fix position counting with internal read shortcut

The primitive `read` uses a shortcut --- a private "ungetc"
implementation --- that did not count position correctly for
non-ASCII characters.

Closes #1599

pkgs/racket-test-core/tests/racket/port.rktl

@@ -920,6 +920,31 @@
                                    void)))
    exn:fail:read?))
 
+;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Check whether `read` counts correctly
+;; (because `read` may cheat internally with a private "unget")
+
+(let ()
+  (define p 
+    (let ([in (open-input-string "(…)abcdef")])
+      (make-input-port
+       "unicode"
+       (lambda (s)
+         (read-bytes-avail!* s in))
+       (lambda (s skip default)
+         (peek-bytes-avail!* s skip #f in))
+       void
+       #f
+       #f
+       #f
+       void
+       1)))
+
+  (test 0 file-position p)
+  (void (read p))
+  (test 5 file-position p)
+  (test  "abcde" read-string 5 p))
+
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 (report-errs)

racket/src/racket/src/port.c

@@ -3662,6 +3662,9 @@ scheme_ungetc (int ch, Scheme_Object *port)
     us[0] = ch;
     len = scheme_utf8_encode_all(us, 1, e);
 
+    if (ip->p.position > (len - 1))
+      ip->p.position -= (len - 1);
+
     if (ip->ungotten_count + len >= 24)
       scheme_signal_error("ungetc overflow");
     while (len) {