chore(api,tests): verify that DNS resolver has at least one DNS server configured

Author: azasypkin

src/network/dns_resolver.rs

@@ -38,7 +38,7 @@ impl DnsResolver for TokioDnsResolver {
 
 #[cfg(test)]
 pub mod tests {
-    use crate::network::DnsResolver;
+    use crate::network::{DnsResolver, TokioDnsResolver};
     use futures::future::BoxFuture;
     use hickory_resolver::{
         lookup::Lookup,
@@ -93,4 +93,29 @@ pub mod tests {
             }
         }
     }
+
+    /// Companion to the test above: actually exercise `create()` end-to-end and assert
+    /// that lookups against a non-existent domain do **not** fail with the empty-
+    /// `name_servers` error mode. We do not assert success because CI/sandbox network
+    /// reachability is out of our control, but the "no connections available" sentinel
+    /// is produced locally by hickory before any network I/O and so is detectable
+    /// regardless of outbound connectivity.
+    #[tokio::test]
+    async fn create_yields_resolver_that_does_not_short_circuit_on_empty_name_servers() {
+        let resolver = TokioDnsResolver::create().expect("TokioDnsResolver should build");
+        // The lookup itself may succeed (NXDOMAIN response) or fail with a network error
+        // depending on outbound DNS reachability of the test environment, neither of
+        // which we want to assert on. What we *do* assert is that the failure mode
+        // (if any) is not the `no connections available` sentinel that hickory raises
+        // synchronously, before any network I/O, when `name_servers` is empty.
+        if let Err(err) = resolver
+            .lookup_ip("nonexistent-retrack-regression-guard.example.")
+            .await
+        {
+            assert!(
+                !err.to_string().contains("no connections available"),
+                "DNS resolver appears to have no configured name servers: {err}"
+            );
+        }
+    }
 }