Add waker and conversion to raw pointer (#16)

Author: Stjepan Glavina

src/join_handle.rs

@@ -4,7 +4,7 @@ use core::marker::{PhantomData, Unpin};
 use core::pin::Pin;
 use core::ptr::NonNull;
 use core::sync::atomic::Ordering;
-use core::task::{Context, Poll};
+use core::task::{Context, Poll, Waker};
 
 use crate::header::Header;
 use crate::state::*;
@@ -92,6 +92,17 @@ impl<R, T> JoinHandle<R, T> {
             &*raw
         }
     }
+
+    /// Returns a waker associated with the task.
+    pub fn waker(&self) -> Waker {
+        let ptr = self.raw_task.as_ptr();
+        let header = ptr as *const Header;
+
+        unsafe {
+            let raw_waker = ((*header).vtable.clone_waker)(ptr);
+            Waker::from_raw(raw_waker)
+        }
+    }
 }
 
 impl<R, T> Drop for JoinHandle<R, T> {

src/raw.rs

@@ -35,6 +35,9 @@ pub(crate) struct TaskVTable {
 
     /// Runs the task.
     pub(crate) run: unsafe fn(*const ()),
+
+    /// Creates a new waker associated with the task.
+    pub(crate) clone_waker: unsafe fn(ptr: *const ()) -> RawWaker,
 }
 
 /// Memory layout of a task.
@@ -131,6 +134,7 @@ where
                     drop_task: Self::drop_task,
                     destroy: Self::destroy,
                     run: Self::run,
+                    clone_waker: Self::clone_waker,
                 },
             });
 

src/task.rs

@@ -4,7 +4,7 @@ use core::marker::PhantomData;
 use core::mem::{self, ManuallyDrop};
 use core::pin::Pin;
 use core::ptr::NonNull;
-use core::task::{Context, Poll};
+use core::task::{Context, Poll, Waker};
 
 use crate::header::Header;
 use crate::raw::RawTask;
@@ -264,6 +264,41 @@ impl<T> Task<T> {
             &*raw
         }
     }
+
+    /// Converts this task into a raw pointer to the tag.
+    pub fn into_raw(self) -> *const T {
+        let offset = Header::offset_tag::<T>();
+        let ptr = self.raw_task.as_ptr();
+        mem::forget(self);
+
+        unsafe { (ptr as *mut u8).add(offset) as *const T }
+    }
+
+    /// Converts a raw pointer to the tag into a task.
+    ///
+    /// This method should only be used with raw pointers returned from [`into_raw`].
+    ///
+    /// [`into_raw`]: #method.into_raw
+    pub unsafe fn from_raw(raw: *const T) -> Task<T> {
+        let offset = Header::offset_tag::<T>();
+        let ptr = (raw as *mut u8).sub(offset) as *mut ();
+
+        Task {
+            raw_task: NonNull::new_unchecked(ptr),
+            _marker: PhantomData,
+        }
+    }
+
+    /// Returns a waker associated with this task.
+    pub fn waker(&self) -> Waker {
+        let ptr = self.raw_task.as_ptr();
+        let header = ptr as *const Header;
+
+        unsafe {
+            let raw_waker = ((*header).vtable.clone_waker)(ptr);
+            Waker::from_raw(raw_waker)
+        }
+    }
 }
 
 impl<T> Drop for Task<T> {

tests/basic.rs

@@ -332,3 +332,39 @@ fn drop_inside_schedule() {
     );
     task.schedule();
 }
+
+#[test]
+fn waker() {
+    let (s, r) = channel::unbounded();
+    let schedule = move |t| s.send(t).unwrap();
+    let (task, handle) = async_task::spawn(
+        future::poll_fn(|_| Poll::<()>::Pending),
+        schedule,
+        Box::new(0),
+    );
+
+    assert!(r.is_empty());
+    let w = task.waker();
+    task.run();
+    w.wake();
+
+    let task = r.recv().unwrap();
+    task.run();
+    handle.waker().wake();
+
+    r.recv().unwrap();
+}
+
+#[test]
+fn raw() {
+    let (task, _handle) = async_task::spawn(async {}, |_| panic!(), Box::new(AtomicUsize::new(7)));
+
+    let a = task.into_raw();
+    let task = unsafe {
+        (*a).fetch_add(1, Ordering::SeqCst);
+        Task::from_raw(a)
+    };
+
+    assert_eq!(task.tag().load(Ordering::SeqCst), 8);
+    task.run();
+}