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Diffstat (limited to '')
| -rw-r--r-- | xpcom/string/nsStringBuffer.h | 198 |
1 files changed, 198 insertions, 0 deletions
diff --git a/xpcom/string/nsStringBuffer.h b/xpcom/string/nsStringBuffer.h new file mode 100644 index 0000000000..43628d6668 --- /dev/null +++ b/xpcom/string/nsStringBuffer.h @@ -0,0 +1,198 @@ +/* -*- 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/. */ + +#ifndef nsStringBuffer_h__ +#define nsStringBuffer_h__ + +#include <atomic> +#include "mozilla/MemoryReporting.h" +#include "nsStringFwd.h" + +template <class T> +struct already_AddRefed; + +/** + * This structure precedes the string buffers "we" allocate. It may be the + * case that nsTAString::mData does not point to one of these special + * buffers. The mDataFlags member variable distinguishes the buffer type. + * + * When this header is in use, it enables reference counting, and capacity + * tracking. NOTE: A string buffer can be modified only if its reference + * count is 1. + */ +class nsStringBuffer { + private: + friend class CheckStaticAtomSizes; + + std::atomic<uint32_t> mRefCount; + uint32_t mStorageSize; + + public: + /** + * Allocates a new string buffer, with given size in bytes and a + * reference count of one. When the string buffer is no longer needed, + * it should be released via Release. + * + * It is up to the caller to set the bytes corresponding to the string + * buffer by calling the Data method to fetch the raw data pointer. Care + * must be taken to properly null terminate the character array. The + * storage size can be greater than the length of the actual string + * (i.e., it is not required that the null terminator appear in the last + * storage unit of the string buffer's data). + * + * This guarantees that StorageSize() returns aStorageSize if the returned + * buffer is non-null. Some callers like nsAttrValue rely on it. + * + * @return new string buffer or null if out of memory. + */ + static already_AddRefed<nsStringBuffer> Alloc(size_t aStorageSize); + + /** + * Returns a string buffer initialized with the given string on it, or null on + * OOM. + * Note that this will allocate extra space for the trailing null byte, which + * this method will add. + */ + static already_AddRefed<nsStringBuffer> Create(const char16_t* aData, + size_t aLength); + static already_AddRefed<nsStringBuffer> Create(const char* aData, + size_t aLength); + + /** + * Resizes the given string buffer to the specified storage size. This + * method must not be called on a readonly string buffer. Use this API + * carefully!! + * + * This method behaves like the ANSI-C realloc function. (i.e., If the + * allocation fails, null will be returned and the given string buffer + * will remain unmodified.) + * + * @see IsReadonly + */ + static nsStringBuffer* Realloc(nsStringBuffer* aBuf, size_t aStorageSize); + + /** + * Increment the reference count on this string buffer. + */ + void NS_FASTCALL AddRef(); + + /** + * Decrement the reference count on this string buffer. The string + * buffer will be destroyed when its reference count reaches zero. + */ + void NS_FASTCALL Release(); + + /** + * This method returns the string buffer corresponding to the given data + * pointer. The data pointer must have been returned previously by a + * call to the nsStringBuffer::Data method. + */ + static nsStringBuffer* FromData(void* aData) { + return reinterpret_cast<nsStringBuffer*>(aData) - 1; + } + + /** + * This method returns the data pointer for this string buffer. + */ + void* Data() const { + return const_cast<char*>(reinterpret_cast<const char*>(this + 1)); + } + + /** + * This function returns the storage size of a string buffer in bytes. + * This value is the same value that was originally passed to Alloc (or + * Realloc). + */ + uint32_t StorageSize() const { return mStorageSize; } + + /** + * If this method returns false, then the caller can be sure that their + * reference to the string buffer is the only reference to the string + * buffer, and therefore it has exclusive access to the string buffer and + * associated data. However, if this function returns true, then other + * consumers may rely on the data in this buffer being immutable and + * other threads may access this buffer simultaneously. + */ + bool IsReadonly() const { + // This doesn't lead to the destruction of the buffer, so we don't + // need to perform acquire memory synchronization for the normal + // reason that a reference count needs acquire synchronization + // (ensuring that all writes to the object made on other threads are + // visible to the thread destroying the object). + // + // We then need to consider the possibility that there were prior + // writes to the buffer on a different thread: one that has either + // since released its reference count, or one that also has access + // to this buffer through the same reference. There are two ways + // for that to happen: either the buffer pointer or a data structure + // (e.g., string object) pointing to the buffer was transferred from + // one thread to another, or the data structure pointing to the + // buffer was already visible on both threads. In the first case + // (transfer), the transfer of data from one thread to another would + // have handled the memory synchronization. In the latter case + // (data structure visible on both threads), the caller needed some + // sort of higher level memory synchronization to protect against + // the string object being mutated at the same time on multiple + // threads. + + // See bug 1603504. TSan might complain about a race when using + // memory_order_relaxed, so use memory_order_acquire for making TSan + // happy. +#if defined(MOZ_TSAN) + return mRefCount.load(std::memory_order_acquire) > 1; +#else + return mRefCount.load(std::memory_order_relaxed) > 1; +#endif + } + + /** + * The FromString methods return a string buffer for the given string + * object or null if the string object does not have a string buffer. + * The reference count of the string buffer is NOT incremented by these + * methods. If the caller wishes to hold onto the returned value, then + * the returned string buffer must have its reference count incremented + * via a call to the AddRef method. + */ + static nsStringBuffer* FromString(const nsAString& aStr); + static nsStringBuffer* FromString(const nsACString& aStr); + + /** + * The ToString methods assign this string buffer to a given string + * object. If the string object does not support sharable string + * buffers, then its value will be set to a copy of the given string + * buffer. Otherwise, these methods increment the reference count of the + * given string buffer. It is important to specify the length (in + * storage units) of the string contained in the string buffer since the + * length of the string may be less than its storage size. The string + * must have a null terminator at the offset specified by |len|. + * + * NOTE: storage size is measured in bytes even for wide strings; + * however, string length is always measured in storage units + * (2-byte units for wide strings). + */ + void ToString(uint32_t aLen, nsAString& aStr, bool aMoveOwnership = false); + void ToString(uint32_t aLen, nsACString& aStr, bool aMoveOwnership = false); + + /** + * This measures the size only if the StringBuffer is unshared. + */ + size_t SizeOfIncludingThisIfUnshared( + mozilla::MallocSizeOf aMallocSizeOf) const; + + /** + * This measures the size regardless of whether the StringBuffer is + * unshared. + * + * WARNING: Only use this if you really know what you are doing, because + * it can easily lead to double-counting strings. If you do use them, + * please explain clearly in a comment why it's safe and won't lead to + * double-counting. + */ + size_t SizeOfIncludingThisEvenIfShared( + mozilla::MallocSizeOf aMallocSizeOf) const; +}; + +#endif /* !defined(nsStringBuffer_h__ */ |