From cb447bc057fcfe10baf36a5d9a0f49fa93439360 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Onur=20=C3=96zkan?= <work@onurozkan.dev>
Date: Sun, 26 Jul 2026 18:03:46 +0300
Subject: [PATCH] libs/kaptan-tx: cover critical frame admission

---
 libs/kaptan-tx/src/lib.rs | 55 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 55 insertions(+)

diff --git a/libs/kaptan-tx/src/lib.rs b/libs/kaptan-tx/src/lib.rs
index ba34674..48c80a1 100644
--- a/libs/kaptan-tx/src/lib.rs
+++ b/libs/kaptan-tx/src/lib.rs
@@ -176,6 +176,61 @@ mod tests {
         assert_eq!(queue.complete().unwrap().frame, 1);
     }
 
+    #[test]
+    fn critical_frame_displaces_pending_best_effort_traffic() {
+        let mut queue = TransmitQueue::<u8, 2>::new();
+        queue.enqueue(PendingFrame {
+            class: FrameClass::BestEffort,
+            frame: 1,
+        });
+        queue.start_with(|_| Ok::<_, ()>(())).unwrap();
+        queue.enqueue(PendingFrame {
+            class: FrameClass::BestEffort,
+            frame: 2,
+        });
+        queue.enqueue(PendingFrame {
+            class: FrameClass::BestEffort,
+            frame: 3,
+        });
+
+        assert_eq!(
+            queue.enqueue(PendingFrame {
+                class: FrameClass::Critical,
+                frame: 4,
+            }),
+            EnqueueResult::ReplacedBestEffort
+        );
+        queue.complete();
+        assert_eq!(
+            queue.start_with(|frame| {
+                assert_eq!(*frame, 4);
+                Ok::<_, ()>(())
+            }),
+            Ok(StartOutcome::Started)
+        );
+    }
+
+    #[test]
+    fn critical_admission_reports_when_only_critical_frames_are_pending() {
+        let mut queue = TransmitQueue::<u8, 2>::new();
+        queue.enqueue(PendingFrame {
+            class: FrameClass::Critical,
+            frame: 1,
+        });
+        queue.enqueue(PendingFrame {
+            class: FrameClass::Critical,
+            frame: 2,
+        });
+
+        assert_eq!(
+            queue.enqueue(PendingFrame {
+                class: FrameClass::Critical,
+                frame: 3,
+            }),
+            EnqueueResult::CriticalQueueFull
+        );
+    }
+
     #[test]
     fn rejects_start_without_claiming_active_ownership() {
         let mut queue = TransmitQueue::<u8, 1>::new();