zephyr: sync: Create PinWeak type

The pin-weak provides a weak pointer that is compatible with `Pin<Arc>`.
Unfortunately, these are not compatible with the Arc that we use from
portable-atomic-utils.  Provide a simplified implementation of this
until the pin-weak is able to gain this functionality.

Signed-off-by: David Brown <[email protected]>

Author: d3zd3z

zephyr/src/sync.rs

@@ -25,11 +25,39 @@ pub mod atomic {
     pub use portable_atomic::*;
 }
 
+use core::pin::Pin;
+
 #[cfg(CONFIG_RUST_ALLOC)]
 pub use portable_atomic_util::Arc;
 #[cfg(CONFIG_RUST_ALLOC)]
 pub use portable_atomic_util::Weak;
 
+/// Safe Pinned Weak references.
+///
+/// Pin<Arc<T>> can't be converted to/from Weak safely, because there is know way to know if a given
+/// weak reference came from a pinned Arc.  This wraps the weak reference in a new type so we know
+/// that it came from a pinned Arc.
+///
+/// There is a pin-weak crate that provides this for `std::sync::Arc`, but not for the one in the
+/// portable-atomic-utils crate.
+pub struct PinWeak<T>(Weak<T>);
+
+impl<T> PinWeak<T> {
+    /// Downgrade an `Pin<Arc<T>>` into a `PinWeak`.
+    ///
+    /// This would be easier to use if it could be added to Arc.
+    pub fn downgrade(this: Pin<Arc<T>>) -> Self {
+        // SAFETY: we will never return anything other than a Pin<Arc<T>>.
+        Self(Arc::downgrade(& unsafe { Pin::into_inner_unchecked(this) }))
+    }
+
+    /// Upgrade back to a `Pin<Arc<T>>`.
+    pub fn upgrade(&self) -> Option<Pin<Arc<T>>> {
+        // SAFETY: The weak was only constructed from a `Pin<Arc<T>>`.
+        self.0.upgrade().map(|arc| unsafe { Pin::new_unchecked(arc) })
+    }
+}
+
 mod mutex;
 
 pub use mutex::{Condvar, LockResult, Mutex, MutexGuard, TryLockError, TryLockResult};