Prevent read-bytevector from corrupting data

Previously it was possible for an extra by to be introduced between multiple reads

CHANGELOG.md

@@ -14,8 +14,7 @@ Features
 
 Bug Fixes
 
-- WIP - bug fixes to `read-bytevector` (return EOF, issue with sha2 sums)
-
+- Fixed a bug in `read-bytevector` where an extra byte could be introduced when reading multiple chunks of data.
 - Fixed a bug where variables defined within `define-syntax` and `let-syntax` are reported as unbound by the compiler.
 
 ## 0.15 - February 26, 2020

scheme/base.sld

@@ -715,16 +715,17 @@
                          (car _port)))
                (bv (make-bytevector k))
                (loop (lambda (n)
+                       (if (>= n k)
+                           bv
                           (let ((b (read-u8 port)))
                             (cond
                              ((eof-object? b)
                               (if (zero? n)
                                   b ;; EOF
                                   (bytevector-copy bv 0 n)))
-                          ((< n k)
+                             (else
                               (bytevector-u8-set! bv n b)
-                           (loop (+ n 1)))
-                          (else bv))))))
+                              (loop (+ n 1)))))))))
         (loop 0)))
     (define (read-bytevector! vec . o)
       (let* ((in (if (pair? o) (car o) (current-input-port)))