[ruby-core:90900] [Ruby trunk Bug#15509] TCPServer#accept isn't interrupted

Issue #15509 has been reported by larskanis (Lars Kanis).

----------------------------------------
Bug #15509: TCPServer#accept isn't interrupted
https://bugs.ruby-lang.org/issues/15509

* Author: larskanis (Lars Kanis)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.6.0p0 (2018-12-25 revision 66547) [x86_64-linux]
* Backport: 2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: UNKNOWN
----------------------------------------
This is another issue I noticed with [Eventbox](https://github.com/larskanis/eventbox):

This script tries to stop `TCPServer#accept` with `Thread#raise`:
```ruby
require "socket"
Thread.abort_on_exception = true

class Stop < RuntimeError
end

serv = TCPServer.new("localhost", 12345)

th = Thread.handle_interrupt(Exception => :never) do
  Thread.new do
    begin
      Thread.handle_interrupt(Stop => :on_blocking) do
#         sleep      # sleep is aborted
        serv.accept  # but accept isn't
      end
    rescue Stop
    end
  end
end

th.raise Stop
th.join
serv.close
```

The program freezes and if stopped per CTRL-C the output looks similar to:
```sh
$ ruby -d --disable-gems no-interrupt-while-accept.rb
^CException `Interrupt' at no-interrupt-while-accept.rb:23 -
Exception `Stop' at no-interrupt-while-accept.rb:15 - Stop
Traceback (most recent call last):
	1: from no-interrupt-while-accept.rb:23:in `<main>'
no-interrupt-while-accept.rb:23:in `join': Interrupt
```

The debug output shows, that the `Stop` exception is delivered after the `Interrupt` exception has been delivered to the main thread. However it should be delivered immediately, since it's a blocking operation. In contrast `sleep` is aborted as expected.

This issue is present on all older MRI versions. However it doesn't appear on JRuby-9.2.5.0.




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