refactor: extract acquire_inflight, rewrite tests against real code

Move Disposition enum and inflight acquisition logic into a standalone
acquire_inflight() function. Rewrite 4 tests that were exercising tokio
primitives to call the real coalescing code path instead.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/ctx.rs

@@ -178,22 +178,7 @@ pub async fn handle_query(
                 (resp, QueryPath::Cached, cached_dnssec)
             } else if ctx.upstream_mode == UpstreamMode::Recursive {
                 let key = (qname.clone(), qtype);
-
-                enum Disposition {
-                    Leader(broadcast::Sender<Option<DnsPacket>>),
-                    Follower(broadcast::Receiver<Option<DnsPacket>>),
-                }
-
-                let disposition = {
-                    let mut inflight = ctx.inflight.lock().unwrap();
-                    if let Some(tx) = inflight.get(&key) {
-                        Disposition::Follower(tx.subscribe())
-                    } else {
-                        let (tx, _) = broadcast::channel::<Option<DnsPacket>>(1);
-                        inflight.insert(key.clone(), tx.clone());
-                        Disposition::Leader(tx)
-                    }
-                };
+                let disposition = acquire_inflight(&ctx.inflight, key.clone());
 
                 match disposition {
                     Disposition::Follower(mut rx) => {
@@ -431,6 +416,22 @@ fn is_special_use_domain(qname: &str) -> bool {
     qname == "local" || qname.ends_with(".local")
 }
 
+enum Disposition {
+    Leader(broadcast::Sender<Option<DnsPacket>>),
+    Follower(broadcast::Receiver<Option<DnsPacket>>),
+}
+
+fn acquire_inflight(inflight: &Mutex<InflightMap>, key: (String, QueryType)) -> Disposition {
+    let mut map = inflight.lock().unwrap();
+    if let Some(tx) = map.get(&key) {
+        Disposition::Follower(tx.subscribe())
+    } else {
+        let (tx, _) = broadcast::channel::<Option<DnsPacket>>(1);
+        map.insert(key, tx.clone());
+        Disposition::Leader(tx)
+    }
+}
+
 struct InflightGuard<'a> {
     inflight: &'a Mutex<InflightMap>,
     key: (String, QueryType),
@@ -559,39 +560,46 @@ mod tests {
         assert!(m.contains_key(&key_aaaa));
     }
 
-    // ---- Coalescing disposition tests ----
+    // ---- Coalescing disposition tests (via acquire_inflight) ----
 
     #[test]
-    fn leader_follower_disposition() {
-        // First caller becomes leader, second becomes follower
+    fn first_caller_becomes_leader() {
         let map: Mutex<InflightMap> = Mutex::new(HashMap::new());
         let key = ("test.com".to_string(), QueryType::A);
 
-        // First: no entry → insert and become leader
-        let is_leader = {
-            let mut m = map.lock().unwrap();
-            if m.get(&key).is_some() {
-                false
-            } else {
-                let (tx, _) = broadcast::channel::<Option<DnsPacket>>(1);
-                m.insert(key.clone(), tx);
-                true
+        let d = acquire_inflight(&map, key.clone());
+        assert!(matches!(d, Disposition::Leader(_)));
+        assert_eq!(map.lock().unwrap().len(), 1);
     }
-        };
-        assert!(is_leader);
 
-        // Second: entry exists → become follower
-        let is_follower = {
-            let m = map.lock().unwrap();
-            m.get(&key).is_some()
-        };
-        assert!(is_follower);
+    #[test]
+    fn second_caller_becomes_follower() {
+        let map: Mutex<InflightMap> = Mutex::new(HashMap::new());
+        let key = ("test.com".to_string(), QueryType::A);
+
+        let _leader = acquire_inflight(&map, key.clone());
+        let follower = acquire_inflight(&map, key);
+        assert!(matches!(follower, Disposition::Follower(_)));
+        // Map still has exactly 1 entry — follower subscribes, doesn't insert
+        assert_eq!(map.lock().unwrap().len(), 1);
     }
 
     #[tokio::test]
-    async fn broadcast_delivers_result_to_follower() {
-        let (tx, _) = broadcast::channel::<Option<DnsPacket>>(1);
-        let mut rx = tx.subscribe();
+    async fn leader_broadcast_reaches_follower() {
+        let map: Mutex<InflightMap> = Mutex::new(HashMap::new());
+        let key = ("test.com".to_string(), QueryType::A);
+
+        let leader = acquire_inflight(&map, key.clone());
+        let follower = acquire_inflight(&map, key);
+
+        let tx = match leader {
+            Disposition::Leader(tx) => tx,
+            _ => panic!("expected leader"),
+        };
+        let mut rx = match follower {
+            Disposition::Follower(rx) => rx,
+            _ => panic!("expected follower"),
+        };
 
         let mut resp = DnsPacket::new();
         resp.header.id = 42;
@@ -600,29 +608,48 @@ mod tests {
             addr: Ipv4Addr::new(1, 2, 3, 4),
             ttl: 300,
         });
-
         let _ = tx.send(Some(resp));
+
         let received = rx.recv().await.unwrap().unwrap();
         assert_eq!(received.header.id, 42);
         assert_eq!(received.answers.len(), 1);
     }
 
     #[tokio::test]
-    async fn broadcast_none_signals_failure() {
-        let (tx, _) = broadcast::channel::<Option<DnsPacket>>(1);
-        let mut rx = tx.subscribe();
-        let _ = tx.send(None);
+    async fn leader_none_signals_failure_to_follower() {
+        let map: Mutex<InflightMap> = Mutex::new(HashMap::new());
+        let key = ("test.com".to_string(), QueryType::A);
 
-        let received = rx.recv().await.unwrap();
-        assert!(received.is_none());
+        let leader = acquire_inflight(&map, key.clone());
+        let follower = acquire_inflight(&map, key);
+
+        let tx = match leader {
+            Disposition::Leader(tx) => tx,
+            _ => panic!("expected leader"),
+        };
+        let mut rx = match follower {
+            Disposition::Follower(rx) => rx,
+            _ => panic!("expected follower"),
+        };
+
+        let _ = tx.send(None);
+        assert!(rx.recv().await.unwrap().is_none());
     }
 
     #[tokio::test]
-    async fn multiple_followers_all_receive_result() {
-        let (tx, _) = broadcast::channel::<Option<DnsPacket>>(1);
-        let mut rx1 = tx.subscribe();
-        let mut rx2 = tx.subscribe();
-        let mut rx3 = tx.subscribe();
+    async fn multiple_followers_all_receive_via_acquire() {
+        let map: Mutex<InflightMap> = Mutex::new(HashMap::new());
+        let key = ("multi.com".to_string(), QueryType::A);
+
+        let leader = acquire_inflight(&map, key.clone());
+        let f1 = acquire_inflight(&map, key.clone());
+        let f2 = acquire_inflight(&map, key.clone());
+        let f3 = acquire_inflight(&map, key);
+
+        let tx = match leader {
+            Disposition::Leader(tx) => tx,
+            _ => panic!("expected leader"),
+        };
 
         let mut resp = DnsPacket::new();
         resp.answers.push(DnsRecord::A {
@@ -632,7 +659,11 @@ mod tests {
         });
         let _ = tx.send(Some(resp));
 
-        for rx in [&mut rx1, &mut rx2, &mut rx3] {
+        for f in [f1, f2, f3] {
+            let mut rx = match f {
+                Disposition::Follower(rx) => rx,
+                _ => panic!("expected follower"),
+            };
             let r = rx.recv().await.unwrap().unwrap();
             assert_eq!(r.answers.len(), 1);
         }