tracing v2: Add base class for zero-copy protobuf (ProtoZero)

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,
 };
 
+// Maximum message size supported: 256 MiB (4 x 7-bit due to varint encoding).
+constexpr size_t kMessageLengthFieldSize = 4;
+constexpr size_t kMaxMessageLength = (1u << (kMessageLengthFieldSize * 7)) - 1;
+
 // Returns the number of bytes sufficient to encode the largest
 // |int_size_in_bits|-bits integer using a non-redundant varint encoding.
 template <typename T>
 constexpr size_t GetMaxVarIntEncodedSize() {
   return (sizeof(T) * 8 + 6) / 7;
 }
 
 // Variable-length field types: (s)int32, (s)int64, bool, enum.
 inline uint32_t MakeTagVarInt(uint32_t field_id) {
   return (field_id << 3) | kFieldTypeVarInt;
@@ skipping 50 matching lines @@
     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_