rtic-sync: Channel: Sender: rewriter `send` logic to be easier to validate

1 files changed, 39 insertions, 32 deletions

diff --git a/rtic-sync/src/channel.rs b/rtic-sync/src/channel.rs
index d5574d0..00e9476 100644
--- a/rtic-sync/src/channel.rs
+++ b/rtic-sync/src/channel.rs
@@ -333,44 +333,51 @@ impl<T, const N: usize> Sender<'_, T, N> {
             //  Do all this in one critical section, else there can be race conditions
             critical_section::with(|cs| {
                 let wq_empty = self.0.wait_queue.is_empty();
-                let fq_empty = self.0.access(cs).freeq.is_empty();
-
-                if !wq_empty || fq_empty {
-                    // SAFETY: This pointer is only dereferenced here and on drop of the future
-                    // which happens outside this `poll_fn`'s stack frame.
-                    let link = unsafe { link_ptr.get() };
-                    if let Some(link) = link {
-                        if !link.is_popped() {
-                            return Poll::Pending;
+                let freeq_empty = self.0.access(cs).freeq.is_empty();
+
+                // SAFETY: This pointer is only dereferenced here and on drop of the future
+                // which happens outside this `poll_fn`'s stack frame.
+                let link = unsafe { link_ptr.get() };
+
+                // We are already in the wait queue.
+                if let Some(link) = link {
+                    if link.is_popped() {
+                        // If our link is popped, then:
+                        // 1. We were popped by `try_recv` and it provided us with a slot.
+                        // 2. We were popped by `Receiver::drop` and it did not provide us with a slot, and the channel is closed.
+                        let slot = unsafe { free_slot_ptr.replace(None, cs) };
+
+                        if let Some(slot) = slot {
+                            Poll::Ready(Ok(slot))
                         } else {
-                            // Fall through to dequeue
+                            Poll::Ready(Err(()))
                         }
                     } else {
-                        // Place the link in the wait queue on first run.
-                        let link_ref =
-                            link.insert(Link::new((cx.waker().clone(), free_slot_ptr.clone())));
-
-                        // SAFETY(new_unchecked): The address to the link is stable as it is defined
-                        // outside this stack frame.
-                        // SAFETY(push): `link_ref` lifetime comes from `link_ptr` that is shadowed,
-                        // and  we make sure in `dropper` that the link is removed from the queue
-                        // before dropping `link_ptr` AND `dropper` makes sure that the shadowed
-                        // `link_ptr` lives until the end of the stack frame.
-                        unsafe { self.0.wait_queue.push(Pin::new_unchecked(link_ref)) };
-
-                        return Poll::Pending;
+                        Poll::Pending
                     }
                 }
-
-                // SAFETY: `free_slot_ptr` is valid for writes, as `free_slot_ptr` is still alive.
-                let slot = unsafe { free_slot_ptr.replace(None, cs) }
-                    .or_else(|| self.0.access(cs).freeq.pop_back());
-
-                if let Some(slot) = slot {
+                // We are not in the wait queue, but others are, or there is currently no free
+                // slot available.
+                else if !wq_empty || freeq_empty {
+                    // Place the link in the wait queue.
+                    let link_ref =
+                        link.insert(Link::new((cx.waker().clone(), free_slot_ptr.clone())));
+
+                    // SAFETY(new_unchecked): The address to the link is stable as it is defined
+                    // outside this stack frame.
+                    // SAFETY(push): `link_ref` lifetime comes from `link_ptr` that is shadowed,
+                    // and  we make sure in `dropper` that the link is removed from the queue
+                    // before dropping `link_ptr` AND `dropper` makes sure that the shadowed
+                    // `link_ptr` lives until the end of the stack frame.
+                    unsafe { self.0.wait_queue.push(Pin::new_unchecked(link_ref)) };
+
+                    Poll::Pending
+                }
+                // We are not in the wait queue, no one else is waiting, and there is a free slot available.
+                else {
+                    assert!(!self.0.access(cs).freeq.is_empty());
+                    let slot = unsafe { self.0.access(cs).freeq.pop_back_unchecked() };
                     Poll::Ready(Ok(slot))
-                } else {
-                    debug_assert!(self.is_closed());
-                    Poll::Ready(Err(()))
                 }
             })
         })