[perl #111462] Don’t leak eval "" op tree when croaking

This patch only fixes the problem for croaks that occur in the peep-
hole optimiser or in Perl_finalize_optree.

It does this by doing SAVEFREEOP first and then restoring the
savestack index to its previous value afterwards (to void the effect
of SAVEFREEOP).

A more correct fix might be to do op_free in die_unwind before
POPEVAL, but I would have to do a lot more digging through the code
to tell whether that is safe.  I don’t feel comfortable with doing
that for 5.16.

This leak causes this warning on non-threaded debugging builds:

$ PERL_DESTRUCT_LEVEL=1 ./perl -Ilib -e 'BEGIN { $^H{foo} = bar } our %FIELDS; my main $x; eval q[$x->{foo}]'
Unbalanced string table refcount: (1) for "foo" during global destruction.

This problem does not affect the main program, because perl_destruct
frees PL_main_root.  It does not affect subroutines, as the op tree is
attached to the CV first, so freeing the CV frees the op tree.

1 files changed, 6 insertions, 1 deletions

diff --git a/op.c b/op.c
index 2ffe10fa23..1e6addb023 100644
--- a/op.c
+++ b/op.c
@@ -2768,6 +2768,7 @@ Perl_newPROG(pTHX_ OP *o)
 
     if (PL_in_eval) {
 	PERL_CONTEXT *cx;
+	I32 i;
 	if (PL_eval_root)
 		return;
 	PL_eval_root = newUNOP(OP_LEAVEEVAL,
@@ -2791,9 +2792,13 @@ Perl_newPROG(pTHX_ OP *o)
 	PL_eval_root->op_private |= OPpREFCOUNTED;
 	OpREFCNT_set(PL_eval_root, 1);
 	PL_eval_root->op_next = 0;
+	i = PL_savestack_ix;
+	SAVEFREEOP(o);
+	ENTER;
 	CALL_PEEP(PL_eval_start);
 	finalize_optree(PL_eval_root);
-
+	LEAVE;
+	PL_savestack_ix = i;
     }
     else {
 	if (o->op_type == OP_STUB) {