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..8c61f612674ef1a5df050cace35d1b90f26943a4
--- /dev/null
+++ b/components/tracing/core/scattered_stream_writer.h
@@ -0,0 +1,75 @@
+// 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 <inttypes.h>

[oystein (OOO til 10th of July), 2016/06/15 08:35:09]

Can we rely on this? Unsure what the status of C99 is.

[Primiano Tucci (use gerrit), 2016/06/29 09:32:02]

ehm, here I really meant stdint (for uint32_t etc), brain collision.
(Btw lot of the codebase seems to depend on inttypes so I think we de-facto can
:)).
Anyways, removing in favor of stindt, that's what I really meant here.

+
+#include "base/macros.h"
+#include "components/tracing/tracing_export.h"
+
+namespace tracing {
+namespace v2 {
+
+// 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 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

[oystein (OOO til 10th of July), 2016/06/15 08:35:10]

nit: need

[Primiano Tucci (use gerrit), 2016/06/29 09:32:02]

Done.

+// chunks of memory. Similarly, when the buffer is backed by the heap, we want
+// to avoid realloc() calls, as they can cause an expensive memcpy to expand.
+// 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:
+  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;
+  };
+
+  explicit ScatteredStreamWriter(Delegate* delegate);
+  ~ScatteredStreamWriter();
+
+  void WriteByte(uint8_t value);
+  void WriteBytes(const uint8_t* src, size_t size);
+
+  // Reserves a fixed number of bytes, to be backfilled later through
+  // WriteReservedBytes().
+  ContiguousMemoryRange ReserveBytes(size_t size);
+  void WriteReservedBytes(const ContiguousMemoryRange& reserved_range,
+                          const uint8_t* src);
+
+  void Reset(ContiguousMemoryRange range);
+
+  // Number of free bytes in the current range that can be written without
+  // reuqesting a new contiguous buffer.

[oystein (OOO til 10th of July), 2016/06/15 08:35:09]

nit: requesting

[Primiano Tucci (use gerrit), 2016/06/29 09:32:02]

Done.

+  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 Expand();
+
+  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_