[ruby-core:102429] [Ruby master Bug#17618] Exceptions in Fiber Scheduler causes a segv

Issue #17618 has been reported by tenderlovemaking (Aaron Patterson).

----------------------------------------
Bug #17618: Exceptions in Fiber Scheduler causes a segv
https://bugs.ruby-lang.org/issues/17618

* Author: tenderlovemaking (Aaron Patterson)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.1.0dev (2021-02-09T13:22:37Z master e7a831de8e) [x86_64-darwin20]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------
If the fiber scheduler doesn't define an unblock function, Ruby will segv when threads are joined.

Here is an example program:

``` ruby
class Scheduler
  def block blocker, timeout = nil
  end

  def fiber &block
    fiber = Fiber.new blocking: false, &block
    fiber.resume
    fiber
  end
end


Fiber.set_scheduler Scheduler.new

Fiber.schedule do
  Thread.new { }.join
end
```

The backtrace looks like this:

```
(lldb) bt
* thread #3, name = 'test.rb:17', stop reason = EXC_BAD_ACCESS (code=1, address=0xb0)
    frame #0: 0x00000001000dc49a miniruby`rb_ec_tag_jump(ec=0x0000000100a2ec50, st=RUBY_TAG_RAISE) at eval_intern.h:185:20
    frame #1: 0x00000001000dbda7 miniruby`rb_longjmp(ec=0x0000000100a2ec50, tag=6, mesg=0x000000010101b3f8, cause=0x0000000000000008) at eval.c:699:5
    frame #2: 0x00000001000dbb9c miniruby`rb_exc_raise(mesg=0x000000010101b3f8) at eval.c:717:5
    frame #3: 0x000000010037446c miniruby`raise_method_missing(ec=0x0000000100a2ec50, argc=3, argv=0x000070000e6d39e0, obj=0x000000010101b8d0, last_call_status=MISSING_MISSING) at vm_eval.c:955:2
    frame #4: 0x0000000100374288 miniruby`method_missing(ec=0x0000000100a2ec50, obj=0x000000010101b8d0, id=24721, argc=3, argv=0x000070000e6d39e0, call_status=MISSING_NOENTRY, kw_splat=0) at vm_eval.c:1002:5
    frame #5: 0x0000000100385fdd miniruby`rb_call0(ec=0x0000000100a2ec50, recv=0x000000010101b8d0, mid=24721, argc=2, argv=0x000070000e6d3be0, call_scope=CALL_FCALL, self=0x0000000000000008) at vm_eval.c:515:20
    frame #6: 0x0000000100358a02 miniruby`rb_funcallv_scope(recv=0x000000010101b8d0, mid=24721, argc=2, argv=0x000070000e6d3be0, scope=CALL_FCALL) at vm_eval.c:1021:16
    frame #7: 0x0000000100354c71 miniruby`rb_funcallv(recv=0x000000010101b8d0, mid=24721, argc=2, argv=0x000070000e6d3be0) at vm_eval.c:1038:12
    frame #8: 0x000000010035921d miniruby`rb_funcall(recv=0x000000010101b8d0, mid=24721, n=2) at vm_eval.c:1109:12
  * frame #9: 0x0000000100291d23 miniruby`rb_fiber_scheduler_unblock(scheduler=0x000000010101b8d0, blocker=0x000000010107bd70, fiber=0x000000010101b768) at scheduler.c:142:12
    frame #10: 0x00000001002f1445 miniruby`rb_threadptr_join_list_wakeup(thread=0x0000000100a2e9b0) at thread.c:555:13
    frame #11: 0x00000001002f0fd5 miniruby`thread_start_func_2(th=0x0000000100a2e9b0, stack_start=0x000070000e7d3f70) at thread.c:891:9
    frame #12: 0x00000001002f07b5 miniruby`thread_start_func_1(th_ptr=0x0000000100a2e9b0) at thread_pthread.c:1033:9
    frame #13: 0x00007fff2043a950 libsystem_pthread.dylib`_pthread_start + 224
    frame #14: 0x00007fff2043647b libsystem_pthread.dylib`thread_start + 15
```

It seems like the ec is missing a tag:

```
(lldb) f 0
frame #0: 0x00000001000dc49a miniruby`rb_ec_tag_jump(ec=0x0000000100a2ec50, st=RUBY_TAG_RAISE) at eval_intern.h:185:20
   182 	static inline void
   183 	rb_ec_tag_jump(const rb_execution_context_t *ec, enum ruby_tag_type st)
   184 	{
-> 185 	    ec->tag->state = st;
   186 	    ruby_longjmp(ec->tag->buf, 1);
   187 	}
   188 	
(lldb) p ec->tag
(rb_vm_tag *const) $1 = 0x0000000000000000
(lldb) 
```

I tried popping the tag later in `thread_start_func_2`, but it caused the process to go in to an infinite loop.




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