1 files changed, 128 insertions, 0 deletions
diff --git a/src/build_guard.rs b/src/build_guard.rs
new file mode 100644
index 0000000..0c7fed3
--- /dev/null
+++ b/src/build_guard.rs
@@ -0,0 +1,128 @@
+use dashmap::DashSet;
+use std::sync::Arc;
+
+/// Manages per-site build scheduling: immediate execution and a depth-1 queue.
+///
+/// When a build is already in progress, a single rebuild can be queued.
+/// Subsequent requests while a rebuild is already queued are collapsed (no-op).
+pub struct BuildScheduler {
+    pub in_progress: DashSet<String>,
+    pub queued: DashSet<String>,
+}
+
+impl BuildScheduler {
+    #[must_use]
+    pub fn new() -> Self {
+        Self {
+            in_progress: DashSet::new(),
+            queued: DashSet::new(),
+        }
+    }
+
+    /// Queue a rebuild for a site that is currently building.
+    /// Returns `true` if newly queued, `false` if already queued (collapse).
+    pub(crate) fn try_queue(&self, site_name: &str) -> bool {
+        self.queued.insert(site_name.to_owned())
+    }
+
+    /// Check and clear queued rebuild. Returns `true` if there was one.
+    pub(crate) fn take_queued(&self, site_name: &str) -> bool {
+        self.queued.remove(site_name).is_some()
+    }
+}
+
+impl Default for BuildScheduler {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+/// RAII guard for per-site build exclusion.
+/// Inserting into the scheduler's `in_progress` set acquires the lock;
+/// dropping removes it.
+pub(crate) struct BuildGuard {
+    site_name: String,
+    scheduler: Arc<BuildScheduler>,
+}
+
+impl BuildGuard {
+    pub(crate) fn try_acquire(site_name: String, scheduler: &Arc<BuildScheduler>) -> Option<Self> {
+        if scheduler.in_progress.insert(site_name.clone()) {
+            Some(Self {
+                site_name,
+                scheduler: Arc::clone(scheduler),
+            })
+        } else {
+            None
+        }
+    }
+}
+
+impl Drop for BuildGuard {
+    fn drop(&mut self) {
+        self.scheduler.in_progress.remove(&self.site_name);
+    }
+}
+
+#[cfg(test)]
+#[allow(clippy::unwrap_used, clippy::indexing_slicing)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn build_guard_try_acquire_success() {
+        let scheduler = Arc::new(BuildScheduler::new());
+        let guard = BuildGuard::try_acquire("my-site".to_owned(), &scheduler);
+        assert!(guard.is_some());
+        assert!(scheduler.in_progress.contains("my-site"));
+    }
+
+    #[test]
+    fn build_guard_try_acquire_fails_when_held() {
+        let scheduler = Arc::new(BuildScheduler::new());
+        let _guard = BuildGuard::try_acquire("my-site".to_owned(), &scheduler);
+        let second = BuildGuard::try_acquire("my-site".to_owned(), &scheduler);
+        assert!(second.is_none());
+    }
+
+    #[test]
+    fn build_guard_drop_releases_lock() {
+        let scheduler = Arc::new(BuildScheduler::new());
+        {
+            let _guard = BuildGuard::try_acquire("my-site".to_owned(), &scheduler);
+            assert!(scheduler.in_progress.contains("my-site"));
+        }
+        // Guard dropped — lock released
+        assert!(!scheduler.in_progress.contains("my-site"));
+        let again = BuildGuard::try_acquire("my-site".to_owned(), &scheduler);
+        assert!(again.is_some());
+    }
+
+    #[test]
+    fn scheduler_try_queue_succeeds() {
+        let scheduler = BuildScheduler::new();
+        assert!(scheduler.try_queue("my-site"));
+        assert!(scheduler.queued.contains("my-site"));
+    }
+
+    #[test]
+    fn scheduler_try_queue_collapse() {
+        let scheduler = BuildScheduler::new();
+        assert!(scheduler.try_queue("my-site"));
+        assert!(!scheduler.try_queue("my-site"));
+    }
+
+    #[test]
+    fn scheduler_take_queued_clears_flag() {
+        let scheduler = BuildScheduler::new();
+        scheduler.try_queue("my-site");
+        assert!(scheduler.take_queued("my-site"));
+        assert!(!scheduler.queued.contains("my-site"));
+    }
+
+    #[test]
+    fn scheduler_take_queued_returns_false_when_empty() {
+        let scheduler = BuildScheduler::new();
+        assert!(!scheduler.take_queued("my-site"));
+    }
+}