Require source-region coverage across supported systems.

.github/workflows/test.yml

@@ -12,9 +12,14 @@ permissions:
 
 jobs:
   coverage:
-    name: Formatting, Clippy, and coverage
-    runs-on: ubuntu-latest
+    name: Coverage (${{ matrix.runner }})
+    runs-on: ${{ matrix.runner }}
     timeout-minutes: 20
+    strategy:
+      fail-fast: false
+      matrix:
+        # Native selectors and positioned file I/O are target-specific.
+        runner: [ubuntu-24.04, macos-15, windows-latest]
     steps:
       - uses: actions/checkout@v7
       - uses: actions-rust-lang/setup-rust-toolchain@v2
@@ -26,9 +31,11 @@ jobs:
       - name: Install coverage tool
         run: cargo install cargo-llvm-cov --locked
       - run: cargo fmt --all -- --check
+        if: matrix.runner == 'ubuntu-24.04'
       - run: cargo clippy --workspace --all-targets --locked -- -D warnings
-      - name: Run tests and require complete line coverage
-        run: cargo bake --locked test:coverage --all-targets true
+        if: matrix.runner == 'ubuntu-24.04'
+      - name: Run tests and require complete source-region coverage
+        run: cargo bake --locked test:coverage --all-targets true --features tokio
 
   test:
     name: ${{ matrix.name }}

crates/executor/src/scheduler/tokio.rs

@@ -65,6 +65,19 @@ impl Shared {
         owner: &Arc<Owner>,
         future: FutureType,
     ) -> Result<TaskHandle<FutureType::Output>, SpawnError>
+    where
+        FutureType: Future + Send + 'static,
+        FutureType::Output: Send + 'static,
+    {
+        self.spawn_with_registration_hook(owner, future, || {})
+    }
+
+    fn spawn_with_registration_hook<FutureType>(
+        self: &Arc<Self>,
+        owner: &Arc<Owner>,
+        future: FutureType,
+        after_registration: impl FnOnce(),
+    ) -> Result<TaskHandle<FutureType::Output>, SpawnError>
     where
         FutureType: Future + Send + 'static,
         FutureType::Output: Send + 'static,
@@ -96,6 +109,7 @@ impl Shared {
         );
         owner.remaining.fetch_add(1, Ordering::Release);
         drop(registry);
+        after_registration();
         // Publish ownership before spawn, but do not hold the registry lock:
         // a shut-down runtime can destroy the submitted future immediately.
         let inner = self.runtime.spawn(TrackedFuture {

crates/executor/src/scheduler/tokio/tests.rs

@@ -3,7 +3,11 @@
 
 use super::*;
 use std::future::ready;
+use std::io::Read;
+use std::sync::mpsc;
 use std::task::{Context, Poll, Waker};
+use std::thread;
+use std::time::Duration;
 
 fn runtime() -> ::tokio::runtime::Runtime {
     ::tokio::runtime::Builder::new_current_thread()
@@ -191,6 +195,61 @@ fn task_identifier_exhaustion_is_reported() {
     ));
 }
 
+#[test]
+fn shutdown_cancels_a_task_registered_before_tokio_spawn() {
+    let runtime = runtime();
+    let scheduler = Scheduler::new(runtime.handle().clone());
+    let shared = Arc::clone(&scheduler.handle.shared);
+    let owner = Arc::new(Owner::new());
+    let (registered, wait_for_registration) = mpsc::sync_channel(0);
+    let (resume, wait_to_resume) = mpsc::sync_channel(0);
+
+    let spawning = {
+        let shared = Arc::clone(&shared);
+        let owner = Arc::clone(&owner);
+        thread::spawn(move || {
+            shared.spawn_with_registration_hook(&owner, std::future::pending::<()>(), || {
+                registered.send(()).unwrap();
+                wait_to_resume.recv().unwrap();
+            })
+        })
+    };
+
+    wait_for_registration.recv().unwrap();
+    {
+        let registry = shared.registry.lock().unwrap();
+        let registration = registry.tasks.values().next().unwrap();
+        assert!(registration.abort.is_none());
+        assert!(!owner.is_empty());
+    }
+
+    shared.close(None, true);
+
+    assert!(shared.closed.load(Ordering::Acquire));
+    assert!(
+        shared
+            .registry
+            .lock()
+            .unwrap()
+            .tasks
+            .values()
+            .next()
+            .unwrap()
+            .state
+            .cancelled
+            .load(Ordering::Acquire)
+    );
+
+    resume.send(()).unwrap();
+    let task = match spawning.join().unwrap() {
+        Ok(task) => task,
+        Err(error) => panic!("task registration failed: {error}"),
+    };
+
+    assert!(matches!(runtime.block_on(task), Err(TaskError::Cancelled)));
+    assert!(owner.is_empty());
+}
+
 #[test]
 fn socket_registration_preserves_configuration_and_conversion_errors() {
     let runtime = runtime();
@@ -359,6 +418,72 @@ fn write_retries_interrupted_and_would_block_operations() {
     });
 }
 
+#[test]
+fn socket_write_waits_until_the_send_buffer_is_writable() {
+    let runtime = runtime();
+    let scheduler = Scheduler::new(runtime.handle().clone());
+    let handle = scheduler.handle();
+    let (client, mut server) = socket_pair();
+    server.set_nonblocking(true).unwrap();
+    let socket = handle.register_socket(client).unwrap();
+    let fill = vec![0; 64 * 1024];
+
+    runtime.block_on(async {
+        socket.inner.writable().await.unwrap();
+        let mut written = 0;
+        loop {
+            match socket.inner.try_write(&fill) {
+                Ok(0) => panic!("socket write made no progress"),
+                Ok(count) => written += count,
+                Err(error) if error.kind() == io::ErrorKind::WouldBlock => break,
+                Err(error) => panic!("failed to fill socket send buffer: {error}"),
+            }
+        }
+        assert!(written > 0);
+    });
+
+    let (start_reader, wait_for_start) = mpsc::sync_channel(0);
+    let (finish_reader, wait_for_finish) = mpsc::sync_channel(0);
+    let reader = thread::spawn(move || {
+        wait_for_start.recv().unwrap();
+        let mut buffer = [0; 64 * 1024];
+        let mut total = 0;
+        loop {
+            match wait_for_finish.try_recv() {
+                Ok(()) | Err(mpsc::TryRecvError::Disconnected) => break,
+                Err(mpsc::TryRecvError::Empty) => {}
+            }
+
+            match server.read(&mut buffer) {
+                Ok(0) => break,
+                Ok(count) => total += count,
+                Err(error) if error.kind() == io::ErrorKind::WouldBlock => {
+                    thread::sleep(Duration::from_millis(1));
+                }
+                Err(error) => panic!("failed to drain socket receive buffer: {error}"),
+            }
+        }
+        total
+    });
+
+    let mut write = Box::pin(handle.io_write(&socket, vec![42]));
+    runtime.block_on(async {
+        std::future::poll_fn(|context| {
+            assert!(write.as_mut().poll(context).is_pending());
+            start_reader.send(()).unwrap();
+            Poll::Ready(())
+        })
+        .await;
+
+        let (result, buffer) = write.await;
+        assert_eq!(result.unwrap(), 1);
+        assert_eq!(buffer, [42]);
+    });
+
+    finish_reader.send(()).unwrap();
+    assert!(reader.join().unwrap() > 0);
+}
+
 #[test]
 fn write_returns_readiness_errors_and_non_retryable_errors() {
     let runtime = runtime();

crates/executor/src/worker.rs

@@ -133,6 +133,10 @@ fn search(shared: &Shared, local: &LocalWorker, iteration: usize) -> Steal<Runna
             return Steal::Success(runnable);
         }
     }
+    search_result(retry)
+}
+
+fn search_result(retry: bool) -> Steal<Runnable> {
     if retry { Steal::Retry } else { Steal::Empty }
 }
 

crates/executor/src/worker/tests.rs

@@ -2,8 +2,8 @@
 // Copyright, 2026, by Samuel Williams.
 
 use super::{
-    IdleSearch, SearchResult, WorkerState, classify_search, finish_idle_search, take_idle_runnable,
-    take_stolen,
+    IdleSearch, SearchResult, WorkerState, classify_search, finish_idle_search, search_result,
+    take_idle_runnable, take_stolen,
 };
 use crate::owner::Owner;
 use crate::task::{Runnable, TaskState, UNASSIGNED_WORKER};
@@ -45,6 +45,12 @@ fn records_retry_separately_from_empty_and_success() {
     assert!(retry);
 }
 
+#[test]
+fn search_reports_retry_only_when_a_queue_raced() {
+    assert!(matches!(search_result(true), Steal::Retry));
+    assert!(matches!(search_result(false), Steal::Empty));
+}
+
 #[test]
 fn worker_yields_after_a_concurrent_steal_retry() {
     assert!(matches!(