[ruby-core:100680] [Ruby master Bug#17300] The Fiber scheduler does not work with ConditionVariable

Issue #17300 has been reported by Eregon (Benoit Daloze).

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Bug #17300: The Fiber scheduler does not work with ConditionVariable
https://bugs.ruby-lang.org/issues/17300

* Author: Eregon (Benoit Daloze)
* Status: Open
* Priority: Normal
* Assignee: ioquatix (Samuel Williams)
* ruby -v: ruby 3.0.0dev (2020-10-31T02:56:41Z master 4f8d9b0db8) [x86_64-linux]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
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When looking at replacing `kernel_sleep` by `blocking`, I found an independent bug.
ConditionVariable does not seem to work with the Fiber scheduler currently.
There is an existing test in https://github.com/ruby/ruby/blob/4f8d9b0db84c42c8d37f75de885de1c0a5cb542c/test/fiber/test_mutex.rb#L105-L140 on which I based this reproduction example.
The test should always have signalled==3, but the check is only > 1.
The test is also racy, as ConditionVariable#signal has no effect if no other Thread/Fiber is in ConditionVariable#wait.

Here is the reproduction, by default it runs without the scheduler. Pass it `scheduler` as an argument to use the test Scheduler.
I save the script under `test/fiber` for convenience.
```ruby
require_relative 'scheduler'

USE_SCHEDULER = ARGV.delete('scheduler')

mutex = Mutex.new
condition = ConditionVariable.new

signalled = 0

q = Queue.new

a = Thread.new do
  Thread.current.scheduler = Scheduler.new if USE_SCHEDULER
  
  body = -> do
    mutex.synchronize do
      3.times do |i|
        q << :ready
        p [:wait, i]
        condition.wait(mutex)
        raise unless mutex.owned?
        signalled += 1
      end
    end
  end
  
  USE_SCHEDULER ? Fiber.schedule(&body) : body.call
end

b = Thread.new do
  Thread.current.scheduler = Scheduler.new if USE_SCHEDULER
  
  body = -> do
    puts "Thread 2 starting"
    3.times do |i|
      q.pop # Only acquire Mutex once the other thread is in wait
      puts "Thread 2 locking Mutex"
      mutex.synchronize do
        p [:signal, i]
        condition.signal
      end

      sleep 1 # 0.1
    end
  end
  
  USE_SCHEDULER ? Fiber.schedule(&body) : body.call
end

a.join
b.join

p signalled
```

```
$ ruby condvar2.rb          
Thread 2 starting
[:wait, 0]
Thread 2 locking Mutex
[:signal, 0]
[:wait, 1]
Thread 2 locking Mutex
[:signal, 1]
[:wait, 2]
Thread 2 locking Mutex
[:signal, 2]
3
```

```
ruby condvar2.rb scheduler
Thread 2 starting
Thread 2 locking Mutex
[:wait, 0]
[:signal, 0]
# hangs
```



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