@TestOnly
  public void setRetryIntervalInMs(long retryIntervalInMs) {
    this.retryIntervalInMs = retryIntervalInMs;
  }
this is set to true after calling isBlocked() at least once which indicates that this
SourceHandle needs to output data.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/exchange/source/SourceHandle.java