tracing v2: Introduce TraceBufferReader to read-back the trace buffer

components/tracing/core/trace_buffer_reader.cc

Index: components/tracing/core/trace_buffer_reader.cc
diff --git a/components/tracing/core/trace_buffer_reader.cc b/components/tracing/core/trace_buffer_reader.cc
new file mode 100644
index 0000000000000000000000000000000000000000..8607fc26cf86532a023bb83c37989117d1642502
--- /dev/null
+++ b/components/tracing/core/trace_buffer_reader.cc
@@ -0,0 +1,223 @@
+// 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_reader.h"
+
+#include <cctype>
+#include <deque>
+#include <utility>
+
+#include "base/memory/ptr_util.h"
+#include "base/strings/string_piece.h"
+#include "components/tracing/core/proto_utils.h"
+#include "components/tracing/core/string_interning.h"
+#include "components/tracing/proto/event.pbzero.h"
+#include "components/tracing/proto/events_chunk.pbzero.h"
+
+namespace tracing {
+namespace v2 {
+
+TraceBufferReader::TraceBufferReader(uint32_t options) : options_(options) {
+  DCHECK((options & kBuildInternedStringsTable) ||
+         (options & kBuildLegacyJSONEvents));
+  if (options & kBuildLegacyJSONEvents)
+    legacy_events_ = base::MakeUnique<base::ListValue>();
+}
+
+TraceBufferReader::~TraceBufferReader() {}
+
+void TraceBufferReader::ParseChunk(const uint8_t* const start, size_t length) {
+  using ChunkProto = pbzero::tracing::proto::EventsChunk;
+  uint32_t writer_id = 0;
+  bool has_writer_id = false;
+  uint32_t seq_id = 0;
+  bool has_seq_id = false;
+  bool first_event_continues_from_prev_chunk = false;
+  bool last_event_continues_on_next_chunk = false;
+  using Range = std::pair<const uint8_t*, const uint8_t*>;  // (begin, end)
+  std::deque<Range> events;
+
+  const uint8_t* const end = start + length;
+  if (end == start)
+    return;  // Empty chunk.
+
+  // Iterate over all the proto fields in the chunk. Each chunk proto message
+  // contains some global properties for the chunk and one submessage for each
+  // trace event.
+  const uint8_t *cur, *next;
+  for (cur = start, next = nullptr; cur < end; cur = next) {
+    uint32_t field_id;
+    proto::FieldType field_type;
+    uint64_t field_intvalue;
+    const uint8_t* field_payload = nullptr;
+    next = proto::ParseField(cur, end, &field_id, &field_type, &field_intvalue);
+    DCHECK(next && next <= end);
+    if (field_type == proto::kFieldTypeLengthDelimited) {
+      // In the case of a length-delimited field, |field_intvalue| contains
+      // the size of the field. |field_payload| points to the start of the
+      // payload (either a string or a submessage).
+      field_payload = next - field_intvalue;
+      DCHECK(cur < field_payload && field_payload <= next);
+    }
+    switch (field_id) {
+      case ChunkProto::kWriterIdFieldNumber:
+        DCHECK_EQ(proto::kFieldTypeVarInt, field_type);
+        writer_id = static_cast<uint32_t>(field_intvalue);
+        has_writer_id = true;
+        break;
+      case ChunkProto::kSeqIdFieldNumber:
+        DCHECK_EQ(proto::kFieldTypeVarInt, field_type);
+        has_seq_id = true;
+        seq_id = static_cast<uint32_t>(field_intvalue);
+        break;
+      case ChunkProto::kFirstEventContinuesFromPrevChunkFieldNumber:
+        DCHECK_EQ(proto::kFieldTypeVarInt, field_type);
+        first_event_continues_from_prev_chunk = field_intvalue ? true : false;
+        break;
+      case ChunkProto::kLastEventContinuesOnNextChunkFieldNumber:
+        DCHECK_EQ(proto::kFieldTypeVarInt, field_type);
+        last_event_continues_on_next_chunk = field_intvalue ? true : false;
+        break;
+      case ChunkProto::kEventsFieldNumber:
+        DCHECK_EQ(proto::kFieldTypeLengthDelimited, field_type);
+        events.push_back(std::make_pair(field_payload, next));
+        break;
+      default:
+        NOTREACHED();
+    }
+  }
+  DCHECK_EQ(end, cur);
+  DCHECK(has_seq_id);
+  DCHECK(has_writer_id);
+
+  WriterState* writer_state = &writer_state_[writer_id];
+
+  // When |first_event_continues_from_prev_chunk| is true, the first event is
+  // a fragment. The fragment will be parsed only if we saw all the previous
+  // fragments AND there are no more fragments upcoming.
+  if (first_event_continues_from_prev_chunk) {
+    DCHECK(!events.empty());
+    if (writer_state->last_fragment_is_valid &&
+        seq_id == writer_state->last_seq_id + 1) {
+      // The chunk sequence is valid (no drops).
+      writer_state->partial_event_data.insert(
+          writer_state->partial_event_data.end(), events[0].first,
+          events[0].second);
+      writer_state->last_seq_id = seq_id;
+
+      // If:
+      // - The current chunk contains > 1 event, in which case the current
+      //   fragment was the last one.
+      // OR
+      // - The current chunks contains 1 event, but is not flagged to to
+      //   continue in the next chunk.
+      // All the sequence of fragments that have been merged in
+      // |partial_event_data| is valid and can be processed now.
+      if (events.size() > 1 || !last_event_continues_on_next_chunk) {
+        ParseEvent(&*writer_state->partial_event_data.begin(),
+                   &*writer_state->partial_event_data.end());
+        writer_state->Reset();
+      }
+    } else {
+      // The previous chunk has been missed, dropping a previous fragment. This
+      // can happen if the trace ring buffer wraps over and the previous chunk
+      // is recycled.
+      writer_state->Reset();
+    }
+    events.pop_front();
+  }
+
+  size_t event_count = 1;
+  for (const Range& event : events) {
+    if (event_count++ == events.size() && last_event_continues_on_next_chunk) {
+      writer_state->last_fragment_is_valid = true;
+      writer_state->last_seq_id = seq_id;
+      writer_state->partial_event_data.insert(
+          writer_state->partial_event_data.end(), event.first, event.second);
+      break;
+    }
+    ParseEvent(event.first, event.second);
+  }
+}
+
+void TraceBufferReader::ParseEvent(const uint8_t* const start,
+                                   const uint8_t* const end) {
+  using EventProto = pbzero::tracing::proto::Event;
+  std::unique_ptr<base::DictionaryValue> event_value;
+  if (options_ & kBuildLegacyJSONEvents)
+    event_value = base::MakeUnique<base::DictionaryValue>();
+
+  DCHECK_GT(end, start);
+  const uint8_t *cur, *next;
+  for (cur = start, next = nullptr; cur < end; cur = next) {
+    uint32_t field_id;
+    proto::FieldType field_type;
+    uint64_t field_intvalue;
+    next = proto::ParseField(cur, end, &field_id, &field_type, &field_intvalue);
+    base::StringPiece field_strvalue;
+    if (field_type == proto::kFieldTypeLengthDelimited) {
+      const size_t str_len = static_cast<size_t>(field_intvalue);
+      const uint8_t* const str_start = next - str_len;
+      DCHECK(cur < str_start && str_start <= next);
+      field_strvalue =
+          base::StringPiece(reinterpret_cast<const char*>(str_start), str_len);
+    }
+    const char* interned_string = nullptr;
+    if (field_id == EventProto::kNameIdFieldNumber ||
+        field_id == EventProto::kCategoryIdFieldNumber) {
+      const int64_t str_id = static_cast<int64_t>(field_intvalue);
+      interned_string = GetInternedStringValue(str_id);
+      if (options_ & kBuildInternedStringsTable)
+        interned_strings_[str_id] = interned_string;

[DmitrySkiba, 2016/11/29 18:09:56]

Hmm, str_id is int64_t here, but interned_strings_ is keyed by int32_t.

+    }
+
+    if (options_ & kBuildLegacyJSONEvents) {
+      switch (field_id) {
+        case EventProto::kTypeFieldNumber: {
+          char event_phase[] = {static_cast<char>(field_intvalue), '\0'};
+          DCHECK(std::isalnum(event_phase[0]));
+          event_value->SetString("ph", base::StringPiece(event_phase, 1));
+          break;
+        }
+        case EventProto::kNameStrFieldNumber:
+          // For the rare cases of TRACE_STR_COPY.
+          event_value->SetString("name", field_strvalue);
+          break;
+        case EventProto::kNameIdFieldNumber:
+          // For the most common case of event name being a const char* string.
+          event_value->SetString("name", interned_string);
+          break;
+        case EventProto::kCategoryIdFieldNumber:
+          event_value->SetString("cat", interned_string);
+          break;
+        case EventProto::kTimestampFieldNumber:
+          event_value->SetInteger("ts", static_cast<int>(field_intvalue));
+          break;
+        case EventProto::kThreadTimestampFieldNumber:
+          event_value->SetInteger("tts", static_cast<int>(field_intvalue));
+          break;
+        default:
+          NOTREACHED();
+      }
+    }
+  }
+  DCHECK_EQ(end, cur);
+  if (options_ & kBuildLegacyJSONEvents)
+    legacy_events_->Append(std::move(event_value));
+}
+
+TraceBufferReader::WriterState::WriterState() {
+  Reset();
+}
+
+TraceBufferReader::WriterState::~WriterState() {}
+
+void TraceBufferReader::WriterState::Reset() {
+  last_seq_id = 0;
+  last_fragment_is_valid = false;
+  partial_event_data.clear();
+}
+
+}  // namespace v2
+}  // namespace tracing