Reland of tracing v2: Introduce TraceBufferWriter (https://codereview.chromium.org/2196663002)

components/tracing/core/proto_utils.h

Index: components/tracing/core/proto_utils.h
diff --git a/components/tracing/core/proto_utils.h b/components/tracing/core/proto_utils.h
index 016975d298757b8ccba71af153afd3194c067004..fdadf1a18df0b98f0f269b23c02c978c659a78c0 100644
--- a/components/tracing/core/proto_utils.h
+++ b/components/tracing/core/proto_utils.h
@@ -15,9 +15,6 @@ namespace tracing {
 namespace v2 {
 namespace proto {
 
-// TODO(kraynov): Change namespace to tracing::proto::internal.
-// This is required in headers and it's too low-level to be exposed.
-
 // See https://developers.google.com/protocol-buffers/docs/encoding wire types.
 
 enum : uint32_t {
@@ -35,7 +32,7 @@ constexpr size_t kMaxMessageLength = (1u << (kMessageLengthFieldSize * 7)) - 1;
 // Largest value of simple (not length-delimited) field is 64-bit varint
 // (10 bytes at most). 15 bytes buffer is enough to store a simple field.
 constexpr size_t kMaxTagEncodedSize = 5;
-constexpr size_t kMaxSimpleFieldEncodedSize = 15;
+constexpr size_t kMaxSimpleFieldEncodedSize = kMaxTagEncodedSize + 10;
 
 // Proto types: (int|uint|sint)(32|64), bool, enum.
 constexpr uint32_t MakeTagVarInt(uint32_t field_id) {
@@ -76,21 +73,27 @@ inline uint8_t* WriteVarInt(T value, uint8_t* target) {
 }
 
 // Writes a fixed-size redundant encoding of the given |value|. This is
-// used to backfill fixed-size reservations for the length of field using a
+// 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. |kMessageLengthFieldSize|
 // bytes are reserved to encode the size field and backfilled at the end.
-inline void WriteRedundantLength(uint32_t value, uint8_t* buf) {
-  DCHECK_LE(value, kMaxMessageLength) << "Message is too long";
+inline void WriteRedundantVarInt(uint32_t value, uint8_t* buf) {
   for (size_t i = 0; i < kMessageLengthFieldSize; ++i) {
     const uint8_t msb = (i < kMessageLengthFieldSize - 1) ? 0x80 : 0;
     buf[i] = static_cast<uint8_t>(value) | msb;
     value >>= 7;
   }
+  DCHECK_EQ(0u, value) << "Message is too long";
 }
 
+template <uint32_t field_id>
+void StaticAssertSingleBytePreamble() {
+  static_assert(field_id < 16,
+                "Proto field id too big to fit in a single byte preamble");
+};
+
 }  // namespace proto
 }  // namespace v2
 }  // namespace tracing