tracing v2: Add proto_utils.h

components/tracing/core/proto_utils.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_PROTO_UTILS_H_
+#define COMPONENTS_TRACING_CORE_PROTO_UTILS_H_
+
+#include <inttypes.h>
+
+#include <type_traits>
+
+#include "base/logging.h"
+#include "base/macros.h"
+#include "components/tracing/tracing_export.h"
+
+namespace tracing {
+namespace v2 {
+namespace proto {
+
+// See https://developers.google.com/protocol-buffers/docs/encoding wire types.
+
+enum : uint32_t {
+  kFieldTypeVarInt = 0,
+  kFieldTypeFixed64 = 1,
+  kFieldTypeLengthDelimited = 2,
+  kFieldTypeFixed32 = 5,
+};
+
+// Variable-length field types: (s)int32, (s)int64, bool, enum.
+inline uint32_t MakeTagVarInt(uint32_t field_id) {
+  return (field_id << 3) | kFieldTypeVarInt;
+}
+
+// Length-limited field types: string, bytes, embedded messages.
+inline uint32_t MakeTagLengthLimited(uint32_t field_id) {
+  return (field_id << 3) | kFieldTypeLengthDelimited;
+}
+
+// 32-bit fixed-length field types: fixed32, sfixed32, float.
+inline uint32_t MakeTagFixed32(uint32_t field_id) {
+  return (field_id << 3) | kFieldTypeFixed32;
+}
+
+// 64-bit fixed-length field types: fixed64, sfixed64, double.
+inline uint32_t MakeTagFixed64(uint32_t field_id) {
+  return (field_id << 3) | kFieldTypeFixed64;
+}
+
+template <typename T>
+inline uint8_t* WriteVarIntInternal(T value, uint8_t* target) {
+  static_assert(std::is_unsigned<T>::value, "value must be unsigned");
+  while (value >= 0x80) {
+    *target++ = static_cast<uint8_t>(value | 0x80);
+    value >>= 7;
+  }
+  *target = static_cast<uint8_t>(value);
+  return target + 1;
+}
+
+inline uint8_t* WriteVarIntU32(uint32_t value, uint8_t* target) {
+  return WriteVarIntInternal<uint32_t>(value, target);
+}
+
+inline uint8_t* WriteVarIntU64(uint64_t value, uint8_t* target) {
+  return WriteVarIntInternal<uint64_t>(value, target);
+}
+
+// TODO(kraynov): add support for signed integers and zig-zag encoding.
+
+// Writes a fixed-size redundant encoding of the given |value|. This is
+// used to backfill fixed-size reservations for the length field using a
+// non-canonical varint encoding (e.g. \x81\x80\x80\x00 instead of \x01).
+// See https://github.com/google/protobuf/issues/1530.
+// In particular, this is used for nested messages. The size of a nested message
+// is not known until all its field have been written. A fixed amount of bytes
+// are reserved to encode the size field and backfilled at the end.

[petrcermak, 2016/07/07 13:08:17]

Is the semantics "reserved (...) and backfilled (...)" or "reserved to (encode
(...) and backfilled (...))"? If it's the latter, then it should be "backfill".

[petrcermak, 2016/07/07 13:08:17]

nit: "... amount ... IS reserved ... " (this is a tricky one, I'm fine either
way).

[Primiano Tucci (use gerrit), 2016/07/07 15:48:38]

The former: reserved (to encode the size field) and (backfilled at the end)

[Primiano Tucci (use gerrit), 2016/07/07 15:48:38]

I knew you would have pointed this out. I am not sure either, but yeah "is"
sounds a bit better.

+template <size_t LENGTH>
+void WriteRedundantVarIntU32(uint32_t value, uint8_t* buf) {
+  for (size_t i = 0; i < LENGTH; ++i) {
+    const uint8_t msb = (i < LENGTH - 1) ? 0x80 : 0;
+    buf[i] = static_cast<uint8_t>((value & 0x7F) | msb);
+    value >>= 7;
+  }
+  DCHECK_EQ(0u, value) << "Buffer too short to encode the given value";
+}
+
+}  // namespace proto
+}  // namespace v2
+}  // namespace tracing
+
+#endif  // COMPONENTS_TRACING_CORE_PROTO_UTILS_H_