[BUGFIX] Resolver: fallback for invalid SRV CNAME entries

src/main/java/eu/siacs/conversations/utils/Resolver.java

@@ -172,6 +172,9 @@ public class Resolver {
         ResolverResult<SRV> result = resolveWithFallback(dnsName, SRV.class);
         final List<Result> results = new ArrayList<>();
         final List<Thread> threads = new ArrayList<>();
+
+        final List<Result> fallbackResults = new ArrayList<>();
+        final List<Thread> fallbackThreads = new ArrayList<>();
         for (SRV record : result.getAnswersOrEmptySet()) {
             if (record.name.length() == 0) {
                 continue;
@@ -188,6 +191,12 @@ public class Resolver {
                     results.addAll(ipv4s);
                 }
             }));
+            fallbackThreads.add(new Thread(() -> {
+                final List<Result> cnames = resolveIp(record, CNAME.class, result.isAuthenticData(), directTls);
+                synchronized (fallbackResults) {
+                    fallbackResults.addAll(cnames);
+                }
+            }));
         }
         for (Thread thread : threads) {
             thread.start();
@@ -199,7 +208,21 @@ public class Resolver {
                 return Collections.emptyList();
             }
         }
-        return results;
+        if (results.size() > 0) {
+            return results;
+        }
+
+        for (Thread thread : fallbackThreads) {
+            thread.start();
+        }
+        for (Thread thread : fallbackThreads) {
+            try {
+                thread.join();
+            } catch (InterruptedException e) {
+                return Collections.emptyList();
+            }
+        }
+        return fallbackResults;
     }
 
     private static <D extends InternetAddressRR> List<Result> resolveIp(SRV srv, Class<D> type, boolean authenticated, boolean directTls) {