Stop pos() from being confused by changing utf8ness

The value of pos() is stored as a byte offset.  If it is stored on a
tied variable or a reference (or glob), then the stringification could
change, resulting in pos() now pointing to a different character off-
set or pointing to the middle of a character:

$ ./perl -Ilib -le '$x = bless [], chr 256; pos $x=1; bless $x, a; print pos $x'
2
$ ./perl -Ilib -le '$x = bless [], chr 256; pos $x=1; bless $x, "\x{1000}"; print pos $x'
Malformed UTF-8 character (unexpected end of string) in match position at -e line 1.
0

So pos() should be stored as a character offset.

The regular expression engine expects byte offsets always, so allow it
to store bytes when possible (a pure non-magical string) but use char-
acters otherwise.

This does result in more complexity than I should like, but the alter-
native (always storing a character offset) would slow down regular
expressions, which is a big no-no.

1 files changed, 2 insertions, 6 deletions

diff --git a/mg.c b/mg.c
index f18a98a33a..c2d2186e96 100644
--- a/mg.c
+++ b/mg.c
@@ -2098,7 +2098,7 @@ Perl_magic_getpos(pTHX_ SV *sv, MAGIC *mg)
 
     if (found && found->mg_len != -1) {
 	    STRLEN i = found->mg_len;
-	    if (DO_UTF8(lsv))
+	    if (found->mg_flags & MGf_BYTES && DO_UTF8(lsv))
 		i = sv_pos_b2u_flags(lsv, i, SV_GMAGIC|SV_CONST_RETURN);
 	    sv_setuv(sv, i);
 	    return 0;
@@ -2149,12 +2149,8 @@ Perl_magic_setpos(pTHX_ SV *sv, MAGIC *mg)
     else if (pos > (SSize_t)len)
 	pos = len;
 
-    if (ulen) {
-	pos = sv_or_pv_pos_u2b(lsv, s, pos, 0);
-    }
-
     found->mg_len = pos;
-    found->mg_flags &= ~MGf_MINMATCH;
+    found->mg_flags &= ~(MGf_MINMATCH|MGf_BYTES);
 
     return 0;
 }