refactor: extract maybe_update_primary for testable upstream re-detection

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

src/forward.rs

@@ -85,6 +85,20 @@ impl UpstreamPool {
         self.primary = primary;
     }
 
+    /// Update the primary upstream if `new_addr` (parsed with `port`) differs
+    /// from the current preferred upstream. Returns `true` if the pool changed.
+    pub fn maybe_update_primary(&mut self, new_addr: &str, port: u16) -> bool {
+        let Ok(new_sock) = format!("{}:{}", new_addr, port).parse::<SocketAddr>() else {
+            return false;
+        };
+        let new_upstream = Upstream::Udp(new_sock);
+        if self.preferred() == Some(&new_upstream) {
+            return false;
+        }
+        self.primary = vec![new_upstream];
+        true
+    }
+
     pub fn label(&self) -> String {
         match self.preferred() {
             Some(u) => {
@@ -469,4 +483,33 @@ mod tests {
         assert_eq!(result.header.id, 0xABCD);
         assert_eq!(result.answers.len(), 1);
     }
+
+    #[test]
+    fn maybe_update_primary_swaps_when_different() {
+        let mut pool = UpstreamPool::new(
+            vec![Upstream::Udp("1.2.3.4:53".parse().unwrap())],
+            vec![Upstream::Udp("8.8.8.8:53".parse().unwrap())],
+        );
+        assert!(pool.maybe_update_primary("5.6.7.8", 53));
+        assert_eq!(pool.preferred().unwrap().to_string(), "5.6.7.8:53");
+    }
+
+    #[test]
+    fn maybe_update_primary_noop_when_same() {
+        let mut pool = UpstreamPool::new(
+            vec![Upstream::Udp("1.2.3.4:53".parse().unwrap())],
+            vec![],
+        );
+        assert!(!pool.maybe_update_primary("1.2.3.4", 53));
+    }
+
+    #[test]
+    fn maybe_update_primary_rejects_invalid_addr() {
+        let mut pool = UpstreamPool::new(
+            vec![Upstream::Udp("1.2.3.4:53".parse().unwrap())],
+            vec![],
+        );
+        assert!(!pool.maybe_update_primary("not-an-ip", 53));
+        assert_eq!(pool.preferred().unwrap().to_string(), "1.2.3.4:53");
+    }
 }

src/main.rs

@@ -610,19 +610,12 @@ async fn network_watch_loop(ctx: Arc<numa::ctx::ServerCtx>) {
                 .default_upstream
                 .or_else(numa::system_dns::detect_dhcp_dns)
                 .unwrap_or_else(|| QUAD9_IP.to_string());
-            if let Ok(new_sock) =
-                format!("{}:{}", new_addr, ctx.upstream_port).parse::<SocketAddr>()
-            {
-                let new_upstream = Upstream::Udp(new_sock);
             let mut pool = ctx.upstream_pool.lock().unwrap();
-                let current = pool.preferred().cloned();
-                if current.as_ref() != Some(&new_upstream) {
-                    info!("upstream changed: {} → {}", pool.label(), new_upstream);
-                    pool.set_primary(vec![new_upstream]);
+            if pool.maybe_update_primary(&new_addr, ctx.upstream_port) {
+                info!("upstream changed → {}", pool.label());
                 changed = true;
             }
         }
-        }
 
         // Flush stale LAN peers on any network change
         if changed {