Remove the requirement for nightly on wasmer-wasix when the js feature is activated

[Michael-F-Bryan]

Describe the bug

As of #4119, compiling wasmer-wasix with the js feature requires a nightly compiler in order to do multi-processing correctly. This is a tracking issue for removing that requirement.

Why is nightly required?

TL;DR: We need to enable target_features = "atomics" so compiling to wasm32-unknown-unknown gives us an implementation of std::sync::Condvar (and therefore locks and channels) that works (src).

The wasmer-wasix crate implements multi-threading in the browser by spinning up some web workers and running Rust code (compiled to WebAssembly) on them to implement a basic thread pool. All workers share the same SharedArrayBuffer for their linear memory, which requires the WebAssembly threading proposal. From there, messages get sent to worker threads using a tokio::sync::mpsc channel, which uses Condvar under the hood.

The way you enable atomics in Rust is with the -Ctarget-feature=+atomics,+bulk-memory rustflags.

Unfortunately, std comes pre-compiled for wasm32-unknown-unknown (that's what rustup target add ... downloads), so even if you set $RUSTFLAGS or put the flags in .cargo/config.toml, things like Condvar will use the "unsupported" implementation which always panics.

You can fix this by telling cargo to rebuild the standard library with the correct $RUSTFLAGS. You can set this automatically by modifying your .cargo/config.toml file:

# .cargo/config.toml

[target.'cfg(target_arch = "wasm32")']
# Use "wasmer" for running tests when compiled to WebAssembly
runner = ["wasmer", "run"]
# This is needed so the module is compiled with atomics support (shared memory)
# We add the `-no-check-features` linker args because otherwise one of the modules fails to link
rustflags = '-Ctarget-feature=+atomics,+bulk-memory -Clink-args=--no-check-features'

[unstable]
# We want to make sure std gets built with atomics, too
build-std = ['std', 'panic_abort']

The build-std feature is unstable, and therefore only available on nightly.

You can run wasm-pack tests on nightly using the following

Steps to reproduce

There are a couple ways you can reproduce this.

If you are running a web app that uses the wasmer-wasix crate under the hood, you may see an exception like this from one of the worker threads:

wasm-bindgen-test_bg.wasm:0x14ec439 Uncaught (in promise) RuntimeError: unreachable
    at __rust_start_panic (wasm-bindgen-test_bg.wasm:0x14ec439)
    at rust_panic (wasm-bindgen-test_bg.wasm:0x14d173c)
    at std::panicking::rust_panic_with_hook::h7f7102b82d51338f (wasm-bindgen-test_bg.wasm:0x11d1215)
    at std::panicking::begin_panic_handler::{{closure}}::hfa40135feb109919 (wasm-bindgen-test_bg.wasm:0x12a11e3)
    at std::sys_common::backtrace::__rust_end_short_backtrace::haf21bfec9a028e09 (wasm-bindgen-test_bg.wasm:0x14ec069)
    at rust_begin_unwind (wasm-bindgen-test_bg.wasm:0x13d842a)
    at core::panicking::panic_fmt::he4489d678d6570d5 (wasm-bindgen-test_bg.wasm:0x14989ee)
    at std::sys::wasm::locks::condvar::Condvar::wait::h2cd023b14d678b96 (wasm-bindgen-test_bg.wasm:0x14578f1)
    at std::sync::condvar::Condvar::wait::h3d3df320bb943f6e (wasm-bindgen-test_bg.wasm:0x10164f3)
    at tokio::runtime::park::Inner::park::hb4fb5fafc6db5d36 (wasm-bindgen-test_bg.wasm:0xb889dd)
    at tokio::runtime::park::CachedParkThread::park::{{closure}}::hdf0900dc017d5606 (wasm-bindgen-test_bg.wasm:0x14778f7)
    at tokio::runtime::park::CachedParkThread::with_current::{{closure}}::hf332e1883859450c (wasm-bindgen-test_bg.wasm:0x14573a8)
    at std::thread::local::LocalKey<T>::try_with::hab7631a84070acb6 (wasm-bindgen-test_bg.wasm:0x106c34f)
    at tokio::runtime::park::CachedParkThread::with_current::hefde44dbf608510f (wasm-bindgen-test_bg.wasm:0x1421199)
    at tokio::runtime::park::CachedParkThread::park::h827bcc2f49db8e95 (wasm-bindgen-test_bg.wasm:0x140fd9a)
    at tokio::sync::mpsc::chan::Rx<T,S>::try_recv::{{closure}}::h7c77c7c4726b2128 (wasm-bindgen-test_bg.wasm:0xc6a5b2)
    at tokio::loom::std::unsafe_cell::UnsafeCell<T>::with_mut::h6f8736a8a89ffe62 (wasm-bindgen-test_bg.wasm:0x13e7084)
    at tokio::sync::mpsc::chan::Rx<T,S>::try_recv::h2ba6aef56be7dc02 (wasm-bindgen-test_bg.wasm:0x13b0a49)
    at tokio::sync::mpsc::unbounded::UnboundedReceiver<T>::try_recv::hac2f62278a7b5734 (wasm-bindgen-test_bg.wasm:0x1488636)
    at wasmer_wasix::runtime::task_manager::web_thread_pool::ThreadStateSync::work::hebed6819631e4e3c (wasm-bindgen-test_bg.wasm:0x24ef65)
    at wasmer_wasix::runtime::task_manager::web_thread_pool::worker_entry_point::h9ae066ccd0ea5cdf (wasm-bindgen-test_bg.wasm:0x8d3968)
    at worker_entry_point (wasm-bindgen-test_bg.wasm:0x1448069)
    at worker_entry_point (wasm-bindgen-test:403:10)
    at globalThis.onmessage (bca07945-deed-4069-aa93-c62e622829dd:15:9)

This happens because creating a wasmer_wasix::runtime::task_manager::WebTaskManager requires setting up a thread pool, and the newly spawned web worker hit that panic!("condvar wait not supported") when it started listening for events. The threadpool has no way of knowing this, so from a WebTaskManager's perspective it's the task just looks like it takes forever to run.

A simpler way to reproduce this is using wasm-pack test and these tests from the wasmer-wasix crate:

// lib/wasix/tests/web.rs

use std::sync::{Condvar, Mutex};

#[wasm_bindgen_test::wasm_bindgen_test]
fn atomics_are_enabled() {
    assert!(cfg!(target_feature = "atomics"));
}

#[wasm_bindgen_test::wasm_bindgen_test]
fn condvar_works() {
    let mutex = Mutex::new("asdf");
    let condvar = Condvar::new();

    let _guard = condvar.wait(mutex.lock().unwrap()).unwrap();
}

Those tests should fail with the following:

$ wasm-pack test --chrome  --features=wasmer/js,wasmer/std,js --no-default-features

---- web::condvar_works output ----
    error output:
        panicked at 'condvar wait not supported', library/std/src/sys/wasm/../unsupported/locks/condvar.rs:20:9
        
        Stack:
        
        Error
            at http://127.0.0.1:8000/wasm-bindgen-test:725:21
            at logError (http://127.0.0.1:8000/wasm-bindgen-test:246:18)
            at imports.wbg.__wbg_new_abda76e883ba8a5f (http://127.0.0.1:8000/wasm-bindgen-test:724:66)
            at console_error_panic_hook::Error::new::h02613b9d5c17cd5c (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[40711]:0x143f1a7)
            at console_error_panic_hook::hook_impl::hcd34ac60637e48a5 (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[9972]:0xf98960)
            at console_error_panic_hook::hook::h1b163b7a28e26082 (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[44094]:0x146eb4d)
            at core::ops::function::Fn::call::hcea8424ff554cdf9 (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[39022]:0x1424698)
            at std::panicking::rust_panic_with_hook::h7f7102b82d51338f (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[17459]:0x11b092c)
            at std::panicking::begin_panic_handler::{{closure}}::hfa40135feb109919 (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[21954]:0x127d17e)
            at std::sys_common::backtrace::__rust_end_short_backtrace::haf21bfec9a028e09 (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[52494]:0x14beabc)
        
    JS exception that was thrown:
        RuntimeError: unreachable
            at __rust_start_panic (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[52684]:0x14bee88)
            at rust_panic (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[48523]:0x14a474e)
            at std::panicking::rust_panic_with_hook::h7f7102b82d51338f (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[17459]:0x11b0957)
            at std::panicking::begin_panic_handler::{{closure}}::hfa40135feb109919 (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[21954]:0x127d17e)
            at std::sys_common::backtrace::__rust_end_short_backtrace::haf21bfec9a028e09 (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[52494]:0x14beabc)
            at rust_begin_unwind (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[32883]:0x13b01ae)
            at core::panicking::panic_fmt::he4489d678d6570d5 (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[43963]:0x146cfa5)
            at std::sys::wasm::locks::condvar::Condvar::wait::h2cd023b14d678b96 (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[39586]:0x142d639)
            at std::sync::condvar::Condvar::wait::h976de1002f01548d (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[10821]:0xfe7d02)
            at web::condvar_works::h039bf24c11969b5f (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[13848]:0x10d8e21)

---- web::atomics_are_enabled output ----
    error output:
        panicked at 'assertion failed: cfg!(target_feature = \"atomics\")', lib/wasix/tests/web.rs:32:5
        
        Stack:
        
        Error
            at http://127.0.0.1:8000/wasm-bindgen-test:725:21
            at logError (http://127.0.0.1:8000/wasm-bindgen-test:246:18)
            at imports.wbg.__wbg_new_abda76e883ba8a5f (http://127.0.0.1:8000/wasm-bindgen-test:724:66)
            at console_error_panic_hook::Error::new::h02613b9d5c17cd5c (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[40711]:0x143f1a7)
            at console_error_panic_hook::hook_impl::hcd34ac60637e48a5 (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[9972]:0xf98960)
            at console_error_panic_hook::hook::h1b163b7a28e26082 (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[44094]:0x146eb4d)
            at core::ops::function::Fn::call::hcea8424ff554cdf9 (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[39022]:0x1424698)
            at std::panicking::rust_panic_with_hook::h7f7102b82d51338f (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[17459]:0x11b092c)
            at std::panicking::begin_panic_handler::{{closure}}::hfa40135feb109919 (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[21954]:0x127d17e)
            at std::sys_common::backtrace::__rust_end_short_backtrace::haf21bfec9a028e09 (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[52494]:0x14beabc)
    
    JS exception that was thrown:
        RuntimeError: unreachable
            at std::panicking::rust_panic_with_hook::h7f7102b82d51338f (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[17459]:0x11b0951)
            at std::panicking::begin_panic_handler::{{closure}}::hfa40135feb109919 (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[21954]:0x127d17e)
            at std::sys_common::backtrace::__rust_end_short_backtrace::haf21bfec9a028e09 (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[52494]:0x14beabc)
            at rust_begin_unwind (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[32883]:0x13b01ae)
            at core::panicking::panic_fmt::he4489d678d6570d5 (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[43963]:0x146cfa5)
            at core::panicking::panic::hea78c4e8b318407d (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[35601]:0x13e75db)
            at web::atomics_are_enabled::h8380fe219deaec1c (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[51163]:0x14b920c)
            at core::ops::function::FnOnce::call_once::h8d535a8698ab7927 (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[47227]:0x14968f0)
            at wasm_bindgen_test::__rt::Context::execute_sync::{{closure}}::h9d40de4d3d7aa4aa (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[17462]:0x11b0b63)
            at <wasm_bindgen_test::__rt::TestFuture<F> as core::future::future::Future>::poll::{{closure}}::{{closure}}::h7e923488e6ab9bbd (http://127.0.0.1:8000/wasm-bindgen-test_bg.wasm:wasm-function[19530]:0x1214d28)

failures:

    web::condvar_works
    web::atomics_are_enabled

If you made the above changes to .cargo/config.toml and have a nightly compiler installed, tests using WebTaskManager will now pass if you run wasm-pack test in a nightly environment.

$ rustup run nightly wasm-pack test --chrome  --features=wasmer/js,wasmer/std,js --no-default-features

Additional context

See the original conversation with me and @john-sharratt on Slack.