0.18 primitive: crate::halt::Halt cancellation token

One-bit cooperative cancellation flag. Replaces ad-hoc Arc<AtomicBool>
patterns scattered across libfreemkv (DiscStream::set_halt) and the
HALT_FLAGS global registry in autorip. See
(internal)/memory/0_18_redesign.md.

Single contributor: MattJackson (no attribution trailers).

src/halt.rs

@@ -0,0 +1,107 @@
+//! One-bit cooperative cancellation flag.
+//!
+//! `Halt` is a clonable token wrapping `Arc<AtomicBool>`. Pass clones into
+//! every long-running loop; the loop polls `is_cancelled()` and bails out
+//! cleanly. Calling `cancel()` from any clone flips the shared flag, and
+//! every other clone observes it on its next poll.
+//!
+//! Why: `Ordering::Relaxed` is sufficient on both load and store because
+//! this flag is purely advisory — no other memory operations piggyback on
+//! it for happens-before ordering. Callers that need to publish data
+//! across threads do so via channels or other synchronization, not via
+//! this bit.
+
+use std::sync::Arc;
+use std::sync::atomic::{AtomicBool, Ordering};
+
+/// Clonable, infallible cooperative-cancellation token.
+///
+/// Clones share the same underlying flag. `cancel()` is one-way; there is
+/// no `reset()` by design — construct a fresh `Halt` for a fresh
+/// operation.
+#[derive(Clone, Debug, Default)]
+pub struct Halt(Arc<AtomicBool>);
+
+impl Halt {
+    /// Construct a fresh, uncancelled token.
+    pub fn new() -> Self {
+        Self(Arc::new(AtomicBool::new(false)))
+    }
+
+    /// Flip the shared flag to cancelled. Idempotent.
+    pub fn cancel(&self) {
+        self.0.store(true, Ordering::Relaxed);
+    }
+
+    /// Read the shared flag.
+    pub fn is_cancelled(&self) -> bool {
+        self.0.load(Ordering::Relaxed)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn fresh_is_not_cancelled() {
+        let h = Halt::new();
+        assert!(!h.is_cancelled());
+    }
+
+    #[test]
+    fn cancel_flips_state() {
+        let h = Halt::new();
+        assert!(!h.is_cancelled());
+        h.cancel();
+        assert!(h.is_cancelled());
+    }
+
+    #[test]
+    fn cancel_is_idempotent() {
+        let h = Halt::new();
+        h.cancel();
+        h.cancel();
+        assert!(h.is_cancelled());
+    }
+
+    #[test]
+    fn clone_shares_state() {
+        let original = Halt::new();
+        let cloned = original.clone();
+        assert!(!original.is_cancelled());
+        assert!(!cloned.is_cancelled());
+
+        // Cancel via the clone; the original observes it.
+        cloned.cancel();
+        assert!(original.is_cancelled());
+        assert!(cloned.is_cancelled());
+    }
+
+    #[test]
+    fn clone_shares_state_reverse_direction() {
+        let original = Halt::new();
+        let cloned = original.clone();
+
+        // Cancel via the original; the clone observes it.
+        original.cancel();
+        assert!(cloned.is_cancelled());
+    }
+
+    #[test]
+    fn clone_shares_state_across_threads() {
+        let h = Halt::new();
+        let h2 = h.clone();
+        let handle = std::thread::spawn(move || {
+            h2.cancel();
+        });
+        handle.join().unwrap();
+        assert!(h.is_cancelled());
+    }
+
+    #[test]
+    fn default_impl_is_uncancelled() {
+        let h = Halt::default();
+        assert!(!h.is_cancelled());
+    }
+}

src/lib.rs

@@ -79,6 +79,7 @@ pub mod disc;
 pub mod drive;
 pub mod error;
 pub mod event;
+pub mod halt;
 pub(crate) mod identity;
 pub(crate) mod ifo;
 pub(crate) mod io;
@@ -116,6 +117,15 @@ pub use drive::{Drive, DriveStatus, find_drive};
 // codes to localized messages. See `error.rs` for the full taxonomy.
 pub use error::{Error, Result};
 
+// ─── 0.18 primitives ────────────────────────────────────────────────────────
+//
+// One-bit cooperative cancellation token, shared by every long-running loop
+// in libfreemkv (sweep, patch, mux). Replaces the ad-hoc `Arc<AtomicBool>`
+// flags scattered across 0.17 (`DiscStream::set_halt`, autorip's
+// `HALT_FLAGS` registry). Clone it cheaply; pass it by value into each
+// component; poll `is_cancelled()` inside the loop body.
+pub use halt::Halt;
+
 // ─── Drive events (low-level callbacks) ─────────────────────────────────────
 pub use event::{Event, EventKind};
 pub use identity::DriveId;