[perl #108994] Stop SvPVutf8 from coercing SVs

In shouldn’t destroy globs or references passed to it, or try to
coerce them if they are read-only or incoercible.

I added tests for SvPVbyte at the same time, even though it was not
exhibiting the same problems, as sv_utf8_downgrade doesn’t try to
coerce anything.  (SvPVbyte has its own set of bugs, which I hope to
fix in fifthcoming commits.)

1 files changed, 3 insertions, 1 deletions

diff --git a/sv.c b/sv.c
index be5aec808c..4b37aabbff 100644
--- a/sv.c
+++ b/sv.c
@@ -3050,10 +3050,12 @@ Usually accessed via the C<SvPVutf8> macro.
 */
 
 char *
-Perl_sv_2pvutf8(pTHX_ register SV *const sv, STRLEN *const lp)
+Perl_sv_2pvutf8(pTHX_ register SV *sv, STRLEN *const lp)
 {
     PERL_ARGS_ASSERT_SV_2PVUTF8;
 
+    if ((SvTHINKFIRST(sv) && !SvIsCOW(sv)) || isGV_with_GP(sv))
+	sv = sv_mortalcopy(sv);
     sv_utf8_upgrade(sv);
     return lp ? SvPV(sv,*lp) : SvPV_nolen(sv);
 }