Try #1019:

.github/workflows/ci.yaml

@@ -65,16 +65,24 @@ jobs:
       - run: cargo test --verbose --package rayon
       - run: cargo test --verbose --package rayon-core
 
-  # wasm won't actually work without threading, but it builds
+  # wasm32-unknown-unknown builds, and even has the runtime fallback for
+  # unsupported threading, but we don't have an environment to execute in.
+  # wasm32-wasi can test the fallback by running in wasmtime.
   wasm:
     name: WebAssembly
     runs-on: ubuntu-latest
+    env:
+      CARGO_TARGET_WASM32_WASI_RUNNER: /home/runner/.wasmtime/bin/wasmtime
     steps:
       - uses: actions/checkout@v3
       - uses: dtolnay/rust-toolchain@stable
         with:
-          target: wasm32-unknown-unknown
+          targets: wasm32-unknown-unknown,wasm32-wasi
       - run: cargo check --verbose --target wasm32-unknown-unknown
+      - run: cargo check --verbose --target wasm32-wasi
+      - run: curl https://wasmtime.dev/install.sh -sSf | bash
+      - run: cargo test --verbose --target wasm32-wasi --package rayon
+      - run: cargo test --verbose --target wasm32-wasi --package rayon-core
 
   fmt:
     name: Format

rayon-core/src/broadcast/test.rs

@@ -12,6 +12,7 @@ fn broadcast_global() {
 }
 
 #[test]
+#[cfg_attr(any(target_os = "emscripten", target_family = "wasm"), ignore)]
 fn spawn_broadcast_global() {
     let (tx, rx) = crossbeam_channel::unbounded();
     crate::spawn_broadcast(move |ctx| tx.send(ctx.index()).unwrap());
@@ -22,13 +23,15 @@ fn spawn_broadcast_global() {
 }
 
 #[test]
+#[cfg_attr(any(target_os = "emscripten", target_family = "wasm"), ignore)]
 fn broadcast_pool() {
     let pool = ThreadPoolBuilder::new().num_threads(7).build().unwrap();
     let v = pool.broadcast(|ctx| ctx.index());
     assert!(v.into_iter().eq(0..7));
 }
 
 #[test]
+#[cfg_attr(any(target_os = "emscripten", target_family = "wasm"), ignore)]
 fn spawn_broadcast_pool() {
     let (tx, rx) = crossbeam_channel::unbounded();
     let pool = ThreadPoolBuilder::new().num_threads(7).build().unwrap();
@@ -40,13 +43,15 @@ fn spawn_broadcast_pool() {
 }
 
 #[test]
+#[cfg_attr(any(target_os = "emscripten", target_family = "wasm"), ignore)]
 fn broadcast_self() {
     let pool = ThreadPoolBuilder::new().num_threads(7).build().unwrap();
     let v = pool.install(|| crate::broadcast(|ctx| ctx.index()));
     assert!(v.into_iter().eq(0..7));
 }
 
 #[test]
+#[cfg_attr(any(target_os = "emscripten", target_family = "wasm"), ignore)]
 fn spawn_broadcast_self() {
     let (tx, rx) = crossbeam_channel::unbounded();
     let pool = ThreadPoolBuilder::new().num_threads(7).build().unwrap();
@@ -58,6 +63,7 @@ fn spawn_broadcast_self() {
 }
 
 #[test]
+#[cfg_attr(any(target_os = "emscripten", target_family = "wasm"), ignore)]
 fn broadcast_mutual() {
     let count = AtomicUsize::new(0);
     let pool1 = ThreadPoolBuilder::new().num_threads(3).build().unwrap();
@@ -73,6 +79,7 @@ fn broadcast_mutual() {
 }
 
 #[test]
+#[cfg_attr(any(target_os = "emscripten", target_family = "wasm"), ignore)]
 fn spawn_broadcast_mutual() {
     let (tx, rx) = crossbeam_channel::unbounded();
     let pool1 = Arc::new(ThreadPoolBuilder::new().num_threads(3).build().unwrap());
@@ -90,6 +97,7 @@ fn spawn_broadcast_mutual() {
 }
 
 #[test]
+#[cfg_attr(any(target_os = "emscripten", target_family = "wasm"), ignore)]
 fn broadcast_mutual_sleepy() {
     let count = AtomicUsize::new(0);
     let pool1 = ThreadPoolBuilder::new().num_threads(3).build().unwrap();
@@ -108,6 +116,7 @@ fn broadcast_mutual_sleepy() {
 }
 
 #[test]
+#[cfg_attr(any(target_os = "emscripten", target_family = "wasm"), ignore)]
 fn spawn_broadcast_mutual_sleepy() {
     let (tx, rx) = crossbeam_channel::unbounded();
     let pool1 = Arc::new(ThreadPoolBuilder::new().num_threads(3).build().unwrap());
@@ -130,6 +139,7 @@ fn spawn_broadcast_mutual_sleepy() {
 }
 
 #[test]
+#[cfg_attr(not(panic = "unwind"), ignore)]
 fn broadcast_panic_one() {
     let count = AtomicUsize::new(0);
     let pool = ThreadPoolBuilder::new().num_threads(7).build().unwrap();
@@ -146,6 +156,7 @@ fn broadcast_panic_one() {
 }
 
 #[test]
+#[cfg_attr(not(panic = "unwind"), ignore)]
 fn spawn_broadcast_panic_one() {
     let (tx, rx) = crossbeam_channel::unbounded();
     let (panic_tx, panic_rx) = crossbeam_channel::unbounded();
@@ -166,6 +177,7 @@ fn spawn_broadcast_panic_one() {
 }
 
 #[test]
+#[cfg_attr(not(panic = "unwind"), ignore)]
 fn broadcast_panic_many() {
     let count = AtomicUsize::new(0);
     let pool = ThreadPoolBuilder::new().num_threads(7).build().unwrap();
@@ -182,6 +194,7 @@ fn broadcast_panic_many() {
 }
 
 #[test]
+#[cfg_attr(not(panic = "unwind"), ignore)]
 fn spawn_broadcast_panic_many() {
     let (tx, rx) = crossbeam_channel::unbounded();
     let (panic_tx, panic_rx) = crossbeam_channel::unbounded();
@@ -202,6 +215,7 @@ fn spawn_broadcast_panic_many() {
 }
 
 #[test]
+#[cfg_attr(any(target_os = "emscripten", target_family = "wasm"), ignore)]
 fn broadcast_sleep_race() {
     let test_duration = time::Duration::from_secs(1);
     let pool = ThreadPoolBuilder::new().num_threads(7).build().unwrap();
@@ -214,3 +228,35 @@ fn broadcast_sleep_race() {
         });
     }
 }
+
+#[test]
+fn broadcast_after_spawn_broadcast() {
+    let (tx, rx) = crossbeam_channel::unbounded();
+
+    // Queue a non-blocking spawn_broadcast.
+    crate::spawn_broadcast(move |ctx| tx.send(ctx.index()).unwrap());
+
+    // This blocking broadcast runs after all prior broadcasts.
+    crate::broadcast(|_| {});
+
+    // The spawn_broadcast **must** have run by now on all threads.
+    let mut v: Vec<_> = rx.try_iter().collect();
+    v.sort_unstable();
+    assert!(v.into_iter().eq(0..crate::current_num_threads()));
+}
+
+#[test]
+fn broadcast_after_spawn() {
+    let (tx, rx) = crossbeam_channel::bounded(1);
+
+    // Queue a regular spawn on a thread-local deque.
+    crate::registry::in_worker(move |_, _| {
+        crate::spawn(move || tx.send(22).unwrap());
+    });
+
+    // Broadcast runs after the local deque is empty.
+    crate::broadcast(|_| {});
+
+    // The spawn **must** have run by now.
+    assert_eq!(22, rx.try_recv().unwrap());
+}

rayon-core/src/join/test.rs

@@ -47,6 +47,7 @@ fn sort() {
 }
 
 #[test]
+#[cfg_attr(any(target_os = "emscripten", target_family = "wasm"), ignore)]
 fn sort_in_pool() {
     let rng = seeded_rng();
     let mut data: Vec<u32> = rng.sample_iter(&Standard).take(12 * 1024).collect();
@@ -77,6 +78,7 @@ fn panic_propagate_both() {
 }
 
 #[test]
+#[cfg_attr(not(panic = "unwind"), ignore)]
 fn panic_b_still_executes() {
     let mut x = false;
     match unwind::halt_unwinding(|| join(|| panic!("Hello, world!"), || x = true)) {
@@ -86,6 +88,7 @@ fn panic_b_still_executes() {
 }
 
 #[test]
+#[cfg_attr(any(target_os = "emscripten", target_family = "wasm"), ignore)]
 fn join_context_both() {
     // If we're not in a pool, both should be marked stolen as they're injected.
     let (a_migrated, b_migrated) = join_context(|a| a.migrated(), |b| b.migrated());
@@ -94,6 +97,7 @@ fn join_context_both() {
 }
 
 #[test]
+#[cfg_attr(any(target_os = "emscripten", target_family = "wasm"), ignore)]
 fn join_context_neither() {
     // If we're already in a 1-thread pool, neither job should be stolen.
     let pool = ThreadPoolBuilder::new().num_threads(1).build().unwrap();
@@ -104,6 +108,7 @@ fn join_context_neither() {
 }
 
 #[test]
+#[cfg_attr(any(target_os = "emscripten", target_family = "wasm"), ignore)]
 fn join_context_second() {
     use std::sync::Barrier;
 
@@ -127,6 +132,7 @@ fn join_context_second() {
 }
 
 #[test]
+#[cfg_attr(any(target_os = "emscripten", target_family = "wasm"), ignore)]
 fn join_counter_overflow() {
     const MAX: u32 = 500_000;
 

rayon-core/src/lib.rs

@@ -26,7 +26,23 @@
 //! [`join()`]: struct.ThreadPool.html#method.join
 //! [`ThreadPoolBuilder`]: struct.ThreadPoolBuilder.html
 //!
-//! ## Restricting multiple versions
+//! # Global fallback when threading is unsupported
+//!
+//! Rayon uses `std` APIs for threading, but some targets have incomplete implementations that
+//! always return `Unsupported` errors. The WebAssembly `wasm32-unknown-unknown` and `wasm32-wasi`
+//! targets are notable examples of this. Rather than panicking on the unsupported error when
+//! creating the implicit global threadpool, Rayon configures a fallback mode instead.
+//!
+//! This fallback mode mostly functions as if it were using a single-threaded "pool", like setting
+//! `RAYON_NUM_THREADS=1`. For example, `join` will execute its two closures sequentially, since
+//! there is no other thread to share the work. However, since the pool is not running independent
+//! of the main thread, non-blocking calls like `spawn` may not execute at all, unless a lower-
+//! priority call like `broadcast` gives them an opening. The fallback mode does not try to emulate
+//! anything like thread preemption or `async` task switching.
+//!
+//! Explicit `ThreadPoolBuilder` methods always report their error without any fallback.
+//!
+//! # Restricting multiple versions
 //!
 //! In order to ensure proper coordination between threadpools, and especially
 //! to make sure there's only one global threadpool, `rayon-core` is actively
@@ -707,6 +723,10 @@ impl ThreadPoolBuildError {
     fn new(kind: ErrorKind) -> ThreadPoolBuildError {
         ThreadPoolBuildError { kind }
     }
+
+    fn is_unsupported(&self) -> bool {
+        matches!(&self.kind, ErrorKind::IOError(e) if e.kind() == io::ErrorKind::Unsupported)
+    }
 }
 
 const GLOBAL_POOL_ALREADY_INITIALIZED: &str =

rayon-core/src/registry.rs

@@ -50,7 +50,7 @@ impl ThreadBuilder {
     /// Executes the main loop for this thread. This will not return until the
     /// thread pool is dropped.
     pub fn run(self) {
-        unsafe { main_loop(self.worker, self.stealer, self.registry, self.index) }
+        unsafe { main_loop(self) }
     }
 }
 
@@ -164,7 +164,7 @@ static THE_REGISTRY_SET: Once = Once::new();
 /// initialization has not already occurred, use the default
 /// configuration.
 pub(super) fn global_registry() -> &'static Arc<Registry> {
-    set_global_registry(|| Registry::new(ThreadPoolBuilder::new()))
+    set_global_registry(default_global_registry)
         .or_else(|err| unsafe { THE_REGISTRY.as_ref().ok_or(err) })
         .expect("The global thread pool has not been initialized.")
 }
@@ -198,6 +198,46 @@ where
     result
 }
 
+fn default_global_registry() -> Result<Arc<Registry>, ThreadPoolBuildError> {
+    let result = Registry::new(ThreadPoolBuilder::new());
+
+    // If we're running in an environment that doesn't support threads at all, we can fall back to
+    // using the current thread alone. This is crude, and probably won't work for non-blocking
+    // calls like `spawn` or `broadcast_spawn`, but a lot of stuff does work fine.
+    //
+    // Notably, this allows current WebAssembly targets to work even though their threading support
+    // is stubbed out, and we won't have to change anything if they do add real threading.
+    let unsupported = matches!(&result, Err(e) if e.is_unsupported());
+    if unsupported && WorkerThread::current().is_null() {
+        let builder = ThreadPoolBuilder::new()
+            .num_threads(1)
+            .spawn_handler(|thread| {
+                // Rather than starting a new thread, we're just taking over the current thread
+                // *without* running the main loop, so we can still return from here.
+                // The WorkerThread is leaked, but we never shutdown the global pool anyway.
+                let worker_thread = Box::leak(Box::new(WorkerThread::from(thread)));
+                let registry = &*worker_thread.registry;
+                let index = worker_thread.index;
+
+                unsafe {
+                    WorkerThread::set_current(worker_thread);
+
+                    // let registry know we are ready to do work
+                    Latch::set(&registry.thread_infos[index].primed);
+                }
+
+                Ok(())
+            });
+
+        let fallback_result = Registry::new(builder);
+        if fallback_result.is_ok() {
+            return fallback_result;
+        }
+    }
+
+    result
+}
+
 struct Terminator<'a>(&'a Arc<Registry>);
 
 impl<'a> Drop for Terminator<'a> {
@@ -655,6 +695,19 @@ thread_local! {
     static WORKER_THREAD_STATE: Cell<*const WorkerThread> = Cell::new(ptr::null());
 }
 
+impl From<ThreadBuilder> for WorkerThread {
+    fn from(thread: ThreadBuilder) -> Self {
+        Self {
+            worker: thread.worker,
+            stealer: thread.stealer,
+            fifo: JobFifo::new(),
+            index: thread.index,
+            rng: XorShift64Star::new(),
+            registry: thread.registry,
+        }
+    }
+}
+
 impl Drop for WorkerThread {
     fn drop(&mut self) {
         // Undo `set_current`
@@ -851,22 +904,11 @@ impl WorkerThread {
 
 /// ////////////////////////////////////////////////////////////////////////
 
-unsafe fn main_loop(
-    worker: Worker<JobRef>,
-    stealer: Stealer<JobRef>,
-    registry: Arc<Registry>,
-    index: usize,
-) {
-    let worker_thread = &WorkerThread {
-        worker,
-        stealer,
-        fifo: JobFifo::new(),
-        index,
-        rng: XorShift64Star::new(),
-        registry,
-    };
+unsafe fn main_loop(thread: ThreadBuilder) {
+    let worker_thread = &WorkerThread::from(thread);
     WorkerThread::set_current(worker_thread);
     let registry = &*worker_thread.registry;
+    let index = worker_thread.index;
 
     // let registry know we are ready to do work
     Latch::set(&registry.thread_infos[index].primed);
@@ -924,7 +966,7 @@ where
             // invalidated until we return.
             op(&*owner_thread, false)
         } else {
-            global_registry().in_worker_cold(op)
+            global_registry().in_worker(op)
         }
     }
 }