private void completeAsync(CompletableFuture<Void> first, CompletableFuture<Void> second) {
            first.handle((unused, cause) -> {
                if (cause != null) {
                    second.completeExceptionally(cause);
                } else {
                    second.complete(null);
                }
                return null;
            });
        }
Reactive processor that encodes a stream of {@link BodyPart}s into an HTTP payload.
core/src/main/java/com/linecorp/armeria/common/multipart/MultipartEncoder.java