in flink-connector-gcp-pubsub/src/main/java/org/apache/flink/streaming/connectors/gcp/pubsub/common/AcknowledgeOnCheckpoint.java [65:68]
public void addAcknowledgeId(ACKID id) {
acknowledgeIdsForPendingCheckpoint.add(id);
outstandingAcknowledgements.incrementAndGet();
}