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Eliza Weisman e699d46534 compat: add a compat runtime (#1663 )

## Motivation

The `futures` crate's [`compat` module][futures-compat] provides
interoperability between `futures` 0.1 and `std::future` _future types_
(e.g. implementing `std::future::Future` for a type that implements the
`futures` 0.1 `Future` trait). However, this on its own is insufficient
to run code written against `tokio` 0.1 on a `tokio` 0.2 runtime, if
that code also relies on `tokio`'s runtime services. If legacy tasks are
executed that rely on `tokio::timer`, perform IO using `tokio`'s
reactor, or call `tokio::spawn`, those API calls will fail unless there
is also a runtime compatibility layer.

## Solution

As proposed in #1549, this branch introduces a new `tokio-compat` crate,
with implementations of the thread pool and current-thread runtimes that
are capable of running both tokio 0.1 and tokio 0.2 tasks. The compat
runtime creates a background thread that runs a `tokio` 0.1 timer and
reactor, and sets itself as the `tokio` 0.1 executor as well as the
default 0.2 executor. This allows 0.1 futures that use 0.1 timer,
reactor, and executor APIs may run alongside `std::future` tasks on the
0.2 runtime.

### Examples

Spawning both `tokio` 0.1 and `tokio` 0.2 futures:

```rust
use futures_01::future::lazy;

tokio_compat::run(lazy(|| {
    // spawn a `futures` 0.1 future using the `spawn` function from the
    // `tokio` 0.1 crate:
    tokio_01::spawn(lazy(|| {
        println!("hello from tokio 0.1!");
        Ok(())
    }));

    // spawn an `async` block future on the same runtime using `tokio`
    // 0.2's `spawn`:
    tokio_02::spawn(async {
        println!("hello from tokio 0.2!");
    });

    Ok(())
}))
```

Futures on the compat runtime can use `timer` APIs from both 0.1 and 0.2
versions of `tokio`:

```rust
use std::time::{Duration, Instant};
use futures_01::future::lazy;
use tokio_compat::prelude::*;

tokio_compat::run_03(async {
    // Wait for a `tokio` 0.1 `Delay`...
    let when = Instant::now() + Duration::from_millis(10);
    tokio_01::timer::Delay::new(when)
        // convert the delay future into a `std::future` that we can `await`.
        .compat()
        .await
        .expect("tokio 0.1 timer should work!");
    println!("10 ms have elapsed");

    // Wait for a `tokio` 0.2 `Delay`...
    let when = Instant::now() + Duration::from_millis(20);
    tokio_02::timer::delay(when).await;
    println!("20 ms have elapsed");
});
```

## Future Work

This is just an initial implementation of a `tokio-compat` crate; there
are more compatibility layers we'll want to provide before that crate is
complete. For example, we should also provide compatibility between
`tokio` 0.2's `AsyncRead` and `AsyncWrite` traits and the `futures` 0.1
and `futures` 0.3 versions of those traits. In #1549, @carllerche also
suggests that the `compat` crate provide reimplementations of APIs that
were removed from `tokio` 0.2 proper, such as the `tcp::Incoming`
future.

Additionally, there is likely extra work required to get the 
`tokio-threadpool` 0.1 `blocking` APIs to work on the compat runtime.
This will be addressed in a follow-up PR.

Fixes: #1605
Fixes: #1552
Refs: #1549

[futures-compat]: https://rust-lang-nursery.github.io/futures-api-docs/0.3.0-alpha.19/futures/compat/index.html

2019-11-01 10:35:02 -07:00

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Tokio Compat

Compatibility layers between tokio 0.2 and legacy versions.

Website | Guides | API Docs | Chat

Overview

This crate provides compatibility runtimes that allow running both futures 0.1 futures that use tokio 0.1 runtime services and std::future futures that use tokio 0.2 runtime services.

Examples

Spawning both tokio 0.1 and tokio 0.2 futures:

use futures_01::future::lazy;

tokio_compat::run(lazy(|| {
    // spawn a `futures` 0.1 future using the `spawn` function from the
    // `tokio` 0.1 crate:
    tokio_01::spawn(lazy(|| {
        println!("hello from tokio 0.1!");
        Ok(())
    }));

    // spawn an `async` block future on the same runtime using `tokio`
    // 0.2's `spawn`:
    tokio_02::spawn(async {
        println!("hello from tokio 0.2!");
    });

    Ok(())
}))

Futures on the compat runtime can use timer APIs from both 0.1 and 0.2 versions of tokio:

use std::time::{Duration, Instant};
use futures_01::future::lazy;
use tokio_compat::prelude::*;

tokio_compat::run_std(async {
    // Wait for a `tokio` 0.1 `Delay`...
    let when = Instant::now() + Duration::from_millis(10);
    tokio_01::timer::Delay::new(when)
        // convert the delay future into a `std::future` that we can `await`.
        .compat()
        .await
        .expect("tokio 0.1 timer should work!");
    println!("10 ms have elapsed");

    // Wait for a `tokio` 0.2 `Delay`...
    let when = Instant::now() + Duration::from_millis(20);
    tokio_02::timer::delay(when).await;
    println!("20 ms have elapsed");
});

License

This project is licensed under the MIT license.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in Tokio by you, shall be licensed as MIT, without any additional terms or conditions.

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