Tracing V2 prototype [NOT FOR REVIEW]

base/trace_event/v2/scattered_buffer.h

Index: base/trace_event/v2/scattered_buffer.h
diff --git a/base/trace_event/v2/scattered_buffer.h b/base/trace_event/v2/scattered_buffer.h
new file mode 100644
index 0000000000000000000000000000000000000000..5a06f49ba876ac5eaa6b283fababd0cb80afb985
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+++ b/base/trace_event/v2/scattered_buffer.h
@@ -0,0 +1,74 @@
+// Copyright 2016 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 BASE_TRACE_EVENT_V2_SCATTERED_BUFFER_H_
+#define BASE_TRACE_EVENT_V2_SCATTERED_BUFFER_H_
+
+#include <inttypes.h>
+
+#include <array>
+#include <memory>
+
+#include "base/base_export.h"
+#include "base/macros.h"
+
+namespace base {
+namespace trace_event {
+namespace v2 {
+
+// This class deals with the following problem: AppendOnlyProtoMessage wants
+// to write a stream of bytes, without caring about the implementation of the
+// underlying buffer, which concretely will be either the trace RingBuffer
+// or a heap-allocated buffer. The main deal is: AppendOnlyProtoMessage does not
+// know in advance what the size of the message will be.
+// Due to the tracing buffer being split into fixed-size chunks, in some
+// occasions these writes nees to be spread over two (or more) non contiguous
+// chunks of memory. Similarly, when the buffer is backed by the heap, we want
+// to avoid realloc() calls (which can require a memcpy of the full contents).
+// The purpose of this class is to abtract away the non-contiguous write logic.
+// This class knows how to deal with writes as long as they fall in the same
+// ContiguousMemoryRange, and defers the chunk-chaining logic to the Delegate.
+class BASE_EXPORT ScatteredBuffer {
+ public:
+  struct ContiguousMemoryRange {
+   uint8_t* begin;
+   uint8_t* end;  // STL style: one byte past the end of the buffer.
+  };
+
+  class Delegate {
+   public:
+    virtual ContiguousMemoryRange GetNewContiguousMemoryBuffer() = 0;
+  };
+
+  ScatteredBuffer(Delegate* delegate);
+  virtual ~ScatteredBuffer();
+
+  void Reset(ContiguousMemoryRange range);
+  void WriteByte(uint8_t value);
+  void WriteBytes(const uint8_t* src, size_t size);
+  ContiguousMemoryRange ReserveBytes(size_t size);
+  void WriteReservedBytes(const ContiguousMemoryRange& reserved_range, const uint8_t* src, size_t size);
+
+  // Number of free bytes in the current range that can be written without
+  // reuqesting a new contiguous buffer.
+  size_t contiguous_bytes_available() const { return static_cast<size_t>(cur_range_.end - wrptr_); }
+
+  // ContiguousMemoryRange cur_range() const { return cur_range_; }
+  uint8_t* write_ptr() const { return wrptr_; }
+
+ private:
+  void Expand();
+
+  Delegate* delegate_;
+  ContiguousMemoryRange cur_range_;
+  uint8_t* wrptr_;
+
+  DISALLOW_COPY_AND_ASSIGN(ScatteredBuffer);
+};
+
+}  // namespace v2
+}  // namespace trace_event
+}  // namespace base
+
+#endif  // BASE_TRACE_EVENT_V2_SCATTERED_BUFFER_H_