unblocked once both conditions
        (server is closed and all connections have been dropped)
        have become true, in either order.

        Historical note: In 3.11 and before, this was broken, returning
        immediately if the server was already closed, even if there
        were still active connections. An attempted fix in 3.12.0 was
        still broken, returning immediately if the server was still
        open and there were no active connections. Hopefully in 3.12.1
        we have it right.
        N)