Support building yt/cpp and yt/yt/core with vanilla protobuf

After this PR yt/cpp and yt/yt/core are possible to be built both with Arcadia protobuf (that uses TString as a string) and vanilla protobuf (that uses std::string as a string). To achieve so, a couple of interoperability primitives are introduced.

* `TProtobufString` is an alias to protobuf string type, i.e. it can be `TString` or `std::string` depending on the protobuf implementation.
* `IsVanillaProtobuf` and `IsArcadiaProtobuf` are the constexpr boolean values that allow to check protobuf implementation both in the compile time and runtime.

The most challenging interoperability issue solved here is a string copy between protobuf message and C++ code that has a form of `TString str = msg.str()`. This code works perfect with Arcadia protobuf but does not work with vanilla protobuf. To solve it, a previously introduced primitive `FromProto<TString>` is used. This expression makes the most efficient cast possible between protobuf string and C++ string. Internally, it is just a copy in both cases. Since TString is CoW by default, this expression is almost zero-cost (actually it's just one atomic operation), so no degradation is expected for YTsaurus server builds. The most hot code is handled differently to avoid even atomic operations (see `GetRequestTargetYPath`). In case of vanilla protobuf string is copied, however there are no places in C++ SDK where it might be a problem. If such issues would appear, performance-critial code can be rewritten in `GetRequestTargetYPath`-style.

---
1a6f3e02cb6e83915102c24b73bc8734f6a48e74

Pull Request resolved: https://github.com/ytsaurus/ytsaurus/pull/466

1 files changed, 11 insertions, 6 deletions

diff --git a/yt/cpp/mapreduce/io/proto_table_reader.cpp b/yt/cpp/mapreduce/io/proto_table_reader.cpp
index 555c5345a7d..c0bf90d2536 100644
--- a/yt/cpp/mapreduce/io/proto_table_reader.cpp
+++ b/yt/cpp/mapreduce/io/proto_table_reader.cpp
@@ -4,6 +4,8 @@
 
 #include "proto_helpers.h"
 
+#include <yt/yt/core/misc/protobuf_helpers.h>
+
 #include <yt/yt_proto/yt/formats/extension.pb.h>
 
 #include <util/string/escape.h>
@@ -15,16 +17,19 @@ using ::google::protobuf::Descriptor;
 using ::google::protobuf::FieldDescriptor;
 using ::google::protobuf::EnumValueDescriptor;
 
-const TString& GetFieldColumnName(const FieldDescriptor* fieldDesc) {
-    const auto& columnName = fieldDesc->options().GetExtension(column_name);
+using NYT::FromProto;
+
+TString GetFieldColumnName(const FieldDescriptor* fieldDesc)
+{
+    auto columnName = FromProto<TString>(fieldDesc->options().GetExtension(column_name));
     if (!columnName.empty()) {
         return columnName;
     }
-    const auto& keyColumnName = fieldDesc->options().GetExtension(key_column_name);
+    auto keyColumnName = FromProto<TString>(fieldDesc->options().GetExtension(key_column_name));
     if (!keyColumnName.empty()) {
         return keyColumnName;
     }
-    return fieldDesc->name();
+    return FromProto<TString>(fieldDesc->name());
 }
 
 void ReadMessageFromNode(const TNode& node, Message* row)
@@ -48,10 +53,10 @@ void ReadMessageFromNode(const TNode& node, Message* row)
             continue; // null field
         }
 
-        auto checkType = [&columnName] (TNode::EType expected, TNode::EType actual) {
+        auto checkType = [fieldDesc] (TNode::EType expected, TNode::EType actual) {
             if (expected != actual) {
                 ythrow TNode::TTypeError() << "expected node type " << expected
-                    << ", actual " << actual << " for node " << columnName.data();
+                    << ", actual " << actual << " for node " << GetFieldColumnName(fieldDesc);
             }
         };