schedule

fn schedule(k: *Kqueue, thread: *Thread, ready_queue: Fiber.Queue) void {
    {
        var fiber = ready_queue.head;
        while (true) {
            std.log.debug("scheduling {*}", .{fiber});
            fiber = fiber.queue_next orelse break;
        }
        assert(fiber == ready_queue.tail);
    }
    // shared fields of previous `Thread` must be initialized before later ones are marked as active
    const new_thread_index = @atomicLoad(u32, &k.threads.active, .acquire);
    for (0..@min(max_idle_search, new_thread_index)) |_| {
        defer thread.idle_search_index += 1;
        if (thread.idle_search_index == new_thread_index) thread.idle_search_index = 0;
        const idle_search_thread = &k.threads.allocated[0..new_thread_index][thread.idle_search_index];
        if (idle_search_thread == thread) continue;
        if (@cmpxchgWeak(
            ?*Fiber,
            &idle_search_thread.ready_queue,
            null,
            ready_queue.head,
            .release,
            .monotonic,
        )) |_| continue;
        const changes = [_]posix.Kevent{
            .{
                .ident = 0,
                .filter = std.c.EVFILT.USER,
                .flags = std.c.EV.ADD | std.c.EV.ONESHOT,
                .fflags = std.c.NOTE.TRIGGER,
                .data = 0,
                .udata = @backingInt(Completion.UserData.wakeup),
            },
        };
        // If an error occurs it only pessimises scheduling.
        _ = kevent(idle_search_thread.kq_fd, &changes, &.{}, null) catch |err| {
            // TODO handle EINTR for cancellation purposes
            @panic(@errorName(err)); // TODO
        };
        return;
    }
    spawn_thread: {
        // previous failed reservations must have completed before retrying
        if (new_thread_index == k.threads.allocated.len or @cmpxchgWeak(
            u32,
            &k.threads.reserved,
            new_thread_index,
            new_thread_index + 1,
            .acquire,
            .monotonic,
        ) != null) break :spawn_thread;
        const new_thread = &k.threads.allocated[new_thread_index];
        const next_thread_index = new_thread_index + 1;
        new_thread.* = .{
            .thread = undefined,
            .idle_context = undefined,
            .current_context = &new_thread.idle_context,
            .ready_queue = ready_queue.head,
            .kq_fd = createFileDescriptor() catch |err| {
                @atomicStore(u32, &k.threads.reserved, new_thread_index, .release);
                // no more access to `thread` after giving up reservation
                std.log.warn("unable to create worker thread due to kqueue init failure: {t}", .{err});
                break :spawn_thread;
            },
            .idle_search_index = 0,
            .steal_ready_search_index = 0,
            .wait_queues = .empty,
        };
        new_thread.thread = std.Thread.spawn(.{
            .stack_size = idle_stack_size,
            .allocator = k.gpa,
        }, threadEntry, .{ k, new_thread_index }) catch |err| {
            closeFd(new_thread.kq_fd);
            @atomicStore(u32, &k.threads.reserved, new_thread_index, .release);
            // no more access to `thread` after giving up reservation
            std.log.warn("unable to create worker thread due spawn failure: {s}", .{@errorName(err)});
            break :spawn_thread;
        };
        // shared fields of `Thread` must be initialized before being marked active
        @atomicStore(u32, &k.threads.active, next_thread_index, .release);
        return;
    }
    // nobody wanted it, so just queue it on ourselves
    while (@cmpxchgWeak(
        ?*Fiber,
        &thread.ready_queue,
        ready_queue.tail.queue_next,
        ready_queue.head,
        .acq_rel,
        .acquire,
    )) |old_head| ready_queue.tail.queue_next = old_head;
}