Crate 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. (We re-export those of tokio_util.

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 provide several traits that that encapsulate different runtime capabilities.

• A runtime is a SpawnBlocking if it can block on a future.
• A runtime if 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, or 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 create_async_std_runtime or 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 TokioRuntimeHandle.

Cargo features

tokio – (Default) Build with Tokio support.

async-std – Build with async_std support.

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 diffferent 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 Runtimes. 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.)

Modules

task	Functions for task management that don’t belong inside the Runtime trait.
tls	Traits used to describe TLS connections and objects that can create them.

Structs

Timeout	A timeout returned by SleepProviderExt::timeout.
TimeoutError	An error value given when a function times out.
TokioRuntimeHandle	Wrapper around a Handle to a tokio runtime.

Traits

CertifiedConn	An object with a peer certificate: typically a TLS connection.
Runtime	A runtime that we can use to run Tor as a client.
SleepProvider	Trait for a runtime that can wait until a timer has expired.
SleepProviderExt	An extension trait on SleepProvider for timeouts and clock delays.
SpawnBlocking	Trait for a runtime that can block on a future.
TcpListener	Trait for a local socket that accepts incoming TCP streams.
TcpProvider	Trait for a runtime that can create and accept TCP connections.
TlsProvider	Trait for a runtime that knows how to create TLS connections.

Functions

create_runtime	Return a new instance of the default Runtime.
create_tokio_runtime	Create and return a new Tokio multithreaded runtime.
current_user_runtime	Try to return an instance of the currently running Runtime.
test_with_runtime	Run a given asynchronous function, which takes a runtime as an argument, using the default runtime.