tracing v2: Add ScatteredStreamWriter

components/tracing/core/scattered_stream_writer.h

Index: components/tracing/core/scattered_stream_writer.h
diff --git a/components/tracing/core/scattered_stream_writer.h b/components/tracing/core/scattered_stream_writer.h
new file mode 100644
index 0000000000000000000000000000000000000000..f5b0b770d852c3b16fb8449de2fbe10293b77da6
--- /dev/null
+++ b/components/tracing/core/scattered_stream_writer.h
@@ -0,0 +1,76 @@
+// 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 COMPONENTS_TRACING_CORE_SCATTERED_STREAM_WRITER_H_
+#define COMPONENTS_TRACING_CORE_SCATTERED_STREAM_WRITER_H_
+
+#include <stdint.h>
+
+#include "base/macros.h"
+#include "components/tracing/tracing_export.h"
+
+namespace tracing {
+namespace v2 {
+
+struct ContiguousMemoryRange {
+  uint8_t* begin;
+  uint8_t* end;  // STL style: one byte past the end of the buffer.
+};
+
+// This class deals with the following problem: append-only proto messages want
+// to write a stream of bytes, without caring about the implementation of the
+// underlying buffer (which concretely will be either the trace ring buffer
+// or a heap-allocated buffer). The main deal is: proto messages don't know in
+// advance what their size will be.
+// Due to the tracing buffer being split into fixed-size chunks, on some
+// occasions, these writes need 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, as they might cause a full copy of the contents
+// of the buffer.
+// 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 TRACING_EXPORT ScatteredStreamWriter {
+ public:
+  class Delegate {
+   public:
+    virtual ContiguousMemoryRange GetNewBuffer() = 0;
+  };
+
+  explicit ScatteredStreamWriter(Delegate* delegate);
+  ~ScatteredStreamWriter();
+
+  void WriteByte(uint8_t value);
+  void WriteBytes(const uint8_t* src, size_t size);
+
+  // Reserves a fixed amount of bytes to be backfilled later. The reserved range
+  // is guaranteed to be contiguous and not span across chunks.
+  ContiguousMemoryRange ReserveBytes(size_t size);
+
+  // Resets the buffer boundaries and the write pointer to the given |range|.
+  // Subsequent WriteByte(s) will write into |range|.
+  void Reset(ContiguousMemoryRange range);
+
+  // Number of contiguous free bytes in |cur_range_| that can be written without
+  // requesting a new buffer.
+  size_t bytes_available() const {
+    return static_cast<size_t>(cur_range_.end - write_ptr_);
+  }
+
+  uint8_t* write_ptr() const { return write_ptr_; }
+
+ private:
+  void Extend();
+
+  Delegate* const delegate_;
+  ContiguousMemoryRange cur_range_;
+  uint8_t* write_ptr_;
+
+  DISALLOW_COPY_AND_ASSIGN(ScatteredStreamWriter);
+};
+
+}  // namespace v2
+}  // namespace tracing
+
+#endif  // COMPONENTS_TRACING_CORE_SCATTERED_STREAM_WRITER_H_