Improve OfflineMessages.getMessages()

poll the result before waiting for a message (with nextResult()), in
order to prevent waiting unnecessarily.

Add PacketCollector.pollResultOrThrow()

smack-core/src/main/java/org/jivesoftware/smack/PacketCollector.java

@@ -112,6 +112,25 @@ public class PacketCollector {
         return (P) resultQueue.poll();
     }
 
+    /**
+     * Polls to see if a packet is currently available and returns it, or
+     * immediately returns <tt>null</tt> if no packets are currently in the
+     * result queue.
+     * <p>
+     * Throws an XMPPErrorException in case the polled stanzas did contain an XMPPError.
+     * </p>
+     * 
+     * @return the next available packet.
+     * @throws XMPPErrorException in case an error response.
+     */
+    public <P extends Packet> P pollResultOrThrow() throws XMPPErrorException {
+        P result = pollResult();
+        if (result != null) {
+            XMPPErrorException.ifHasErrorThenThrow(result);
+        }
+        return result;
+    }
+
     /**
      * Returns the next available packet. The method call will block (not return) until a packet is
      * available.

smack-extensions/src/main/java/org/jivesoftware/smackx/offline/OfflineMessageManager.java

@@ -194,6 +194,17 @@ public class OfflineMessageManager {
             Message message = messageCollector.nextResult();
             while (message != null) {
                 messages.add(message);
+                // It is important that we query the resultCollector before the messageCollector
+                Packet result = resultCollector.pollResultOrThrow();
+                message = messageCollector.pollResult();
+                if (message == null && result != null) {
+                    // No new messages, but we have a non-error IQ response, we are done
+                    return messages;
+                } else if (message != null) {
+                    // We have received a message without waiting, great, continue to add this message and proceed with
+                    // the loop
+                    continue;
+                }
                 message = messageCollector.nextResult();
             }
             resultCollector.nextResultOrThrow();