Import 'futures-channel' rust crate version 0.3.4
Bug: b/151760391
Test: m crosvm.experimental
Change-Id: I40daaec86f1c422d2bc563057f04384d4fda7740
diff --git a/src/lock.rs b/src/lock.rs
new file mode 100644
index 0000000..e477369
--- /dev/null
+++ b/src/lock.rs
@@ -0,0 +1,105 @@
+//! A "mutex" which only supports `try_lock`
+//!
+//! As a futures library the eventual call to an event loop should be the only
+//! thing that ever blocks, so this is assisted with a fast user-space
+//! implementation of a lock that can only have a `try_lock` operation.
+
+use core::cell::UnsafeCell;
+use core::ops::{Deref, DerefMut};
+use core::sync::atomic::Ordering::SeqCst;
+use core::sync::atomic::AtomicBool;
+
+/// A "mutex" around a value, similar to `std::sync::Mutex<T>`.
+///
+/// This lock only supports the `try_lock` operation, however, and does not
+/// implement poisoning.
+#[derive(Debug)]
+pub(crate) struct Lock<T> {
+ locked: AtomicBool,
+ data: UnsafeCell<T>,
+}
+
+/// Sentinel representing an acquired lock through which the data can be
+/// accessed.
+pub(crate) struct TryLock<'a, T> {
+ __ptr: &'a Lock<T>,
+}
+
+// The `Lock` structure is basically just a `Mutex<T>`, and these two impls are
+// intended to mirror the standard library's corresponding impls for `Mutex<T>`.
+//
+// If a `T` is sendable across threads, so is the lock, and `T` must be sendable
+// across threads to be `Sync` because it allows mutable access from multiple
+// threads.
+unsafe impl<T: Send> Send for Lock<T> {}
+unsafe impl<T: Send> Sync for Lock<T> {}
+
+impl<T> Lock<T> {
+ /// Creates a new lock around the given value.
+ pub(crate) fn new(t: T) -> Lock<T> {
+ Lock {
+ locked: AtomicBool::new(false),
+ data: UnsafeCell::new(t),
+ }
+ }
+
+ /// Attempts to acquire this lock, returning whether the lock was acquired or
+ /// not.
+ ///
+ /// If `Some` is returned then the data this lock protects can be accessed
+ /// through the sentinel. This sentinel allows both mutable and immutable
+ /// access.
+ ///
+ /// If `None` is returned then the lock is already locked, either elsewhere
+ /// on this thread or on another thread.
+ pub(crate) fn try_lock(&self) -> Option<TryLock<'_, T>> {
+ if !self.locked.swap(true, SeqCst) {
+ Some(TryLock { __ptr: self })
+ } else {
+ None
+ }
+ }
+}
+
+impl<T> Deref for TryLock<'_, T> {
+ type Target = T;
+ fn deref(&self) -> &T {
+ // The existence of `TryLock` represents that we own the lock, so we
+ // can safely access the data here.
+ unsafe { &*self.__ptr.data.get() }
+ }
+}
+
+impl<T> DerefMut for TryLock<'_, T> {
+ fn deref_mut(&mut self) -> &mut T {
+ // The existence of `TryLock` represents that we own the lock, so we
+ // can safely access the data here.
+ //
+ // Additionally, we're the *only* `TryLock` in existence so mutable
+ // access should be ok.
+ unsafe { &mut *self.__ptr.data.get() }
+ }
+}
+
+impl<T> Drop for TryLock<'_, T> {
+ fn drop(&mut self) {
+ self.__ptr.locked.store(false, SeqCst);
+ }
+}
+
+#[cfg(test)]
+mod tests {
+ use super::Lock;
+
+ #[test]
+ fn smoke() {
+ let a = Lock::new(1);
+ let mut a1 = a.try_lock().unwrap();
+ assert!(a.try_lock().is_none());
+ assert_eq!(*a1, 1);
+ *a1 = 2;
+ drop(a1);
+ assert_eq!(*a.try_lock().unwrap(), 2);
+ assert_eq!(*a.try_lock().unwrap(), 2);
+ }
+}