Potential deadlock of `.join()`

[CPunisher]

use std::thread;

use napi_derive::napi;

pub fn panic123() {
    panic!("123")
}

#[napi]
pub fn test() {
    println!("{:?}", thread::spawn(|| panic123()).join());
}

1. The main js thread is waiting for the child thread with .join
2. Child thread gets panic and throws error in the js side
3. worker.onerror cannot be executed since the main thread is stuck
   

   emnapi/packages/wasi-threads/src/thread-manager.ts

   Line 231 in 004474c

   const handleError = function (e: Event | Error): void {

[toyobayashi]

Unlike emscripten, emnapi can't override wasi-libc's implementation. I'll try to find a way to wake up the main thread, but it seems unlikely at the moment. Any ideas?

[toyobayashi]

Found out a way to handle this case, you need to specify --export=pthread_self. However, there are still similar potential deadlock cases may be caused by other synchronization primitives, which are unlikely handled by emnapi.

[toyobayashi]

Do you know what version of wasi-libc is rust toolchain using? Perhaps __wasilibc_futex_wait_maybe_busy introduced by WebAssembly/wasi-libc#562 may solve this problem in a better way, like what emscripten does.

Refs:

• https://github.com/emscripten-core/emscripten/blob/6f625bd62ee64f4f8cd11476774741b961e4a2d5/src/runtime_pthread.js#L192
• https://github.com/emscripten-core/emscripten/blob/6f625bd62ee64f4f8cd11476774741b961e4a2d5/system/lib/pthread/emscripten_yield.c#L13
• https://github.com/emscripten-core/emscripten/blob/6f625bd62ee64f4f8cd11476774741b961e4a2d5/system/lib/pthread/emscripten_yield.c#L30-L39
• https://github.com/emscripten-core/emscripten/blob/6f625bd62ee64f4f8cd11476774741b961e4a2d5/system/lib/libc/musl/src/thread/__timedwait.c#L94
• https://github.com/emscripten-core/emscripten/blob/6f625bd62ee64f4f8cd11476774741b961e4a2d5/system/lib/pthread/emscripten_futex_wait.c#L125

[CPunisher]

Oh thanks for your so much investigate. I’m not too familiar but I’m willing to help tomorrow.

[toyobayashi]

I have found the rust prebuilt wasi-libc static library in .rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/wasm32-wasip1-threads/lib/self-contained/libc.a. But I couldn't determine which commit it is built from. If it contains the weak symbol __wasilibc_futex_wait_maybe_busy then #160 will work.

[CPunisher]

Investigation:
wasi-sdk synchronizes that change of wasi-libc in wasi-sdk-26: https://github.com/WebAssembly/wasi-sdk/releases/tag/wasi-sdk-26.
rust upgraded wasi-sdk from 25 to 27 two weeks ago (07-29): https://github.com/rust-lang/rust/blob/577166503aee7290e09374da21f4045c455acfd5/src/ci/docker/host-x86_64/dist-various-2/Dockerfile#L86.

So the latest stable rust, which is 1.89.0 branched from master on (06-20), does not support this change yet. From my test, nightly-2025-07-31 is the first version with it avaliable, and it will be stablized on 09-18 with rust 1.90.0

Validation:

[CPunisher]

@toyobayashi Does this mean that emnapi will be only available with newer rustc? Otherwise users should compile the wasi-libc by themselves?

[toyobayashi]

For this issue only, yes. Old rustc still can link against emnapi prebuilt library but will experience this issue. No breaking changes for other features.