Call FETCH once on handle passed as 3rd arg to open()

When open() has three arguments and the second ends with & the third
argument is treated as a handle.

In some cases get-magic was being skipped; in others, it was being
called three times.

This commit fixes it by modifying sv_2io.

In 5.8.x (before commit 7a5fd60d4), sv_2io did not call get-magic at
all except when croaking ("Bad filehandle: %"SVf).  In 5.10.0 (after
commit 7a5fd60d4), it started calling get-magic only if the sv was
neither a glob, a reference, or undef.  So it has never been reliable
in its invocation of get-magic.

sv_2io now consistently skips get-magic on the sv passed in directly
to it (open(), the only caller in the core, has already called get-
magic before passing it in).  It now calls get-magic on SvRV(sv) if
what is passed in is a reference, so open(fh, ">&", \$tied) will work.

Interestingly, open supports multiple levels of references:

\\\\\\\\\\\\open+f,">&",\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\
 \\\\\\\\\\\*STDOUT;print{f}"Just another Perl hacker,\n",.\\\\\\\y\\\

1 files changed, 12 insertions, 4 deletions

diff --git a/sv.c b/sv.c
index d9153b6a36..03aff4eb79 100644
--- a/sv.c
+++ b/sv.c
@@ -8842,15 +8842,23 @@ Perl_sv_2io(pTHX_ SV *const sv)
     default:
 	if (!SvOK(sv))
 	    Perl_croak(aTHX_ PL_no_usym, "filehandle");
-	if (SvROK(sv))
+	if (SvROK(sv)) {
+	    SvGETMAGIC(SvRV(sv));
 	    return sv_2io(SvRV(sv));
-	gv = gv_fetchsv(sv, 0, SVt_PVIO);
+	}
+	gv = gv_fetchsv_nomg(sv, 0, SVt_PVIO);
 	if (gv)
 	    io = GvIO(gv);
 	else
 	    io = 0;
-	if (!io)
-	    Perl_croak(aTHX_ "Bad filehandle: %"SVf, SVfARG(sv));
+	if (!io) {
+	    SV *newsv = sv;
+	    if (SvGMAGICAL(sv)) {
+		newsv = sv_newmortal();
+		sv_setsv_nomg(newsv, sv);
+	    }
+	    Perl_croak(aTHX_ "Bad filehandle: %"SVf, SVfARG(newsv));
+	}
 	break;
     }
     return io;