;; Don't need active-writer; if a writer has the lock then its active.
;; If writers-waiting > 0 then readers should defer.

(defmacro write-with-rw-lock ((reader-writer-lock) &body body)
  (let ((retval (gensym))
        (lock (gensym)))
    `(let ((,retval nil)
           (,lock ,reader-writer-lock))
        (with-accessors ((g g-lock)
                         (ra readers-active)
                         (ww writers-waiting)
                         (c-var condition-var))
            ,lock
          (bt:with-lock-held (g)
            (unwind-protect
                (incf ww)
              (unwind-protect
                  (progn
                    (while-loop (> ra 0)
                        (bt:condition-wait c-var g))
                    (setf ,retval (multiple-value-list (progn ,@body))))
                (decf ww)
                (bt:condition-notify c-var)))))
        (values-list ,retval))))