[PATCH v2 7/8] rust: workqueue: add `try_spawn` helper method

[Alice Ryhl <aliceryhl@google.com>]

This adds a convenience method that lets you spawn a closure for
execution on a workqueue. This will be the most convenient way to use
workqueues, but it is fallible because it needs to allocate memory.

Co-developed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 rust/kernel/workqueue.rs | 43 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/rust/kernel/workqueue.rs b/rust/kernel/workqueue.rs
index f06a2f036d8b..c302e8b8624b 100644
--- a/rust/kernel/workqueue.rs
+++ b/rust/kernel/workqueue.rs
@@ -29,6 +29,7 @@
 //! C header: [`include/linux/workqueue.h`](../../../../include/linux/workqueue.h)
 
 use crate::{bindings, prelude::*, sync::Arc, types::Opaque};
+use alloc::alloc::AllocError;
 use alloc::boxed::Box;
 use core::marker::{PhantomData, PhantomPinned};
 use core::pin::Pin;
@@ -87,6 +88,44 @@ pub fn enqueue<W, const ID: u64>(&self, w: W) -> W::EnqueueOutput
             })
         }
     }
+
+    /// Tries to spawn the given function or closure as a work item.
+    ///
+    /// This method can fail because it allocates memory to store the work item.
+    pub fn try_spawn<T: 'static + Send + FnOnce()>(&self, func: T) -> Result<(), AllocError> {
+        let init = pin_init!(ClosureWork {
+            work <- Work::new(),
+            func: Some(func),
+        });
+
+        self.enqueue(Box::pin_init(init).map_err(|_| AllocError)?);
+        Ok(())
+    }
+}
+
+/// A helper type used in `try_spawn`.
+#[pin_data]
+struct ClosureWork<T> {
+    #[pin]
+    work: Work<ClosureWork<T>>,
+    func: Option<T>,
+}
+
+impl<T> ClosureWork<T> {
+    fn project(self: Pin<&mut Self>) -> &mut Option<T> {
+        // SAFETY: The `func` field is not structurally pinned.
+        unsafe { &mut self.get_unchecked_mut().func }
+    }
+}
+
+impl<T: FnOnce()> WorkItem for ClosureWork<T> {
+    type Pointer = Pin<Box<Self>>;
+
+    fn run(mut this: Pin<Box<Self>>) {
+        if let Some(func) = this.as_mut().project().take() {
+            (func)()
+        }
+    }
 }
 
 /// A raw work item.
@@ -325,6 +364,10 @@ unsafe fn raw_get_work(ptr: *mut Self) -> *mut $crate::workqueue::Work<$work_typ
     )*};
 }
 
+impl_has_work! {
+    impl<T> HasWork<Self> for ClosureWork<T> { self.work }
+}
+
 unsafe impl<T, const ID: u64> WorkItemPointer<ID> for Arc<T>
 where
     T: WorkItem<ID, Pointer = Self>,
-- 
2.41.0.rc0.172.g3f132b7071-goog