runtime/cgo: misuse of an invalid Handle

[brodo]

Hi there!
First of all, thank you for maintaining this project!

Steps to reproduce:

1. Clone https://github.com/brodo/goBoids
2. Check out the bugreport tag
3. Run the project
4. Hit localhost:3000 a couple of times. (Refresh until it crashes.)
   Here is the complete error log:

panic: runtime/cgo: misuse of an invalid Handle
	panic: runtime/cgo: misuse of an invalid Handle

goroutine 1 [running, locked to thread]:
runtime/cgo.Handle.Delete(0x140000021c0?)
	/opt/homebrew/opt/go/libexec/src/runtime/cgo/handle.go:137 +0x60
panic({0x100ea0180?, 0x100f098c0?})
	/opt/homebrew/opt/go/libexec/src/runtime/panic.go:792 +0x124
runtime/cgo.Handle.Value(0x1400047d8c8?)
	/opt/homebrew/opt/go/libexec/src/runtime/cgo/handle.go:124 +0x60
github.com/cogentcore/webgpu/wgpu.gowebgpu_buffer_map_callback_go(0x0, 0x1006f7b5c?)
	/Users/brodo/go/pkg/mod/github.com/cogentcore/webgpu@v0.23.0/wgpu/buffer.go:65 +0x4c
github.com/cogentcore/webgpu/wgpu._Cfunc_wgpuQueueSubmitForIndex(0x600001a8c9b0, 0x1, 0x600001894030)
	_cgo_gotypes.go:2442 +0x34
github.com/cogentcore/webgpu/wgpu.(*Queue).Submit.func3(...)
	/Users/brodo/go/pkg/mod/github.com/cogentcore/webgpu@v0.23.0/wgpu/queue.go:79
github.com/cogentcore/webgpu/wgpu.(*Queue).Submit(0x1400010a050, {0x1400047dd98, 0x1, 0x0?})
	/Users/brodo/go/pkg/mod/github.com/cogentcore/webgpu@v0.23.0/wgpu/queue.go:79 +0x168
main.(*State).Render(0x14000102000)
	/Users/brodo/repos/goBoids/main.go:410 +0x970
main.main()
	/Users/brodo/repos/goBoids/main.go:518 +0x174

[kkoreilly]

Thanks for reporting this issue. You probably need a mutex; if you have a mutex that you lock around s.Render and around the logic in your http.HandleFunc, then it should probably work (have one sync.Mutex on State and lock and unlock the same mutex in both s.Render and your http.HandleFunc). Please let me know if that fixes the issue.

[oliverbestmann]

I was wrong. You're not allowed to do unsafe.Pointer(handle) in go, as that would put the handles value (a small integer) as a pointer on the stack, which would lead to an error if the garbage collector encounters that value on the stack. I've modified my local branch to go with uintptr_t on the c-side and working with uintptr values on the go side. This should now be correct. I'll update my PR soonish.