Revert of tracing v2: Introduce TraceBufferWriter

components/tracing/core/trace_buffer_writer.cc

-// 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.
-
-#include "components/tracing/core/trace_buffer_writer.h"
-
-#include "base/compiler_specific.h"
-#include "base/logging.h"
-#include "components/tracing/core/proto_utils.h"
-
-namespace tracing {
-namespace v2 {
-
-namespace {
-
-// TODO(primiano) remove this in next CLs. This should just be taken from the
-// C++ class autogenerated from events_chunk.proto (crbug.com/608721).
-struct ChunkProto {
-  enum : uint32_t {
-    kWriterIdFieldNumber = 1,
-    kSeqIdInStreamFieldNumber = 2,
-    kEventsFieldNumber = 3,
-    kFirstEventContinuesFromPrevChunkFieldNumber = 4,
-    kLastEventContinuesOnNextChunkFieldNumber = 5
-  };
-};
-
-const size_t kEventPreambleSize = 1 + proto::kMessageLengthFieldSize;
-
-// TODO(primiano): replace 16 with a more reasonable size, that is, the size
-// of a simple trace event with no args.
-const size_t kMinEventSize = 16;
-
-}  // namespace
-
-TraceBufferWriter::TraceBufferWriter(TraceRingBuffer* trace_ring_buffer,
-                                     uint32_t writer_id)
-    : trace_ring_buffer_(trace_ring_buffer),
-      writer_id_(writer_id),
-      chunk_seq_id_(0),
-      chunk_(nullptr),
-      event_data_start_in_current_chunk_(nullptr),
-      stream_writer_(this) {
-  event_.Reset(&stream_writer_);
-}
-
-TraceBufferWriter::~TraceBufferWriter() {}
-
-void TraceBufferWriter::FinalizeCurrentEvent() {
-  if (UNLIKELY(!chunk_))
-    return;
-
-  // Finalize the last event added. This ensures that it and all its nested
-  // fields are committed to the ring buffer and sealed. No further changes to
-  // the chunks's memory can be made from the |event_| after this point.
-  event_.Finalize();
-
-  // In the unlikely event that the last event did wrap over one or more chunks,
-  // is is now time to return those chunks (all but the active one) back.
-  TraceRingBuffer::Chunk* retained_chunk = chunk_->next_in_owner_list();
-  if (UNLIKELY(retained_chunk)) {
-    while (retained_chunk) {
-      TraceRingBuffer::Chunk* next = retained_chunk->next_in_owner_list();
-      retained_chunk->set_next_in_owner_list(nullptr);
-      trace_ring_buffer_->ReturnChunk(retained_chunk);
-      retained_chunk = next;
-    }
-    chunk_->set_next_in_owner_list(nullptr);
-  }
-}
-
-TraceEventHandle TraceBufferWriter::AddEvent() {
-  FinalizeCurrentEvent();
-
-  // In order to start a new event at least kMessageLengthFieldSize + 1 bytes
-  // are required in the chunk to write the preamble and size of the event
-  // itself. We take a bit more room here, it doesn't make a lot of sense
-  // starting a partial event that will fragment immediately after.
-  static_assert(kMinEventSize >= proto::kMessageLengthFieldSize + 1,
-                "kMinEventSize too small");
-  if (stream_writer_.bytes_available() < kMinEventSize)
-    stream_writer_.Reset(AcquireNewChunk(false /* is_fragmenting_event */));
-
-  event_.Reset(&stream_writer_);
-  WriteEventPreambleForNewChunk(
-      stream_writer_.ReserveBytesUnsafe(kEventPreambleSize));
-  DCHECK_EQ(stream_writer_.write_ptr(), event_data_start_in_current_chunk_);
-  return TraceEventHandle(static_cast<::tracing::proto::Event*>(&event_));
-}
-
-// This is invoked by the ProtoZeroMessage write methods when reaching the
-// end of the current chunk during a write.
-ContiguousMemoryRange TraceBufferWriter::GetNewBuffer() {
-  return AcquireNewChunk(true /* is_fragmenting_event */);
-}
-
-void TraceBufferWriter::FinalizeCurrentChunk(bool is_fragmenting_event) {
-  DCHECK(!is_fragmenting_event || chunk_);
-  if (!chunk_)
-    return;
-  uint8_t* write_ptr = stream_writer_.write_ptr();
-  DCHECK_GE(write_ptr, chunk_->payload());
-  DCHECK_LE(write_ptr, chunk_->end() - 2);
-
-  if (is_fragmenting_event) {
-    proto::StaticAssertSingleBytePreamble<
-        ChunkProto::kLastEventContinuesOnNextChunkFieldNumber>();
-    *write_ptr++ = static_cast<uint8_t>(proto::MakeTagVarInt(
-        ChunkProto::kLastEventContinuesOnNextChunkFieldNumber));
-    *write_ptr++ = 1;  // = true.
-  }
-
-  DCHECK_LT(static_cast<uintptr_t>(write_ptr - chunk_->payload()), kChunkSize);
-  chunk_->set_used_size(static_cast<uint32_t>(write_ptr - chunk_->payload()));
-}
-
-// There are paths that lead to AcquireNewChunk():
-// When |is_fragmenting_event| = false:
-//   AddEvent() is called and there isn't enough room in the current chunk to
-//   start a new event (or we don't have a chunk yet).
-// When |is_fragmenting_event| = true:
-//   The client is writing an event, a ProtoZeroMessage::Append* method hits
-//   the boundary of the chunk and requests a new one via GetNewBuffer().
-ContiguousMemoryRange TraceBufferWriter::AcquireNewChunk(
-    bool is_fragmenting_event) {
-  FinalizeCurrentChunk(is_fragmenting_event);
-  TraceRingBuffer::Chunk* new_chunk = trace_ring_buffer_->TakeChunk(writer_id_);
-  if (is_fragmenting_event) {
-    // Backfill the size field of the event with the partial size accumulated
-    // so far in the old chunk. WriteEventPreambleForNewChunk() will take care
-    // of resetting the |size_field| of the event to the new chunk.
-    DCHECK_GE(event_data_start_in_current_chunk_, chunk_->payload());
-    DCHECK_LE(event_data_start_in_current_chunk_,
-              chunk_->end() - proto::kMessageLengthFieldSize);
-    const uint32_t event_partial_size = static_cast<uint32_t>(
-        stream_writer_.write_ptr() - event_data_start_in_current_chunk_);
-    proto::WriteRedundantVarIntU32<proto::kMessageLengthFieldSize>(
-        event_partial_size, event_.size_field().begin);
-    event_.inc_size_already_written(event_partial_size);
-
-    // If this is a continuation of an event, this writer needs to retain the
-    // old chunk. The client might still be able to write to it. This is to deal
-    // with the case of a nested message which is started in one chunk and
-    // ends in another one. The finalization needs to write-back the size field
-    // in the old chunk.
-    new_chunk->set_next_in_owner_list(chunk_);
-  } else if (chunk_) {
-    // Otherwise, if this is a new event, the previous chunk can be returned.
-    trace_ring_buffer_->ReturnChunk(chunk_);
-  }
-  chunk_ = new_chunk;
-
-  // Write the protobuf for the chunk header. The generated C++ stub for
-  // events_chunk.proto cannot be used here because that would re-enter this
-  // class and make this code extremely hard to reason about.
-  uint8_t* chunk_proto = new_chunk->payload();
-
-  proto::StaticAssertSingleBytePreamble<ChunkProto::kWriterIdFieldNumber>();
-  *chunk_proto++ = static_cast<uint8_t>(
-      proto::MakeTagVarInt(ChunkProto::kWriterIdFieldNumber));
-  chunk_proto = proto::WriteVarIntU32(writer_id_, chunk_proto);
-
-  proto::StaticAssertSingleBytePreamble<
-      ChunkProto::kSeqIdInStreamFieldNumber>();
-  *chunk_proto++ = static_cast<uint8_t>(
-      proto::MakeTagVarInt(ChunkProto::kSeqIdInStreamFieldNumber));
-  chunk_proto = proto::WriteVarIntU32(chunk_seq_id_, chunk_proto);
-
-  if (is_fragmenting_event) {
-    proto::StaticAssertSingleBytePreamble<
-        ChunkProto::kFirstEventContinuesFromPrevChunkFieldNumber>();
-    *chunk_proto++ = static_cast<uint8_t>(proto::MakeTagVarInt(
-        ChunkProto::kFirstEventContinuesFromPrevChunkFieldNumber));
-    *chunk_proto++ = 1;  // = true.
-  }
-
-  ++chunk_seq_id_;
-
-  // If the new chunk was requested while writing an event (the event spans
-  // across chunks) write a new preamble for the partial event in the new chunk.
-  if (is_fragmenting_event)
-    chunk_proto = WriteEventPreambleForNewChunk(chunk_proto);
-
-  // We reserve 2 bytes from the end, so that FinalizeCurrentChunk() can use
-  // them to write the |last_event_continues_on_next_chunk| field.
-  return {chunk_proto, new_chunk->end() - 2};
-}
-
-// Writes the one-byte preamble for the start of either a new or a partial
-// event and reserves kMessageLengthFieldSize bytes for its length. Also
-// keeps size-field the bookkeeping up to date. Returns the pointer in the chunk
-// past the event preamble, where the event proto should be written.
-uint8_t* TraceBufferWriter::WriteEventPreambleForNewChunk(uint8_t* begin) {
-  // The caller must have ensured to have enough room in the chunk. The event
-  // preamble itself cannot be fragmented.
-  uint8_t* const end = begin + kEventPreambleSize;
-  proto::StaticAssertSingleBytePreamble<ChunkProto::kEventsFieldNumber>();
-  *begin++ = static_cast<uint8_t>(
-      proto::MakeTagLengthDelimited(ChunkProto::kEventsFieldNumber));
-  ContiguousMemoryRange range = {begin, end};
-  event_.set_size_field(range);
-  event_data_start_in_current_chunk_ = end;
-  return end;
-}
-
-void TraceBufferWriter::Flush() {
-  FinalizeCurrentEvent();
-  FinalizeCurrentChunk(false /* is_fragmenting_event */);
-  trace_ring_buffer_->ReturnChunk(chunk_);
-  chunk_ = nullptr;
-}
-
-}  // namespace v2
-}  // namespace tracing