in hadoop-api-shim/src/main/java/org/apache/hadoop/fs/shim/functional/FutureIO.java [206:216]
FSBuilder<T, U> propagateOptions(
final FSBuilder<T, U> builder,
final Configuration conf,
final String optionalPrefix,
final String mandatoryPrefix) {
propagateOptions(builder, conf,
optionalPrefix, false);
propagateOptions(builder, conf,
mandatoryPrefix, true);
return builder;
}