Add a ref-counted data holder to ArrayBufferContents

Source/wtf/ArrayBufferContents.cpp

 /*
  * Copyright (C) 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 19 matching lines @@
 #include "wtf/Assertions.h"
 #include "wtf/PartitionAlloc.h"
 #include "wtf/Partitions.h"
 #include <string.h>
 
 namespace WTF {
 
 AdjustAmountOfExternalAllocatedMemoryFunction ArrayBufferContents::s_adjustAmountOfExternalAllocatedMemoryFunction;
 
 ArrayBufferContents::ArrayBufferContents()
-    : m_data(0)
-    , m_sizeInBytes(0) { }
+    : m_holder(adoptRef(new DataHolder())) { }
 
-ArrayBufferContents::ArrayBufferContents(unsigned numElements, unsigned elementByteSize, ArrayBufferContents::InitializationPolicy policy)
-    : m_data(0)
-    , m_sizeInBytes(0)
+ArrayBufferContents::ArrayBufferContents(unsigned numElements, unsigned elementByteSize, SharingType isShared, ArrayBufferContents::InitializationPolicy policy)
+    : m_holder(adoptRef(new DataHolder()))
 {
     // Do not allow 32-bit overflow of the total size.
+    unsigned totalSize = numElements * elementByteSize;
     if (numElements) {
-        unsigned totalSize = numElements * elementByteSize;
         if (totalSize / numElements != elementByteSize) {
-            m_data = 0;
             return;
         }
     }
-    allocateMemory(numElements * elementByteSize, policy, m_data);
-    m_sizeInBytes = numElements * elementByteSize;
+
+    m_holder->allocateNew(totalSize, isShared, policy);
 }
 
-ArrayBufferContents::ArrayBufferContents(void* data, unsigned sizeInBytes)
-    : m_data(data)
-    , m_sizeInBytes(sizeInBytes)
+ArrayBufferContents::ArrayBufferContents(
+    void* data, unsigned sizeInBytes, SharingType isShared)
+    : m_holder(adoptRef(new DataHolder()))
 {
-    if (!m_data) {
-        ASSERT(!m_sizeInBytes);
-        m_sizeInBytes = 0;
-        // Allow null data if size is 0 bytes, make sure m_data is valid pointer.
-        // (partitionAllocGeneric guarantees valid pointer for size 0)
-        allocateMemory(0, ZeroInitialize, m_data);
+    if (data) {
+        m_holder->adopt(data, sizeInBytes, isShared);
+    } else {
+        ASSERT(!sizeInBytes);
+        sizeInBytes = 0;
+        // Allow null data if size is 0 bytes, make sure data is valid pointer.
+        // (PartitionAlloc guarantees valid pointer for size 0)
+        m_holder->allocateNew(sizeInBytes, isShared, ZeroInitialize);
     }
 }
 
 ArrayBufferContents::~ArrayBufferContents()
 {
-    freeMemory(m_data, m_sizeInBytes);
-    clear();
+    neuter();
 }
 
-void ArrayBufferContents::clear()
+void ArrayBufferContents::neuter()
 {
-    m_data = 0;
-    m_sizeInBytes = 0;
+    m_holder.clear();
 }
 
 void ArrayBufferContents::transfer(ArrayBufferContents& other)
 {
-    ASSERT(!other.m_data);
-    other.m_data = m_data;
-    other.m_sizeInBytes = m_sizeInBytes;
-    clear();
+    ASSERT(!other.m_holder->data());
+    other.m_holder = m_holder;
+    if (!m_holder->isShared())
+        neuter();

[haraken, 2015/06/20 00:59:31]

I'm wondering if we should call neuter() for a shared buffer as well.

Imagine that you have shared ArrayBufferContents X1 and X2 which share the
DataHolder C. When X1 is transfered to another ArrayBufferContents, we want to
drop a reference to C (meaning that we want to call neuter()). When X2 is
transfered to another ArrayBufferContents, we want to drop another reference to
C, at which point the DataHolder loses all referece counts and thus the
DataHolder gets collected.

So I think what we want to do is something like:

ArrayBufferContents::~ArrayBufferContents()
{
} // m_holder.clear() is implicitly called.

void ArrayBufferContents::transfer(ArrayBufferContents& other)
{
  ...;
  m_holder.clear();
}

[binji, 2015/06/20 01:36:21]

Right, but we shouldn't ever be transferring a shared ArrayBufferContents and
neutering it; they should never be neutered.

I think the problem is that the ArrayBufferContents::transfer() function is
actually doing two things:
1) share this data holder with another ArrayBufferContents
2) neuter this data holder

And I'm explicitly blocking the second one in transfer. So now it is confusing
when you call transfer(). Is it neutering or not?

It's clearer if I make transfer() only do step #1 above, and have an explicit
call to neuter afterward, only in the case when the ArrayBuffer is not shared.

I've implemented that in patchset #3.

 }
 
 void ArrayBufferContents::copyTo(ArrayBufferContents& other)
 {
-    ASSERT(!other.m_sizeInBytes);
-    other.freeMemory(other.m_data, other.m_sizeInBytes);
-    allocateMemory(m_sizeInBytes, DontInitialize, other.m_data);
-    if (!other.m_data)
-        return;
-    memcpy(other.m_data, m_data, m_sizeInBytes);
-    other.m_sizeInBytes = m_sizeInBytes;
+    ASSERT(!m_holder->isShared() && !other.m_holder->isShared());
+    m_holder->copyMemoryTo(*other.m_holder);
 }
 
 void ArrayBufferContents::allocateMemory(size_t size, InitializationPolicy policy, void*& data)
 {
     if (s_adjustAmountOfExternalAllocatedMemoryFunction)
         s_adjustAmountOfExternalAllocatedMemoryFunction(static_cast<int>(size));
     data = partitionAllocGenericFlags(WTF::Partitions::bufferPartition(), PartitionAllocReturnNull, size);
     if (policy == ZeroInitialize && data)
         memset(data, '\0', size);
 }
 
 void ArrayBufferContents::freeMemory(void* data, size_t size)
 {
     partitionFreeGeneric(WTF::Partitions::bufferPartition(), data);
     if (s_adjustAmountOfExternalAllocatedMemoryFunction)
         s_adjustAmountOfExternalAllocatedMemoryFunction(-static_cast<int>(size));
 }
 
+ArrayBufferContents::DataHolder::DataHolder()
+    : m_data(nullptr)
+    , m_sizeInBytes(0)
+    , m_isShared(NotShared) { }
+
+ArrayBufferContents::DataHolder::~DataHolder()
+{
+    ArrayBufferContents::freeMemory(m_data, m_sizeInBytes);
+
+    m_data = nullptr;
+    m_sizeInBytes = 0;
+    m_isShared = NotShared;
+}
+
+void ArrayBufferContents::DataHolder::allocateNew(unsigned sizeInBytes, SharingType isShared, InitializationPolicy policy)
+{
+    ASSERT(!m_data);
+    void* data = nullptr;
+    allocateMemory(sizeInBytes, policy, data);
+    m_data = data;
+    m_sizeInBytes = sizeInBytes;
+    m_isShared = isShared;
+}
+
+void ArrayBufferContents::DataHolder::adopt(void* data, unsigned sizeInBytes, SharingType isShared)
+{
+    ASSERT(!m_data);
+    m_data = data;
+    m_sizeInBytes = sizeInBytes;
+    m_isShared = isShared;
+}
+
+void ArrayBufferContents::DataHolder::copyMemoryTo(DataHolder& other)
+{
+    ASSERT(!other.m_sizeInBytes);
+    ArrayBufferContents::freeMemory(other.m_data, other.m_sizeInBytes);
+    ArrayBufferContents::allocateMemory(m_sizeInBytes, DontInitialize, other.m_data);
+    if (!other.m_data)
+        return;
+    memcpy(other.m_data, m_data, m_sizeInBytes);
+    other.m_sizeInBytes = m_sizeInBytes;
+}
+
 } // namespace WTF