runtime: immediate lock-up when using go library in Unity 6

[arjenveenhuizen]

Go version

go1.21.1 darwin/arm64 (we have observed identical behaviour when using go 1.23.1 and 1.23.4)

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='arm64'
GOBIN=''
GOCACHE='Library/Caches/go-build'
GOENV='Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='temp/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/$HOME/temp/go/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.21.1'
GCCGO='gccgo'
AR='ar'
CC='clang'
CXX='clang++'
CGO_ENABLED='1'
GOMOD='/dev/null'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/f8/0nlswm8x445_jl3720jx6kmw0000gn/T/go-build2270444360=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

When running on Apple Silicon hardware:

Prerequisites:

1. Extract plugin code: TestPlugin.zip
2. Extract Unity 2022.3.43f1 repro project: Unity2022ReproProject.zip. Do not open the project yet.
3. Extract Unity 6000.0.28f1 repro project: Unity6ReproProject.zip. Do not open the project yet.
4. Install dylibbundler required for dylib signing (brew install dylibbundler)
5. Build attached TestPlugin go lib as .dylib on MacOS (using cgo). You can use the included magefile.
6. Copy build/TestPlugin.dylib to the extracted Unity 2022 project in Assets/Plugins/TestPlugin.dylib.
7. Copy the same build/TestPlugin.dylib to the extracted Unity 6 project in Assets/Plugins/TestPlugin.dylib.

What did you see happen?

In Unity 2022

Unity 2022 (and also other versions like Unity 2021 and Unity 2023) is working as expected.

Open the Unity 2022 project, open the Scenes/SampleScene and click the Play button. Observe that the Unity Console shows:

Calling HelloWorld on plugin...
Successfully called HelloWorld on plugin...

If you open ~/Library/Logging/Unity/Editor.log it will show:

Calling HelloWorld on plugin...
UnityEngine.StackTraceUtility:ExtractStackTrace () (at /Users/bokken/build/output/unity/unity/Runtime/Export/Scripting/StackTrace.cs:37)
UnityEngine.DebugLogHandler:LogFormat (UnityEngine.LogType,UnityEngine.Object,string,object[])
UnityEngine.Logger:Log (UnityEngine.LogType,object)
UnityEngine.Debug:Log (object)
MacOSPluginLoader:Start () (at Assets/Scripts/MacOSPluginLoader.cs:17)

(Filename: Assets/Scripts/MacOSPluginLoader.cs Line: 17)

Hello from Go!
Hello from Go 2!
Hello from Go 3!
Successfully called HelloWorld on plugin...
UnityEngine.StackTraceUtility:ExtractStackTrace () (at /Users/bokken/build/output/unity/unity/Runtime/Export/Scripting/StackTrace.cs:37)
UnityEngine.DebugLogHandler:LogFormat (UnityEngine.LogType,UnityEngine.Object,string,object[])
UnityEngine.Logger:Log (UnityEngine.LogType,object)
UnityEngine.Debug:Log (object)
MacOSPluginLoader:Start () (at Assets/Scripts/MacOSPluginLoader.cs:19)

In Unity 6

Unity 6 is immediately locking up (showing a beachball spinner) as soon as any go API is called.

Open the Unity 6 project, open the Scenes/SampleScene and click the Play button. Observe that the Unity Console shows:

Calling HelloWorld on plugin...

If you open ~/Library/Logging/Unity/Editor.log it will show:

Calling HelloWorld on plugin...
UnityEngine.StackTraceUtility:ExtractStackTrace () (at /Users/bokken/build/output/unity/unity/Runtime/Export/Scripting/StackTrace.cs:37)
UnityEngine.DebugLogHandler:LogFormat (UnityEngine.LogType,UnityEngine.Object,string,object[])
UnityEngine.Logger:Log (UnityEngine.LogType,object)
UnityEngine.Debug:Log (object)
MacOSPluginLoader:Start () (at Assets/Scripts/MacOSPluginLoader.cs:17)

What did you expect to see?

Unity 2022 and Unity 6 to behave the same. Instead, Unity 2022 is behaving as expected wheras Unity 6 immediately locks-up the Unity Editor.

[ianlancetaylor]

I'm sorry, I don't know what Unity is. But from your description, this seems like a problem with Unity, not a problem with Go. Have you raised this with the developers of Unity?

[arjenveenhuizen]

I'm sorry, I don't know what Unity is.

Unity is a 3D game engine that is used abundantly.

But from your description, this seems like a problem with Unity, not a problem with Go. Have you raised this with the developers of Unity?

Plugins written in plain C/C++ work just fine in the Unity Editor. When you add (c-)go in the mix Unity locks-up, hence our believe that this (c-)go related.

In the meantime we have a couple more findings:

1. The exact same go code works fine in Unity 6000.0.28f1 when compiled as a Windows DLL.
2. If you build the Unity 6 project into a stand-alone app on MacOS it also works OK.

[ianlancetaylor]

Just to set expectations, while the problem may be Go related, since we don't know anything about Unity it's unlikely that we'll be able to figure this out.

[arjenveenhuizen]

Just to set expectations, while the problem may be Go related, since we don't know anything about Unity it's unlikely that we'll be able to figure this out.

Appreciated that. We are willing to contribute. Any pointer on where to start debugging this issue would be appreciated.

To set the scene, the following minimal example (from the ZIP files attached to the first post) is causing the lock-up:

package main

// #include <stdio.h>
// #include <stdlib.h>
import "C"
import (
	"fmt"
)

//export HelloWorld
func HelloWorld() {
	fmt.Println("Hello from Go!")
}

func main() {}

Build:

go build -o ./TestPlugin.dylib -buildmode=c-shared -ldflags="-s -w" TestPlugin.go

C#:

    private const string PluginRelativePath =  "TestPlugin";
    [DllImport(PluginRelativePath, EntryPoint = "HelloWorld", CallingConvention = CallingConvention.Cdecl)]
    private static extern void HelloWorld();

    void Start() {
            Debug.Log("Calling HelloWorld on plugin...");
            HelloWorld();
            Debug.Log("Successfully called HelloWorld on plugin...");
    }

Unity locks up when executing HelloWorld();

[ianlancetaylor]

I'm sorry, I have no idea. From your description, it works with one version of Unity and fails with a different one. So something must have changed in Unity, but I have no idea what.

[arjenveenhuizen]

We have reported this issue to Unity (issue 91704, I do not have a public link (yet?)).

Our own investigation (by sprinkling log lines in the go source code) has shown that the go runtime is at least reaching x_cgo_init:

go/src/runtime/cgo/gcc_darwin_arm64.c

Line 129 in 05d8984

x_cgo_init(G *g, void (*setg)(void*))

and we see a number of threads spinning up in _cgo_sys_thread_start:

go/src/runtime/cgo/gcc_darwin_arm64.c

Line 27 in 05d8984

_cgo_sys_thread_start(ThreadStart *ts)

when the plugin is loaded by the unity editor.

But when we call the exported function, it immediately hangs. At this point, I have no clue where to look next. Are there other spots where I can add some log lines to see what is going on? Any pointer is greatly appreciated.

[ianlancetaylor]

When a Go library is built with -buildmode=c-shared, then when the library is loaded the Go initializers are run automatically in a separate thread. Because Go initializers must be complete before any ordinary Go code can run, if the non-Go code calls a Go function, we wait for all Go initializers to be complete before permitting the Go function call to proceed. This means that we can have a deadlock in the case where code in a Go initializer calls a non-Go function that then calls a Go function. The call to the Go function will wait for initializers to complete, but that will never happen because the initializer is waiting for the non-Go function to complete. Perhaps that is what is happening here, somehow.