Cello v1.2.4 — Critical Async Handler Fix

Release Date: June 14, 2026 License: MIT Python: 3.12+

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Overview

Cello v1.2.4 fixes a critical regression introduced in v1.2.1 where all async def route handlers silently returned HTTP 500 and emitted:

RuntimeWarning: coroutine 'handler_name' was never awaited

Drop-in upgrade from v1.2.3. No API changes.

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Root Cause

In v1.2.1 the server startup was changed from pyo3_asyncio::tokio::run to py.allow_threads + tokio::block_on to fix port binding on Python 3.12+. That fix was correct, but it had an unintended side effect: pyo3_asyncio was no longer initialised, so pyo3_asyncio::tokio::into_future (used in handler.rs to drive async handlers) failed silently on every request — returning 500 and dropping the unawaited coroutine.

v1.2.1 change (lib.rs):
  - pyo3_asyncio::tokio::run(py, server_future)   ← initialises pyo3_asyncio
  + py.allow_threads(|| rt.block_on(server_future))  ← does NOT initialise it

handler.rs (unchanged):
  pyo3_asyncio::tokio::into_future(coro)  ← fails: pyo3_asyncio not ready → 500

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Fix

handler.rs Phase 2 now drives coroutines via tokio::task::spawn_blocking + asyncio.run() instead of pyo3_asyncio::tokio::into_future:

// Before (broken since v1.2.1):
let future = Python::with_gil(|py| {
    pyo3_asyncio::tokio::into_future(raw_result.as_ref(py))  // ← failed silently
})?;
future.await?

// After (v1.2.4):
let (tx, rx) = tokio::sync::oneshot::channel();
let coro = raw_result;
tokio::task::spawn_blocking(move || {
    let result = Python::with_gil(|py| {
        let asyncio = py.import("asyncio")?;
        asyncio.call_method1("run", (coro.as_ref(py),))
            .map(|r| r.into_py(py))
    });
    let _ = tx.send(result);
});
rx.await??

The Tokio thread is released during spawn_blocking, so other requests can be served while the coroutine runs. asyncio.run() creates a fresh event loop per call (~0.1 ms overhead).

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Files Changed

File	Change
src/handler.rs	Replace pyo3_asyncio::tokio::into_future with spawn_blocking + asyncio.run()
tests/verify_async_client.py	New test — all HTTP methods via local echo server
Cargo.toml, pyproject.toml, python/cello/__init__.py	Version → 1.2.4

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Upgrade

pip install --upgrade cello-framework==1.2.4

No code changes required — all async def handlers that were silently returning 500 will now work correctly after upgrading.

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Impact

Any application using async def handlers on Cello v1.2.1, v1.2.2, or v1.2.3 was affected. Sync (def) handlers were not impacted.