groupCancel

fn groupCancel(userdata: ?*anyopaque, type_erased: *Io.Group, initial_token: *anyopaque) void {
    _ = initial_token;
    if (builtin.single_threaded) unreachable; // nothing to cancel
    const t: *Threaded = @ptrCast(@alignCast(userdata));
    const g: Group = .{ .ptr = type_erased };

    var num_completed: std.atomic.Value(u32) = .init(0);
    g.awaiter().* = &num_completed;

    const pre_cancel_status = g.status().fetchOr(.{
        .num_running = 0,
        .have_awaiter = true,
        .canceled = true,
    }, .acq_rel); // acquire results if complete; release `g.awaiter()`

    assert(!pre_cancel_status.have_awaiter);
    assert(!pre_cancel_status.canceled);
    if (pre_cancel_status.num_running == 0) {
        // Already done. Since the group is finished, it's illegal to spawn more tasks in it
        // until we return, so we can access `g.status()` non-atomically.
        g.status().raw.have_awaiter = false;
        g.status().raw.canceled = false;
        return;
    }

    g.waitForCancelWithSignaling(t, &num_completed, false);

    g.status().raw = .{ .num_running = 0, .have_awaiter = false, .canceled = false };
}