+open my $tmp, '+>>', undef or BAIL_OUT $!;
+$tmp->autoflush(1);
+$od = PublicInbox::OnDestroy->new(1, sub { print $tmp "$$ DESTROY\n" });
+undef $od;
+is(-s $tmp, 0, '$tmp is empty on pid mismatch');
+$od = PublicInbox::OnDestroy->new($$, sub { $tmp = $$ });
+undef $od;
+is($tmp, $$, '$tmp set to $$ by callback');
+
+$od = PublicInbox::OnDestroy->new($$, sub { $tmp = 'foo' });
+$od->cancel;
+$od = undef;
+isnt($tmp, 'foo', '->cancel');
+