[ruby-core:88860] [Ruby trunk Bug#14999] ConditionVariable doesn't reacquire the Mutex if Thread#kill-ed

Issue #14999 has been updated by Eregon (Benoit Daloze).


@larskanis Thanks for the report.
I also noticed it happens on RubyCI for Ruby 2.4 and earlier
(example: https://rubyci.org/logs/rubyci.s3.amazonaws.com/arch/ruby-2.4/log/20180905T105615Z.fail.html.gz)

I'm not sure what's best.

@normalperson Do you think we could backport the fix (or the logic behind the fix since the code changed quite a bit) to Ruby 2.4 and 2.3?
Interestingly enough, it seems to not fail on Ruby 2.5 at least in recent RubyCI builds.

For now, I'll mark the spec as ruby_bug on Ruby < 2.5.

----------------------------------------
Bug #14999: ConditionVariable doesn't reacquire the Mutex if Thread#kill-ed
https://bugs.ruby-lang.org/issues/14999#change-73899

* Author: Eregon (Benoit Daloze)
* Status: Closed
* Priority: Normal
* Assignee: normalperson (Eric Wong)
* Target version: 
* ruby -v: ruby 2.6.0dev (2018-08-16 trunk 64394) [x86_64-linux]
* Backport: 2.3: UNKNOWN, 2.4: UNKNOWN, 2.5: UNKNOWN
----------------------------------------
These specs reproduce the issue:
https://raw.githubusercontent.com/oracle/truffleruby/master/spec/ruby/library/conditionvariable/wait_spec.rb

I can commit them, but of course they will fail.
Feel free to just copy it to MRI's spec/ruby/library/conditionvariable/wait_spec.rb
(I would do it when synchronizing specs anyway)

I'd guess it's caused by the recent timer thread changes or so.
This spec works fine on 2.5.1 and older.

Just copy-pasting the spec out allows for a standalone reproducer:

~~~ ruby
m = Mutex.new
cv = ConditionVariable.new
in_synchronize = false
owned = nil

th = Thread.new do
  m.synchronize do
    in_synchronize = true
    begin
      cv.wait(m)
    ensure
      owned = m.owned?
      $stderr.puts "\nThe Thread doesn't own the Mutex!" unless owned
    end
  end
end

# wait for m to acquire the mutex
Thread.pass until in_synchronize
# wait until th is sleeping (ie waiting)
Thread.pass while th.status and th.status != "sleep"

m.synchronize {
  cv.signal
  # Wait that the thread is blocked on acquiring the Mutex
  sleep 0.001
  # Kill the thread, yet the thread should first acquire the Mutex before going on
  th.kill
}

th.join
~~~

Result on trunk:
~~~
The Thread doesn't own the Mutex!
#<Thread:0x0000565216f4ba28@cv_bug.rb:6 aborting> terminated with exception (report_on_exception is true):
Traceback (most recent call last):
	1: from cv_bug.rb:7:in `block in <main>'
cv_bug.rb:7:in `synchronize': Attempt to unlock a mutex which is not locked (ThreadError)
Traceback (most recent call last):
	1: from cv_bug.rb:7:in `block in <main>'
cv_bug.rb:7:in `synchronize': Attempt to unlock a mutex which is not locked (ThreadError)
~~~



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