Add tree walking debug function

test/test_refcount.c

@@ -46,6 +46,16 @@ void reref_destructor(void *a, void *ctx_raw) {
     refcount_context_ref(ctx, a);
 }
 
+UNUSED bool print_tree_walk_callback(void *obj, const RefcountList *trail,
+                                     void *ignored) {
+    size_t len = refcount_list_length(trail);
+    for (size_t i = 0; i < len; ++i) {
+        fprintf(stderr, " ");
+    }
+    fprintf(stderr, "- %p\n", obj);
+    return false;
+}
+
 int main(int argc, const char **argv) {
     struct ContextAndFlag ctx_and_flag = {.should_be_doing_gc = false};
 
@@ -153,6 +163,8 @@ int main(int argc, const char **argv) {
     assert(refcount_context_garbage_collect(c) == 0);
     assert(refcount_context_unref(c, a_with_destructor));
     assert(!refcount_context_is_doing_gc(c));
+    /* refcount_debug_context_walk_tree(c, a_with_destructor, */
+    /*                                  print_tree_walk_callback, NULL); */
     ctx_and_flag.should_be_doing_gc = true;
     assert(refcount_context_garbage_collect(c) == 26);
     ctx_and_flag.should_be_doing_gc = false;