Fix GH Issue #19472: read warnings from open($fh,">",\(my $x))

We produce all kinds of warnings if someone opens a scalar reference
that is undef. Prior to this we handled write operations ok, at
least in blead, but read operations would produce a plethora of
warnings. To me this analogous to treating an undef var as hash
and trying to read from it, we autovivify the undef var to be
a hash. So in this case we should just "autovivify" the referenced
scalar to be an empty string.

Eg. before this patch:

./perl -Ilib -wle'open my $fh,"+>", \(my $v); my @x=<$fh>; print 0+@x'
Use of uninitialized value $fh in <HANDLE> at -e line 1.
Use of uninitialized value $fh in <HANDLE> at -e line 1.
Use of uninitialized value $fh in <HANDLE> at -e line 1.
Use of uninitialized value $fh in <HANDLE> at -e line 1.
Use of uninitialized value $fh in <HANDLE> at -e line 1.
Use of uninitialized value $fh in <HANDLE> at -e line 1.
Use of uninitialized value $fh in <HANDLE> at -e line 1.
0

After it:
./perl -Ilib -wle'open my $fh,"+>", \(my $v); my @x=<$fh>; print 0+@x'
0

3 files changed, 43 insertions, 3 deletions

diff --git a/doio.c b/doio.c
index 87790da137..4009545ffc 100644
--- a/doio.c
+++ b/doio.c
@@ -639,6 +639,16 @@ Perl_do_open6(pTHX_ GV *gv, const char *oname, STRLEN len,
                 goto say_false;
             }
 #endif /* USE_STDIO */
+            if (SvROK(*svp) && !sv_isobject(*svp)) {
+                /* if they pass in a reference and its not an object
+                 * and the reference is to undef, "autovivify" it to
+                 * the empty string. See GH Issue #19472
+                 */
+                SV *sv= SvRV(*svp);
+                if (!SvOK(sv) && !SvREADONLY(sv))
+                    sv_setpvs(MUTABLE_SV(sv),"");
+            }
+
             p = (SvOK(*svp) || SvGMAGICAL(*svp)) ? SvPV(*svp, nlen) : NULL;
 
             if (p && !IS_SAFE_PATHNAME(p, nlen, "open")) {
diff --git a/pod/perlfunc.pod b/pod/perlfunc.pod
index 26d59a580e..1950e95cd0 100644
--- a/pod/perlfunc.pod
+++ b/pod/perlfunc.pod
@@ -4657,8 +4657,8 @@ L<C<seek>|/seek FILEHANDLE,POSITION,WHENCE> to do the reading.
 =item Opening a filehandle into an in-memory scalar
 
 You can open filehandles directly to Perl scalars instead of a file or
-other resource external to the program. To do so, provide a reference to
-that scalar as the third argument to C<open>, like so:
+other resource external to the program. To do so, provide an unblessed
+reference to that scalar as the third argument to C<open>, like so:
 
  open(my $memory, ">", \$var)
      or die "Can't open memory file: $!";
@@ -4674,6 +4674,14 @@ The scalars for in-memory files are treated as octet strings: unless
 the file is being opened with truncation the scalar may not contain
 any code points over 0xFF.
 
+Be aware that attempting to open a reference to a readonly scalar will
+cause a warning and the open to fail.
+
+Prior to Perl version 5.36.0, passing in a reference to an undef scalar
+could cause strange warnings. As of Perl version 5.36.0, provided the
+reference is unblessed, the scalar will be "autovivified" to be an empty
+string, even for read mode open operations.
+
 Opening in-memory files I<can> fail for a variety of reasons.  As with
 any other C<open>, check the return value for success.
 
diff --git a/t/io/open.t b/t/io/open.t
index 7fa492342d..86ca8a8498 100644
--- a/t/io/open.t
+++ b/t/io/open.t
@@ -10,7 +10,7 @@ $|  = 1;
 use warnings;
 use Config;
 
-plan tests => 188;
+plan tests => 193;
 
 sub ok_cloexec {
     SKIP: {
@@ -410,6 +410,28 @@ SKIP: {
         or unlink \*STDOUT;
 }
 
+# [GH Issue #19472] Opening a reference to an undef scalar
+SKIP: {
+    skip_if_miniperl("no dynamic loading on miniperl, so can't load PerlIO::scalar", 5);
+    use strict;
+    my $var;
+    open my $fh, "+>", \$var;
+    is defined($var) ? "defined" : "undef", "defined",
+        '[GH Issue #19472]: open $fh, ">", \$undef_var leaves var defined';
+    is $var, "",
+        '[GH Issue #19472]: open $fh, ">", \$undef_var leaves var empty string';
+    my $warn_count= 0;
+    my $warn_text= "";
+    local $SIG{__WARN__}= sub { $warn_count++; $warn_text .= shift; };
+    my @x= <$fh>;
+    is $warn_count, 0, '[GH Issue #19472]: read after open $fh, ">", ' .
+                       '\$undef_var produces 0 warnings';
+    is $warn_text, "", '[GH Issue #19472]: read after open $fh, ">", ' .
+                       '\$undef_var produces no warning text';
+    is 0+@x, 0,        '[GH Issue #19472]: <$fh> after open $fh, ">", ' .
+                       '\$undef_var returns nothing';
+}
+
 # check that we can call methods on filehandles auto-magically
 # and have IO::File loaded for us
 SKIP: {