private void setSource(Properties p, ConfigSource source) {
    if (source == null) {
      throw new IllegalArgumentException("Valid ConfigSource must be specified instead of null.");
    }
    for (Object k : p.keySet()) {
      sourceMap.put((String) k, source);
    }
  }
For every key in p mark it as being from the given source.
geode-core/src/main/java/org/apache/geode/distributed/internal/DistributionConfigImpl.java