The Tokio Upgrade from 0.2 to 1.x

Summary

Fly.io’s Rust-based load balancer handles almost all of its traffic and stands on Tokio and Hyper, motivating an upgrade from Tokio 0.2 to 1.0. The team waited to upgrade until related crates including hyper, reqwest, async-compression, tokio-io-timeout, tokio-tungstenite, and sentry were ready; after a few weeks, most of the ecosystem had upgraded. The migration foregoes Tokio’s Stream implementations pending stabilization in the standard library, relaxes mutable-access requirements, changes AsyncRead and AsyncWrite polling to return Result<()>, and requires more explicit pinning for !Unpin futures. Because runtime::Handle lacked block_on, the team worked around the restriction with Arc; standard TcpListener values also required set_nonblocking(true) before TcpListener::from_std. Other changes included renamed APIs and revised Hyper WebSocket upgrades, and the author says the major upgrade was not hard.

Takeaways

1. 1

   Tokio 1.0 foregoes Stream implementations until standard-library stabilization; tokio-stream supplies implementations for TcpListener, Interval, and more.

2. 2

   AsyncRead::poll_read and AsyncWrite::poll_write now return Result<()> rather than byte counts, with ReadBuf::filled() used to calculate bytes read when needed.

3. 3

   A standard listener passed to tokio::net::TcpListener::from_std must be set nonblocking first; otherwise accepting connections can hang.