Common async pitfalls

Level: 201 · working knowledge

Stub — an outline, not a lesson. There is no runnable example behind this page yet, so nothing on it has been through the check that backs every other claim in this library. The bullets below are the questions the finished page has to answer.

One line: Most async bugs compile — a future nobody awaits, a worker thread stuck in a blocking call, a lock held across a suspension point — so each one needs a name and a symptom you can recognise.

What it has to cover

Each pitfall with its symptom, and the chapter where the fix is built:

Pitfall	What you see	Fixed in
A future created and never awaited	nothing happens; one warning	What a future is
A blocking call (std::thread::sleep, file IO, a long computation) inside a task	every other task on that worker stalls	Tasks, Diagnosing a stuck runtime
A std::sync::MutexGuard held across .await	tokio::spawn refuses the future as not Send; unspawned, it can deadlock	Who owns the state
Borrowing a local into a spawned task	the 'static error	Tasks
A task spawned and never joined	its panic reaches stderr and nothing else — no error, no restart	Shutdown and supervision
A future dropped mid-way inside select!	data read but never processed	Cancellation
An unbounded channel	memory grows until the process dies	Backpressure
block_on called from inside the runtime	a panic, at run time	Diagnosing a stuck runtime
A test that depends on scheduling order	passes locally, flakes in CI	Testing async code

• For each row: the smallest program that shows it, and whether the compiler, Clippy, or nothing at all catches it
• Why most of these are silent: the compiler checks types across .await, not timing

See also

• async fn and .await — the suspension point most of these are about
• Blocking the event loop ↗ and task leak ↗ — two of these rows, across languages
• Forgotten unlock — the C version of the lock row
• A #[retry] attribute — a macro that sleeps between attempts, and the error it gives an async fn rather than block inside one

Po polsku

Najgroźniejsze błędy asynchroniczne w Ruscie kompilują się bez ostrzeżeń: blokujące wywołanie w zadaniu zatrzymuje wszystkie inne zadania na tym samym wątku roboczym, blokada ze std::sync trzymana przez .await kończy się zakleszczeniem albo błędem Send, a panika w zadaniu, na które nikt nie czeka, znika bez śladu. Ta strona zbiera je w jednej tabeli i odsyła do rozdziału, który każdy z nich naprawia.

Szukaj po polsku: pułapki programowania asynchronicznego · zakleszczenie w async · rust async pitfalls · tokio blocking in async