C# threads / tasks don't work on the web (async / await)

[wihrl]

Godot version:
3.2 beta 4
OS/device including version:
Windows 10
Issue description:
Threading and tasks don't seem to work at all when exporting mono projects to the web.
Steps to reproduce:

1. Create a new project.
2. Create a new thread or task in a c# script that does something ( I tried printing to the console via GD.Print )
3. Export for the web
4. Run the exported project in the web browser.
   Minimal reproduction project:
   https://1drv.ms/u/s!AvsEaC7oMEMagstwXvyFlCMdOOMO2A?e=uuytsD
   The label should change text to "it worked" when the program starts.

[akien-mga]

Browsers don't enable multithreading in WebAssembly for now, so this is normal.

Duplicate of #12699.

[neikeq]

There is a runtime-threads target in Mono's wasm sdks: https://github.com/mono/mono/blob/master/sdks/wasm/README.md#threading-support
I haven't tested this yet. We're using the runtime target for now.

I'll re-open this as this is about Mono's threading while #12699 is about Godot's.

[NovemberDev]

Apparently in Chrome 70, webassembly will be able to support threads

Source: https://developers.google.com/web/updates/2018/10/wasm-threads

[robbertzzz]

According to the article linked by @NovemberDev Google Chrome 74+ should have threads for webassembly enabled by default, but doesn't work with my Mono build in Chrome 79

[Flavelius]

It should be possible to schedule the work to run on the main thread. I think unity does this aswell. This would make it more compatible for browsers that don't support threads.

[lenscas]

I wouldn't mind checking myself but had trouble compiling godot myself (ran both into an error and finding time is also not the easiest)

because threaded wasm is a thing in 3.2.4 due to: #12699 does that mean that threads and/or tasks now also work in C#?

if it doesn't work in the normal C# way, is there a (possible) work around because that feature got closed? (Maybe through communication with gdscript to have that spawn the thread?)

thanks in advance :)

[Faless]

because threaded wasm is a thing in 3.2.4 due to: #12699 does that mean that threads and/or tasks now also work in C#?

@lenscas Sadly, we were not yet able to build a mono version with threads support.
So at least the upcoming beta will only ship the threads-enabled and the gdnative-enabled versions for the "classical" builds and not the "mono" one.
We still plan to expand both threads support (hopefully soon) and gdnative support (probably later on) to "mono" builds too.

[akien-mga]

@neikeq Now that #12699 is solved (threads support in Godot's WebAssembly port), we could indeed try https://github.com/mono/mono/blob/master/sdks/wasm/README.md#threading-support to see if it can work with Mono.

[animanmaster]

@neikeq Now that #12699 is solved (threads support in Godot's WebAssembly port), we could indeed try https://github.com/mono/mono/blob/master/sdks/wasm/README.md#threading-support to see if it can work with Mono.

Just curious if anyone has tried a custom build with this yet or if this is working in 4.0? Would I just have to compile a new wasm export template with threads on in order to try this myself, or would I have to recompile the engine too?

[Beliar83]

@animanmaster I tried compiling a mono wasm export template with threads for 3.4, but I am always running into some problems. For example, I get several duplicate symbols between libmonosgen and libstubs.a (from emscripten) (getpwnam_r, getpwuid_r, getgrnam, getgrgid, pthread_sigmask), but maybe someone with more knowledge has better luck.

Edit: I got it to compile by removing the stub functions from "mono\mono\mini\mini-wasm.c". Have to test if everything works now.
Edit 2: I keep getting into problems either running or compiling. Not sure if it is just because of me, my setup or, something else. I'll give up for now.

[YuriSizov]

Regarding the milestone change: Godot 4 C# doesn't support the web platform at all at the moment, so this problem is not really relevant there.

[Bip901]

The symptoms of this issue are:

• You use a System.Threading.Tasks related function
• It seems to work, but it internally breaks something in the engine
• The next time Godot and Mono interact, a seemingly random error will occur. This can even be in a line of code as simple as new Node() - the glue between Godot and Mono breaks so it fails. Here's a list of the errors I've experienced so far:
  • FATAL: Condition "!klass" is true - this is caused by the assert in managed_type.cpp
  • NullReferenceException in your code on things that aren't really null (even if they're guarded by if != null)
  • timeout callback threw a System.NullReferenceException at System.Threading.WasmRuntime.TimeoutCallback (System.Int32 id)

Thus, the following issues are really just duplicates of this one:

• Mono to HTML5 Error FATAL: Condition "!klass" is true. #58538
• Mono to HTML5: Fatal Error and Unhandled Exception #61412
• [3.x] Task.Delay throws NullReferenceException on HTML5 exports #100740

I suggest we merge them into this one.