Function monarch_with_gil 

pub async fn monarch_with_gil<F, R>(f: F) -> R
where
    F: for<'py> FnOnce(Python<'py>) -> R + Send,

Async wrapper around Python::attach intended for async call sites.

Why: under high concurrency, many async tasks can simultaneously try to acquire the GIL. Each call blocks the current tokio worker thread, which can cause runtime starvation / apparent deadlocks (nothing else gets polled).

This wrapper serializes GIL acquisition among async callers so at most one tokio task is blocked in Python::attach at a time, preventing runtime starvation under GIL contention.

Note: this does not globally prevent other sync code from calling Python::attach directly. Use this wrapper for Python interaction that occurs on async hot paths.

Re-entrancy Safety

This function is re-entrant safe. If called while already inside a monarch_with_gil or monarch_with_gil_blocking call (e.g., from a Python→Rust callback), it bypasses the GIL_LOCK to avoid deadlocks.

Example

let result = monarch_with_gil(|py| {
    // Do work with Python GIL
    Ok(42)
})
.await?;