add a parking lot impl

src/busy_lot.rs

@@ -0,0 +1,40 @@
+use std::sync::Arc;
+use std::sync::atomic::{AtomicUsize, Ordering};
+
+/// Shared, lock-free counter of active jobs.
+/// Cloning is cheap (Arc); dropping a ticket auto-decrements.
+#[derive(Clone)]
+pub struct BusyLot {
+    inner: Arc<AtomicUsize>,
+}
+
+/// RAII ticket returned from `BusyLot::park()`.
+/// When the ticket is dropped (even on panic) the lot counter goes down.
+pub struct Ticket {
+    lot: BusyLot,
+}
+
+impl BusyLot {
+    pub fn new() -> Self {
+        Self {
+            inner: Arc::new(AtomicUsize::new(0)),
+        }
+    }
+
+    /// Takes a parking space and returns a ticket.
+    pub fn park(&self) -> Ticket {
+        self.inner.fetch_add(1, Ordering::AcqRel);
+        Ticket { lot: self.clone() }
+    }
+
+    /// `true` if at least one ticket is still parked.
+    pub fn is_busy(&self) -> bool {
+        self.inner.load(Ordering::Acquire) != 0
+    }
+}
+
+impl Drop for Ticket {
+    fn drop(&mut self) {
+        self.lot.inner.fetch_sub(1, Ordering::AcqRel);
+    }
+}