arti/crates/tor-rtcompat

tor-rtcompat

Compatibility between different async runtimes for Arti

Overview

Rust's support for asynchronous programming is powerful, but still a bit immature: there are multiple powerful runtimes you can use, but they do not expose a consistent set of interfaces.

The [futures] API abstracts much of the differences among these runtime libraries, but there are still areas where no standard API yet exists, including:

• Network programming.
• Time and delays.
• Launching new tasks
• Blocking until a task is finished.

Additionally, the AsyncRead and AsyncWrite traits provide by [futures] are not the same as those provided by tokio, and require compatibility wrappers to use.

To solve these problems, the tor-rtcompat crate provides a set of traits that represent a runtime's ability to perform these tasks, along with implementations for these traits for the tokio and async-std runtimes. In the future we hope to add support for other runtimes as needed.

This crate is part of Arti, a project to implement Tor in Rust. As such, it does not currently include (or plan to include) any functionality beyond what Arti needs to implement Tor.

We hope that in the future this crate can be replaced (or mostly replaced) with standardized and general-purpose versions of the traits it provides.

Using tor-rtcompat

The tor-rtcompat crate provides several traits that encapsulate different runtime capabilities.

• A runtime is a [SpawnBlocking] if it can block on a future.
• A runtime is a [SleepProvider] if it can make timer futures that become Ready after a given interval of time.
• A runtime is a [TcpProvider] if it can make and receive TCP connections
• A runtime is a [TlsProvider] if it can make TLS connections.

For convenience, the [Runtime] trait derives from all the traits above, plus [futures::task::Spawn] and [Send].

You can get a [Runtime] in several ways:

• If you already have an asynchronous backend (for example, one that you built with tokio by running with #[tokio::main]), you can wrap it as a [Runtime] with [current_user_runtime()].

• If you want to construct a default runtime that you won't be using for anything besides Arti, you can use [create_runtime()].

• If you want to explicitly construct a runtime with a specific backend, you can do so with [async_std::create_async_std_runtime] or [tokio::create_tokio_runtime]. Or if you have already constructed a tokio runtime that you want to use, you can wrap it as a [Runtime] explicitly with [tokio::TokioRuntimeHandle].

Cargo features

tokio -- (Default) Build with Tokio support.

async-std -- Build with async_std support.

static -- Try to link with a static copy of our native TLS library, if possible.

Design FAQ

Why support async_std?

Although Tokio currently a more popular and widely supported asynchronous runtime than async_std is, we believe that it's critical to build Arti against multiple runtimes.

By supporting multiple runtimes, we avoid making tokio-specific assumptions in our code, which we hope will make it easier to port to other environments (like WASM) in the future.

Why a Runtime trait, and not a set of functions?

We could simplify this code significantly by removing most of the traits it exposes, and instead just exposing a single implementation. For example, instead of exposing a [SpawnBlocking] trait to represent blocking until a task is done, we could just provide a single global block_on function.

That simplification would come at a cost, however. First of all, it would make it harder for us to use Rust's "feature" system correctly. Current features are supposed to be additive only, but if had a single global runtime, then support for different backends would be mutually exclusive. (That is, you couldn't have both the tokio and async-std features building at the same time.)

Secondly, much of our testing in the rest of Arti relies on the ability to replace [Runtime]s. By treating a runtime as an object, we can override a runtime's view of time, or of the network, in order to test asynchronous code effectively. (See the [tor-rtmock] crate for examples.)

License: MIT OR Apache-2.0