private InternalSource getChild(final String streamTypeName) {
        if (closed) {
            throw new RuntimeException("Closed");
        }

        if (streamTypeName == null) {
            return this;
        }

        return childMap.computeIfAbsent(streamTypeName, this::child);
    }
Creates a new file system stream source.

@return A new file system source.
stroom-data/stroom-data-store-impl-fs/src/main/java/stroom/data/store/impl/fs/FsSource.java