Issue #15992 has been reported by naruse (Yui NARUSE).

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Bug #15992: An exception breaks monitor state and cause deadlock
https://bugs.ruby-lang.org/issues/15992

* Author: naruse (Yui NARUSE)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 
* Backport: 2.5: REQUIRED, 2.6: REQUIRED
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lib/monitor.rb provides Monitor.
But its state handling is weak for interrupts caused by Thread.kill for example timeout libraries.

Timeout exception may happen everywhere. If it raised when the thread is executing

```ruby
  def mon_exit
    mon_check_owner
    @mon_count -=1
    if @mon_count == 0
      @mon_owner = nil
      # HERE!!!
      @mon_mutex.unlock
    end
  end
``` 

It breaks the state of the monitor and it causes deadlock.



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