Concurrent set fix when encountering garbage obj in find_or_insert

When CASing the garbage key to EMPTY, if we succeed we should decrease
set->size because we're trying to re-insert into the same slot right after.
The insertion increases set->size if it succeeds. This issue can lead to
set->size being overcounted, which can lead to more table resizes.

Author: luke-gruber

concurrent_set.c

@@ -399,7 +399,12 @@ rb_concurrent_set_find_or_insert(VALUE *set_obj_ptr, VALUE key, void *data)
                 if (continuation) {
                     goto probe_next;
                 }
-                rbimpl_atomic_value_cas(&entry->key, curr_key, CONCURRENT_SET_EMPTY, RBIMPL_ATOMIC_RELEASE, RBIMPL_ATOMIC_RELAXED);
+                {
+                    VALUE prev = rbimpl_atomic_value_cas(&entry->key, curr_key, CONCURRENT_SET_EMPTY, RBIMPL_ATOMIC_RELEASE, RBIMPL_ATOMIC_RELAXED);
+                    if (prev == curr_key) {
+                        rbimpl_atomic_sub(&set->size, 1, RBIMPL_ATOMIC_RELAXED);
+                    }
+                }
                 continue;
             }