Adding partially circular memory buffer wrapper.

content/common/partial_circular_buffer.h

+// Copyright (c) 2013 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CONTENT_COMMON_PARTIAL_CIRCULAR_BUFFER_H_
+#define CONTENT_COMMON_PARTIAL_CIRCULAR_BUFFER_H_
+
+#include "base/basictypes.h"
+#include "base/gtest_prod_util.h"
+
+namespace content {
+
+// A wrapper around a memory buffer that allows circular read and write with a
+// selectable wrapping position. Buffer layout (after wrap; H is header):
+// -----------------------------------------------------------
+// | H | Beginning           | End           | Middle        |
+// -----------------------------------------------------------
+//  ^---- Non-wrapping -----^ ^--------- Wrapping ----------^
+// The non-wrapping part is never overwritten. The wrapping part will be
+// circular. The very first part is the header (see the BufferData struct
+// below). It consists of the following information:
+// - Length written to the buffer (not including header).
+// - Wrapping position.
+// - End position of buffer. (If the last byte is at x, this will be x + 1.)
+// Users of wrappers around the same underlying buffer must ensure that writing
+// is finished before reading is started.
+class PartialCircularBuffer {
+ public:
+  // Use for reading. |buffer_size| is in bytes and must be larger than the
+  // header size (see above).
+  PartialCircularBuffer(void* buffer, size_t buffer_size);
+
+  // Use for writing. |buffer_size| is in bytes and must be larger than the
+  // header size (see above).
+  PartialCircularBuffer(void* buffer,
+                        size_t buffer_size,
+                        size_t wrap_position);
+
+  size_t Read(void* buffer, size_t buffer_size);
+  void Write(const void* buffer, size_t buffer_size);
+
+ private:
+  FRIEND_TEST_ALL_PREFIXES(PartialCircularBufferTest, TestHeaderSize);
+
+// The reason of packing the struct is to simplify test code since the header
+// and data size will the same for all platforms.
+#pragma pack(push)

[tommi (sloooow) - chröme, 2013/04/11 16:37:39]

I still think we should have the packing in there.  For anything IPC related we
use fixed sizes that are equal across all platforms.

[Henrik Grunell, 2013/04/14 07:06:49]

Does this fall in the "IPC related" bin? It's an interpretation of the buffer
contents that will take place in two places. The actual data isn't sent over
IPC. And don't we get an unnecessary performance penalty?

+#pragma pack(4)
+  struct BufferData {
+    uint32 total_written;
+    uint32 wrap_position;
+    uint32 end_position;
+    uint8 data[1];
+  };
+#pragma pack(pop)
+
+  void DoWrite(void* dest, const void* src, size_t num);
+
+  // Used for reading and writing.
+  BufferData* buffer_data_;
+  size_t memory_buffer_size_;
+  size_t data_size_;
+  size_t position_;
+
+  // Used for reading.
+  size_t total_read_;
+};
+
+}  // namespace content
+
+#endif  // CONTENT_COMMON_PARTIAL_CIRCULAR_BUFFER_H_