fix: replace stale magic DNS records on IP change (#1906)

Magic DNS updates are full snapshots, so appending routes keeps old IPs and returns duplicate A records. Replace each client's previous routes on update and add a regression test to ensure hostname resolution keeps only the latest IP.

easytier/src/instance/dns_server/server_instance.rs

@@ -140,6 +140,19 @@ impl MagicDnsServerInstanceData {
         }
     }
 
+    async fn keep_zone_authoritative(&self, zone: &str) {
+        if let Err(e) = self
+            .update_dns_records(std::iter::empty::<&Route>(), zone)
+            .await
+        {
+            tracing::error!(
+                "Failed to keep DNS zone {} authoritative after route prune: {:?}",
+                zone,
+                e
+            );
+        }
+    }
+
     fn do_system_config(&self, zone: &str) -> Result<(), anyhow::Error> {
         if let Some(c) = &self.system_config {
             c.set_dns(&OSConfig {
@@ -183,10 +196,25 @@ impl MagicDnsServerRpc for MagicDnsServerInstanceData {
             return Err(anyhow::anyhow!("No remote addr").into());
         };
         let zone = input.zone.clone();
-        self.route_infos
-            .entry(zone.clone())
-            .or_default()
-            .insert_many(remote_addr.clone().into(), input.routes);
+        let remote_addr: url::Url = remote_addr.clone().into();
+        let mut zone_removed = false;
+
+        if let Some(mut routes_by_addr) = self.route_infos.get_mut(&zone) {
+            routes_by_addr.remove(&remote_addr);
+            if !input.routes.is_empty() {
+                routes_by_addr.insert_many(remote_addr, input.routes);
+            }
+            zone_removed = routes_by_addr.is_empty();
+        } else if !input.routes.is_empty() {
+            let mut routes_by_addr = MultiMap::new();
+            routes_by_addr.insert_many(remote_addr, input.routes);
+            self.route_infos.insert(zone.clone(), routes_by_addr);
+        }
+
+        if zone_removed {
+            self.route_infos.remove(&zone);
+            self.keep_zone_authoritative(&zone).await;
+        }
 
         self.update().await;
         Ok(Default::default())
@@ -438,11 +466,19 @@ impl RpcServerHook for MagicDnsServerInstanceData {
             return;
         };
         let remote_addr = remote_addr.into();
+        let mut removed_zones = vec![];
         for mut item in self.route_infos.iter_mut() {
             item.value_mut().remove(&remote_addr);
+            if item.value().is_empty() {
+                removed_zones.push(item.key().clone());
+            }
+        }
+        for zone in &removed_zones {
+            self.route_infos.remove(zone);
+        }
+        for zone in removed_zones {
+            self.keep_zone_authoritative(&zone).await;
         }
-        self.route_infos.retain(|_, v| !v.is_empty());
-        self.route_infos.shrink_to_fit();
         self.update().await;
     }
 }