rb_provide_feature: Prevent $LOADED_FEATURES from being copied

[Bug #18599]

`vm->loaded_features` and `vm->loaded_features_snapshot` both share the
same root. When a feature is pushed into `loaded_features`, the sharing
is broken and `loaded_features` is copied.

So an application requiring 1000 files, will allocate 1000 arrays of
increasing size, which is very wasteful.

To avoid this, we first clear the snapshot, so that `loaded_features`
can directly be pushed into.

Co-Authored-By: Peter Zhu <peter.zhu@shopify.com>

Author: byroot

load.c

@@ -625,6 +625,10 @@ rb_provide_feature(rb_vm_t *vm, VALUE feature)
     rb_str_freeze(feature);
 
     get_loaded_features_index(vm);
+    // If loaded_features and loaded_features_snapshot share the same backing
+    // array, pushing into it would cause the whole array to be copied.
+    // To avoid this we first clear loaded_features_snapshot.
+    rb_ary_clear(vm->loaded_features_snapshot);
     rb_ary_push(features, rb_fstring(feature));
     features_index_add(vm, feature, INT2FIX(RARRAY_LEN(features)-1));
     reset_loaded_features_snapshot(vm);