[PATCH v2 4/6] scm_i_utf8_string_hash: don't overrun when len is zero

[Rob Browning <rlb@defaultvalue.org>]

When the length is zero, the previous code would include the byte after
the end of the string in the hash.  Fix that (the wide an narrow hashers
also guard against it via "case 0"), and while we're there, switch to
u8_mbtouc since the unsafe variant is now the same (see the info pages),
and don't bother mutating length for the trailing bytes, since we don't
need to.

libguile/hash.c (scm_i_utf8_string_hash): switch to u8_mbtouc, and avoid
overrun when len == 0.
---
 libguile/hash.c | 28 +++++++++++++++-------------
 1 file changed, 15 insertions(+), 13 deletions(-)

diff --git a/libguile/hash.c b/libguile/hash.c
index a038a11bf..d92f60df8 100644
--- a/libguile/hash.c
+++ b/libguile/hash.c
@@ -195,32 +195,34 @@ scm_i_utf8_string_hash (const char *str, size_t len)
   /* Handle most of the key.  */
   while (length > 3)
     {
-      ustr += u8_mbtouc_unsafe (&u32, ustr, end - ustr);
+      ustr += u8_mbtouc (&u32, ustr, end - ustr);
       a += u32;
-      ustr += u8_mbtouc_unsafe (&u32, ustr, end - ustr);
+      ustr += u8_mbtouc (&u32, ustr, end - ustr);
       b += u32;
-      ustr += u8_mbtouc_unsafe (&u32, ustr, end - ustr);
+      ustr += u8_mbtouc (&u32, ustr, end - ustr);
       c += u32;
       mix (a, b, c);
       length -= 3;
     }
 
   /* Handle the last 3 elements's.  */
-  ustr += u8_mbtouc_unsafe (&u32, ustr, end - ustr);
-  a += u32;
-  if (--length)
+  if (length)
     {
-      ustr += u8_mbtouc_unsafe (&u32, ustr, end - ustr);
-      b += u32;
-      if (--length)
+      ustr += u8_mbtouc (&u32, ustr, end - ustr);
+      a += u32;
+      if (length > 1)
         {
-          ustr += u8_mbtouc_unsafe (&u32, ustr, end - ustr);
-          c += u32;
+          ustr += u8_mbtouc (&u32, ustr, end - ustr);
+          b += u32;
+          if (length > 2)
+            {
+              ustr += u8_mbtouc (&u32, ustr, end - ustr);
+              c += u32;
+            }
         }
+      final (a, b, c);
     }
 
-  final (a, b, c);
-
   if (sizeof (unsigned long) == 8)
     ret = (((unsigned long) c) << 32) | b;
   else
-- 
2.43.0