feat: minimal async runtime on top of the NGINX event loop

[bavshin-f5]

This change introduces a general infrastructure for spawing async tasks on NGINX event loop. The only utility offered for now is a timer support via ngx::async_::sleep, with async IO and other scenarios being planned for future.

An incomplete connection IO example is available here: https://github.com/bavshin-f5/ngx-rust/tree/async

[Copilot]

Pull Request Overview

This PR introduces a minimal async runtime on top of the NGINX event loop, providing task spawning and timer-based sleep support under a new async feature.

• Adds a feature-gated async_ module with spawn and sleep utilities
• Implements a single-threaded scheduler (spawn.rs) that posts tasks via NGINX events
• Provides sleep (sleep.rs) as an async primitive using ngx_add_timer/ngx_del_timer

Reviewed Changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/lib.rs	Expose async_ module when async feature is set
src/async_/spawn.rs	Scheduler implementation and spawn function
src/async_/sleep.rs	sleep future implementation using NGINX timers
src/async_/mod.rs	Re-export spawn and sleep from the async_ module
Cargo.toml	Added optional async-task dependency and async feature flag

Comments suppressed due to low confidence (4)

src/async_/spawn.rs:135

• [nitpick] There are no tests covering the spawn function and scheduling logic. Consider adding unit or integration tests to verify tasks are enqueued and executed correctly on the NGINX event loop.

pub fn spawn<F, T>(future: F) -> Task<T>

src/async_/sleep.rs:17

• [nitpick] The sleep function isn't covered by any tests. Add tests to ensure it completes after the expected delays, including edge cases around the maximum timer duration.

pub async fn sleep(duration: Duration) {

src/async_/sleep.rs:19

• The log macro uses a formatting placeholder {duration:?} without passing the duration argument, so it will log the literal string. Change to:

ngx_log_debug!(timer.event.log, "async: sleep for {:?}", duration);

    ngx_log_debug!(timer.event.log, "async: sleep for {duration:?}");

src/async_/sleep.rs:29

• Same issue here: the placeholder {duration:?} isn’t substituted. Update to:

ngx_log_debug!(timer.event.log, "async: sleep for {:?}", duration);

    ngx_log_debug!(timer.event.log, "async: sleep for {duration:?}");

[pchickey]

Looks excellent. Thank you for iterating on this!

[xeioex]

Look good.