Decoding errors do not advance read pointer

* libguile/ports.c (scm_getc): If the port conversion strategy is
  'error, signal an error before advancing the read pointer.  This is a
  change from previous behavior; before, we advanced the read pointer
  under an understanding that that was what R6RS required.  But, that
  seems to be not the case.
* test-suite/tests/ports.test ("string ports"): Update decoding-error
  tests to assume that read-char with an error doesn't advance the read
  pointer.
* test-suite/tests/rdelim.test ("read-line"): Likewise.

libguile/ports.c

@@ -1811,34 +1811,22 @@ peek_codepoint (SCM port, scm_t_wchar *codepoint, size_t *len)
   return err;
 }
 
-static SCM_C_INLINE int
-get_codepoint (SCM port, scm_t_wchar *codepoint)
-{
-  int err;
-  size_t len = 0;
-  scm_t_port *pt = SCM_PTAB_ENTRY (port);
-
-  err = peek_codepoint (port, codepoint, &len);
-  scm_port_buffer_did_take (pt->read_buf, len);
-  if (*codepoint == EOF)
-    scm_i_clear_pending_eof (port);
-  update_port_lf (*codepoint, port);
-  return err;
-}
-
 /* Read a codepoint from PORT and return it.  */
 scm_t_wchar
 scm_getc (SCM port)
 #define FUNC_NAME "scm_getc"
 {
   int err;
-  scm_t_wchar codepoint;
+  size_t len = 0;
+  scm_t_wchar codepoint = EOF;
 
-  err = get_codepoint (port, &codepoint);
+  err = peek_codepoint (port, &codepoint, &len);
   if (SCM_UNLIKELY (err != 0))
-    /* At this point PORT should point past the invalid encoding, as per
-       R6RS-lib Section 8.2.4.  */
     scm_decoding_error (FUNC_NAME, err, "input decoding error", port);
+  scm_port_buffer_did_take (SCM_PTAB_ENTRY (port)->read_buf, len);
+  if (codepoint == EOF)
+    scm_i_clear_pending_eof (port);
+  update_port_lf (codepoint, port);
 
   return codepoint;
 }

test-suite/tests/ports.test

@@ -822,21 +822,32 @@
   ;; Mini DSL to test decoding error handling.
   (letrec-syntax ((decoding-error?
                    (syntax-rules ()
-                     ((_ port exp)
+                     ((_ port proc)
                       (catch 'decoding-error
                         (lambda ()
-                          (pk 'exp exp)
+                          (pk 'proc (proc port))
                           #f)
                         (lambda (key subr message errno p)
+                          (define (skip-over-error)
+                            (let ((strategy (port-conversion-strategy p)))
+                              (set-port-conversion-strategy! p 'substitute)
+                              ;; If `proc' is `read-char', this will
+                              ;; skip over the bad bytes.
+                              (let ((c (proc p)))
+                                (unless (eqv? c #\?)
+                                  (error "unexpected char" c))
+                                (set-port-conversion-strategy! p strategy)
+                                #t)))
                           (and (eq? p port)
-                               (not (= 0 errno))))))))
+                               (not (= 0 errno))
+                               (skip-over-error)))))))
                   (make-check
                    (syntax-rules (-> error eof)
                      ((_ port (proc -> error))
                       (if (eq? 'substitute
                                (port-conversion-strategy port))
                           (eqv? (proc port) #\?)
-                          (decoding-error? port (proc port))))
+                          (decoding-error? port proc)))
                      ((_ port (proc -> eof))
                       (eof-object? (proc port)))
                      ((_ port (proc -> char))

test-suite/tests/rdelim.test

@@ -19,7 +19,7 @@
 
 (define-module (test-suite test-rdelim)
   #:use-module (ice-9 rdelim)
-  #:use-module ((rnrs io ports) #:select (open-bytevector-input-port))
+  #:use-module ((rnrs io ports) #:select (open-bytevector-input-port get-u8))
   #:use-module (test-suite lib))
 
 (with-test-prefix "read-line"
@@ -79,8 +79,7 @@
           #f)
         (lambda (key subr message err port)
           (and (eq? port p)
-
+               (eqv? (get-u8 p) 255)
-               ;; PORT should now point past the error.
                (string=? (read-line p) "BCD")
                (eof-object? (read-line p)))))))