check the IO object exists when writing to IO magic variables

pp_select() ensures that the GV in PL_defoutgv has an IO object
when it the default output is set, but can't prevent that GV
being cleared afterwards, resulting in a seg fault when the
variable is written.

To prevent this, check PL_defoutgv has an IO object before trying
to write to it.

Fixes #20733

1 files changed, 15 insertions, 1 deletions

diff --git a/t/io/defout.t b/t/io/defout.t
index aa40666162..2c8af5c80d 100644
--- a/t/io/defout.t
+++ b/t/io/defout.t
@@ -14,7 +14,7 @@ BEGIN {
 
 $|=0;   # test.pl makes it 1, and that conflicts with the below.
 
-plan tests => 16;
+plan tests => 22;
 
 
 my $stdout = *STDOUT;
@@ -48,6 +48,20 @@ $- = 1; pass '$- = 1';
 $% = 1; pass '$% = 1';
 $| = 1; pass '$| = 1';
 
+# test a NULLed GV
+my $t = tempfile;
+open FOO, ">", $t or die;
+select(FOO);
+my $io = *FOO{IO};
+*FOO = 0;
+$^ = 1; pass 'empty GV: $^ = 1';
+$~ = 1; pass 'empty GV: $~ = 1';
+$= = 1; pass 'empty GV: $= = 1';
+$- = 1; pass 'empty GV: $- = 1';
+$% = 1; pass 'empty GV: $% = 1';
+$| = 1; pass 'empty GV: $| = 1';
+close $io;
+
 # Switch to STDERR for this test, so we do not lose our test output
 my $stderr = *STDERR;
 select($stderr);