Segfault when exiting due to `Fiddle::MemoryView`

The following script causes a segfault when exiting:

$workaround = false

require 'fiddle'

Fiddle::MemoryView.new(Fiddle::Pointer["foo"])

GC.start if $workaround

A backtrace from GDB:

Thread 1 received signal SIGSEGV, Segmentation fault.
rb_class_get_superclass (klass=klass@entry=0) at ../object.c:2252
2252        return RCLASS(klass)->super;
(gdb) bt
#0  rb_class_get_superclass (klass=klass@entry=0) at ../object.c:2252
#1  0x0000000589c03ae3 in lookup_memory_view_entry (klass=0) at ../memory_view.c:784
#2  0x0000000589c04921 in rb_memory_view_release (view=view@entry=0x800157770) at ../memory_view.c:836
#3  0x0000000596513c13 in fiddle_memview_free (ptr=0x800157770) at ../../../ext/fiddle/memory_view.c:41
#4  0x0000000589bd9226 in run_final (zombie=123145300985120, objspace=0x800052bc0) at ../gc.c:4040
#5  finalize_list (objspace=objspace@entry=0x800052bc0, zombie=123145300985120) at ../gc.c:4059
#6  0x0000000589bdd6e1 in rb_objspace_call_finalizer (objspace=0x800052bc0) at ../gc.c:4222
#7  0x0000000589bc5f37 in rb_ec_finalize (ec=ec@entry=0x800053520) at ../eval.c:187
#8  0x0000000589bc8e25 in rb_ec_cleanup (ec=ec@entry=0x800053520, ex=<optimized out>) at ../eval.c:298
#9  0x0000000589bc8fa8 in ruby_run_node (n=0x6fffffed3068) at ../eval.c:379
#10 0x00000001004017ad in main (argc=<optimized out>, argv=<optimized out>) at ../main.c:47

Invoking a GC before exiting seems to be a workaround.

ruby -v: ruby 3.1.0dev (2021-05-21T09:28:24Z master 50a534a152) [x86_64-linux]

まず、提供された Ruby スクリプトで終了時のクラッシュを再現し、GDB 下で GC.start の回避策を使った場合と比較します。memory_view.c と ext/fiddle/memory_view.c を通るファイナライズの経路を追跡します。スクリプトが segmentation fault なしで終了し、回帰が適切なテストでカバーされれば完了です。