Running Hermes for VisionOS

[okwasniewski]

Hey!

I'm working on bringing React Native to VisionOS (here). I've been trying to compile the app with Hermes enabled but unfortunately without success.

After bringing this topic on RN Contributors Discord folks from Microsoft working on RN MacOS pointed out that building Hermes from source didn't work for them either and they suggested trying out the prebuilt version.

However, the prebuilt version doesn't support VisionOS (as it supports MacOS and iOS). I don't want to follow the path of React Native TvOS which has its own fork of Hermes. Can we add support for VisionOS in Hermes podspec or is there any different way to make it work for Out Of Tree Platforms?

Coming back to the error I was getting when building from source: Undefined symbols: hermes::vm::NopCrashManager::~NopCrashManager()

Expand the full error log

ld: Undefined symbols:
  hermes::vm::NopCrashManager::~NopCrashManager(), referenced from:
      std::__1::default_delete<hermes::vm::NopCrashManager>::operator()[abi:v160006](hermes::vm::NopCrashManager*) const in libReact-hermes.a[5](HermesExecutorFactory.o)
  facebook::hermes::inspector_modern::chrome::CDPHandler::registerCallbacks(std::__1::function<void (std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>> const&)>, std::__1::function<void ()>), referenced from:
      facebook::hermes::inspector_modern::chrome::ConnectionDemux::addPage(std::__1::shared_ptr<facebook::hermes::inspector_modern::chrome::CDPHandler>)::$_0::operator()(std::__1::unique_ptr<facebook::react::jsinspector_modern::IRemoteConnection, std::__1::default_delete<facebook::react::jsinspector_modern::IRemoteConnection>>) const in libReact-hermes.a[4](ConnectionDemux.o)
  facebook::hermes::inspector_modern::chrome::CDPHandler::unregisterCallbacks(), referenced from:
      facebook::hermes::inspector_modern::chrome::ConnectionDemux::removePage(int) in libReact-hermes.a[4](ConnectionDemux.o)
      facebook::hermes::inspector_modern::chrome::(anonymous namespace)::LocalConnection::disconnect() in libReact-hermes.a[4](ConnectionDemux.o)
  facebook::hermes::inspector_modern::chrome::CDPHandler::create(std::__1::unique_ptr<facebook::hermes::inspector_modern::RuntimeAdapter, std::__1::default_delete<facebook::hermes::inspector_modern::RuntimeAdapter>>, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>> const&, bool), referenced from:
      facebook::hermes::inspector_modern::chrome::ConnectionDemux::enableDebugging(std::__1::unique_ptr<facebook::hermes::inspector_modern::RuntimeAdapter, std::__1::default_delete<facebook::hermes::inspector_modern::RuntimeAdapter>>, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>> const&) in libReact-hermes.a[4](ConnectionDemux.o)
  facebook::hermes::inspector_modern::chrome::CDPHandler::handle(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>), referenced from:
      facebook::hermes::inspector_modern::chrome::(anonymous namespace)::LocalConnection::sendMessage(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>) in libReact-hermes.a[4](ConnectionDemux.o)
  facebook::hermes::makeHermesRuntime(hermes::vm::RuntimeConfig const&), referenced from:
      facebook::react::HermesExecutorFactory::createJSExecutor(std::__1::shared_ptr<facebook::react::ExecutorDelegate>, std::__1::shared_ptr<facebook::react::MessageQueueThread>) in libReact-hermes.a[5](HermesExecutorFactory.o)
  facebook::hermes::inspector_modern::chrome::CDPHandler::getTitle() const, referenced from:
      facebook::hermes::inspector_modern::chrome::ConnectionDemux::enableDebugging(std::__1::unique_ptr<facebook::hermes::inspector_modern::RuntimeAdapter, std::__1::default_delete<facebook::hermes::inspector_modern::RuntimeAdapter>>, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>> const&) in libReact-hermes.a[4](ConnectionDemux.o)
      facebook::hermes::inspector_modern::chrome::ConnectionDemux::removePage(int) in libReact-hermes.a[4](ConnectionDemux.o)
      facebook::hermes::inspector_modern::chrome::ConnectionDemux::addPage(std::__1::shared_ptr<facebook::hermes::inspector_modern::chrome::CDPHandler>) in libReact-hermes.a[4](ConnectionDemux.o)
      facebook::hermes::inspector_modern::chrome::(anonymous namespace)::LocalConnection::LocalConnection(std::__1::shared_ptr<facebook::hermes::inspector_modern::chrome::CDPHandler>, std::__1::shared_ptr<std::__1::unordered_set<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>, std::__1::hash<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>, std::__1::equal_to<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>>>) in libReact-hermes.a[4](ConnectionDemux.o)
      facebook::hermes::inspector_modern::chrome::(anonymous namespace)::LocalConnection::disconnect() in libReact-hermes.a[4](ConnectionDemux.o)
  vtable for hermes::vm::CrashManager, referenced from:
      hermes::vm::CrashManager::CrashManager() in libReact-hermes.a[5](HermesExecutorFactory.o)
  vtable for hermes::vm::NopCrashManager, referenced from:
      hermes::vm::NopCrashManager::NopCrashManager() in libReact-hermes.a[5](HermesExecutorFactory.o)
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Feel free to let me know if you need more details!

[Saadnajmi]

Just to chime in, we saw this error building Hermes from source for RN macOS too. Looks like something broke recently on main.

[tmikov]

We will look into this. FWIW, I am building Hermes on MacOS daily and it certainly builds without error. However I am not using CocoaPods. Unfortunately no one on our team really understands or uses CocoaPods. Last time I tried even installing it, it was a challenge (see https://x.com/tmikov/status/1705991622100676812?s=20).

FWIW, I don't understand why TvOS needs a fork of Hermes. IIRC, there was a PR which needed some minor improvements, but they didn't seem interested in working on it.

[Saadnajmi]

@shwanton might have more context on the macOS + cocoapods hit

[okwasniewski]

Okay I will try to open a PR to support VisionOS based on the PR created for tvOS: #1000

[neildhar]

Hey @okwasniewski, could you share repro steps for observing this linker error on macOS? These symbols are exported by libhermes, so they should be resolved when you link against libhermes.

[neildhar]

The failure here is in HermesExecutorFactory and in the parts of the Hermes inspector that live on the RN side. These take direct dependencies on functions exported by the Hermes framework.

[Saadnajmi]

Ah, so native RN components. I thought you meant like, a JS react component. Makes sense!

[dannysu]

Does the same issue occur when not building for Debug?
In the upstream RN repo, there are various places that check for HERMES_ENABLE_DEBUGGER, which should be true only for Debug.

[neildhar]

@okwasniewski Given what @Saadnajmi said above, a good starting point for investigating this may be to bisect the problem in both Hermes/RN and identify when/what change caused this build to stop working.

If you could share the results of your bisect, we can help figure out what to do next.

[okwasniewski]

Thanks for the insights! I will check it out and provide details of my investigation here.

[okwasniewski]

I've spent a significant amount of time debugging what's actually happening here. And it turns out for MacOS Hermes wasn't properly linked for local Xcode builds. I've adjusted the build script here to fix the issue for MacOS: facebook/react-native#41387

Regarding visionOS first issue was the same - Hermes wasn't linked properly. But it also wasn't built targeting visionOS (it was built for MacOS as it's the default for CMake).

The issue was laying inside of CMake and flag that it passes (-mtargetos=xros) it was passing it for both XROS and XRSimulator builds. And it wasn't properly building the app.

Only after temporarily passing correct target flag (and compiling CMake from source removing previous flags), it started to work:

if(HERMES_APPLE_TARGET_PLATFORM MATCHES "xrsimulator")
  set(CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS} "-target arm64-apple-xros1.0-simulator")
endif()
if(HERMES_APPLE_TARGET_PLATFORM MATCHES "xros")
  set(CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS} "-target arm64-apple-xros1.0")
endif()

Here is the corresponding issue in CMake repo: https://gitlab.kitware.com/cmake/cmake/-/issues/25188

I'll wait with adding visionOS support to Hermes until that issue is resolved and a stable version of CMake 3.28 will be released.

Screenshot of visionOS running Hermes: