cancel

fn cancel(
    userdata: ?*anyopaque,
    any_future: *Io.AnyFuture,
    result: []u8,
    result_alignment: Alignment,
) void {
    _ = result_alignment;
    if (builtin.single_threaded) unreachable; // nothing to cancel
    const t: *Threaded = @ptrCast(@alignCast(userdata));
    const future: *Future = @ptrCast(@alignCast(any_future));

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

    const pre_cancel_status = future.status.fetchOr(.{
        .tag = .pending_canceled,
        .thread = .null,
    }, .acq_rel); // acquire results if complete; release `future.awaiter`
    switch (pre_cancel_status.tag) {
        .pending => {
            const working_thread = pre_cancel_status.thread.unpack();
            future.waitForCancelWithSignaling(t, &num_completed, @alignCast(working_thread));
        },
        .pending_awaited => unreachable, // `await` raced with `await`
        .pending_canceled => unreachable, // `await` raced with `cancel`
        .done => {},
    }
    @memcpy(result, future.resultPointer());
    future.destroy(t.allocator);
}