Properly handle UTF8-flagged strings when assigning to $0.

Issue #19331: Use of SvPV_const and SvPV_force in S_set_dollarzero()
wrote the PV internals directly to argv, which causes an improper UTF-8
encode if the SV is UTF8-flagged/upgraded.

This fixes that doing a downgrade prior to those SvPV* calls. If the
string contains wide characters (and thus cannot be downgraded) a
warning is thrown; this mirrors preexisting behavior with %ENV, print,
and other output channels that convert Perl SVs to bytes.

The relevant documentation is also updated.

1 files changed, 10 insertions, 0 deletions

diff --git a/mg.c b/mg.c
index 10813a7316..5f3eeae4fe 100644
--- a/mg.c
+++ b/mg.c
@@ -3367,6 +3367,16 @@ Perl_magic_set(pTHX_ SV *sv, MAGIC *mg)
         else sv_setiv(mg->mg_obj, (IV)PerlProc_getpid());
         break;
     case '0':
+        if (!sv_utf8_downgrade(sv, /* fail_ok */ TRUE)) {
+
+            /* Since we are going to set the string's UTF8-encoded form
+               as the process name we should update $0 itself to contain
+               that same (UTF8-encoded) value. */
+            sv_utf8_encode(GvSV(mg->mg_obj));
+
+            Perl_ck_warner_d(aTHX_ packWARN(WARN_UTF8), "Wide character in %s", "$0");
+        }
+
         LOCK_DOLLARZERO_MUTEX;
         S_set_dollarzero(aTHX_ sv);
         UNLOCK_DOLLARZERO_MUTEX;