C++ client producer sendAsync() method will be blocked forever, if enough...

C++ client producer sendAsync() method will be blocked forever, if enough batched messages sent timeout. (#4569) (#4657)

pulsar-client-cpp/lib/ProducerImpl.cc

@@ -232,6 +232,14 @@ void ProducerImpl::failPendingMessages(Result result) {
     // without holding producer mutex.
     for (MessageQueue::const_iterator it = pendingMessagesQueue_.begin(); it != pendingMessagesQueue_.end();
          it++) {
+        // When dealing any failure message, if the current message is a batch one, we should also release
+        // the reserved spots in the pendingMessageQueue_, for all individual messages inside this batch
+        // message. See 'ProducerImpl::sendAsync' for more details.
+        if (it->msg_.impl_->metadata.has_num_messages_in_batch()) {
+            // batch message - need to release more spots
+            // -1 since the pushing batch message into the queue already released a spot
+            pendingMessagesQueue_.release(it->msg_.impl_->metadata.num_messages_in_batch() - 1);
+        }
         messagesToFail.push_back(*it);
     }