Add CopyOnWriteBuffer class

webrtc/base/buffer.h

Index: webrtc/base/buffer.h
diff --git a/webrtc/base/buffer.h b/webrtc/base/buffer.h
index 5c9380afee03a5e0825d92475e9ed578244314ae..e7924cc78b0c221aeb0457b22c2c3d5c8794fc05 100644
--- a/webrtc/base/buffer.h
+++ b/webrtc/base/buffer.h
@@ -12,16 +12,20 @@
 #define WEBRTC_BASE_BUFFER_H_
 
 #include <algorithm>  // std::swap (pre-C++11)
-#include <cassert>
 #include <cstring>
 #include <memory>
 #include <utility>  // std::swap (C++11 and later)
 
+#include "webrtc/base/checks.h"
 #include "webrtc/base/constructormagic.h"
 #include "webrtc/base/deprecation.h"
+#include "webrtc/base/refcount.h"
+#include "webrtc/base/scoped_ref_ptr.h"
 
 namespace rtc {
 
+class Buffer;

[kwiberg-webrtc, 2016/02/15 12:33:16]

This forward declaration should no longer be necessary.

+
 namespace internal {
 
 // (Internal; please don't use outside this file.) ByteType<T>::t is int if T
@@ -47,6 +51,8 @@ struct ByteType {
 
 // Basic buffer class, can be grown and shrunk dynamically.
 // Unlike std::string/vector, does not initialize data when expanding capacity.
+// The underlying memory is shared between copies of buffers and cloned if a
+// shared buffer is being modified.
 class Buffer {
  public:
   Buffer();                   // An empty buffer.
@@ -65,12 +71,12 @@ class Buffer {
   template <typename T, typename internal::ByteType<T>::t = 0>
   Buffer(const T* data, size_t size, size_t capacity)
       : Buffer(size, capacity) {
-    std::memcpy(data_.get(), data, size);
+    std::memcpy(data_->data_.get(), data, size);
   }
 
   // Construct a buffer from the contents of an array.
   template <typename T, size_t N, typename internal::ByteType<T>::t = 0>
-  Buffer(const T(&array)[N])
+  Buffer(const T(&array)[N])  // NOLINT: runtime/explicit
       : Buffer(array, N) {}
 
   ~Buffer();
@@ -79,42 +85,51 @@ class Buffer {
   // but you may also use .data<int8_t>() and .data<char>().
   template <typename T = uint8_t, typename internal::ByteType<T>::t = 0>
   const T* data() const {
-    assert(IsConsistent());
-    return reinterpret_cast<T*>(data_.get());
+    if (!data_) {
+      return nullptr;
+    }
+    RTC_DCHECK(IsConsistent());
+    return reinterpret_cast<T*>(data_->data_.get());
   }
+  // Get writable pointer to the data. This will create a copy of the underlying
+  // data if it is shared with other buffers.
   template <typename T = uint8_t, typename internal::ByteType<T>::t = 0>
   T* data() {
-    assert(IsConsistent());
-    return reinterpret_cast<T*>(data_.get());
+    if (!data_) {
+      return nullptr;
+    }
+    CloneDataIfReferenced();
+    return reinterpret_cast<T*>(data_->data_.get());
   }
 
   size_t size() const {
-    assert(IsConsistent());
-    return size_;
+    RTC_DCHECK(IsConsistent());
+    return data_ ? data_->size_ : 0;
   }
   size_t capacity() const {
-    assert(IsConsistent());
-    return capacity_;
+    RTC_DCHECK(IsConsistent());
+    return data_ ? data_->capacity_ : 0;
   }
 
   Buffer& operator=(const Buffer& buf) {
-    if (&buf != this)
-      SetData(buf.data(), buf.size());
+    if (&buf != this) {
+      SetData(buf);
+    }
     return *this;
   }
   Buffer& operator=(Buffer&& buf) {
-    assert(IsConsistent());
-    assert(buf.IsConsistent());
-    size_ = buf.size_;
-    capacity_ = buf.capacity_;
-    data_ = std::move(buf.data_);
-    buf.OnMovedFrom();
+    if (&buf != this) {
+      RTC_DCHECK(IsConsistent());
+      RTC_DCHECK(buf.IsConsistent());
+      data_ = std::move(buf.data_);
+      buf.OnMovedFrom();
+    }
     return *this;
   }
 
   bool operator==(const Buffer& buf) const {
-    assert(IsConsistent());
-    return size_ == buf.size() && memcmp(data_.get(), buf.data(), size_) == 0;
+    return data_ == buf.data_ ||
+           (size() == buf.size() && memcmp(data(), buf.data(), size()) == 0);
   }
 
   bool operator!=(const Buffer& buf) const { return !(*this == buf); }
@@ -123,25 +138,35 @@ class Buffer {
   // constructors.
   template <typename T, typename internal::ByteType<T>::t = 0>
   void SetData(const T* data, size_t size) {
-    assert(IsConsistent());
-    size_ = 0;
-    AppendData(data, size);
+    RTC_DCHECK(IsConsistent());
+    if (!data_ || !data_->HasOneRef()) {
+      data_ = new BufferData(data, size, size);
+    } else {
+      data_->SetData(data, size);
+    }
   }
   template <typename T, size_t N, typename internal::ByteType<T>::t = 0>
   void SetData(const T(&array)[N]) {
     SetData(array, N);
   }
-  void SetData(const Buffer& buf) { SetData(buf.data(), buf.size()); }
+  void SetData(const Buffer& buf) {
+    if (&buf != this) {
+      RTC_DCHECK(IsConsistent());
+      RTC_DCHECK(buf.IsConsistent());
+      data_ = buf.data_;
+    }
+  }
 
   // Append data to the buffer. Accepts the same types as the constructors.
   template <typename T, typename internal::ByteType<T>::t = 0>
   void AppendData(const T* data, size_t size) {
-    assert(IsConsistent());
-    const size_t new_size = size_ + size;
-    EnsureCapacity(new_size);
-    std::memcpy(data_.get() + size_, data, size);
-    size_ = new_size;
-    assert(IsConsistent());
+    if (!data_) {
+      data_ = new BufferData(data, size, size);
+      return;
+    }
+
+    CloneDataIfReferenced();
+    data_->AppendData(data, size);

[kwiberg-webrtc, 2016/02/15 12:33:16]

Here you'll potentially clone the data, then immediately reallocate it because
of insufficient capacity. Solve by passing required capacity to
CloneDataIfReferenced?

   }
   template <typename T, size_t N, typename internal::ByteType<T>::t = 0>
   void AppendData(const T(&array)[N]) {
@@ -154,76 +179,168 @@ class Buffer {
   // the existing contents will be kept and the new space will be
   // uninitialized.
   void SetSize(size_t size) {
-    EnsureCapacity(size);
-    size_ = size;
+    if (!data_) {
+      data_ = new BufferData(nullptr, size, size);
+      return;
+    }
+
+    CloneDataIfReferenced();
+    data_->SetSize(size);

[kwiberg-webrtc, 2016/02/15 12:33:16]

Here you'll potentially clone the data, then immediately reallocate it because
of insufficient capacity.

   }
 
   // Ensure that the buffer size can be increased to at least capacity without
   // further reallocation. (Of course, this operation might need to reallocate
   // the buffer.)
   void EnsureCapacity(size_t capacity) {
-    assert(IsConsistent());
-    if (capacity <= capacity_)
+    RTC_DCHECK(IsConsistent());
+    if (!data_) {
+      data_ = new BufferData(nullptr, 0, capacity);
       return;
-    std::unique_ptr<uint8_t[]> new_data(new uint8_t[capacity]);
-    std::memcpy(new_data.get(), data_.get(), size_);
-    data_ = std::move(new_data);
-    capacity_ = capacity;
-    assert(IsConsistent());
+    } else if (capacity <= data_->capacity_) {
+      return;
+    }
+
+    CloneDataIfReferenced();
+    data_->EnsureCapacity(capacity);

[kwiberg-webrtc, 2016/02/15 12:33:16]

Here you'll potentially clone the data, then immediately reallocate it because
of insufficient capacity.

   }
 
   // b.Pass() does the same thing as std::move(b).
   // Deprecated; remove in March 2016 (bug 5373).
   RTC_DEPRECATED Buffer&& Pass() { return DEPRECATED_Pass(); }
   Buffer&& DEPRECATED_Pass() {
-    assert(IsConsistent());
+    RTC_DCHECK(IsConsistent());
     return std::move(*this);
   }
 
-  // Resets the buffer to zero size and capacity. Works even if the buffer has
-  // been moved from.
+  // Resets the buffer to zero size and capacity.
   void Clear() {
-    data_.reset();
-    size_ = 0;
-    capacity_ = 0;
-    assert(IsConsistent());
+    if (!data_ || !data_->HasOneRef()) {
+      data_ = nullptr;
+    } else {
+      data_->Clear();
+    }
+    RTC_DCHECK(IsConsistent());
   }
 
-  // Swaps two buffers. Also works for buffers that have been moved from.
+  // Swaps two buffers.
   friend void swap(Buffer& a, Buffer& b) {
-    using std::swap;
-    swap(a.size_, b.size_);
-    swap(a.capacity_, b.capacity_);
-    swap(a.data_, b.data_);
+    std::swap(a.data_, b.data_);
   }
 
  private:
+  // Data stored in a buffer. RefCounted and will be shared between buffers that
+  // are cloned to avoid unnecessary allocations / copies.
+  class BufferData : public RefCountedObject<RefCountInterface> {
+   public:
+    // Construct data and copy the specified number of bytes into it.
+    BufferData(const void* data, size_t size, size_t capacity);
+
+    void SetData(const void* data, size_t size) {
+      RTC_DCHECK(IsConsistent());
+      size_ = 0;
+      AppendData(data, size);
+    }
+
+    void AppendData(const void* data, size_t size) {
+      RTC_DCHECK(IsConsistent());
+      const size_t new_size = size_ + size;
+      EnsureCapacity(new_size);
+      std::memcpy(data_.get() + size_, data, size);
+      size_ = new_size;
+      RTC_DCHECK(IsConsistent());
+    }
+
+    // Sets the size of the data. If the new size is smaller than the old, the
+    // data contents will be kept but truncated; if the new size is greater,
+    // the existing contents will be kept and the new space will be
+    // uninitialized.
+    void SetSize(size_t size) {
+      EnsureCapacity(size);
+      size_ = size;
+    }
+
+    // Ensure that the data size can be increased to at least capacity without
+    // further reallocation. (Of course, this operation might need to reallocate
+    // the buffer.)
+    void EnsureCapacity(size_t capacity) {
+      RTC_DCHECK(IsConsistent());
+      if (capacity <= capacity_) {
+        return;
+      }
+
+      std::unique_ptr<uint8_t[]> new_data(new uint8_t[capacity]);
+      std::memcpy(new_data.get(), data_.get(), size_);
+      data_ = std::move(new_data);
+      capacity_ = capacity;
+      RTC_DCHECK(IsConsistent());
+    }
+
+    // Resets the data to zero size and capacity.
+    void Clear() {
+      data_.reset();
+      size_ = 0;
+      capacity_ = 0;
+      RTC_DCHECK(IsConsistent());
+    }
+
+    // Precondition for all methods except Clear and the destructor.
+    // Postcondition for all methods except move construction and move
+    // assignment, which leave the moved-from object in a possibly inconsistent
+    // state.
+    bool IsConsistent() const {
+      return (data_ || capacity_ == 0) && capacity_ >= size_;

[kwiberg-webrtc, 2016/02/15 12:33:16]

Perhaps better to make this

  data_ && capacity_ > 0 && capacity_ >= size_;

and completely eliminate any remaining cases where you have a BufferData with
zero capacity? That way, you eliminate one of two ways to construct a
zero-capacity Buffer.

+    }
+
+#ifndef NDEBUG
+    // Called when *this has been moved from. Conceptually it's a no-op, but we
+    // can mutate the state slightly to help subsequent sanity checks catch
+    // bugs.
+    void OnMovedFrom() {
+      // Ensure that *this is always inconsistent, to provoke bugs.
+      size_ = 1;
+      capacity_ = 0;
+    }
+#endif
+
+    size_t size_;
+    size_t capacity_;
+    std::unique_ptr<uint8_t[]> data_;
+  };
+
   // Precondition for all methods except Clear and the destructor.
   // Postcondition for all methods except move construction and move
   // assignment, which leave the moved-from object in a possibly inconsistent
   // state.
   bool IsConsistent() const {
-    return (data_ || capacity_ == 0) && capacity_ >= size_;
+    return data_ == nullptr || data_->IsConsistent();
   }
 
   // Called when *this has been moved from. Conceptually it's a no-op, but we
   // can mutate the state slightly to help subsequent sanity checks catch bugs.
   void OnMovedFrom() {
 #ifdef NDEBUG
-    // Make *this consistent and empty. Shouldn't be necessary, but better safe
-    // than sorry.
-    size_ = 0;
-    capacity_ = 0;
+    data_ = nullptr;
 #else
     // Ensure that *this is always inconsistent, to provoke bugs.
-    size_ = 1;
-    capacity_ = 0;
+    data_ = new BufferData(nullptr, 0, 0);
+    data_->OnMovedFrom();
 #endif
   }
 
-  size_t size_;
-  size_t capacity_;
-  std::unique_ptr<uint8_t[]> data_;
+  // Create a copy of the underlying data if it is referenced from other Buffer
+  // objects.
+  void CloneDataIfReferenced() {
+    RTC_DCHECK(data_ != nullptr);
+    RTC_DCHECK(IsConsistent());
+    if (data_->HasOneRef()) {
+      return;
+    }
+
+    data_ = new BufferData(data_->data_.get(), data_->size_,
+        data_->capacity_);
+  }
+
+  scoped_refptr<BufferData> data_;
 };
 
 }  // namespace rtc