CONCEPT Cited by 1 source

Rust Waker

Definition

A Waker (std::task::Waker) is the abstract wake-up handle an async-Rust executor passes into every call to Future::poll. When a Future returns Poll::Pending (not ready yet), it stashes a clone of the Waker with whatever will eventually unblock it — a socket's readiness notification, a timer, an inner Future — and is guaranteed to get re-polled only after that thing fires the Waker.

From the Fly.io 2025-02 incident post:

"The Waker is an abstract handle that allows the Future to instruct the Tokio runtime to call poll, because something has happened."

The Waker is the contract point between userland Futures and the executor: if the Waker is signalled when it shouldn't be, the executor re-polls; if it isn't signalled when it should be, the executor never re-polls.

Two footguns

Fly.io's post enumerates them crisply:

1. "A poll of a Future that isn't Ready wastes cycles, and, if you have a bug in your code and that Pending poll happens to trip a Waker, you'll slip into an infinite loop." — the Future says "not ready, wake me later" and in the same breath wakes itself — cycle.
2. "An AsyncRead can poll_read to a Ready that doesn't actually progress its underlying state machine." — the Future says "I'm ready!" but polling it again yields the same non-progressing Ready — the caller spins on it.

Both collapse into 100% CPU burn ("samples that almost terminate in libc, but spend next to no time in the kernel doing actual I/O") — canonical spurious-wakeup busy-loop signature.

Why Waker bugs are hard to debug

The Waker indirection is the whole point of async Rust — computations advance only when the executor is told to advance them. Getting one wrong produces a symptom (busy-polling) that is structurally different from the bug's location (mis-issued Waker wake-up at the state-machine boundary). Fly.io's diagnostic move was a flamegraph: the Future's fully-qualified type in the flamegraph pointed at the guilty layer (tokio-rustls::TlsStream).

Seen in

• sources/2025-02-26-flyio-taming-a-voracious-rust-proxy — the rustls TLS state machine mis-handled its Waker on orderly close_notify shutdown with buffered socket data, dragging the containing Future into a busy-poll loop on systems/fly-proxy in IAD. Fixed upstream at rustls PR #1950.

Related

• concepts/async-rust-future — what the Waker wakes up.
• concepts/asyncread-contract — the streaming-IO variant.
• concepts/spurious-wakeup-busy-loop — the failure mode.
• systems/tokio — the canonical executor passing Wakers into poll.
• systems/rustls / systems/tokio-rustls — the library that had the 2025-02 Waker bug.
• companies/flyio