public static <T> Mono<Long> writeAll(Writer writer, Flux<T> values) throws IOException {
        return writeAll(DEFAULT_OBJECT_MAPPER, writer, values);
    }
@Test
    void writeAll_fromEmptySource() throws IOException {
        final Path outputTempFilePath = createTempFile();

        final Long outputCount = FileSerde.writeAll(Files.newBufferedWriter(outputTempFilePath), Flux.empty()).block();
        assertThat(outputCount, is(0L));
    }