fix position counting in `read-byte'

Merge to 5.2.1
(cherry picked from commit c723aeeb6a)

collects/tests/racket/port.rktl

@@ -717,6 +717,27 @@
   (test t2 sync t2)
   (test oc sync/timeout 0 oc))
 
+;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; eof should not advance port position
+
+(let ()
+  (define (check read-byte)
+    (define-values (i o) (make-pipe))
+    (test 0 file-position i)
+    (write-byte 10 o)
+    (close-output-port o)
+    (test 0 file-position i)
+    (test 10 read-byte i)
+    (test 1 file-position i)
+    (test eof read-byte i)
+    (test 1 file-position i))
+  (check read-byte)
+  (check (lambda (i) 
+           (define c (read-char i))
+           (if (char? c)
+               (char->integer c)
+               c))))
+
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 (report-errs)

src/racket/src/port.c

@@ -3201,10 +3201,11 @@ static MZ_INLINE intptr_t get_one_byte(GC_CAN_IGNORE const char *who,
         if (v == SCHEME_SPECIAL) {
           scheme_bad_time_for_special(who, port);
         }
-
-        ip = (Scheme_Input_Port *)port; /* since ignored by GC */
+        if (v != EOF) {
+          ip = (Scheme_Input_Port *)port; /* since `ip is ignored by GC */
           if (ip->p.position >= 0)
             ip->p.position++;
+        }
 
         return v;
       }