[ruby-core:103618] [Ruby master Bug#17827] Monitor is not fiber safe

Issue #17827 has been updated by Eregon (Benoit Daloze).


Fixed in https://github.com/ruby/ruby/commit/3a3b19b2bba49e5d6f1cf13764eb6dd701397be9.
This should be backported to 3.0 (I marked it as such).

I noticed something else while writing a test for this:

On 2.7:
```
ruby -e 'm=Mutex.new; m.lock; p 1; Fiber.new { m.lock }.resume'
1
Traceback (most recent call last):
	1: from -e:1:in `block in <main>'
-e:1:in `lock': deadlock; recursive locking (ThreadError)
```
On 3.0:
```
$ ruby -e 'm=Mutex.new; m.lock; p 1; Fiber.new { m.lock }.resume'           
1
-e:1:in `lock': No live threads left. Deadlock? (fatal)
1 threads, 1 sleeps current:0x000000000214f080 main thread:0x000000000214f080
* #<Thread:0x00000000021bb488 sleep_forever>
   rb_thread_t:0x000000000214f080 native:0x00007f05edc32b80 int:0   
	from -e:1:in `block in <main>'

$ ruby -e 'Thread.new {sleep}; sleep 0.1; m=Mutex.new; m.lock; p 1; Fiber.new { m.lock }.resume'
1
=> hangs
```
Same behavior on master for `Monitor.new` with my fix (`ruby -rmonitor -e 'm=Monitor.new; m.enter; p 1; Fiber.new { p m.enter }.resume'` => fatal, or hang if there is another Thread).

It would be nice to raise a ThreadError when detecting in Mutex#lock that the Mutex is owned by the same Thread but a different Fiber, if there is no Fiber scheduler.
That's more of a nice-to-have though.

----------------------------------------
Bug #17827: Monitor is not fiber safe
https://bugs.ruby-lang.org/issues/17827#change-91715

* Author: ioquatix (Samuel Williams)
* Status: Closed
* Priority: Normal
* Assignee: Eregon (Benoit Daloze)
* Backport: 2.6: DONTNEED, 2.7: DONTNEED, 3.0: REQUIRED
----------------------------------------
According to our discussion here https://github.com/rspec/rspec-support/issues/501 it seems like typical implementation of per-thread reentrant mutex is no longer valid and can lead to some deadlock situation.

```
#!/usr/bin/env ruby

require 'monitor'

def monitor_failure
	m = Monitor.new
	
	f1 = Fiber.new do
		m.synchronize do
			puts "f1 A"
			Fiber.yield
			puts "f1 B"
		end
	end
	
	f2 = Fiber.new do
		m.synchronize do
			puts "f2 A"
			# Fiber.yield
			f1.resume
			puts "f2 B"
		end
	end
	
	f1.resume
	f2.resume
end

monitor_failure
```



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