tracing v2: Introduce TraceBufferWriter

components/tracing/core/trace_buffer_writer.h

+// 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_TRACE_BUFFER_WRITER_H_
+#define COMPONENTS_TRACING_CORE_TRACE_BUFFER_WRITER_H_
+
+#include "base/macros.h"
+#include "components/tracing/core/proto_zero_message.h"
+#include "components/tracing/core/proto_zero_message_handle.h"
+#include "components/tracing/core/scattered_stream_writer.h"
+#include "components/tracing/core/trace_ring_buffer.h"
+#include "components/tracing/tracing_export.h"
+
+namespace tracing {
+
+// TODO(primiano): in next CL the Event class will just come from the C++
+// header generated by the ProtoZero plugin.
+namespace proto {
+class Event : public v2::ProtoZeroMessage {};
+}
+
+namespace v2 {
+
+using TraceEventHandle = ProtoZeroMessageHandle<proto::Event>;
+
+// This class is the entry-point to add events to the TraceRingBuffer.
+// It acts as a glue layer between the protobuf classes (ProtoZeroMessage) and
+// the chunked trace ring buffer (TraceRingBuffer). This class is deliberately
+// NOT thread safe. The expected design is that each thread owns an instance of
+// TraceBufferWriter and that trace events produced on one thread are not passed
+// to other threads.
+class TRACING_EXPORT TraceBufferWriter
+    : public ScatteredStreamWriter::Delegate {
+ public:
+  // |trace_buffer| is the underlying ring buffer for taking and returning

[alph, 2016/08/04 19:06:52]

you're referencing the arg name which is missing in the declaration.

[Primiano Tucci (use gerrit), 2016/08/05 11:23:38]

Done.

+  // chunks. |writer_id| is an identifier, unique for each instance, which is
+  // appended to the header of each chunk. Its purpose is to allow the importer
+  // to reconstruct the logical sequence of chunks for a given writer. Think to
+  // this as a thread-id (just, in rare cases, some threads can have more than
+  // one writer).
+  TraceBufferWriter(TraceRingBuffer*, uint32_t writer_id);
+  ~TraceBufferWriter();

[alph, 2016/08/04 19:06:52]

Could you please make the Delegate destructor virtual? Just to be on the safe
side.

[Primiano Tucci (use gerrit), 2016/08/05 11:23:38]

oh yeah sure.

+
+  // Adds a new event and returns a handle to it. The new event is valid (can be
+  // populated) until the next call to AddEvent(). The new event is finalized
+  // and fully committed to the ring buffer on the next call to AddEvent() or
+  // when the returned handle goes out of scope, whichever comes first.
+  TraceEventHandle AddEvent();
+
+  // ScatteredStreamWriter::Delegate implementation.
+  // Called by the ProtoZeroMessage's ScatteredStreamWriter when the caller
+  // tries to append a new field and the write overflows the current chunk.
+  ContiguousMemoryRange GetNewBuffer() override;
+
+  // Finalize the pending event (if any) and returns all chunks.

[alph, 2016/08/04 19:06:52]

doesn't seem to return anything.

[Primiano Tucci (use gerrit), 2016/08/05 11:23:38]

Reworded the comment. I meant "returns back" the chunks to the main buffer. Not
returns as in C return :)

+  void Flush();
+
+  const ScatteredStreamWriter& stream_writer() const { return stream_writer_; }
+
+  uint32_t writer_id() const { return writer_id_; }
+
+ private:
+  void FinalizeCurrentEvent();
+  void FinalizeCurrentChunk();
+  ContiguousMemoryRange AcquireNewChunk(bool event_continues_from_prev_chunk);
+  uint8_t* WriteEventPrambleForNewChunk(uint8_t* begin);
+
+  TraceRingBuffer* trace_ring_buffer_;
+
+  // Unique id of this writer (see comment in the ctor).
+  const uint32_t writer_id_;
+
+  // Monotonic counter (within the scope of writer_id_) of chunks.
+  uint32_t chunk_seq_id_;
+
+  // The last chunk acquired from the ring buffer. nullptr before the first call
+  // to AddEvent(). Each instance can own more than one chunk at any given time
+  // (to deal with messages larger than a chunk). This is being tracked through
+  // the singly linked list Chunk::next_in_owner_list().
+  TraceRingBuffer::Chunk* chunk_;
+
+  // Pointer to the byte which represents the Chunk's proto field
+  // |continue_on_next_chunk| (a boolean).
+  uint8_t* continue_on_next_chunk_ptr_;
+
+  // Used to work out how many bytes for the current |event_| lie in the current
+  // |chunk_|.
+  uint8_t* event_data_start_in_current_chunk_;
+
+  ScatteredStreamWriter stream_writer_;
+
+  // This field should be a proto::Event. However, ProtoZeroMessage subclasses
+  // are stateless by design. This avoids to pull the full tree of autogenerated
+  // headers for the stub classes and reduce build time.
+  ProtoZeroMessage event_;
+
+  DISALLOW_COPY_AND_ASSIGN(TraceBufferWriter);
+};
+
+}  // namespace v2
+}  // namespace tracing
+
+#endif  // COMPONENTS_TRACING_CORE_TRACE_BUFFER_WRITER_H_