Add panic propagation

Cargo.toml

@@ -23,6 +23,7 @@ atomic-waker = "1"
 easy-parallel = "3"
 flaky_test = "0.1"
 flume = { version = "0.10", default-features = false }
+futures-lite = "1.12.0"
 once_cell = "1"
 smol = "1"
 

src/header.rs

@@ -31,6 +31,10 @@ pub(crate) struct Header<M> {
     ///
     /// This metadata may be provided to the user.
     pub(crate) metadata: M,
+
+    /// Whether or not a panic that occurs in the task should be propagated.
+    #[cfg(feature = "std")]
+    pub(crate) propagate_panic: bool,
 }
 
 impl<M> Header<M> {

src/raw.rs

@@ -13,6 +13,12 @@ use crate::state::*;
 use crate::utils::{abort, abort_on_panic, max, Layout};
 use crate::Runnable;
 
+#[cfg(feature = "std")]
+pub(crate) type Panic = alloc::boxed::Box<dyn core::any::Any + Send + 'static>;
+
+#[cfg(not(feature = "std"))]
+pub(crate) type Panic = core::convert::Infallible;
+
 /// The vtable for a task.
 pub(crate) struct TaskVTable {
     /// Schedules the task.
@@ -76,7 +82,7 @@ pub(crate) struct RawTask<F, T, S, M> {
     pub(crate) future: *mut F,
 
     /// The output of the future.
-    pub(crate) output: *mut T,
+    pub(crate) output: *mut Result<T, Panic>,
 }
 
 impl<F, T, S, M> Copy for RawTask<F, T, S, M> {}
@@ -97,7 +103,7 @@ impl<F, T, S, M> RawTask<F, T, S, M> {
         let layout_header = Layout::new::<Header<M>>();
         let layout_s = Layout::new::<S>();
         let layout_f = Layout::new::<F>();
-        let layout_r = Layout::new::<T>();
+        let layout_r = Layout::new::<Result<T, Panic>>();
 
         // Compute the layout for `union { F, T }`.
         let size_union = max(layout_f.size(), layout_r.size());
@@ -138,7 +144,7 @@ where
     pub(crate) fn allocate<'a, Gen: FnOnce(&'a M) -> F>(
         future: Gen,
         schedule: S,
-        metadata: M,
+        builder: crate::Builder<M>,
     ) -> NonNull<()>
     where
         F: 'a,
@@ -158,6 +164,12 @@ where
 
             let raw = Self::from_ptr(ptr.as_ptr());
 
+            let crate::Builder {
+                metadata,
+                #[cfg(feature = "std")]
+                propagate_panic,
+            } = builder;
+
             // Write the header as the first field of the task.
             (raw.header as *mut Header<M>).write(Header {
                 state: AtomicUsize::new(SCHEDULED | TASK | REFERENCE),
@@ -173,6 +185,8 @@ where
                     layout_info: &Self::TASK_LAYOUT,
                 },
                 metadata,
+                #[cfg(feature = "std")]
+                propagate_panic,
             });
 
             // Write the schedule function as the third field of the task.
@@ -199,7 +213,7 @@ where
                 header: p as *const Header<M>,
                 schedule: p.add(task_layout.offset_s) as *const S,
                 future: p.add(task_layout.offset_f) as *mut F,
-                output: p.add(task_layout.offset_r) as *mut T,
+                output: p.add(task_layout.offset_r) as *mut Result<T, Panic>,
             }
         }
     }
@@ -525,8 +539,30 @@ where
 
         // Poll the inner future, but surround it with a guard that closes the task in case polling
         // panics.
+        // If available, we should also try to catch the panic so that it is propagated correctly.
         let guard = Guard(raw);
-        let poll = <F as Future>::poll(Pin::new_unchecked(&mut *raw.future), cx);
+
+        // Panic propagation is not available for no_std.
+        #[cfg(not(feature = "std"))]
+        let poll = <F as Future>::poll(Pin::new_unchecked(&mut *raw.future), cx).map(Ok);
+
+        #[cfg(feature = "std")]
+        let poll = {
+            // Check if we should propagate panics.
+            if (*raw.header).propagate_panic {
+                // Use catch_unwind to catch the panic.
+                match std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
+                    <F as Future>::poll(Pin::new_unchecked(&mut *raw.future), cx)
+                })) {
+                    Ok(Poll::Ready(v)) => Poll::Ready(Ok(v)),
+                    Ok(Poll::Pending) => Poll::Pending,
+                    Err(e) => Poll::Ready(Err(e)),
+                }
+            } else {
+                <F as Future>::poll(Pin::new_unchecked(&mut *raw.future), cx).map(Ok)
+            }
+        };
+
         mem::forget(guard);
 
         match poll {

src/runnable.rs

@@ -17,7 +17,11 @@ use crate::Task;
 #[derive(Debug)]
 pub struct Builder<M> {
     /// The metadata associated with the task.
-    metadata: M,
+    pub(crate) metadata: M,
+
+    /// Whether or not a panic that occurs in the task should be propagated.
+    #[cfg(feature = "std")]
+    pub(crate) propagate_panic: bool,
 }
 
 impl<M: Default> Default for Builder<M> {
@@ -40,7 +44,11 @@ impl Builder<()> {
     /// let (runnable, task) = Builder::new().spawn(|()| async {}, |_| {});
     /// ```
     pub fn new() -> Builder<()> {
-        Builder { metadata: () }
+        Builder {
+            metadata: (),
+            #[cfg(feature = "std")]
+            propagate_panic: false,
+        }
     }
 
     /// Adds metadata to the task.
@@ -123,11 +131,63 @@ impl Builder<()> {
     /// # });
     /// ```
     pub fn metadata<M>(self, metadata: M) -> Builder<M> {
-        Builder { metadata }
+        Builder {
+            metadata,
+            #[cfg(feature = "std")]
+            propagate_panic: self.propagate_panic,
+        }
     }
 }
 
 impl<M> Builder<M> {
+    /// Propagates panics that occur in the task.
+    ///
+    /// When this is `true`, panics that occur in the task will be propagated to the caller of
+    /// the [`Task`]. When this is false, no special action is taken when a panic occurs in the
+    /// task, meaning that the caller of [`Runnable::run`] will observe a panic.
+    ///
+    /// This is only available when the `std` feature is enabled. By default, this is `false`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use async_task::Builder;
+    /// use futures_lite::future::poll_fn;
+    /// use std::future::Future;
+    /// use std::panic;
+    /// use std::pin::Pin;
+    /// use std::task::{Context, Poll};
+    ///
+    /// fn did_panic<F: FnOnce()>(f: F) -> bool {
+    ///     panic::catch_unwind(panic::AssertUnwindSafe(f)).is_err()
+    /// }
+    ///
+    /// # smol::future::block_on(async {
+    /// let (runnable1, mut task1) = Builder::new()
+    ///    .propagate_panic(true)
+    ///    .spawn(|()| async move { panic!() }, |_| {});
+    ///
+    /// let (runnable2, mut task2) = Builder::new()
+    ///    .propagate_panic(false)
+    ///    .spawn(|()| async move { panic!() }, |_| {});
+    ///
+    /// assert!(!did_panic(|| { runnable1.run(); }));
+    /// assert!(did_panic(|| { runnable2.run(); }));
+    ///
+    /// let waker = poll_fn(|cx| Poll::Ready(cx.waker().clone())).await;
+    /// let mut cx = Context::from_waker(&waker);
+    /// assert!(did_panic(|| { let _ = Pin::new(&mut task1).poll(&mut cx); }));
+    /// assert!(did_panic(|| { let _ = Pin::new(&mut task2).poll(&mut cx); }));
+    /// # });
+    /// ```
+    #[cfg(feature = "std")]
+    pub fn propagate_panic(self, propagate_panic: bool) -> Builder<M> {
+        Builder {
+            metadata: self.metadata,
+            propagate_panic,
+        }
+    }
+
     /// Creates a new task.
     ///
     /// The returned [`Runnable`] is used to poll the `future`, and the [`Task`] is used to await its
@@ -313,18 +373,16 @@ impl<M> Builder<M> {
         S: Fn(Runnable<M>),
         M: 'a,
     {
-        let Self { metadata } = self;
-
         // Allocate large futures on the heap.
         let ptr = if mem::size_of::<Fut>() >= 2048 {
             let future = |meta| {
                 let future = future(meta);
                 Box::pin(future)
             };
 
-            RawTask::<_, Fut::Output, S, M>::allocate(future, schedule, metadata)
+            RawTask::<_, Fut::Output, S, M>::allocate(future, schedule, self)
         } else {
-            RawTask::<Fut, Fut::Output, S, M>::allocate(future, schedule, metadata)
+            RawTask::<Fut, Fut::Output, S, M>::allocate(future, schedule, self)
         };
 
         let runnable = Runnable {

src/task.rs

@@ -8,6 +8,7 @@ use core::sync::atomic::Ordering;
 use core::task::{Context, Poll};
 
 use crate::header::Header;
+use crate::raw::Panic;
 use crate::state::*;
 
 /// A spawned task.
@@ -226,7 +227,7 @@ impl<T, M> Task<T, M> {
     }
 
     /// Puts the task in detached state.
-    fn set_detached(&mut self) -> Option<T> {
+    fn set_detached(&mut self) -> Option<Result<T, Panic>> {
         let ptr = self.ptr.as_ptr();
         let header = ptr as *const Header<M>;
 
@@ -256,8 +257,10 @@ impl<T, M> Task<T, M> {
                         ) {
                             Ok(_) => {
                                 // Read the output.
-                                output =
-                                    Some((((*header).vtable.get_output)(ptr) as *mut T).read());
+                                output = Some(
+                                    (((*header).vtable.get_output)(ptr) as *mut Result<T, Panic>)
+                                        .read(),
+                                );
 
                                 // Update the state variable because we're continuing the loop.
                                 state |= CLOSED;
@@ -382,8 +385,22 @@ impl<T, M> Task<T, M> {
                         }
 
                         // Take the output from the task.
-                        let output = ((*header).vtable.get_output)(ptr) as *mut T;
-                        return Poll::Ready(Some(output.read()));
+                        let output = ((*header).vtable.get_output)(ptr) as *mut Result<T, Panic>;
+                        let output = output.read();
+
+                        // Propagate the panic if the task panicked.
+                        let output = match output {
+                            Ok(output) => output,
+                            Err(panic) => {
+                                #[cfg(feature = "std")]
+                                std::panic::resume_unwind(panic);
+
+                                #[cfg(not(feature = "std"))]
+                                match panic {}
+                            }
+                        };
+
+                        return Poll::Ready(Some(output));
                     }
                     Err(s) => state = s,
                 }