Bug 1542561 - TRR: Don't return NS_ERROR_UNKNOWN_HOST when a AAAA res…

…ponse comes back first, but the second A response is NXDOMAIN r=dragana

Differential Revision: https://phabricator.services.mozilla.com/D33471

--HG--
extra : moz-landing-system : lando

netwerk/dns/nsHostResolver.cpp

@@ -1741,9 +1741,14 @@ nsHostResolver::LookupStatus nsHostResolver::CompleteLookup(
       // If mFirstTRR is set, merge those addresses into current set!
       if (addrRec->mFirstTRR) {
         if (NS_SUCCEEDED(status)) {
+          LOG(("Merging responses"));
           merge_rrset(newRRSet, addrRec->mFirstTRR);
         } else {
+          LOG(("Will use previous response"));
           newRRSet.swap(addrRec->mFirstTRR);  // transfers
+          // We must use the status of the first response, otherwise we'll
+          // pass an error result to the consumers.
+          status = addrRec->mFirstTRRresult;
         }
         addrRec->mFirstTRR = nullptr;
       }