1 files changed, 112 insertions, 0 deletions
diff --git a/vendor/crossbeam-utils/src/atomic/seq_lock.rs b/vendor/crossbeam-utils/src/atomic/seq_lock.rs
new file mode 100644
index 0000000..ff8defd
--- /dev/null
+++ b/vendor/crossbeam-utils/src/atomic/seq_lock.rs
@@ -0,0 +1,112 @@
+use core::mem;
+use core::sync::atomic::{self, AtomicUsize, Ordering};
+
+use crate::Backoff;
+
+/// A simple stamped lock.
+pub(crate) struct SeqLock {
+    /// The current state of the lock.
+    ///
+    /// All bits except the least significant one hold the current stamp. When locked, the state
+    /// equals 1 and doesn't contain a valid stamp.
+    state: AtomicUsize,
+}
+
+impl SeqLock {
+    pub(crate) const fn new() -> Self {
+        Self {
+            state: AtomicUsize::new(0),
+        }
+    }
+
+    /// If not locked, returns the current stamp.
+    ///
+    /// This method should be called before optimistic reads.
+    #[inline]
+    pub(crate) fn optimistic_read(&self) -> Option<usize> {
+        let state = self.state.load(Ordering::Acquire);
+        if state == 1 {
+            None
+        } else {
+            Some(state)
+        }
+    }
+
+    /// Returns `true` if the current stamp is equal to `stamp`.
+    ///
+    /// This method should be called after optimistic reads to check whether they are valid. The
+    /// argument `stamp` should correspond to the one returned by method `optimistic_read`.
+    #[inline]
+    pub(crate) fn validate_read(&self, stamp: usize) -> bool {
+        atomic::fence(Ordering::Acquire);
+        self.state.load(Ordering::Relaxed) == stamp
+    }
+
+    /// Grabs the lock for writing.
+    #[inline]
+    pub(crate) fn write(&'static self) -> SeqLockWriteGuard {
+        let backoff = Backoff::new();
+        loop {
+            let previous = self.state.swap(1, Ordering::Acquire);
+
+            if previous != 1 {
+                atomic::fence(Ordering::Release);
+
+                return SeqLockWriteGuard {
+                    lock: self,
+                    state: previous,
+                };
+            }
+
+            backoff.snooze();
+        }
+    }
+}
+
+/// An RAII guard that releases the lock and increments the stamp when dropped.
+pub(crate) struct SeqLockWriteGuard {
+    /// The parent lock.
+    lock: &'static SeqLock,
+
+    /// The stamp before locking.
+    state: usize,
+}
+
+impl SeqLockWriteGuard {
+    /// Releases the lock without incrementing the stamp.
+    #[inline]
+    pub(crate) fn abort(self) {
+        self.lock.state.store(self.state, Ordering::Release);
+
+        // We specifically don't want to call drop(), since that's
+        // what increments the stamp.
+        mem::forget(self);
+    }
+}
+
+impl Drop for SeqLockWriteGuard {
+    #[inline]
+    fn drop(&mut self) {
+        // Release the lock and increment the stamp.
+        self.lock
+            .state
+            .store(self.state.wrapping_add(2), Ordering::Release);
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::SeqLock;
+
+    #[test]
+    fn test_abort() {
+        static LK: SeqLock = SeqLock::new();
+        let before = LK.optimistic_read().unwrap();
+        {
+            let guard = LK.write();
+            guard.abort();
+        }
+        let after = LK.optimistic_read().unwrap();
+        assert_eq!(before, after, "aborted write does not update the stamp");
+    }
+}