Add support for scoped threads

Add loom::thread::scope to mirror std::thread::scope provided by the
standard library.

src/thread.rs

@@ -14,11 +14,7 @@ use std::{fmt, io};
 use tracing::trace;
 
 /// Mock implementation of `std::thread::JoinHandle`.
-pub struct JoinHandle<T> {
-    result: Arc<Mutex<Option<std::thread::Result<T>>>>,
-    notify: rt::Notify,
-    thread: Thread,
-}
+pub struct JoinHandle<T>(JoinHandleInner<'static, T>);
 
 /// Mock implementation of `std::thread::Thread`.
 #[derive(Clone, Debug)]
@@ -128,7 +124,7 @@ where
     F: 'static,
     T: 'static,
 {
-    spawn_internal(f, None, location!())
+    JoinHandle(spawn_internal_static(f, None, location!()))
 }
 
 /// Mock implementation of `std::thread::park`.
@@ -142,38 +138,6 @@ pub fn park() {
     rt::park(location!());
 }
 
-fn spawn_internal<F, T>(f: F, name: Option<String>, location: Location) -> JoinHandle<T>
-where
-    F: FnOnce() -> T,
-    F: 'static,
-    T: 'static,
-{
-    let result = Arc::new(Mutex::new(None));
-    let notify = rt::Notify::new(true, false);
-
-    let id = {
-        let name = name.clone();
-        let result = result.clone();
-        rt::spawn(move || {
-            rt::execution(|execution| {
-                init_current(execution, name);
-            });
-
-            *result.lock().unwrap() = Some(Ok(f()));
-            notify.notify(location);
-        })
-    };
-
-    JoinHandle {
-        result,
-        notify,
-        thread: Thread {
-            id: ThreadId { id },
-            name,
-        },
-    }
-}
-
 impl Builder {
     /// Generates the base configuration for spawning a thread, from which
     /// configuration methods can be chained.
@@ -206,21 +170,40 @@ impl Builder {
         F: Send + 'static,
         T: Send + 'static,
     {
-        Ok(spawn_internal(f, self.name, location!()))
+        Ok(JoinHandle(spawn_internal_static(f, self.name, location!())))
+    }
+}
+
+impl Builder {
+    /// Spawns a new scoped thread using the settings set through this `Builder`.
+    pub fn spawn_scoped<'scope, 'env, F, T>(
+        self,
+        scope: &'scope Scope<'scope, 'env>,
+        f: F,
+    ) -> io::Result<ScopedJoinHandle<'scope, T>>
+    where
+        F: FnOnce() -> T + Send + 'scope,
+        T: Send + 'scope,
+    {
+        Ok(ScopedJoinHandle(
+            // Safety: the call to this function requires a `&'scope Scope`
+            // which can only be constructed by `scope()`, which ensures that
+            // all spawned threads are joined before the `Scope` is destroyed.
+            unsafe { spawn_internal(f, self.name, Some(scope.data.clone()), location!()) },
+        ))
     }
 }
 
 impl<T> JoinHandle<T> {
     /// Waits for the associated thread to finish.
     #[track_caller]
     pub fn join(self) -> std::thread::Result<T> {
-        self.notify.wait(location!());
-        self.result.lock().unwrap().take().unwrap()
+        self.0.join()
     }
 
     /// Gets a handle to the underlying [`Thread`]
     pub fn thread(&self) -> &Thread {
-        &self.thread
+        self.0.thread()
     }
 }
 
@@ -301,3 +284,220 @@ impl<T: 'static> fmt::Debug for LocalKey<T> {
         f.pad("LocalKey { .. }")
     }
 }
+
+/// A scope for spawning scoped threads.
+///
+/// See [`scope`] for more details.
+#[derive(Debug)]
+pub struct Scope<'scope, 'env: 'scope> {
+    data: Arc<ScopeData>,
+    scope: PhantomData<&'scope mut &'scope ()>,
+    env: PhantomData<&'env mut &'env ()>,
+}
+
+/// An owned permission to join on a scoped thread (block on its termination).
+///
+/// See [`Scope::spawn`] for details.
+#[derive(Debug)]
+pub struct ScopedJoinHandle<'scope, T>(JoinHandleInner<'scope, T>);
+
+/// Create a scope for spawning scoped threads.
+///
+/// Mock implementation of [`std::thread::scope`].
+#[track_caller]
+pub fn scope<'env, F, T>(f: F) -> T
+where
+    F: for<'scope> FnOnce(&'scope Scope<'scope, 'env>) -> T,
+{
+    let scope = Scope {
+        data: Arc::new(ScopeData {
+            running_threads: Mutex::default(),
+            main_thread: current(),
+        }),
+        env: PhantomData,
+        scope: PhantomData,
+    };
+
+    // Run `f`, but catch panics so we can make sure to wait for all the threads to join.
+    let result = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| f(&scope)));
+
+    // Wait until all the threads are finished. This is required to fulfill
+    // the safety requirements of `spawn_internal`.
+    let running = loop {
+        {
+            let running = scope.data.running_threads.lock().unwrap();
+            if running.count == 0 {
+                break running;
+            }
+        }
+        park();
+    };
+
+    for notify in &running.notify_on_finished {
+        notify.wait(location!())
+    }
+
+    // Throw any panic from `f`, or the return value of `f` if no thread panicked.
+    match result {
+        Err(e) => std::panic::resume_unwind(e),
+        Ok(result) => result,
+    }
+}
+
+impl<'scope, 'env> Scope<'scope, 'env> {
+    /// Spawns a new thread within a scope, returning a [`ScopedJoinHandle`] for it.
+    ///
+    /// See [`std::thread::Scope`] and [`std::thread::scope`] for details.
+    pub fn spawn<F, T>(&'scope self, f: F) -> ScopedJoinHandle<'scope, T>
+    where
+        F: FnOnce() -> T + Send + 'scope,
+        T: Send + 'scope,
+    {
+        Builder::new()
+            .spawn_scoped(self, f)
+            .expect("failed to spawn thread")
+    }
+}
+
+impl<'scope, T> ScopedJoinHandle<'scope, T> {
+    /// Extracts a handle to the underlying thread.
+    pub fn thread(&self) -> &Thread {
+        self.0.thread()
+    }
+
+    /// Waits for the associated thread to finish.
+    pub fn join(self) -> std::thread::Result<T> {
+        self.0.join()
+    }
+}
+
+/// Handle for joining on a thread with a scope.
+#[derive(Debug)]
+struct JoinHandleInner<'scope, T> {
+    data: Arc<ThreadData<'scope, T>>,
+    notify: rt::Notify,
+    thread: Thread,
+}
+
+/// Spawns a thread without a local scope.
+fn spawn_internal_static<F, T>(
+    f: F,
+    name: Option<String>,
+    location: Location,
+) -> JoinHandleInner<'static, T>
+where
+    F: FnOnce() -> T,
+    F: 'static,
+    T: 'static,
+{
+    // Safety: the requirements of `spawn_internal` are trivially satisfied
+    // since there is no `scope`.
+    unsafe { spawn_internal(f, name, None, location) }
+}
+
+/// Spawns a thread with an optional scope.
+///
+/// The caller must ensure that if `scope` is not None, the provided closure
+/// finishes before `'scope` ends.
+unsafe fn spawn_internal<'scope, F, T>(
+    f: F,
+    name: Option<String>,
+    scope: Option<Arc<ScopeData>>,
+    location: Location,
+) -> JoinHandleInner<'scope, T>
+where
+    F: FnOnce() -> T,
+    F: 'scope,
+    T: 'scope,
+{
+    let scope_notify = scope
+        .clone()
+        .map(|scope| (scope.add_running_thread(), scope));
+    let thread_data = Arc::new(ThreadData::new());
+    let notify = rt::Notify::new(true, false);
+
+    let id = {
+        let name = name.clone();
+        let thread_data = thread_data.clone();
+        let body: Box<dyn FnOnce() + 'scope> = Box::new(move || {
+            rt::execution(|execution| {
+                init_current(execution, name);
+            });
+
+            *thread_data.result.lock().unwrap() = Some(Ok(f()));
+            notify.notify(location);
+
+            if let Some((notifier, scope)) = scope_notify {
+                notifier.notify(location!());
+                scope.remove_running_thread()
+            }
+        });
+        rt::spawn(std::mem::transmute::<_, Box<dyn FnOnce()>>(body))
+    };
+
+    JoinHandleInner {
+        data: thread_data,
+        notify,
+        thread: Thread {
+            id: ThreadId { id },
+            name,
+        },
+    }
+}
+
+/// Data for a running thread.
+#[derive(Debug)]
+struct ThreadData<'scope, T> {
+    result: Mutex<Option<std::thread::Result<T>>>,
+    _marker: PhantomData<Option<&'scope ScopeData>>,
+}
+
+impl<'scope, T> ThreadData<'scope, T> {
+    fn new() -> Self {
+        Self {
+            result: Mutex::new(None),
+            _marker: PhantomData,
+        }
+    }
+}
+
+impl<'scope, T> JoinHandleInner<'scope, T> {
+    fn join(self) -> std::thread::Result<T> {
+        self.notify.wait(location!());
+        self.data.result.lock().unwrap().take().unwrap()
+    }
+
+    fn thread(&self) -> &Thread {
+        &self.thread
+    }
+}
+
+#[derive(Default, Debug)]
+struct ScopeThreads {
+    count: usize,
+    notify_on_finished: Vec<rt::Notify>,
+}
+
+#[derive(Debug)]
+struct ScopeData {
+    running_threads: Mutex<ScopeThreads>,
+    main_thread: Thread,
+}
+
+impl ScopeData {
+    fn add_running_thread(&self) -> rt::Notify {
+        let mut running = self.running_threads.lock().unwrap();
+        running.count += 1;
+        let notify = rt::Notify::new(true, false);
+        running.notify_on_finished.push(notify);
+        notify
+    }
+
+    fn remove_running_thread(&self) {
+        let mut running = self.running_threads.lock().unwrap();
+        running.count -= 1;
+        if running.count == 0 {
+            self.main_thread.unpark()
+        }
+    }
+}