reproduce error stack overflow from . as MX target

stop additional lookup on Root name

crates/server/src/store/file/authority.rs

@@ -92,6 +92,7 @@ impl FileAuthority {
             origin,
             records.len()
         );
+        debug!("zone: {:#?}", records);
 
         FileAuthority::new(origin, records, zone_type, allow_axfr)
     }

crates/server/src/store/in_memory/authority.rs

@@ -227,7 +227,8 @@ impl InMemoryAuthority {
         and_rrsigs: bool,
         supported_algorithms: SupportedAlgorithms,
     ) -> Option<Arc<RecordSet>> {
-        let wildcard = if name.is_wildcard() {
+        // if this is a wildcard or a root, both should break continued lookups
+        let wildcard = if name.is_wildcard() || name.is_root() {
             return None;
         } else {
             name.clone().into_wildcard()

crates/server/tests/authority_battery/basic.rs

@@ -146,6 +146,29 @@ pub fn test_mx<A: Authority<Lookup = AuthLookup>>(authority: A) {
     assert_eq!(Ipv6Addr::new(0, 0, 0, 0, 0, 0, 0, 1), *aaaa);
 }
 
+pub fn test_mx_to_null<A: Authority<Lookup = AuthLookup>>(authority: A) {
+    let query = Query::query(
+        Name::from_str("no-service.example.com.").unwrap(),
+        RecordType::MX,
+    );
+
+    let mut lookup =
+        block_on(authority.search(&query.into(), false, SupportedAlgorithms::new())).unwrap();
+
+    // In this case there should be no additional records
+    assert!(lookup.take_additionals().is_none());
+
+    let mx = lookup
+        .into_iter()
+        .next()
+        .expect("MX record not found in authority")
+        .rdata()
+        .as_mx()
+        .expect("Not an MX record");
+
+    assert_eq!(Name::from_str(".").unwrap(), *mx.exchange());
+}
+
 pub fn test_cname<A: Authority<Lookup = AuthLookup>>(authority: A) {
     let query = Query::query(
         Name::from_str("alias.example.com.").unwrap(),
@@ -565,6 +588,9 @@ macro_rules! define_basic_test {
         $(
             #[test]
             fn $f () {
+                // Useful for getting debug logs
+                // env_logger::init();
+
                 let authority = crate::$new("../../tests/test-data/named_test_configs/example.com.zone", module_path!(), stringify!($f));
                 crate::authority_battery::basic::$f(authority);
             }
@@ -583,6 +609,7 @@ macro_rules! basic_battery {
                     test_ns,
                     test_ns_lookup,
                     test_mx,
+                    test_mx_to_null,
                     test_cname,
                     test_cname_alias,
                     test_cname_chain,

tests/test-data/named_test_configs/example.com.zone

@@ -27,3 +27,5 @@ aname-chain     ANAME   alias
 server          SRV     1 1 443 alias
 
 *.wildcard      CNAME   www
+
+no-service 86400 IN MX 0 .