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, 2 deletions

diff --git a/pp_hot.c b/pp_hot.c
index 6068d21973..afecce8e11 100644
--- a/pp_hot.c
+++ b/pp_hot.c
@@ -1392,7 +1392,7 @@ PP(pp_match)
     if (global) {
         mg = mg_find_mglob(TARG);
         if (mg && mg->mg_len >= 0) {
-            curpos = mg->mg_len;
+            curpos = MgBYTEPOS(mg, TARG, truebase, len);
             /* last time pos() was set, it was zero-length match */
             if (mg->mg_flags & MGf_MINMATCH)
                 had_zerolen = 1;
@@ -1448,7 +1448,7 @@ PP(pp_match)
     if (global && (gimme != G_ARRAY || (dynpm->op_pmflags & PMf_CONTINUE))) {
         if (!mg)
             mg = sv_magicext_mglob(TARG);
-        mg->mg_len = RX_OFFS(rx)[0].end;
+        MgBYTEPOS_set(mg, TARG, truebase, RX_OFFS(rx)[0].end);
         if (RX_ZERO_LEN(rx))
             mg->mg_flags |= MGf_MINMATCH;
         else