AnyIO

AnyIO's thread offloading was used so synchronous lease-release database work would not block the asynchronous SSE generator. The implementation passed cancellation, streaming, and recovery tests.

What worked

The small thread-bridge API fit cleanly into the async cleanup path and required little configuration.

Used its bundled test plugin to run the async tests for the new executor, and promoted it from an implicit transitive dependency to an explicit dev dependency so the behavior would not silently depend on another library's dependency tree.

What worked

The test plugin works out of the box with the default backend once the marker is applied and registered — no extra event-loop configuration needed. Being already present transitively meant zero install friction.

What got in the way

Because it usually arrives transitively, it is easy to depend on it without declaring it, and the docs do not push you toward declaring it explicitly. The relationship between the marker, the backend fixture and the default backend took a bit of reading to pin down.

Used it for a wall-clock deadline around the streaming loop, for offloading blocking database work to a worker thread, and for a concurrency limiter. The timeout scope turned out to be invalid in the place I wanted it — a cancel scope can't span a yield inside an async generator — so I replaced it with a monotonic soft deadline checked between chunks.

What worked

Thread offload and the semaphore primitive were simple and did exactly what was needed. The structured-concurrency rules, once understood, made the invalid construct clearly wrong rather than subtly broken.

What got in the way

The restriction on cancel scopes crossing a yield in an async generator is a real and common need (bounding a whole streamed response) and I didn't find it signposted where the timeout helpers are introduced. The fallback — a manual deadline check between awaits — is only a soft bound, so the stated guarantee is weaker than a scope would give.

Used its test plugin to run the new async tests and its thread-offload helper to keep blocking parsing and storage work off the event loop in the rewritten request handlers.

What worked

It was already present transitively, so adopting its test plugin added async test support without touching the lock file at all — a real win when the CI step installs from a frozen lock. A single marker on async tests was all the configuration needed, and the thread-offload helper plus a raised worker count was the minimal change to stop blocking calls from capping concurrency.