cs: repair sync on empty byte-string port

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

@@ -968,6 +968,12 @@
   (test #f file-position* p2)
   (err/rt-test (file-position p2) exn:fail:filesystem?))
 
+(let ([i (open-input-bytes #"")])
+  (test i sync/timeout 0 i)
+  (test i sync/timeout #f i)
+  (test #t byte-ready? i)
+  (test #t char-ready? i))
+
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; Text mode, file positions, and buffers
 

racket/src/ChezScheme/c/gc.c

@@ -1769,10 +1769,10 @@ ptr GCENTRY(ptr tc_in, ptr count_roots_ls) {
     }                                             \
   } while (0)
 
-#if 1
+#if 0
 # define COUNT_SWEPT_BYTES(start, end) num_swept_bytes += ((uptr)TO_PTR(end) - (uptr)TO_PTR(start))
 #else
-# define COUNT_SWEPT_BYTES(start, end) do { } while (0);
+# define COUNT_SWEPT_BYTES(start, end) do { } while (0)
 #endif
 
 #ifdef ENABLE_PARALLEL

racket/src/io/port/bytes-port.rkt

@@ -102,7 +102,8 @@
 
   [byte-ready
    (lambda (work-done!)
-     ((in-buffer-pos) . < . (direct-end buffer)))]
+     ;; byte or EOF is always ready:
+     #t)]
   
   [get-progress-evt
    (lambda ()