Fix Iteration (#117)

* Fix nextAuthority()

This was remarkibly broken. In iterative mode, we would never find
an authority deeper than example.com. For example, if the authority
for a.b.example.com was b.example.com, we failed, as we'd keep hitting
example.com

Sigh.

* Properly report caching on a specific code path

modules/miekg/miekg.go

@@ -575,6 +575,7 @@ func (s *Lookup) cachedRetryingLookup(dnsType uint16, dnsClass uint16, name stri
 		s.VerboseLog(depth+2, "Cache auth check for ", authName)
 		cachedResult, ok = s.Factory.Factory.GetCachedResult(authName, dns.TypeNS, dnsType, depth+2, s.Factory.ThreadID)
 		if ok {
+			isCached = true
 			return cachedResult, isCached, zdns.STATUS_NOERROR, nil
 		}
 	}
@@ -602,17 +603,34 @@ func nameIsBeneath(name string, layer string) (bool, string) {
 }
 
 func nextAuthority(name string, layer string) string {
+	// We are our own authority for PTRs
+	// (This is dealt with elsewhere)
+	if strings.HasSuffix(layer, "in-addr.arpa") {
+		return name
+	}
+
 	idx := strings.LastIndex(name, ".")
 	if idx < 0 || (idx+1) >= len(name) {
-		return ""
+		return name
 	}
-	if layer != "." {
-		idx = strings.LastIndex(name[0:idx], ".")
-		if idx < 0 || (idx+1) >= len(name) {
-			return name
-		}
+	if layer == "." {
+		return name[idx+1:]
+	}
+
+	if !strings.HasSuffix(name, layer) {
+		panic("Layers by definition are suffixes of names")
+	}
+
+	// Limit the search space to the prefix of the string that isnt layer
+	idx = strings.LastIndex(name, layer) - 1
+	if idx < 0 || (idx+1) >= len(name) {
+		// Out of bounds. We are our own authority
+		return name
 	}
-	return name[idx+1:]
+	// Find the next step in the layer
+	idx = strings.LastIndex(name[0:idx], ".")
+	next := name[idx+1:]
+	return next
 }
 
 func (s *Lookup) checkGlue(server string, depth int, result Result) (Result, zdns.Status) {