Handle non-PV $_ in @INC filters

@INC filters (code refs returned by code refs in @INC) are given the
current line of code in $_ and can modify it.  The C code that invokes
the Perl filter is in pp_ctl.c:S_run_user_filter.  It was not taking
into account that $_ might not have a PV pointer when it is returned,
and so this could result in crashes or assertion failures.

This commit forces the scalar to be a string before returning it to
the lexer, unless it is undef.  If we force it to be a string when it
is undef, then existing tests start producing uninitialized warnings.

The logic is still faulty in places.  Subsequent commits will
address that.

1 files changed, 1 insertions, 0 deletions

diff --git a/pp_ctl.c b/pp_ctl.c
index 85149fe2f3..aa11d58872 100644
--- a/pp_ctl.c
+++ b/pp_ctl.c
@@ -5506,6 +5506,7 @@ S_run_user_filter(pTHX_ int idx, SV *buf_sv, int maxlen)
         (SvOK(upstream) || SvGMAGICAL(upstream))) {
 	sv_catsv(buf_sv, upstream);
     }
+    else if (SvOK(upstream)) (void)SvPV_force_nolen(buf_sv);
 
     if (status <= 0) {
 	IoLINES(datasv) = 0;