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, 22 insertions, 1 deletions

diff --git a/inline.h b/inline.h
index 48cc187e4a..b33cd3fd6f 100644
--- a/inline.h
+++ b/inline.h
@@ -62,6 +62,26 @@ S_strip_spaces(pTHX_ const char * orig, STRLEN * const len)
 }
 #endif
 
+/* ------------------------------- mg.h ------------------------------- */
+
+#if defined(PERL_CORE) || defined(PERL_EXT)
+/* assumes get-magic and stringification have already occurred */
+PERL_STATIC_INLINE STRLEN
+S_MgBYTEPOS(pTHX_ MAGIC *mg, SV *sv, const char *s, STRLEN len)
+{
+    assert(mg->mg_type == PERL_MAGIC_regex_global);
+    assert(mg->mg_len != -1);
+    if (mg->mg_flags & MGf_BYTES || !DO_UTF8(sv))
+	return (STRLEN)mg->mg_len;
+    else {
+	const STRLEN pos = (STRLEN)mg->mg_len;
+	/* Without this check, we may read past the end of the buffer: */
+	if (pos > sv_or_pv_len_utf8(sv, s, len)) return len+1;
+	return sv_or_pv_pos_u2b(sv, s, pos, NULL);
+    }
+}
+#endif
+
 /* ----------------------------- regexp.h ----------------------------- */
 
 PERL_STATIC_INLINE struct regexp *
@@ -151,10 +171,11 @@ S_SvPADSTALE_off(SV *sv)
     assert(SvFLAGS(sv) & SVs_PADMY);
     return SvFLAGS(sv) &= ~SVs_PADSTALE;
 }
-#ifdef PERL_CORE
+#if defined(PERL_CORE) || defined (PERL_EXT)
 PERL_STATIC_INLINE STRLEN
 S_sv_or_pv_pos_u2b(pTHX_ SV *sv, const char *pv, STRLEN pos, STRLEN *lenp)
 {
+    PERL_ARGS_ASSERT_SV_OR_PV_POS_U2B;
     if (SvGAMAGIC(sv)) {
 	U8 *hopped = utf8_hop((U8 *)pv, pos);
 	if (lenp) *lenp = (STRLEN)(utf8_hop(hopped, *lenp) - hopped);