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// 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"
#include "components/tracing/proto/event.pbzero.h"
#include "components/tracing/proto/events_chunk.pbzero.h"
namespace tracing {
namespace v2 {
namespace {
using ChunkProto = pbzero::tracing::proto::EventsChunk;
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<pbzero::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::WriteRedundantVarInt(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::WriteVarInt(writer_id_, chunk_proto);
proto::StaticAssertSingleBytePreamble<ChunkProto::kSeqIdFieldNumber>();
*chunk_proto++ =
static_cast<uint8_t>(proto::MakeTagVarInt(ChunkProto::kSeqIdFieldNumber));
chunk_proto = proto::WriteVarInt(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() {
if (!chunk_)
return;
FinalizeCurrentEvent();
FinalizeCurrentChunk(false /* is_fragmenting_event */);
trace_ring_buffer_->ReturnChunk(chunk_);
chunk_ = nullptr;
}
} // namespace v2
} // namespace tracing
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