Wrong initialization of fixed_address_empty_string causes ShutdownProtobufLibrary to fail

[danieljoos]

What version of protobuf and what language are you using?
Version: v3.14.0, master
Language: C++

What operating system (Linux, Windows, ...) and version?
Windows

What runtime / compiler are you using (e.g., python version or gcc version)
MSVC 14.27.29110
Platform Toolset v142 (Visual Studio 2019)
Debug Runtime

What did you do?
Steps to reproduce the behavior:

#include "addressbook.pb.h"

int main() {
	::google::protobuf::ShutdownProtobufLibrary();
}

with addressbook.proto (it really doesn't matter at all):

syntax = "proto3";

package tutorial;

message AddressBook {
  string name = 1;
}

What did you expect to see

No exception when execution ShutdownProtobufLibrary(). The program should just run-through without any issues.

What did you see instead?

The program throws an exception when run in Debug mode on executing ShutdownProtobufLibrary().

It is caused by attempting to destruct a broken std::string stored ::google::protobuf::internal::fixed_address_empty_string.value.
The reason is that the implementation relies on a static initialization of that variable. The variable fixed_address_empty_string is default constructed to dummy (here

protobuf/src/google/protobuf/message_lite.h

Line 158 in bd9a710

constexpr EmptyString() : dummy{} {}

).
However, as the initialization of static global variables is undefined across translation units in C++, it is possible that InitProtobufDefaultsImpl() (

protobuf/src/google/protobuf/generated_message_util.cc

Line 82 in bd9a710

::new (static_cast<void*>(&fixed_address_empty_string.value)) std::string();

) is called before the initialization of the fixed_address_empty_string (

protobuf/src/google/protobuf/generated_message_util.cc

Line 77 in bd9a710

fixed_address_empty_string; // NOLINT

). This causes the allocated std::string value to get broken, as the dummy value gets initialized "over" the existing std::string object.

This happens, for example when including a generated protobuf header file as a call to AddDescriptors() is forced there to happen during dynamic initialization time.

The actual exception is thrown by the MSCV STL on destruction as the std::string is now malformed.

Anything else we should know about your project / environment

[great944]

Hello, I see this issue with windows VS2019 16.8.3.61104. is it possible to merge this fix to master please? just updated to latest release and started seeing this issue when shutting down the program with a debug build.

[danieljoos]

Hi. Unfortunately, none of the above linked commit actually fixes the issue. I was playing around in my fork and it looks like Github already associated these commits here. Not sure how it can be unlinked.

The actual issue cannot be fixed easily, imo. The problem is the EmptyString structure that relies on constinit, which is not available on MSVC.

The only viable workaround at the moment is to comment-out the DestroyString call for the fixed_address_empty_string in case of msvc debug builds.

[great944]

thank you for the update

[danieljoos]

Seems to be fixed with v3.15.1