Fix UTF-8 symbol repair

Commit 6a5a3037b4 was not quite right, because it used sightly the
wrong variant among a dozen decoding functions.  The test suite caught
the problem, but I forgot to run it before pushing.

Also, repair the "Inside" documentation on the function that was
incorrectly used, and document the new variant.

pkgs/racket-pkgs/racket-doc/scribblings/inside/strings.scrbl

@@ -57,11 +57,30 @@ If @var{ipos} is non-@cpp{NULL}, it is filled with the first undecoded
  how many bytes were decoded before decoding stopped.
 
 If @var{permissive} is non-zero, it is used as the decoding of bytes
- that are not part of a valid UTF-8 encoding. Thus, the function
+ that are not part of a valid UTF-8 encoding or if the input ends in the
- result can be @cpp{-2} only if @var{permissive} is @cpp{0}.
+ middle of an encoding. Thus, the function
+ result can be @cpp{-1} or @cpp{-2} only if @var{permissive} is @cpp{0}.
 
 This function does not allocate or trigger garbage collection.}
 
+@function[(int scheme_utf8_decode_offset_prefix
+           [const-unsigned-char* s]
+           [int start]
+           [int end]
+           [mzchar* us]
+           [int dstart]
+           [int dend]
+           [intptr_t* ipos]
+           [char utf16]
+           [int permissive])]{
+
+Like @cpp{scheme_utf8_decode}, but returns @cpp{-1} if the input ends
+in the middle of a UTF-8 encoding even if @var{permission} is
+non-zero.
+
+@history[#:added "6.0.1.13"]}
+
+
 @function[(int scheme_utf8_decode_as_prefix
            [const-unsigned-char* s]
            [int start]
@@ -99,7 +118,7 @@ Like @cpp{scheme_utf8_decode}, but with fewer arguments. The
            [int permissive])]{
 
 Like @cpp{scheme_utf8_decode}, but with fewer arguments. The
- decoding produces UCS-4 @cpp{mzchar}s. If the buffer @var{us}
+ decoding produces UCS-4 @cpp{mzchar}s. The buffer @var{us}
  @bold{must} be non-@cpp{NULL}, and it is assumed to be long enough to hold the
  decoding (which cannot be longer than the length of the input, though
  it may be shorter). If @var{len} is negative, @cpp{strlen(@var{s})}

racket/src/racket/include/mzwin.def

@@ -338,6 +338,7 @@ EXPORTS
  scheme_is_cpointer
  scheme_get_proc_name
  scheme_utf8_decode
+ scheme_utf8_decode_offset_prefix
  scheme_utf8_decode_as_prefix
  scheme_utf8_decode_all
  scheme_utf8_decode_prefix

racket/src/racket/include/mzwin3m.def

@@ -353,6 +353,7 @@ EXPORTS
  scheme_is_cpointer
  scheme_get_proc_name
  scheme_utf8_decode
+ scheme_utf8_decode_offset_prefix
  scheme_utf8_decode_as_prefix
  scheme_utf8_decode_all
  scheme_utf8_decode_prefix

racket/src/racket/include/racket.exp

@@ -355,6 +355,7 @@ scheme_make_offset_external_cptr
 scheme_is_cpointer
 scheme_get_proc_name
 scheme_utf8_decode
+scheme_utf8_decode_offset_prefix
 scheme_utf8_decode_as_prefix
 scheme_utf8_decode_all
 scheme_utf8_decode_prefix

racket/src/racket/include/racket3m.exp

@@ -361,6 +361,7 @@ scheme_make_offset_external_cptr
 scheme_is_cpointer
 scheme_get_proc_name
 scheme_utf8_decode
+scheme_utf8_decode_offset_prefix
 scheme_utf8_decode_as_prefix
 scheme_utf8_decode_all
 scheme_utf8_decode_prefix

racket/src/racket/src/schemef.h

@@ -672,6 +672,9 @@ MZ_EXTERN const char *scheme_get_proc_name(Scheme_Object *p, int *len, int for_e
 MZ_EXTERN intptr_t scheme_utf8_decode(const unsigned char *s, intptr_t start, intptr_t end, 
 				      unsigned int *us, intptr_t dstart, intptr_t dend,
 				      intptr_t *ipos, char utf16, int permissive);
+MZ_EXTERN intptr_t scheme_utf8_decode_offset_prefix(const unsigned char *s, intptr_t start, intptr_t end, 
+                                                    unsigned int *us, intptr_t dstart, intptr_t dend,
+                                                    intptr_t *ipos, char utf16, int permissive);
 MZ_EXTERN intptr_t scheme_utf8_decode_as_prefix(const unsigned char *s, intptr_t start, intptr_t end, 
 						unsigned int *us, intptr_t dstart, intptr_t dend,
 						intptr_t *ipos, char utf16, int permissive);

racket/src/racket/src/schemex.h

@@ -541,6 +541,9 @@ const char *(*scheme_get_proc_name)(Scheme_Object *p, int *len, int for_error);
 intptr_t (*scheme_utf8_decode)(const unsigned char *s, intptr_t start, intptr_t end, 
 				      unsigned int *us, intptr_t dstart, intptr_t dend,
 				      intptr_t *ipos, char utf16, int permissive);
+intptr_t (*scheme_utf8_decode_offset_prefix)(const unsigned char *s, intptr_t start, intptr_t end, 
+                                                    unsigned int *us, intptr_t dstart, intptr_t dend,
+                                                    intptr_t *ipos, char utf16, int permissive);
 intptr_t (*scheme_utf8_decode_as_prefix)(const unsigned char *s, intptr_t start, intptr_t end, 
 						unsigned int *us, intptr_t dstart, intptr_t dend,
 						intptr_t *ipos, char utf16, int permissive);

racket/src/racket/src/schemex.inc

@@ -398,6 +398,7 @@
   scheme_extension_table->scheme_is_cpointer = scheme_is_cpointer;
   scheme_extension_table->scheme_get_proc_name = scheme_get_proc_name;
   scheme_extension_table->scheme_utf8_decode = scheme_utf8_decode;
+  scheme_extension_table->scheme_utf8_decode_offset_prefix = scheme_utf8_decode_offset_prefix;
   scheme_extension_table->scheme_utf8_decode_as_prefix = scheme_utf8_decode_as_prefix;
   scheme_extension_table->scheme_utf8_decode_all = scheme_utf8_decode_all;
   scheme_extension_table->scheme_utf8_decode_prefix = scheme_utf8_decode_prefix;

racket/src/racket/src/schemexm.h

@@ -398,6 +398,7 @@
 #define scheme_is_cpointer (scheme_extension_table->scheme_is_cpointer)
 #define scheme_get_proc_name (scheme_extension_table->scheme_get_proc_name)
 #define scheme_utf8_decode (scheme_extension_table->scheme_utf8_decode)
+#define scheme_utf8_decode_offset_prefix (scheme_extension_table->scheme_utf8_decode_offset_prefix)
 #define scheme_utf8_decode_as_prefix (scheme_extension_table->scheme_utf8_decode_as_prefix)
 #define scheme_utf8_decode_all (scheme_extension_table->scheme_utf8_decode_all)
 #define scheme_utf8_decode_prefix (scheme_extension_table->scheme_utf8_decode_prefix)

racket/src/racket/src/string.c

@@ -5476,6 +5476,14 @@ intptr_t scheme_utf8_decode(const unsigned char *s, intptr_t start, intptr_t end
 		       ipos, NULL, utf16, utf16, NULL, 0, permissive);
 }
 
+intptr_t scheme_utf8_decode_offset_prefix(const unsigned char *s, intptr_t start, intptr_t end,
+                                          unsigned int *us, intptr_t dstart, intptr_t dend,
+                                          intptr_t *ipos, char utf16, int permissive)
+{
+  return utf8_decode_x(s, start, end, us, dstart, dend,
+		       ipos, NULL, utf16, utf16, NULL, 1, permissive);
+}
+
 intptr_t scheme_utf8_decode_as_prefix(const unsigned char *s, intptr_t start, intptr_t end,
                                       unsigned int *us, intptr_t dstart, intptr_t dend,
                                       intptr_t *ipos, char utf16, int permissive)

racket/src/racket/src/symbol.c

@@ -613,9 +613,9 @@ const char *scheme_symbol_name_and_size(Scheme_Object *sym, uintptr_t *length, i
       mzchar buf[2];
       int ul = 1;
       while (1) {
-        if (scheme_utf8_decode((unsigned char *)s, i, i + ul,
+        if (scheme_utf8_decode_offset_prefix((unsigned char *)s, i, i + ul,
-                               buf, 0, 1,
+                                             buf, 0, 1,
-                               NULL, 0, '?') > 0)
+                                             NULL, 0, '?') > 0)
           break;
         ul++;
       }