Verify preview admission bounds under contention and worker unwind

apps/desktop/src-tauri/src/media_preview.rs

@@ -413,6 +413,51 @@ pub fn error_response(status: StatusCode) -> Response<Vec<u8>> {
 mod tests {
     use super::*;
 
+    #[test]
+    fn concurrent_admission_stays_bounded_and_unwind_releases_capacity() {
+        let registry = Arc::new(PreviewRegistry::default());
+        let active = Arc::new(AtomicUsize::new(0));
+        let barrier = Arc::new(std::sync::Barrier::new(8));
+        let workers: Vec<_> = (0..8)
+            .map(|_| {
+                let registry = registry.clone();
+                let active = active.clone();
+                let barrier = barrier.clone();
+                std::thread::spawn(move || {
+                    barrier.wait();
+                    for _ in 0..500 {
+                        if let Some(admission) = registry.admit() {
+                            let count = active.fetch_add(1, Ordering::SeqCst) + 1;
+                            assert!(count <= MAX_REQUESTS);
+                            std::thread::yield_now();
+                            active.fetch_sub(1, Ordering::SeqCst);
+                            drop(admission);
+                        }
+                    }
+                })
+            })
+            .collect();
+        for worker in workers {
+            worker.join().unwrap();
+        }
+        assert_eq!(active.load(Ordering::SeqCst), 0);
+        assert_eq!(registry.requests.load(Ordering::SeqCst), 0);
+        let first = registry.admit().unwrap();
+        let interrupted = registry.clone();
+        assert!(std::panic::catch_unwind(move || {
+            let _second = interrupted.admit().unwrap();
+            panic!("injected preview worker panic");
+        })
+        .is_err());
+        let second = registry
+            .admit()
+            .expect("Unwinding must release the second slot");
+        assert!(registry.admit().is_none());
+        drop(first);
+        drop(second);
+        assert_eq!(registry.requests.load(Ordering::SeqCst), 0);
+    }
+
     struct Fixture {
         _directory: tempfile::TempDir,
         database: PathBuf,