QDnsLookup/Unix: handle more error conditions from res_nsend()

It does set ECONNREFUSED explicitly. So let's reuse ServerRefusedError
for it too (it was so far only used when we got a REFUSED DNS answer),
which allows tst_QDnsLookup to handle this as an ignorable failure.

For almost everything else, it sets ETIMEDOUT to indicate "no answer",
which isn't useful. So let's inspect the reply to see if it has
something.

Pick-to: 6.7
Change-Id: I455fe22ef4ad4b2f9b01fffd17c771ffa4a998be
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/network/kernel/qdnslookup_unix.cpp

@@ -183,14 +183,22 @@ void QDnsLookupRunnable::query(QDnsLookupReply *reply)
     auto attemptToSend = [&]() {
         std::memset(buffer.data(), 0, HFIXEDSZ);        // the header is enough
         int responseLength = res_nsend(&state, qbuffer.data(), queryLength, buffer.data(), buffer.size());
-        if (responseLength < 0) {
-            // network error of some sort
-            if (errno == ETIMEDOUT)
-                reply->makeTimeoutError();
-            else
-                reply->makeResolverSystemError();
-        }
-        return responseLength;
+        if (responseLength >= 0)
+            return responseLength;  // success
+
+        // libresolv uses ETIMEDOUT for resolver errors ("no answer")
+        if (errno == ECONNREFUSED)
+            reply->setError(QDnsLookup::ServerRefusedError, qt_error_string());
+        else if (errno != ETIMEDOUT)
+            reply->makeResolverSystemError();   // some other error
+
+        auto query = reinterpret_cast<HEADER *>(qbuffer.data());
+        auto header = reinterpret_cast<HEADER *>(buffer.data());
+        if (query->id == header->id && header->qr)
+            reply->makeDnsRcodeError(header->rcode);
+        else
+            reply->makeTimeoutError();      // must really be a timeout
+        return -1;
     };
 
     // strictly use UDP, we'll deal with truncated replies ourselves