variable.c: handle cleared fields_obj in genfields cache

[Bug #21547]

Followup: https://github.com/ruby/ruby/pull/14201

When adding an instance variable and the IMEMO/fields need to be
larger, we allocate a new one and clear the old one.

Since the old one may still be in other ec's cache, on a hit we must
check the IMEMO/fields isn't a stale one.

1 files changed, 3 insertions, 0 deletions

diff --git a/imemo.c b/imemo.c
index fde5b15ad6..2fde22a3db 100644
--- a/imemo.c
+++ b/imemo.c
@@ -3,6 +3,7 @@
 #include "id_table.h"
 #include "internal.h"
 #include "internal/imemo.h"
+#include "internal/object.h"
 #include "internal/st.h"
 #include "vm_callinfo.h"
 
@@ -208,6 +209,8 @@ rb_imemo_fields_clear(VALUE fields_obj)
     else {
         RBASIC_SET_SHAPE_ID(fields_obj, ROOT_SHAPE_ID);
     }
+    // Invalidate the ec->gen_fields_cache.
+    RBASIC_CLEAR_CLASS(fields_obj);
 }
 
 /* =========================================================================