Revert "open: only treat literal undef as special"

This reverts commit 471c834 but also
adds a test which the reverted code would not pass and a perldelta item.

Author: ap

doio.c

@@ -845,9 +845,6 @@ Perl_do_open6(pTHX_ GV *gv, const char *oname, STRLEN len,
                 }
                 else {
                     if (num_svs) {
-                        if (*svp == &PL_sv_undef && PL_op && !(PL_op->op_flags & OPf_SPECIAL)) {
-                            *svp = sv_newmortal();
-                        }
                         fp = PerlIO_openn(aTHX_ type,mode,-1,0,0,NULL,num_svs,svp);
                     }
                     else {
@@ -883,9 +880,6 @@ Perl_do_open6(pTHX_ GV *gv, const char *oname, STRLEN len,
             }
             else {
                 if (num_svs) {
-                    if (*svp == &PL_sv_undef && PL_op && !(PL_op->op_flags & OPf_SPECIAL)) {
-                        *svp = sv_newmortal();
-                    }
                     fp = PerlIO_openn(aTHX_ type,mode,-1,0,0,NULL,num_svs,svp);
                 }
                 else {

op.c

@@ -13901,19 +13901,6 @@ Perl_ck_open(pTHX_ OP *o)
              last->op_private &= ~OPpCONST_STRICT;
          }
     }
-    {
-        /* mark as special if filename is a literal undef */
-        const OP *arg = cLISTOPx(o)->op_first; /* pushmark */
-        if (
-            (arg = OpSIBLING(arg))    /* handle */
-            && (arg = OpSIBLING(arg)) /* mode */
-            && (arg = OpSIBLING(arg)) /* filename */
-        ) {
-            if (arg->op_type == OP_UNDEF && !(arg->op_flags & OPf_KIDS)) {
-                o->op_flags |= OPf_SPECIAL;
-            }
-        }
-    }
     return ck_fun(o);
 }
 
@@ -16113,19 +16100,7 @@ Perl_coresub_op(pTHX_ SV * const coreargssv, const int code,
             }
             return o;
         default:
-            /* For open(), OPf_SPECIAL indicates we saw a literal undef as the
-             * filename argument and thus a &PL_sv_undef argument at runtime
-             * should trigger the creation of a temp file. This is to
-             * distinguish between open(..., ..., undef) and
-             * open(..., ..., delete $hash{key}), which also passes
-             * &PL_sv_undef if $hash{key} does not exist, but which should not
-             * create a temporary file.
-             * In case of a runtime call via &CORE::open(...) or
-             * my $f = \&CORE::open; $f->(...), we cannot distinguish between
-             * those cases. Therefore we always set the flag to interpret
-             * &PL_sv_undef as a temp file.
-             */
-            o = op_convert_list(opnum,OPf_SPECIAL*(opnum == OP_GLOB || opnum == OP_OPEN),argop);
+            o = op_convert_list(opnum,OPf_SPECIAL*(opnum == OP_GLOB),argop);
             if (is_handle_constructor(o, 2))
                 argop->op_private |= OPpCOREARGS_DEREF2;
             if (opnum == OP_SUBSTR) {

op.h

@@ -164,8 +164,6 @@ Deprecated.  Use C<GIMME_V> instead.
                                 /*  On OP_RETURN, module_true is in effect */
                                 /*  On OP_NEXT/OP_LAST/OP_REDO, there is no
                                  *  loop label */
-                                /*  On OP_OPEN, create a temporary file if the
-                                 *  filename argument is &PL_sv_undef */
 /* There is no room in op_flags for this one, so it has its own bit-
    field member (op_folded) instead.  The flag is only used to tell
    op_convert_list to set op_folded.  */

pod/perldelta.pod

@@ -395,6 +395,28 @@ reading and that error is C<EAGAIN> or C<EWOULDBLOCK>.  This allows
 old code that depended on C<readline> to clear all errors to ignore
 these relatively harmless errors.  [GH #22883]
 
+=item *
+
+L<C<open>|perlfunc/open> automatically creates an anonymous temporary file
+when passed C<undef> as a filename:
+
+    open(my $fh, "+>", undef) or die ...
+
+This is supposed to work only when the undefined value is the one returned by
+the C<undef> function.
+
+In perls before 5.41.3, this caused a problem due to the fact that the same
+undefined value can be generated by lookups of non-existent hash keys or array
+elements, which can lead to bugs in user-level code (reported as [GH #22385]).
+
+In 5.41.3, additional checks based on the syntax tree of the call site were
+added, which fixed this issue for some number of common cases, though not all
+of them, at the cost of breaking the ability of APIs that wrap C<open> to
+expose its anonymous file mode. A notable example of such an API is autodie.
+
+This release reverts to the old problem in preference to the new one for the
+time being.
+
 =back
 
 =head1 Known Problems

t/io/perlio_open.t

@@ -11,7 +11,7 @@ BEGIN {
 use strict;
 use warnings;
 
-plan tests => 16;
+plan tests => 11;
 
 use Fcntl qw(:seek);
 
@@ -44,26 +44,8 @@ SKIP:
 }
 
 {
-    my $fn = \&CORE::open;
-    ok($fn->(my $fh, "+>", undef), "(\\&CORE::open)->(my \$fh, '+>', undef)");
-    print $fh "the right write stuff";
-    ok(seek($fh, 0, SEEK_SET), "seek to zero");
-    my $data = <$fh>;
-    is($data, "the right write stuff", "found the right stuff");
-}
-
-{
-    # GH #22385
-    my %hash;
-    my $warnings = '';
-    local $SIG{__WARN__} = sub { $warnings .= $_[0] };
-    my $r = open my $fh, "+>", delete $hash{nosuchkey};
-    my $enoent = $!{ENOENT};
-    is $r, undef, "open(my \$fh, '+>', delete \$hash{nosuchkey}) fails";
-    SKIP: {
-        skip "This system doesn't understand ENOENT", 1
-            unless exists $!{ENOENT};
-        ok $enoent, "\$! is ENOENT";
-    }
-    like $warnings, qr/^Use of uninitialized value in open/, "it warns about undef";
+    # minimal-reproduction moral equivalent of the autodie wrapper for open()
+    # because APIs that wrap open() should be able to expose this behaviour
+    sub wrapped_open (*;$@) { open $_[0], $_[1], $_[2] }
+    ok((wrapped_open my $fh, "+>", undef), "wrapped_open my \$fh, '+>', undef");
 }