tracing v2: Introduce TraceBufferWriter

components/tracing/proto/events_chunk.proto

Index: components/tracing/proto/events_chunk.proto
diff --git a/components/tracing/proto/events_chunk.proto b/components/tracing/proto/events_chunk.proto
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index 0000000000000000000000000000000000000000..837dfec56d6f29e7e61cc1e7656eba7753fc104b
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+++ b/components/tracing/proto/events_chunk.proto
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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.
+
+syntax = "proto2";
+option optimize_for = LITE_RUNTIME;
+
+package tracing.proto;
+
+// An EventsChunk is a fixed portion of the trace ring buffer which is written
+// exclusively by one writer (almost always 1 writer == 1 thread). It contains
+// a variable number of variable-sized events. Events can be fragmented across
+// several chunks.
+message EventsChunk {
+  // Chunks produced by the same trace writer have the same writer_id. This
+  // allows to identify the next chunk for a writer (thread) in the ring buffer.
+  optional uint32 writer_id = 1;
+
+  // A monotonic counter within the writer, to establish sequencing of chunks.
+  optional uint32 seq_id = 2;
+
+  // Each event is a byte sequence that encapsulates the bytes for a single
+  // Event (see event.proto) or an incomplete fragment.
+  // Why this isn't just a "repeated Events" field? An Event can spawn across
+  // several chunks. In this case the reader has to first glue the byte
+  // sequences together and then process the event.
+  repeated bytes events = 3;
+
+  // The first entry of |events| is a fragment which continues from the previous
+  // chunk.
+  optional bool first_event_continues_from_prev_chunk = 4;
+
+  // The last entry of |events| is a fragment and contiues into the next chunk.
+  optional bool last_event_continues_on_next_chunk = 5;
+}