Lessons learnt updating a library to std::future

Updating a multipart-async library to use futures
rust mpart-async 2020-01-22

With the new std::future way of doing things and tokio slowly reaching maturation, it's time to look at updating the libraries out there that are using the old ways. For one of my libraries, tmq, a Tokio ZeroMQ library, there is some awesome work already done to get this updated.

But, I thought it pertinent to at least get my feet in the water to see how hard it would be, from a library maintainer perspective, to update to std::future. For this effort, I chose my small library: mpart-async. You can see the changes I have made by comparing the versions here. This blog is a small collection of notes & gotches I found when porting code across.

mpart-async is a mostly agnostic library for client side multipart/form-data requests. There are existing libraries out there, like multipart, but I found the API a little unwieldy for my taste (That: and async support is still in alpha according to the readme). I wanted something that worked, and was simple, but didn't offer an opinion on the web client. I use both the actix client & hyper to make multipart requests depending on the project.

The challenge is that with multipart requests you mostly have fields & binary files. For binary files, you can appreciate, should not be buffered entirely in memory, but streamed out as the bytes become available. So mpart-async works with multiple internal streams of files and also provides a convenience wrapper (if using tokio) for sending files given a path.

A New Example

All these changes to support async fn and is it actually easier to consume/use async libraries?

Given the hyper example, I would say yes.

Compare the new example:

async fn main() -> Result<(), Error> {
    //Setup a mock server to accept connections.

    let client = Client::new();

    let mut mpart = MultipartRequest::default();

    mpart.add_field("foo", "bar");
    mpart.add_file("test", "Cargo.toml");

    let request = Request::post("http://localhost:3000")
            format!("multipart/form-data; boundary={}", mpart.get_boundary()),



With the older one:

fn main() {
    // current_thread::Runtime can't be used because of the blocking file operations
    let mut rt = Runtime::new().expect("new rt");

    //Setup a mock server to accept connections.

    // Open `Cargo.toml` file and create a request
    let request = File::open("Cargo.toml")
        .map_err(|e| format!("{}", e))
        .and_then(|file| {
            // A Stream of BytesMut decoded from an AsyncRead
            let framed = FramedRead::new(file, BytesCodec::new());

            let mut mpart = MultipartRequest::default();
            mpart.add_field("foo", "bar");
                framed.map(|b| b.freeze()),
                    format!("multipart/form-data; boundary={}", mpart.get_boundary()),
                .map_err(|e| format!("{}", e))

    // Send request
    let task = request.and_then(|request| {
        let client = hyper::Client::new();
            .map_err(|e| format!("{}", e))
            .and_then(|response| {
                    .map_err(|e| format!("{}", e))
                    .and_then(|body| {
                        if let Ok(data) = str::from_utf8(&body) {
                            println!("Response: {}", data);

    rt.block_on(task).expect("request failed");

While there are still a few warts (such as having to manually add the boundary header), I find the code a lot more readable and easy to follow, rather than your standard combinator paths.

No more Error Associated Type

The most drastic change to the Stream trait is that there is no longer an Error associated type. The old Stream/Future traits had both Item and Error, as it assumed that streams were always going to be fallible. The new traits do away with the Error associated type. Instead, if you want your Stream to possibly be an error then you need to return a Result as your Item type.

For the Stream trait, the method you implement has changed to poll_next and uses the std::task::Poll enum as a return type.

The Poll enum did feel a little inside out when I started using it, but makes sense in terms of there being no Error type. You don't return a Result<Async<Option>, _>... you instead return a Poll<Option<Result<_,_>>>.

Generally, this means if you wrote this:

return Ok(Async::Ready(Some(bytes)))

You instead will return:

return Poll::Ready(Some(Ok(bytes)))

StreamExt traits and friends

As a consequence of no Error associated type, and an example of different ergonomics, if you are dealing with Result items, you may want to use the TryStreamExt trait instead of StreamExt. They are basically the same trait but one deals with Result a bit more nicely. I'm not sure whether a simpler solution will be possible with higher kinded types in the future, but for now it is a tiny bit more complex than futures 0.1.

Interestingly, the index page for stream docs doesn't currently mention that the TryStreamExt trait exists & I only found out about it by asking questions in the discord chat.

But wait, there is also StreamExt from tokio which is subtly different, but does as of 0.2.10 allow you to run collect() on Result<Bytes, _> item streams.

When would you use either StreamExt? I would say that if you want to be more general, you should probably use the futures implementation. Bearing in mind you can't run try_collect() on Result<Bytes, _> at the moment due to an outstanding issue with the bytes crate. Why does this matter? Well, hyper passes around streams of Result<Bytes, _> when you are streaming a body in & out of a request. Working with this particular stream signature is a bit clunky still, but I'm sure that this will be resolved in due course.

So we have:

  • StreamExt from futures that works on Stream that aren't Result
  • TryStreamExt from futures that works on Stream that are Result but doesn't work well with Result<Bytes,_> when trying to try_collect()
  • StreamExt from tokio that works with Result<Bytes,_> and allows you to collect() to get the output but now means you have a dependency on tokio

This might end up confusing more than just me, and I do hope that it's simplified in the future.

Luckily in the core of the API I don't need to use them, only for tests & filestream, which means I'm quite happy to depend on the tokio implementation.

Wrapping inner streams in Pin

I got tripped up on this for a while, and needed to again ask in chat for the answer here. If you have an inner stream and you want to call poll_next on it, you need to wrap it in a Pin, otherwise the Stream trait does not stick and you will get some gnarly error message with some weird suggestions:

The my original shot at the code was:


But it needs to be:

Pin::new(&mut stream.stream).poll_next(cx)

The compiler error message is not too helpful here, and insists I restrict S by Stream, even though it is already restricted by Stream.

error[E0599]: no method named `poll_next` found for type `S` in the current scope
   --> src/lib.rs:282:41
282 |                     match stream.stream.poll_next(cx) {
    |                                         ^^^^^^^^^ method not found in `S`
    = help: items from traits can only be used if the type parameter is bounded by the trait
help: the following trait defines an item `poll_next`, perhaps you need to restrict type parameter `S` with it:
254 | impl<E, S: futures::Stream + Stream> Stream for MultipartRequest<S>
    |         ^^^^^^^^^^^^^^^^^^^^

You need to make sure that your inner stream implements Unpin as well to go down this path:

impl<E, S> Stream for MultipartRequest<S>
    S: Stream<Item = Result<Bytes, E>> + Unpin,

Storing the results from an async fn in a struct

Some of the methods in tokio are returns from an async fn , like the File::open method, which I use for FileStream. I found the answer on stack overflow as to how to do this with std::future, since async fn returns an opaque type.

For the older version, the OpenFuture was a concrete type:

pub struct FileStream {
    inner: Option<FramedRead<File, BytesCodec>>,
    file: OpenFuture<PathBuf>,

The newer version is a return from an async fn and is opaque, so we wrap it using Box::pin:

pub struct FileStream {
    inner: Option<FramedRead<File, BytesCodec>>,
    file: Pin<Box<dyn Future<Output = Result<File, Error>> + Send + Sync>>,

You can then instantiate this with:


Then it's easy to call passing on the context from an existing poll():


Notifying a task

FileStream has two stages. One is the future to open the file. The second is streaming out the bytes of the file. When the future resolves to open the file, I want to notify the context that it should be polled again to start streaming.

The old way of doing this was task::current().notify():

self.inner = Some(FramedRead::new(file, BytesCodec::new()));

The new way appears to be using cx.waker().wake_by_ref() where cx is the context received from the poll:

self.inner = Some(FramedRead::new(file, BytesCodec::new()));

I say appears because the test written works without wake_by_ref() being called. Requires a bit more investigation here I think to know exactly what's going on. My simple test example works either way, strangely.


It is not too onerous to convert to std::future for an existing library. I would assume the leap from the old tokio-core would be harder, as the changes feel mostly cosmetic here.

The omission of the Error associated type to me actually makes things less ergonomic and things a little more fragmented (as evidenced by 3 *StreamExt traits..). I was an advocate of this initially, but there probably needs to be a bit more work in making this nicer.

There are still a lot of libraries out there that will be required to be updated, a lot of old blogs that are no longer relevant, and a lot of exploration that needs to be done to see how the async ecosystem falls out. But considering the friction of updating is quite small, I am quitely optimistic!