[ruby-core:102189] [Ruby master Bug#17568] Thread.handle_interrupt is per-Thread but should probably be per-Fiber

Issue #17568 has been reported by Eregon (Benoit Daloze).

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Bug #17568: Thread.handle_interrupt is per-Thread but should probably be per-Fiber
https://bugs.ruby-lang.org/issues/17568

* Author: Eregon (Benoit Daloze)
* Status: Open
* Priority: Normal
* ruby -v: ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [x86_64-linux]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
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Thread.handle_interrupt and the pending interrupts is currently kept as state on the thread (rb_thread_t).
However, that seems potentially confusing when Fibers are used.

For instance, this innocent-looking Fiber will forever disable interrupts in the main thread:
```ruby
main_thread = Thread.current

never_die = Fiber.new do
  Thread.handle_interrupt(RuntimeError => :never) do
    Fiber.yield
  end
end

begin
  Thread.new { main_thread.raise "interrupt1" }.join
rescue => e
  puts "raised: #{e}"
end

never_die.resume

Thread.new { main_thread.raise "interrupt2" }.join

Thread.new { main_thread.raise "interrupt3" }.join

p :after
```

Output is:
```
raised: interrupt1
:after
```

(I noticed these weird semantics when implementing `Thread.handle_interrupt` on TruffleRuby, and got a fairly subtle bug due to multiple Fibers of the same Thread queuing the same interrupt multiple times in the Thread's interrupt queue).



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