async

fn async(
    userdata: ?*anyopaque,
    result: []u8,
    result_alignment: Alignment,
    context: []const u8,
    context_alignment: Alignment,
    start: *const fn (context: *const anyopaque, result: *anyopaque) void,
) ?*Io.AnyFuture {
    const t: *Threaded = @ptrCast(@alignCast(userdata));
    if (builtin.single_threaded) {
        start(context.ptr, result.ptr);
        return null;
    }

    const gpa = t.allocator;
    const future = Future.create(gpa, result.len, result_alignment, context, context_alignment, start) catch |err| switch (err) {
        error.OutOfMemory => {
            start(context.ptr, result.ptr);
            return null;
        },
    };

    mutexLock(&t.mutex);

    const busy_count = t.busy_count;

    if (busy_count >= @backingInt(t.async_limit)) {
        mutexUnlock(&t.mutex);
        future.destroy(gpa);
        start(context.ptr, result.ptr);
        return null;
    }

    t.busy_count = busy_count + 1;

    const pool_size = t.wait_group.value();
    if (pool_size - busy_count == 0) {
        t.wait_group.start();
        const thread = std.Thread.spawn(.{ .stack_size = t.stack_size }, worker, .{t}) catch {
            t.wait_group.finish();
            t.busy_count = busy_count;
            mutexUnlock(&t.mutex);
            future.destroy(gpa);
            start(context.ptr, result.ptr);
            return null;
        };
        thread.detach();
    }

    t.run_queue.prepend(&future.runnable.node);

    mutexUnlock(&t.mutex);
    condSignal(&t.cond);
    return @ptrCast(future);
}