Returning value of custom binary output port

custom-bout.scm

#!r6rs
(import (rnrs))

(define (print . args) (for-each display args) (newline))

(define (make-predefined-buffer-output-port buf)
  (define left (bytevector-length buf))
  (define index 0)
  (define (write! bv start count)
    (cond ((zero? left) 0)
	  ((> count left)
	   (let ((size left))
	     (bytevector-copy! bv start buf index size)
	     (set! index (+ index left))
	     (set! left 0)
	     size))
	  (else
	   (bytevector-copy! bv start buf index count)
	   (set! index (+ index count))
	   (set! left (- left count))
	   count)))
  (define (close!) #t)
  (define (get-position) index)
  (make-custom-binary-output-port "predefined buffer port"
				  write! get-position #f close!))

(define buf (make-bytevector 1))
(define out (make-predefined-buffer-output-port buf))
(print (put-bytevector out #vu8(1 2 3)))
(flush-output-port out)

(print buf)
(flush-output-port (current-output-port))
#|
;; Updated to support port-position
- Mosh and Sagittarius print #vu8(1)
- Vicare went into inifinite loop
- Racket (v6.1.1) raised an error (even port-position is supported)
- Guile also goes to infinte loop (picked up from the comment)
|#

Here is my final take on this:

Scenario: In the case where not bytes written is less than count.

If get-position is #f, then an error should be raised like it does with Racket.

If get-position is not #f, the procedure calling write! should check if get-position has advanced the same amount as the value returned from write!. If not, raise an error.