recursor: make the test helpers more capable

they did not previously allow any way to mock DNS query sequences
in a manner compatible with the RecursorPool, which prefers to create
new NameServers itself, rather than via anything injectable by the test.

crates/recursor/src/recursor.rs

@@ -40,35 +40,12 @@ type NameServerCache<P> = LruCache<Name, RecursorPool<P>>;
 /// A top down recursive resolver which operates off a list of roots for initial recursive requests.
 ///
 /// This is the well known root nodes, referred to as hints in RFCs. See the IANA [Root Servers](https://www.iana.org/domains/root/servers) list.
-pub struct Recursor<P: ConnectionProvider> {
+pub struct Recursor<P: ConnectionProvider=TokioConnectionProvider> {
     roots: RecursorPool<P>,
     name_server_cache: Mutex<NameServerCache<P>>,
     record_cache: DnsLru,
 }
 
-impl Recursor<GenericConnector<TokioRuntimeProvider>> {
-    /// Construct a new recursor using the list of NameServerConfigs for the root node list
-    ///
-    /// # Panics
-    ///
-    /// This will panic if the roots are empty.
-    pub fn new(
-        roots: impl Into<NameServerConfigGroup>,
-        ns_cache_size: usize,
-        record_cache_size: usize,
-    ) -> Result<Self, ResolveError> {
-        // configure the hickory-resolver
-        let roots: NameServerConfigGroup = roots.into();
-
-        assert!(!roots.is_empty(), "roots must not be empty");
-
-        let opts = recursor_opts();
-        let roots =
-            GenericNameServerPool::from_config(roots, opts, TokioConnectionProvider::default());
-
-        Self::new_with_pool(roots, ns_cache_size, record_cache_size)
-    }
-}
 
 impl<P: ConnectionProvider> Recursor<P> {
     /// Construct a new recursor using a custom name server pool.
@@ -92,6 +69,27 @@ impl<P: ConnectionProvider> Recursor<P> {
 }
 
 impl<P: ConnectionProvider + Default> Recursor<P> {
+    /// Construct a new recursor using the list of NameServerConfigs for the root node list
+    ///
+    /// # Panics
+    ///
+    /// This will panic if the roots are empty.
+    pub fn new(
+        roots: impl Into<NameServerConfigGroup>,
+        ns_cache_size: usize,
+        record_cache_size: usize,
+    ) -> Result<Self, ResolveError> {
+        // configure the hickory-resolver
+        let roots: NameServerConfigGroup = roots.into();
+
+        assert!(!roots.is_empty(), "roots must not be empty");
+
+        let opts = recursor_opts();
+        let roots = NameServerPool::from_config(roots, opts, P::default());
+
+        Self::new_with_pool(roots, ns_cache_size, record_cache_size)
+    }
+
     /// Perform a recursive resolution
     ///
     /// [RFC 1034](https://datatracker.ietf.org/doc/html/rfc1034#section-5.3.3), Domain Concepts and Facilities, November 1987