[ruby-core:94456] [Ruby master Bug#15992] An exception breaks monitor state and cause deadlock

Issue #15992 has been updated by nagachika (Tomoyuki Chikanaga).

Backport changed from 2.5: REQUIRED, 2.6: DONE to 2.5: REQUIRED, 2.6: REQUIRED

9557069299ac3b96691040a541afa65761a724ad is a follow up commit to ease memory consumption impact of f91879a7b548284c93743168acfd11e3d2aeefac. I think it should be backported too.

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Bug #15992: An exception breaks monitor state and cause deadlock
https://bugs.ruby-lang.org/issues/15992#change-80887

* Author: naruse (Yui NARUSE)
* Status: Closed
* 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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