(defparameter *iters* 10000)
(defparameter *array-size* 1025) ;on abcl, *array-size* >= 1024 => fail

(defun main ()
  (with-open-file (stream "io_test.tmp"
                       :element-type '(unsigned-byte 8)
                       :direction :io
                       :if-exists :overwrite
                       :if-does-not-exist :create)
    (let ((b (make-array *array-size*
                         :element-type '(unsigned-byte 8)
                         :initial-element 0)))
      (do ((i 0 (1+ i))
           (nbytes (random (length b)) (random (length b))))
          ((>= i *iters*))
        (if (or (zerop (random 2)) (zerop (file-length stream)))
          (let ((before (file-length stream)))
            (assert before)
            (assert (file-position stream (file-length stream)))
            ;(assert (file-position stream :end))
            (write-sequence b stream :end nbytes)
            (finish-output stream)
            (assert (= nbytes (- (file-length stream) before))
                    nil
                    "iter ~A: expected file-length to change by ~A bytes not ~A"
                    i
                    nbytes
                    (- (file-length stream) before)))
          (progn
            (assert (file-position stream (random (file-length stream))))
            (read-byte stream)))))))

;abcl (1.3.2)
;#<THREAD "interpreter" {4997BAB}>: Debugger invoked on condition of type SIMPLE-ERROR
;  iter 179: expected file-length to change by 1642 bytes not 760
;Restarts:
;  0: CONTINUE  Retry assertion.
;  1: TOP-LEVEL Return to top level.
;
;the finish-output fix for other lisps did not help here
;using symbol :end instead of file-length when calling #'file-position
;  didn't help either

;ecl fails (even for 1024 byte array)
;Condition of type: SIMPLE-ERROR
;iter 2: expected file-length to change by 1246 bytes not 1024
;
;fixed by calling finish-output before assertion

;sbcl also fails
;debugger invoked on a SIMPLE-ERROR in thread
;#<THREAD "main thread" RUNNING {1002A01083}>:
;  iter 0: expected file-length to change by 860 bytes not 0
;
;fixed by calling #'finish-output

;cmucl fails
;Error in function LISP::ASSERT-ERROR:
;   iter 0: expected file-length to change by 24 bytes not 0
;   [Condition of type SIMPLE-ERROR]
;
;fixed by calling #'finish-output

;ccl is okay (even without needing to call #'finish-output)
;clisp is okay (even without needing to call #'finish-output)