fix B::walkoptree_debug()

It turns out that this method has been mostly broken since its
introduction in 1998. It will normally successfully turn debugging on
with a 'true' argument but will fail to disable again with a 'false'
argument.

This is for two reasons. First the XS code only ever sets the internal
debugging flag, never disables it, and second, it was checking the
truthfulness of the arg one too high on the stack and thus was actually
checking the CV which had just been popped off the stack, which happened
to be true.

2 files changed, 7 insertions, 3 deletions

diff --git a/ext/B/B.xs b/ext/B/B.xs
index c317ed43f2..7c60e4ad10 100644
--- a/ext/B/B.xs
+++ b/ext/B/B.xs
@@ -703,8 +703,8 @@ walkoptree_debug(...)
     CODE:
 	dMY_CXT;
 	RETVAL = walkoptree_debug;
-	if (items > 0 && SvTRUE(ST(1)))
-	    walkoptree_debug = 1;
+	if (items > 0)
+	    walkoptree_debug = SvTRUE(ST(0));
     OUTPUT:
 	RETVAL
 
diff --git a/ext/B/t/walkoptree.t b/ext/B/t/walkoptree.t
index e7a39b1624..29aaf47409 100644
--- a/ext/B/t/walkoptree.t
+++ b/ext/B/t/walkoptree.t
@@ -48,7 +48,11 @@ foreach (qw(substcont split leavesub)) {
 is_deeply ([keys %debug], [], 'walkoptree_debug was not called');
 
 B::walkoptree_debug(2);
-is (B::walkoptree_debug, 1, 'walkoptree_debug() is 1');
+is (B::walkoptree_debug(), 1, 'walkoptree_debug() is 1');
+B::walkoptree_debug(0);
+is (B::walkoptree_debug(), 0, 'walkoptree_debug() is 0');
+B::walkoptree_debug(1);
+is (B::walkoptree_debug(), 1, 'walkoptree_debug() is 1 again');
 %seen = ();
 
 B::walkoptree(B::svref_2object($victim)->ROOT, "pie");