Protobuf's constant initialization is not actually constant on Windows

[davidben]

What version of protobuf and what language are you using?
Version: main
Language: C++

What operating system (Linux, Windows, ...) and version?
Windows 10 20H2

What runtime / compiler are you using (e.g., python version or gcc version)
clang-cl from Visual Studio 2019

What did you do?
Steps to reproduce the behavior:

1. Install Visual Studio 2019
2. In the installer, include "C++ Clang tools for Windows"
3. git clone https://github.com/protocolbuffers/protobuf.git
4. cd protobuf
5. git submodule update --init --recursive
6. cmake -B build-shared -D BUILD_SHARED_LIBS=1 -T ClangCL
7. cmake --build build-shared --parallel 16

What did you expect to see
The build should succeed

What did you see instead?
The build fails with errors like:

...\protobuf\src\google\protobuf\compiler\plugin.pb.cc(75,31): message : required by 'require_constant_initializatio
n' attribute here [...\protobuf\build-shared\libprotoc.vcxproj]
...\protobuf\src\google/protobuf/port_def.inc(656,30): message : expanded from macro 'PROTOBUF_CONSTINIT' [...\pr
otobuf\build-shared\libprotoc.vcxproj]
...\protobuf\src\google\protobuf\compiler\plugin.pb.cc(91,125): error : variable does not have a constant initialize
r [...\protobuf\build-shared\libprotoc.vcxproj]

Anything else we should know about your project / environment
This was an issue updating protobuf in Chromium. We've applied a workaround by patching out the PROTOBUF_CONSTINIT in some configurations, but the root problem is that protobuf's new constant initialization was not actually cross-platform, and relies on assumption that aren't actually universally true.

While, for the repro steps, I used clang-cl, this is only because protobuf has a pre-C++20 version of PROTOBUF_CONSTINIT for Clang, [[clang::require_constant_initialization]]. When protobuf moves to C++20, I expect it will have the same problem on MSVC.

This happens because the protobuf constant initialization assumes it can reference &fixed_address_empty_string. fixed_address_empty_string lives in the protobuf dll, so when referenced from another dll or exe, is __declspec(dllimport). But pointers to dllimport variables require a static initializer on Windows. See https://godbolt.org/z/5rn4xWrhE. As I understand it, Windows does not have a suitable relocation to express this.

As a result, the new constant initialization actually requires a per-message static initializer, which trips PROTOBUF_CONSTINIT and fails to build.

[davidben]

When protobuf moves to C++20, I expect it will have the same problem on MSVC.

Yup! See https://godbolt.org/z/qTxb8vocb

[goldenbull]

I have the same problem. I have to use c++20 because std::format is only available in c++20. Currently I have to hack the header file google\protobuf\port_def.inc, change #define PROTOBUF_CONSTINIT constinit to #define PROTOBUF_CONSTINIT

[github-actions]

We triage inactive PRs and issues in order to make it easier to find active work. If this issue should remain active or becomes active again, please add a comment.

This issue is labeled inactive because the last activity was over 90 days ago.

[github-actions]

We triage inactive PRs and issues in order to make it easier to find active work. If this issue should remain active or becomes active again, please reopen it.

This issue was closed and archived because there has been no new activity in the 14 days since the inactive label was added.

[dmiller423]

Ran into this upon update to [4?.] 25.0, building w. clang-cl.
Also many invalid offsetof() warnings..