From 69d7636770acc968fc448bc896595e66d8ff570b Mon Sep 17 00:00:00 2001
From: Matthew Flatt <mflatt@racket-lang.org>
Date: Mon, 30 Jan 2017 20:00:07 -0700
Subject: [PATCH] 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 | 25 ++++++++++++++++++++
 racket/src/racket/src/port.c                 |  3 +++
 2 files changed, 28 insertions(+)

diff --git a/pkgs/racket-test-core/tests/racket/port.rktl b/pkgs/racket-test-core/tests/racket/port.rktl
index ada754c2a3..6b06706ae5 100644
--- a/pkgs/racket-test-core/tests/racket/port.rktl
+++ b/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)
diff --git a/racket/src/racket/src/port.c b/racket/src/racket/src/port.c
index 7f5b1130a4..5613de63dc 100644
--- a/racket/src/racket/src/port.c
+++ b/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) {