lib.rs

//! Implement [`wasi-threads`].
//!
//! [`wasi-threads`]: https://github.com/WebAssembly/wasi-threads

use anyhow::{anyhow, bail, Result};
use rand::Rng;
use std::panic::{catch_unwind, AssertUnwindSafe};
use std::sync::Arc;
use std::thread;
use wasmtime::{Caller, Linker, Module, SharedMemory, Store, ValType};
use wasmtime_wasi::maybe_exit_on_error;

// This name is a function export designated by the wasi-threads specification:
// https://github.com/WebAssembly/wasi-threads/#detailed-design-discussion
const WASI_ENTRY_POINT: &str = "wasi_thread_start";

pub struct WasiThreadsCtx<T> {
    module: Module,
    linker: Arc<Linker<T>>,
}

impl<T: Clone + Send + 'static> WasiThreadsCtx<T> {
    pub fn new(module: Module, linker: Arc<Linker<T>>) -> Result<Self> {
        if !has_wasi_entry_point(&module) {
            bail!(
                "failed to find wasi-threads entry point function: {}",
                WASI_ENTRY_POINT
            );
        }
        Ok(Self { module, linker })
    }

    pub fn spawn(&self, host: T, thread_start_arg: i32) -> Result<i32> {
        let module = self.module.clone();
        let linker = self.linker.clone();

        // Start a Rust thread running a new instance of the current module.
        let wasi_thread_id = random_thread_id();
        let builder = thread::Builder::new().name(format!("wasi-thread-{}", wasi_thread_id));
        builder.spawn(move || {
            // Catch any panic failures in host code; e.g., if a WASI module
            // were to crash, we want all threads to exit, not just this one.
            let result = catch_unwind(AssertUnwindSafe(|| {
                // Each new instance is created in its own store.
                let mut store = Store::new(&module.engine(), host);

                // Ideally, we would have already checked much earlier (e.g.,
                // `new`) whether the module can be instantiated. Because
                // `Linker::instantiate_pre` requires a `Store` and that is only
                // available now. TODO:
                // https://github.com/bytecodealliance/wasmtime/issues/5675.
                let instance = linker.instantiate(&mut store, &module).expect(&format!(
                    "wasi-thread-{} exited unsuccessfully: failed to instantiate",
                    wasi_thread_id
                ));
                let thread_entry_point = instance
                    .get_typed_func::<(i32, i32), ()>(&mut store, WASI_ENTRY_POINT)
                    .unwrap();

                // Start the thread's entry point. Any traps or calls to
                // `proc_exit`, by specification, should end execution for all
                // threads. This code uses `process::exit` to do so, which is what
                // the user expects from the CLI but probably not in a Wasmtime
                // embedding.
                log::trace!(
                    "spawned thread id = {}; calling start function `{}` with: {}",
                    wasi_thread_id,
                    WASI_ENTRY_POINT,
                    thread_start_arg
                );
                match thread_entry_point.call(&mut store, (wasi_thread_id, thread_start_arg)) {
                    Ok(_) => log::trace!("exiting thread id = {} normally", wasi_thread_id),
                    Err(e) => {
                        log::trace!("exiting thread id = {} due to error", wasi_thread_id);
                        let e = maybe_exit_on_error(e);
                        eprintln!("Error: {:?}", e);
                        std::process::exit(1);
                    }
                }
            }));

            if let Err(e) = result {
                eprintln!("wasi-thread-{} panicked: {:?}", wasi_thread_id, e);
                std::process::exit(1);
            }
        })?;

        Ok(wasi_thread_id)
    }
}

/// Helper for generating valid WASI thread IDs (TID).
///
/// Callers of `wasi_thread_spawn` expect a TID >=0 to indicate a successful
/// spawning of the thread whereas a negative return value indicates an
/// failure to spawn.
fn random_thread_id() -> i32 {
    let tid: u32 = rand::thread_rng().gen();
    (tid >> 1) as i32
}

/// Manually add the WASI `thread_spawn` function to the linker.
///
/// It is unclear what namespace the `wasi-threads` proposal should live under:
/// it is not clear if it should be included in any of the `preview*` releases
/// so for the time being its module namespace is simply `"wasi"` (TODO).
pub fn add_to_linker<T: Clone + Send + 'static>(
    linker: &mut wasmtime::Linker<T>,
    store: &wasmtime::Store<T>,
    module: &Module,
    get_cx: impl Fn(&mut T) -> &WasiThreadsCtx<T> + Send + Sync + Copy + 'static,
) -> anyhow::Result<SharedMemory> {
    linker.func_wrap(
        "wasi",
        "thread-spawn",
        move |mut caller: Caller<'_, T>, start_arg: i32| -> i32 {
            log::trace!("new thread requested via `wasi::thread_spawn` call");
            let host = caller.data().clone();
            let ctx = get_cx(caller.data_mut());
            match ctx.spawn(host, start_arg) {
                Ok(thread_id) => {
                    assert!(thread_id >= 0, "thread_id = {}", thread_id);
                    thread_id
                }
                Err(e) => {
                    log::error!("failed to spawn thread: {}", e);
                    -1
                }
            }
        },
    )?;

    // Find the shared memory import and satisfy it with a newly-created shared
    // memory import. This currently does not handle multiple memories (TODO).
    for import in module.imports() {
        if let Some(m) = import.ty().memory() {
            if m.is_shared() {
                let mem = SharedMemory::new(module.engine(), m.clone())?;
                linker.define(store, import.module(), import.name(), mem.clone())?;
                return Ok(mem);
            }
        }
    }
    Err(anyhow!(
        "unable to link a shared memory import to the module; a `wasi-threads` \
         module should import a single shared memory as \"memory\""
    ))
}

fn has_wasi_entry_point(module: &Module) -> bool {
    module
        .get_export(WASI_ENTRY_POINT)
        .and_then(|t| t.func().cloned())
        .and_then(|t| {
            let params: Vec<ValType> = t.params().collect();
            Some(params == [ValType::I32, ValType::I32] && t.results().len() == 0)
        })
        .unwrap_or(false)
}