriteValue(aValue))
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{ }
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explicit CopyOnWrite(already_AddRefed<T>&& aValue)
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: mValue(new CopyOnWriteValue(aValue))
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{ }
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explicit CopyOnWrite(const RefPtr<T>& aValue)
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: mValue(new CopyOnWriteValue(aValue))
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{ }
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explicit CopyOnWrite(RefPtr<T>&& aValue)
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: mValue(new CopyOnWriteValue(aValue))
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{ }
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/// @return true if it's safe to read at this time.
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bool CanRead() const { return !mValue->HasWriter(); }
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/**
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* Read from the contained data structure using the function @aReader.
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* @aReader will be passed a pointer of type |const T*|. It's not legal to
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* call this while a writer is active.
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*
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* @return whatever value @aReader returns, or nothing if @aReader is a void
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* function.
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*/
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template <typename ReadFunc>
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auto Read(ReadFunc aReader) const
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-> decltype(aReader(static_cast<const T*>(nullptr)))
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{
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MOZ_ASSERT(NS_IsMainThread());
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MOZ_ASSERT(CanRead());
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// Run the provided function while holding a read lock.
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RefPtr<CopyOnWriteValue> cowValue = mValue;
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typename CopyOnWriteValue::AutoReadLock lock(cowValue);
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return aReader(cowValue->get());
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}
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/**
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* Read from the contained data structure using the function @aReader.
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* @aReader will be passed a pointer of type |const T*|. If it's currently not
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* possible to read because a writer is currently active, @aOnError will be
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* called instead.
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*
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* @return whatever value @aReader or @aOnError returns (their return types
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* must be consistent), or nothing if the provided functions are void.
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*/
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template <typename ReadFunc, typename ErrorFunc>
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auto Read(ReadFunc aReader, ErrorFunc aOnError) const
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-> decltype(aReader(static_cast<const T*>(nullptr)))
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{
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MOZ_ASSERT(NS_IsMainThread());
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if (!CanRead()) {
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return aOnError();
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}
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return Read(aReader);
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}
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/// @return true if it's safe to write at this time.
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bool CanWrite() const { return !mValue->HasWriter(); }
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/**
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* Write to the contained data structure using the function @aWriter.
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* @aWriter will be passed a pointer of type |T*|. It's not legal to call this
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* while another writer is active.
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*
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* If readers are currently active, they will be able to continue reading from
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* a copy of the old version of the data structure. The copy will be destroyed
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* when all its readers finish. Later readers and writers will see the
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* version of the data structure produced by the most recent call to Write().
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*
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* @return whatever value @aWriter returns, or nothing if @aWriter is a void
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* function.
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*/
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template <typename WriteFunc>
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auto Write(WriteFunc aWriter)
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-> decltype(aWriter(static_cast<T*>(nullptr)))
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{
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MOZ_ASSERT(NS_IsMainThread());
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MOZ_ASSERT(CanWrite());
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// If there are readers, we need to copy first.
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if (mValue->HasReaders()) {
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mValue = new CopyOnWriteValue(new T(*mValue->get()));
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}
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// Run the provided function while holding a write lock.
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RefPtr<CopyOnWriteValue> cowValue = mValue;
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typename CopyOnWriteValue::AutoWriteLock lock(cowValue);
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return aWriter(cowValue->get());
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}
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/**
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* Write to the contained data structure using the function @aWriter.
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* @aWriter will be passed a pointer of type |T*|. If it's currently not
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* possible to write because a writer is currently active, @aOnError will be
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* called instead.
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*
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* If readers are currently active, they will be able to continue reading from
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* a copy of the old version of the data structure. The copy will be destroyed
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* when all its readers finish. Later readers and writers will see the
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* version of the data structure produced by the most recent call to Write().
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*
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* @return whatever value @aWriter or @aOnError returns (their return types
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* must be consistent), or nothing if the provided functions are void.
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*/
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template <typename WriteFunc, typename ErrorFunc>
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auto Write(WriteFunc aWriter, ErrorFunc aOnError)
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-> decltype(aWriter(static_cast<T*>(nullptr)))
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{
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MOZ_ASSERT(NS_IsMainThread());
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if (!CanWrite()) {
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return aOnError();
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}
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return Write(aWriter);
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}
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private:
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CopyOnWrite(const CopyOnWrite&) = delete;
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CopyOnWrite(CopyOnWrite&&) = delete;
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RefPtr<CopyOnWriteValue> mValue;
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};
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} // namespace image
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} // namespace mozilla
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#endif // mozilla_image_CopyOnWrite_h
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